Biology 1 and 2

P530 [2020] By Nakapanka J Mayanja 0704716641
2020 form five biology notes

Table of Contents
TOPIC 1: LEVELS OF ORGANISATION AND DIVERSITY OF LIFE .......................................... 7
Syllabus extract.............................................................................................................................................. 7
IMPORTANCE OF STUDYING BIODIVERISTY .................................................................................. 9
SPECIMEN IDENTIFICATION AND KEYS ........................................................................................... 9
THE DOMAIN SYSTEM ........................................................................................................................... 10
THE FIVE KINGDOMS ................................................................................................................................ 10
VIRUSES ...................................................................................................................................................... 10
KINGDOM PROKARYOTAE/MONERA ............................................................................................... 12
KINGDOM PROTISTA ............................................................................................................................. 14
KINGDOM FUNGI ..................................................................................................................................... 16
KINGDOM PLANTAE (plants) ................................................................................................................ 18
ALTERNATION OF GENERATION................................................................................................... 19
KINGDOM ANIMALIA............................................................................................................................. 23
Sample questions ............................................................................................................................................. 32
REFERENCES ................................................................................................................................................ 35
TOPIC 2: CHEMICALS OF LIFE ........................................................................................................... 36
Syllabus extract................................................................................................................................................ 36
Introduction ..................................................................................................................................................... 39
INORGANIC CHEMICALS OF LIFE ......................................................................................................... 39
ACIDS AND BASES ................................................................................................................................... 39
MINERAL ELEMENTS............................................................................................................................. 40
WATER ........................................................................................................................................................ 43
THE ORGANIC CHEMICALS OF LIFE .................................................................................................... 47
VITAMINS................................................................................................................................................... 48
CARBOHYDRATES .................................................................................................................................. 49
LIPIDS (fats and oils) .................................................................................................................................. 58
PROTEINS................................................................................................................................................... 61
ENZYMES ....................................................................................................................................................... 70
REVISION QUESTIONS ............................................................................................................................... 79
REFERENCES ................................................................................................................................................ 85
TOPIC 3: INHERITANCE ......................................................................................................................... 86
SYLLABUS EXTRACT ................................................................................................................................. 86
NUCLEIC ACIDS ........................................................................................................................................... 88
Page 1 of 447
PROTEIN SYNTHESIS ............................................................................................................................. 96

CELL DIVISION............................................................................................................................................. 99
THE CELL CYCLE .................................................................................................................................... 99
MITOSIS ................................................................................................................................................ 100
MEIOSIS ................................................................................................................................................ 103
GENETICS .................................................................................................................................................... 109
MENDEL’S GENETIC EXPERIMENTS AND MONOHYBRID INHERITANCE ......................... 110
MENDEL’S FIRST LAW OF INHERITANCE................................................................................. 112
CO-DOMINANCE ................................................................................................................................ 119
INCOMPLETE DOMINANCE ........................................................................................................... 124
MULTIPLE ALLELES ........................................................................................................................ 125
Inheritance of ABO blood system ........................................................................................................ 126
DIHYBRID INHERITANCE ................................................................................................................... 129
INHERITANCE OF COMPLEMENTARY GENES ............................................................................ 132
MODIFICATION OF 9:3:3:1 PHENOTYPIC RATIO ......................................................................... 134
EPISTASIS............................................................................................................................................. 134
CROSSOVER VALUE AND CHROMOSOME MAPS .................................................................... 139
INHERITANCE OF SEX ......................................................................................................................... 142
Pedigree charts........................................................................................................................................... 147
VARIATION .................................................................................................................................................. 149
OUT OF CLASS EXERCISES ................................................................................................................ 158
TOPIC 4: CYTOLOGY, MICROSCOPY AND HISTOLOGY ......................................................... 177
CELL BIOLOGY .......................................................................................................................................... 178
TYPES OF CELLS.................................................................................................................................... 179
MICROSCOPY ............................................................................................................................................. 184
CELL STRUCTURE................................................................................................................................. 187
The nucleus ................................................................................................................................................ 193
Mitochondria ............................................................................................................................................. 194
Chloroplast ................................................................................................................................................. 195
Ribosomes................................................................................................................................................... 197
Endoplasmic reticulum ............................................................................................................................. 197
Lysosomes (suicide bag) ............................................................................................................................ 198
Golgi apparatus/body ................................................................................................................................ 200
Plant cell wall ............................................................................................................................................. 202
Flagella and cilia ........................................................................................................................................ 204
Page 2 of 447
Microbodies ................................................................................................................................................ 204

Vacuoles ...................................................................................................................................................... 204
Protoplasm ................................................................................................................................................. 205
Microtubules .............................................................................................................................................. 205
HISTOLOGY................................................................................................................................................. 207
HISTOLOGY OF PLANTS ..................................................................................................................... 207
MERISTEMS......................................................................................................................................... 207
PERMANENT TISSUES ...................................................................................................................... 207
THE VASCULAR TISSUE ...................................................................................................................... 210
HISTOLOGY OF ANIMALS .................................................................................................................. 214
EPITHELIAL TISSUE ......................................................................................................................... 214
CONNECTIVE TISSUE ....................................................................................................................... 220
SAMPLE QUESTIONS ................................................................................................................................ 225
REFERENCES .............................................................................................................................................. 229
TOPIC 5: MOVEMENT IN AND OUT OF CELLS ........................................................................... 230
Introduction ................................................................................................................................................... 230
SIMPLE DIFFUSION ............................................................................................................................... 231
FACILITATED DIFFUSION .................................................................................................................. 232
ACTIVE TRANSPORT ............................................................................................................................ 234
OSMOSIS ................................................................................................................................................... 236
Water potential ...................................................................................................................................... 237
Solute potential ( s) .............................................................................................................................. 238
Pressure potential ( p) .......................................................................................................................... 238
Osmosis and plant cells ......................................................................................................................... 239
Plant-water relations ............................................................................................................................. 241
Osmosis and animal cells ...................................................................................................................... 243
BULK TRANSPORT ACROSS THE CELL MEMBRANE................................................................. 245
Cytosis......................................................................................................................................................... 245
Endocytosis............................................................................................................................................. 245
Phagocytosis (cellular eating) ............................................................................................................... 246
Pinocytosis (cellular drinking).............................................................................................................. 246
Receptor mediated endocytosis ............................................................................................................ 247
Exocytosis ............................................................................................................................................... 247
REFERENCES .............................................................................................................................................. 255
Page 3 of 447
TOPIC 6: TRANSPORT IN LIVING ORGANISMS ................................................................................ 256

Need for a transport system .................................................................................................................. 258
BLOOD....................................................................................................................................................... 259
ERYTHROCYTES (Red blood cells) .................................................................................................. 259
LEUCOCYTES (white blood cells) ...................................................................................................... 260
BLOOD PLATELETS (thrombocytes) ............................................................................................... 261
TRANSPORT OF OXYGEN ................................................................................................................... 262
Oxygen tension and oxyhaemoglobin formation ................................................................................ 263
Effect of carbon dioxide on the oxygen dissociation curve (Bohr’s effect) ....................................... 264
TRANSPORT OF CARBON DIOXIDE ............................................................................................. 270
VASCULAR SYSTEMS IN ANIMALS .................................................................................................. 271
MAMMALIAN BLOOD CIRCULATION ......................................................................................... 275
THE MAMMALIAN HEART ................................................................................................................. 275
Initiation of the heart beat .................................................................................................................... 276
Intrinsic control of the heart beat ........................................................................................................ 279
Hormonal control of the heat rate........................................................................................................ 279
Control of the rate of the heart beat .................................................................................................... 279
BLOOD VESSELS .................................................................................................................................... 281
TOPIC 7: DEFENCE AGAINST DISEASES ...................................................................................... 286
Clotting of blood .................................................................................................................................... 286
Mechanism of immune responses......................................................................................................... 293
Cell mediated immune response. .......................................................................................................... 295
THE LYMPHATIC SYSTEM ................................................................................................................. 300
Vaccines .................................................................................................................................................. 301
BLOOD TRANSFUSION ..................................................................................................................... 301
RHESUS FACTOR (D-Antigens) ........................................................................................................ 302
UPTAKE AND TRANSPORT IN PLANTS ............................................................................................... 304
TRANSPIRATION.................................................................................................................................... 304
STOMATA ............................................................................................................................................. 310
LENTICELS .......................................................................................................................................... 314
WATER UPTAKE BY THE ROOTS ................................................................................................. 315
Vascular tissues...................................................................................................................................... 318
THE UPTAKE OF WATER FROM THE ROOTS TO THE LEAVES .......................................... 321
UPTAKE AND TRANSLOCATION OF MINERAL IONS ............................................................. 323
TRASLOCATION OF ORGANIC MOLECULES ........................................................................... 326
Page 4 of 447

Time days ........................................................................................................................................................ 334
REFERENCES .............................................................................................................................................. 344
TOPIC 8: EVOLUTION............................................................................................................................... 345
SYLLABUS EXTRACT ............................................................................................................................... 345
THEORIES FOR THE ORIGIN OF LIFE ................................................................................................ 346
THEORIES TO EXPLAIN THE MECHANISM OF EVOLUTION .................................................. 346
DARWINISM ........................................................................................................................................ 346
LAMARCKCISM.................................................................................................................................. 347
NEODARWINISM (Modern synthetic theory of organic evolution) ............................................... 348
EVIDENCE FOR EVOLUTION ............................................................................................................. 349
COMPARATIVE EMBRYOLOGY ........................................................................................................... 349
PALEONTOLOGY (The study of fossils) ............................................................................................... 350
CELL BIOLOGY ...................................................................................................................................... 351
TAXONOMY (CLASSIFICATION) ....................................................................................................... 351
COMPARATIVE ANATOMY ................................................................................................................ 352
COMPARATIVE BIOCHEMISTRY AND PHYSIOLOGY ............................................................ 354
BIOGEOGRAPHY (SPECIES DISTRIBUTION/GEOGRAPHICAL ISOLATION) ................... 355
INDUSTRIAL MELANISM................................................................................................................. 357
CROSS BREEDING/ ARTIFICIAL SELECTION ........................................................................... 357
RESISTANCE TO DRUGS AND PESTICIDES ............................................................................... 357
SELECTION .................................................................................................................................................. 358
NATURAL SELECTION ......................................................................................................................... 358
ARTIFICIAL SELECTION ..................................................................................................................... 362
SEXUAL SELECTION......................................................................................................................... 362
KIN AND GROUP SELECTION ........................................................................................................ 363
SPECIATION ................................................................................................................................................ 363
INTERSPECIFIC HYBRIDISATION ........................................................................................................ 363
MECHANISM OF SPECIATION .............................................................................................................. 364
EXTINCTION OF SPECIES ....................................................................................................................... 368
THE HARDY-WEINBERG PRINCIPLE .............................................................................................. 370
TOPIC 9: ECOLOGY................................................................................................................................... 376
SYLLABUS EXTRACT ............................................................................................................................... 376
Introduction ................................................................................................................................................... 378
2.0 CONCEPT OF THE ECOSYSTEM...................................................................................................... 381
Page 5 of 447
B.O.D (Biological oxygen demand) ...................................................................................................... 386

ECOLOGICAL SUCCESSIONS ............................................................................................................. 389
Energy flow through an ecosystem .......................................................................................................... 392
Energy budgets ...................................................................................................................................... 394
ECOLOGICAL PYRAMIDS ................................................................................................................... 395
BIOLOGICAL AND GEOCHEMICAL CYCLING (NUTRIENT CYCLING) ................................. 396
3.0 POPULATION AND NATURAL RESOURCES ................................................................................. 400
Population growth ..................................................................................................................................... 401
Population growth and survivorship curves ........................................................................................... 405
Determination of population size of organisms ...................................................................................... 407
4.0 INTERDEPENDENCE ........................................................................................................................... 412
Interaction within the populations ....................................................................................................... 412
Competition ................................................................................................................................................ 412
Predation .................................................................................................................................................... 414
Parasitism ................................................................................................................................................... 417
5.0 EFFECTS OF HUMAN ACTIVITIES ON ECOSYSTEMS .............................................................. 418
POLLUTION ............................................................................................................................................. 418
Chemical pest control ................................................................................................................................ 424
NATURAL RESOURCES ........................................................................................................................ 427
Conservation of natural resource................................................................................................................. 430
Page 6 of 447
TOPIC 1: LEVELS OF ORGANISATION AND DIVERSITY OF

LIFE
Syllabus extract
Specific objectives: The learner should be able to; Content
1.1 Diversity of living things
 Explain the principles of taxonomy.  Principles of taxonomy: identification, classification and
 Explain the principles of classification. nomenclature.
 Explain the importance of studying diversity.  Principles of classification: artificial and natural.
 List 3 criteria for classifying organism.  Importance of studies on diversity.
 State the hierarchy of classification according to Carl  3 criteria of classifying organisms: (morphology, anatomy,
Linnaeus. physiology)
 Hierarchy of classification according to Carl Linnaeus
(kingdom-phylum/ division-class-order-family-genus-
species). Binomial classification (scientific and local
name).
1.2 Classification Practical 
 Distinguish between organisms using the binomial  Differences between organisms using the Binomial
system of nomenclature. nomenclature.
 Construct simple biological keys.  Construction of simple biological keys.
 Explain the need to conserve biodiversity.  Importance of conserving biodiversity.
1.3 Viruses (Akaryotae) 
 Draw and label a diagram to show the structure of a  Structure of viruses
virus.  Characteristics of viruses
 Describe the general structure of a virus.  Economic importance of viruses
 Explain characteristics of viruses.  Methods of preventing the spread of viral diseases.
 Describe the economic importance of viruses.
 Suggest methods of preventing the spread of vital
diseases.
1.4 Kingdom Monera 
 Differentiate between bacteria and viruses.  Differences between bacteria and viruses.
 Make a labeled diagram to show the structure of  Structure of Bacteria.
bacteria.  Characteristics of bacteria: shape, cell wall, reproduction,
 Describe characteristics of bacteria. movements.
 State the role of bacteria in the environment.  Economic importance of bacteria in the environment.
1.5 Kingdom Monera. Practical
1 Draw, label and state the types of bacteria. 1 Types of bacteria.
2 State the role of bacteria in the production of daily 2 Role of bacteria in production of dairy products.
products. 3 Common bacterial diseases.
3 Name common bacterial diseases. 4 Methods of preventing them.
4 Describe methods of preventing the common bacterial
diseases.
1.6 Kingdom Protoctista 
 State characteristics of the Protoctista.  Characteristics of the Protoctista.
 Describe the structure of protozoa and Algae  Structure of Protozoa and Algae
 Outline the role of protozoa and Algae organisms in the  Economic importance of Protozoa and Algae, e.g.Amoeba,
environment. Euglena, Entamoeba, Paramecium, Trypanosome,
 Name common diseases caused by protocol Plasmodium.
 Culture methods of preventing spread of diseases  Common diseases caused by protozoa
caused by protoctists.  Methods to prevent spread of diseases caused by protozoa.
1.7 Kingdom Protoctista Practical 
 Prepare temporary mount of Spirogyra filaments.  Structure of the spirogyra
 Draw and label structure of Spirogyra as seen under a  Characteristics and structure of protozoa
light microscope
 Identify and draw protozoa from prepared slides.
1.8 Kingdom Fungi Practical 
Page 7 of 447
 State characteristics of fungi.  Characteristics of fungi (feeding, reproduction)

 State characteristics of Rhizopus of Mucor, yeast, and  Characteristics of Rhizopus of Mucor, yeast, and the
the mushroom. mushroom.
 State the economic importance of fungi  Economic importance of fungi.
 Name common fungal diseases.  Common fungal diseases.
 Describe the methods of preventing the spread of fungal  Methods of preventing the spread of fungal diseases.
diseases.  Use of yeast in brewing alcohol and bread making.
 Explain the use of yeast in brewing alcohol and bread
making.
1.9 Kingdom Fungi Practical 
 Prepare a temporary mount of yeast Mucor/Rhizopus.  Structure of yeast, Mucor/Rhizopus as seen under the light
 Draw and label structure of Rhizopus or Mucor, yeast, microscope.
and the mushroom.  Structure of the mushroom.
1.10 Kingdom Plantae
 Identify lower plants and higher plants using structural  Structural features of lower plants and higher plant.
features.  Plant divisions/ phyla: Bryophyte and Pteridophyta (Ferns)/
 Name the plant divisions phyla Filicinophyta, Coniferophyta, Spermatophyte.
 Outline briefly the characteristics and structures of the  Characteristics and structures of named plant drown/ phyla,
named plant domain/phyla Bryophyte, Filicinophyta, Coniferophyta, Spermatophyte
 State the role of plants in the environment (gymnosperms and angiosperms to class level).
 Economic importance of plants in the environment
1.11 Kingdom Plantae, Practical 
 Identify distinguishing structural features of plant  Structural features of lower plants; Bryophyta,
groups in lower plants. pteridophytes/ Filicinophyta.
 Identify distinguishing structural features of plant  Structural features of: higher plants; Coniferophyta,
groups in higher plants. Spermatophyta; (gymnosperins, angiosperms)
1.12 Kingdom Animalia 
 State characteristics of invertebrates and vertebrates  Characteristics of invertebrates and vertebrates.
 State the distinguishing structural features of organisms  Distinguishing structural features of the following phyla:
in different animal phyla. - Porifera
 Explain the role of animals in the environment - Coelenterate/ Cnidaria
 Discuss the welfare of domestic and wild animals. - Platyhelminthes
- Nematode
- Annelid
- Mollusca
- Echinodermata
- Arthropoda down to classes, Consider class insect
down to order
- Chordate down to vertebrate classes
 Economic importance of the animal groups
 Welfare of domestic animals (consider care and feeding)
and wild animals (Mention protection and conservation).
1.12 Kingdom Animalia Practical
1 Classify phylum Arthropod to class level using 1 Structural features of Arthropod organisms down to class
structural features. level.
2 Identify structural features of class insecta to order 2 Structural features of other animals other than arthropods.
level.
3 State distinguishing structural features of animal s
other than arthropod
Classification is defined as the grouping of organisms Systematics is the placing of organisms into groups
together basing on the features they have in common. basing on their similarities and differences.
Taxonomy is defined as the science of classification. Binomial nomenclature is the assigning of two Latin
names to each organism. The first name/word is the
Branches of taxonomy generic name and the second name/word is the specific
Nomenclature is the giving of names to organisms. name.
Page 8 of 447
In binomial nomenclature, the following rules are collected and the degree of similarity between different
observed; organisms is usually calculated by computers.
i. The generic name starts with the upper case NOTE: classification today is mostly natural and
(capital) letter while the species name starts with phylogenetic.
a lower case (small) letter.
ii. Unless written in italics, the two words must be IMPORTANCE OF STUDYING BIODIVERISTY
underlined separately e.g. Homo sapiens/ Homo The survival of humanity depends upon properly
sapiens. functioning ecosystems to maintain drinkable water,
breathable air and productive soils to grow food.
THE NEED TO CONSERVE BIODIVERSITY
1. Food. 75% of our food supply comes from just 12

plant species, and more than 90% of global
THE TAXONOMIC HIERARCHY livestock production comes from just 15 species of
This is the descending order in size of the taxonomic mammals and birds. That’s deceptive, though,
group. The order is Kingdom, Phylum (division in because those 27 species-along with many others
plants), Class, Order, Family, Genus and Species. that also provide food for humans couldn’t exist
without help from hundreds of thousands of lesser-
Each taxonomic group is called a taxon or taxa (plural). known species working behind the scenes e.g.
Each taxon possess a diagnostic feature i.e. features pollinators and wild relatives which are gene
which are unique (peculiar) to that group e.g. presence
reserves for the domesticated plants and animals
of the vertebral column is a diagnostic feature for
phylum Chordata. Fur is a diagnostic feature for class 2. Health. By having a diverse mix of plants, fungi
mammalia and feathers are peculiar to birds. and animals to eat, we ensure nutrition that buffers
What is a species? This is a group of organisms having our bodies against diseases and other hardships.
many common physical and other features and if
sexually reproducing, they can breed to produce fertile 3. Ecosystem services. These include clean air
offsprings. (oxygen from photosynthesis of forests as well as
phytoplanktons, these absorb carbon dioxide),
PRINCIPLES OF CLASSIFYING clean water (forests help soil absorb more water
ORGANISMS which reduces flooding, limits erosion, filter out
contaminants and refill aquifers), healthy soil
Artificial classification: this is based on one or a few
(many arthropods and microorganisms play
easily observable characteristics for simplicity and
important roles in maintain soil fertile), raw
convenience.
materials (biodiversity supplies a variety of
Natural classification: this considers natural materials from both wild and cultivated species).
relationships between organisms e.g. internal and
external features. The features considered include; 4. Resilience. Biodiversity provides insurance, it
allows for ecosystems to adjust to disturbances like
 Embryology extreme fires and floods.
 Physiology
 Biochemistry 5. Ethics, aesthetics and awe.
 Cell structure
 Behavior SPECIMEN IDENTIFICATION AND KEYS
Phylogenetic classification: this is based on A specimen key involves listing observable
evolutionary history (phylogeny) of organisms. characteristics of organisms and matching them with
Organisms belonging to the same group are believed to those features which are diagnostic in a particular
share a common ancestor. It bases so much on fossil group.
evidence.
The characteristics used in keys should be readily
Phonetic classification: this is based only on observable morphological characters. They may be
observable characteristics and all characters are qualitative e.g. shape or quantitave e.g. number of
considered to be of importance. A lot of data is segments. The characteristics must be constant for that
species and not subject to variations as a result of
Page 9 of 447
environmental influence, colour and size are highly THE FIVE KINGDOMS
discouraged.
Kingdom Monera:
Dichotomous key
Eubacteria - new bacteria -Prokarya
This is a simple diagnostic key in which pairs of
statements called leads, each dealing with a particular
Archaebacteria – old bacteria
characteristic is numbered e.g. 1, 2, 3, e.t.c. The paired celled organisms- Eukarya
statements of each lead should be contrasting and
mutually exclusive. Such that by considering them in Kingdom Fungi- multicellular fungi/yeast-Eukarya
order, a large group of organisms is broken down into Kingdom Plantae- photosynthetic plants - Eukarya
progressively smaller groups until the unknown
organism is identified. An example of a dichotomous Kingdom Animalia- animals from zygote-Eukarya
key for identifying arthropods is shown below,
1 a)b) Has 8 legs……………………..W

Has 6 legs………………………2
2 a)b) Has long antennae……………..X

Has short antennae……………..3
3 a)b) Has proboscis………………….Y

Has mandibles…………………Z
THE DOMAIN SYSTEM
The domain is the most inclusive taxonomic
category; larger than a Kingdom. The three
domains as proposed by Woese, are
a. Bacteria, which includes kingdom Eubacteria VIRUSES
b. Archaea, which includes Kingdom Viruses do not fit in any of the above kingdoms because
Archaebacteria they are on the border of living and non-living things.
c. Eukarya, which includes Kingdom Protista, Viruses have a simple structure consisting of a small
Fungi, Plantae and Animalia. piece of nucleic acid either DNA or RNA which in most
viruses is surrounded by a protein or a lipoprotein.
Characteristics of viruses
i. They lack a cellular structure i.e. they are acellular
ii. They are the smallest living things 20-300nm in
diameter
iii. They are obligate endoparasites i.e. they can only
live parasitically inside other cells.
iv. They depend on host cells for reproduction
v. Viruses are highly specific i.e. each virus
recognises and infects a particular host.
vi. Most viruses enter their hosts by phagocytosis and
pinocytosis
Reasons why viruses are considered to be
living things
a. They possess genetic material

b. They can mutate and hence evolve
c. They carry out protein synthesis in host cells
Page 10 of 447
d. They are capable self-replication when inside HIV is spherical and about 1000nm in diameter. The
host cells core region contains 2 molecules of single stranded
e. They can transmit characteristics to the next RNA and reverse transcriptase enzyme surrounded by a
generation cone shaped protein capsid. The capsid is enclosed by
an envelope composed of a lipid and glycoprotein.
Reasons why viruses are considered to be The reverse transcriptase enzyme converts single
non-living things stranded RNA into double stranded DNA copies. HIV
a. They can be crystallised is referred to as a retrovirus because the enzyme
reverse transcriptase, found in retroviruses, catalyses
b. They lack enzyme systems
the conversion of viral RNA into DNA i.e. reverse
c. They cannot metabolise unless they are inside
transcription. The viral DNA made is then inserted into
host cells
the host’s DNA where it directs the production of more
d. viral properties.
Generalised structure of a virus
The envelope contains glycoproteins which bind
specifically to helper T-cell receptors, enabling the
virus to enter the helper T-lymphocytes.
Examples of viral diseases;
a. In plants
i. Cassava mosaic disease
ii. Tobacco mosaic disease
iii. Tomato bush stunt disease
iv. Southern bean mosaic disease
b. In animals
i. Small pox
ii. The Acquired Immuno Deficiency
Syndrome (AIDS)
Core This is the inner region in which the iii. Rabies
genetic material (DNA or RNA) is found.
iv. Measles
The DNA or RNA may be single v. New castle disease
stranded or double stranded Economic importance of viruses
Useful roles
Capsid This is the protective coat of protein
surrounding the core. 1. In preparing antidotes/ vaccine: Pox, mumps,
polio, jaundice e.t.c. diseases can be controlled by
The Capsid is made up of subunits called penetrating using or dead virus in human body as
capsomeres. vaccines
2. In controlling harmful animals and
Envelop This is found only in some large viruses
insects: Some animals and insects which are
Structure of HIV harmful for humans can be controlled by some
special virus
3. Control of disease: T2 bacteriophage virus saves
humans from dysentery by spoiling some harmful
bacteria, like, e-coli.
4. In laboratory: Virus is used in lab, as the simplest
living model. In the research of genetics virus used.
It is an important subject in genetic engineering.
5. In the evidence of evolution: Virus plays a vital
role to acquire knowledge about the trend of
evolution and the process of formation of living
organisms because virus contains both living and
non-Iiving characteristics.
Page 11 of 447
Harmful roles: BACTERIA

6. Virus destroys plenty of bacteria useful for humans They are the smallest unicellular organisms and they are
Different diseases like common Cold, Influenza, the most abundant.
Mumps, Pox, Polio, Yellow fever, Harpies, Aids Generalised structure of a bacteria
etc. are caused by the attack of virus.
State the ways of spreading viral diseases
1.
2.
Methods of preventing spread of viral diseases
1. Vaccination: Vaccines are not available for all
viruses. For those that there are, public policy
decisions must be made as to whether at-risk
populations are targeted or whether universal
vaccination is pursued.
2. Hygiene and Sanitation: Proper waste water
treatment is important in keeping viruses out of the
water supply since virus contamination in sewage
is between 103 and 104 particles per liter.
3. Vector control: Mosquito control is often more
effective than vaccination given the sporadic nature
of most arboviral (arthropod-borne) diseases.
4. Lifestyle changes: For diseases transmitted
sexually and by intravenous drug use, lifestyle
change should change transmission patterns if it
can be implemented.
5. Eradication:
Not all viral disease are eradicable. The
necessary features for eradication are:
 An effective vaccine that optimally doesn't CLASSIFICATION OF BACTERIA
require a cold chain and is easily administered
 No animal reservoir This is based on structural and metabolic features.
 Lack of recurrent infection Classification by shape
 One or a few stable serotypes
There are four main shapes of bacteria and they are as
 No infectivity before symptoms and no
follows;
unapparent infections, making early
containment possible a. Spherical shape (cocci, singular = coccus)
They may be clusters e.g. Staphylococcus aureus which
KINGDOM PROKARYOTAE/MONERA causes boils and food poisoning.
Prokaryotes are organisms whose genetic material is not
bound by a nuclear membrane.
All members are unicellular and they belong to two They may occur in pairs enclosed by a capsule,
main groups; diplococci e.g. Diplococcus pneumoniae which causes
pneumonia.
a. Archaea
This group contains organisms that grow under
extreme conditions e.g. halophiles which grow
under extremely high salt concentration
They may occur in chains, streptococci e.g.
b. Hyperthermophiles
Streptococcus thermopiles which gives yoghurt the
This group contains organisms that grow under
creamy flavor
very high temperatures.
Page 12 of 447
b. Rod shaped (bacilli, singular = bacillus) Nitrobacter O2+NO2- NO3 + Energy

They may occur as single rods e.g. Escherichia coli O2
which lives in the guts of humans and Bacillus anthrax b. Heterotrophic bacteria
which causes anthrax. They feed on already made organic food but in different
ways.
Chemo-heterotrophic bacteria obtain energy from
chemicals in food.
They may occur in chains e.g. Azotobacter which fixes Saptrotrophic bacteria obtain their food from dead
nitrogen in the soil. and decayed organic matter.
Such bacteria secrete enzymes into the food, and absorb
the soluble products of extra cellular digestion with the
c. Curved or spiral shaped saptrotrophic body for assimilation.
Spiral shaped bacteria include Spirillum species Parasitic bacteria live on other organisms (hosts) from
which they obtain food as the host suffers harm.
Mutualistic bacteria live in close associations with
other organisms e.g. in the root nodules of legumes
Curved shaped bacteria include the coma shaped Note: Escherichia coli contribute vitamins B and K
(vibrios) bacteria such as Vibrio cholera which causes groups. Rhizobium fixes nitrogen into the plants as it is
cholera. provided with a shelter.
Classification by staining reaction
Gram positive bacteria; they stain purple with a gram

d. Filamentous bacteria stain. The cell wall has an extra outer membrane made
This group includes Actinomyces which occur in the u of lipopolysaccharides.
mouth and may cause dental caries. Gram negative bacteria; they stain pink with a gram
Classification by method of respiration stain. Their cell wall lacks an extra outer membrane
which is made out of lipids and polysaccharides. The
a. Aerobic bacteria
outer membrane gives them protection against penicillin
These bacteria require oxygen for respiration. Obligate
and lysozymes.
aerobes cannot survive without oxygen but facultative
aerobic bacteria can survive in the absence of oxygen. ECONOMIC IMPORTANCE OF BACTERIA
b. Anaerobic bacteria
1. They are cultured for research purposes e.g.
They respire without oxygen and obligate anaerobes are
genetics studies
killed in the presence of oxygen. Facultative anaerobic
2. They facilitate the making of foods like yoghurt,
bacteria can use oxygen but can respire without it.
cheese and vinegar
Classification by methods of nutrition
3. They are used for making antibiotics, amino acids
a. Autotrophic bacteria and enzymes.
These bacteria manufacture their own organic food 4. In humans, vitamin K and B complex are produced
from carbon dioxide. by the symbiotic bacteria (E. Coli) while in animals
Photoautotrophic (photosynthetic) bacteria use energy it is used to break down cellulose.
of sun light to convert carbon dioxide into 5. They cause decomposition of dead organic matter,
carbohydrates. Examples include; the blue-green hence enabling their disposal.
bacteria, sulphur bacteria and cyano bacteria. 6. They take part in nutrient recycling e.g. the
Chemoautotrophic (chemosynthetic) bacteria use nitrogen cycle, carbon cycle and the phosphorous
energy from chemical reactions to convert carbon cycle.
dioxide into carbohydrates. Inorganic substances such 7. On the other hand, bacteria cause food to get spoilt
as ammonia, methane and hydrogen sulphide are 8. Bacteria like Thiobacillus and Disulphovibrio
oxidized to release energy. produces sulphuric acid which destroys
Examples include; underground metal pipes.
Nitrosomonas NH4+ NO2- + Energy
Page 13 of 447
9. Manufacturing processes e.g. making soap PHYLUM RHIZOPODA

powders, tanning leather, making linen.
Examples include;
Examples of bacterial diseases i.
Amoeba proteus which lives in fresh water
1. ii.
Entamoeba histolytica which causes amoebic
dysentery
2.
Characteristics
3.
 They are unicellular and bear pseudopodia
Methods of preventing common bacterial diseases (false feet) which enables movement and
phagocytosis
1.
 They reproduce asexually
2.  They feed heterotrophically

3. Diagram of Amoeba
Differences between bacteria and
viruses
Viruses Bacteria
They are very small They are larger in size as
(ultra-microscopic) compared to viruses
(microscopic)
Non-cellular (acellular) Single-celled
Have no metabolism of Have metabolism of their

their own own
Do not grow and do not Grow in size and divide

divide to produce more bacteria
Take no food by any Take food by adsorption

method Functions of the parts
Command the host cell Can reproduce by their The mitochondria is used in the production of energy
to reproduce virus own for the contractile vacuole
Can be crystallised Cannot be crystallised The cytoplasm is the place where all the important
All produce diseases in Some are harmless, some chemical reactions take place.
man, animals or plants useful and some are The contractile vacuole is used for osmoregulation
disease causing
The nucleus is essential for directing activities
Either DNA or RNA is Both DNA and RNA
present in virus body present in bacteria body PHYLUM CILIOPHORA (CILIATES)
Virus is a true parasite Bacteria is parasite of Examples include Paramecium, Stentor, Vorticella,
saprophytic or
Didinia e.t.c.
photosynthetic
Characteristics
KINGDOM PROTISTA  They are unicellular
Protists are eukaryotes and they may be unicellular or  They feed heterotrophically
multicellular. They are placed under several phyla but  Their cilia has a 9+2 tubule arrangement
those of much importance at this level include the  The cilia collects food and enable locomotion in
following. water
 Their habitat is fresh water and marine water
Page 14 of 447
 They have two types of nuclei, the larger macro PHYLUM APICOMPLEXA (sporozoans)
nucleus which controls all cell metabolic activities
Members include plasmodium which causes malaria in
and the micro nucleus which controls sexual
humans
reproduction called conjugation.
 The macro nucleus is polyploid i.e. it has more than Characteristics
two sets of chromosomes and the micro nucleus is
diploid i.e. it has two sets of chromosomes.  they are unicellular
Diagram of a paramecium  they are heterotrophic
 they lack locomotory structures
 they are spore producing parasites of animals
 they reproduce sexually and asexually
 their lifecycles are complex involving several
animal hosts
Life cycle of plasmodium
An infected anopheles mosquito bites a person,

injecting plasmodium sporozoites in its saliva. The
sporozoites enter the person’s liver cells. After several
days, the sporozoites undergo multiple divisions and
become merozoites, which use the apical complex to
PHYLUM ZOOMASTIGINA (Flagellates) penetrate red blood cells. The merozoites divide
asexually inside the red blood cells. At intervals of 48
Examples include trypanasoma which causes
or 72 hours (depending on the species), large numbers
trypanasomiasis (sleeping sickness), trychomonas
of merozoites break out of the blood cells, causing
Characteristics periodic chills and fever. Some of the merozoites infect
other red blood cells. Some merozoites form
 They bear flagella for locomotion gametophytes. Another anopheles mosquito bites the
 They are heterotrophic infected person and picks up plasmodium gametophytes
 They are unicellular along with blood.
 The reproduce both asexually and sexually Gametes form from the gametophytes; each male
 They bear a 9+2 tubule arrangement gametophyte produces several slender male gametes.
 Fertilisation occurs in the mosquito’s digestive tract,
PHYLUM EUGLENOPHYTA and a zygote forms. An oocyst develops from the
zygote in the wall of the mosquito’s gut. The oocyst
The only member is euglena which lives in an aquatic releases thousands of sporozoites, which migrate to the
environment. mosquito’s salivary glands.
Characteristics PHYLUM CHLOROPHYTA (green algae)
 They are mostly unicellular The members include volvox, chlorella and spirogyra
 They reproduce asexually
 They move by flagella Chlorella which is a unicellular non filamentous alga
that lives in fresh water ponds
 Some are photosynthetic while others are
heterotrophic or autotrophic Chlamydominas which is a motile unicellular algae
Characteristics
 They contain chlorophyll and therefore they are

photosynthetic
 Their cell walls contain cellulose
 They store starch
 They reproduce sexually and asexually
 Spirogyra is a filamentous algae that lives in fresh
water ponds
Page 15 of 447
 It produces agar which is extracted from them for

laboratory purposes
PHYLUM OOMYCOTA
Includes peronospora which grows on grapes and
pythium which causes late potato blight and tomato rot
They are characterized by production of spores that bear

flagella. Such spores are produced both sexually and
asexually.
These are referred to as the lower fungi
Spirogyra is a filamentous algae that lives in fresh water

ponds
PHYLUM PHAEOPHYTA ECONOMIC IMPORTANCE OF ALGAE

These are the brown algae. The members include Fucus,
1. They can be used as fertilisers in farms
Laminaria and Ascophyllum.
2. The carry out photosynthesis in oceans which:
Characteristics  Provide food for other organisms
 Release oxygen
 They posses chlorophyll and therefore carryout
 Reduce carbon dioxide which would cause
photosynthesis
acidity in water
 They are multicellular
3. Some of their products are useful in various
 They are marine and are therefore called the sea industrial processes
weed 4. Algae blooms result in severe pollution of water
 They contain a brown pigment called fucoxanithin bodies.
which gives them a brown colour 5. Chlorella can be cultivated to provide Single Cell
Protein (SCP) for humans and animal consumption
PHYLUM RHODOPHYTA
KINGDOM FUNGI
These are red algae and members include chodris.
Characteristics that distinguish them from
Characteristics plants
 It is marine  Carbohydrates are stored as glycogen but not as

 It contains chlorophyll hence it carries out starch
photosynthesis  Their cell walls contain chitin but not cellulose
 It contains a red pigment called phycoerythin  They have no chlorophyll hence they don’t carry
 It also contains a blue pigment called phycocyanin out photosynthesis
Page 16 of 447
 They reproduce by spores that lack flagella PHYLUM ASCOMYCOTA

 They carry out heterotrophic nutrition as parasites
Members include Penicillium, Aspergillus and Yeast
and saprophytes on microorganisms.
sacchromycetes.
 Their bodies are usually made up of a mycelium of
thread like multi nucleate hyphae without distinct Characteristics
cell bodies (aseptate) or they may have cross walls
 Penicillium species form blue; some are green or
(septate)
sometimes yellow moulds on bread, decaying fruit
 They undergo nuclear mitosis i.e. their cytoplasm,
nuclear membrane and cell membrane never e.t.c.
divided  Their hyphae have cross walls called septa,
 Fungal mitochondria have flattened cristae therefore Penicillium is septate
(whereas plants have tubular ones)  It reproduces asexually by means of spores called
conidia formed at the tip of special hyphae called
conidiophores.
 Penicillium produces penicillin which is an
antibiotic, during aerobic respiration.
CLASSIFICATION OF FUNGI
PHYLUM ZYGOMYCOTA
Members include mucor and rhizopus (bread mould).
They live in damp organic matter e.g. bread
Characteristics of the yeast cell

 It has oval shaped cells
Note;
 It is unicellular
a. The sporangium is black when ripe and colour less  Reproduces by budding
when immature. It produces spores for asexual  Yeast produces ethanol during anaerobic
reproduction respiration
b. The sporagiophore is a vertically growing hypha
that bears the sporangium.
c. Rhizoids which are root like structures
Page 17 of 447
7. Fungi causes decomposition of stored food and

deterioration of natural materials like leather
8. Some fungi cause plant diseases e.g. powdery
mildew caused by Erysphiegraminae
9. Some are poisonous to man
10. They cause skin irritations e.g. ringworms
11. They are eaten as food e.g. mushrooms
KINGDOM PLANTAE (plants)

Characteristics
 Their cell walls contain cellulose

 They reproduce both sexually and asexually
 They are multi cellular
 They are photosynthetic except for some parasites
that lack chlorophyll
 They have alternation of generations i.e. the
haploid and diploid generations alternate in the
lifecycle.
PHYLUM BRYOPHYTA (bryophytes)

The members include mosses and liverworts. The
members live in damp shady soils or tree logs.
Bryophytes are the smallest land plants and they are

thought to have evolved from green algae.
Characteristics
 They lack vascular tissues

PHYLUM BASIDIOMYCOTA  They lack true roots, stems or leaves
 Their body is a thallus which is differentiated into
Members include mushrooms, toad stools, puff balls simple “leaves and stems”
and rusts. Rusts attack cereal crops  Alternation of generation occurs and the
Characteristics gametophyte generation is dominant
 The gametophyte is anchored by thallus rhizoids
 They reproduce asexually by sporulation which grow from the stem.
 They have septate hyphae NOTE: Water and mineral salts are absorbed by the
 whole plant surface because the plant surface lacks a
ECONOMIC IMPORTANCE OF FUNGI cuticle; therefore water uptake occurs by osmosis.
1. They cause decomposition of sewage and organic Phylum bryophyta contains two main classes;
material in soil
1. Class hepaticae (liverworts)
2. Penicillium and Aspergillus form antibiotics during
2. Class Musci (mosses)
aerobic respiration
3. Yeast forms alcohol during anaerobic respiration
4. Yeast is used in bread production
5. Fermentation of Aspergillus forms citric acid used
in lemonade formation
6. Used for experimental purposes especially in
genetic investigations
Page 18 of 447
EXTERNAL FEAUTRES OF A MOSS When mature, the antheridia shade their sperms
(antherozoids) into the archegonia aided by the rain-
splash.
The haploid biflagellate sperms fuse with haploid eggs

(ospheres) to form a diploid zygote (oospheres)
The zygote develops into sporophytes which attach and

survive on the gametophyte
When mature, the sporophyte produces haploid spores

by meiosis. The spores are released by splitting of the
spore capsule when dry.
When the spores land on moisten soils, they germinate

into a protonema which later develops into a new
diploid gametophyte
ALTERNATION OF GENERATION
This is the occurancy of two or more generations within
the lifecycle of an organism, a haploid gametophyte and
a diploid sporophyte.
PHYLUM PTERIDOPHYTA
(Filicinophyta or the ferns)
Description of alternation of
generation in a bryophyte like a moss Members include; Pteridium and Dryopteris
A moss consists of two distinct forms in its lifecycle Characteristics

i.e. the haploid gametophyte which is dominant and the
 The vascular tissue (xylem and phloem) are
diploid sporophyte
present.
The antheridia (sperm producing organ) and archegonia  The leaves are relatively large and are called
(egg producing organ) may be located on the same plant fronds. The large surface area of the leaves
or different plants. increases the photosynthetic surface of the plant
Page 19 of 447
 Spores are produced in sporangia (singular;

sporangium), usually in clusters called sori (sorus,
singular)
 Alternation of generation occurs and the
sporophyte is dominant
 The gametophyte is reduced to a small simple
prothallus
 The sporophyte generation posses true roots, stems
and leaves.
The roots penetrate the soil to absorb water
and dissolved mineral salts
 On rupturing, the ciliated sperms from the

antheridia swim towards the fertile eggs in the
archegonia
 The zygotes develops into sporophytes
COMPARISON BETWEEN A MOSS AND

A FERN
ALTERNATION OF GENERATIONS IN A Similarities
FERN (Pteridophyte/filicinophyte)
 Both form spores
A fern consists of two distinct forms in its life. The  Both grow in damp soils/ organic matter
diploid sporophyte, which is the dominant stage and the  Alternation of generation occurs in both
haploid gametophyte
 In both the gametophyte is anchored by the
 The diploid spore (mother cell) inside the sporangia
rhizoids
divide
Differences
 When mature, the protecting covering (indusium),
shrinks and catapults the spores of the sporangia Moss Fern
 The spores germinate into a heart-shaped prothallus
No vascular tissue Vascular tissue present
which is the gametophyte
 Prothallus bears antheridia which produces haploid No sorus Sorus present at leaf
sperms and archegonia which produces eggs by underside
mitosis Spore capsule present No spore capsule
Saprophyte is attached to Gametophyte is attached
gametophyte to sporophyte
It lacks true roots, leaves True roots, stems and

and stems leaves are present
Page 20 of 447
Rhizoids present No rhizoid SIGNIFICANCE OF ALTERNATION OF

Gametophyte not heart Gametophyte is heart
GENERATION
shaped shaped 1. Spores cause rapid multiplication of species
Leaves are simple and Leaves are relatively 2. Different habitats of the ecosystem are exploited by
small large the different generations
3. It enables plants to cope with adverse
environmental conditions
COMPARISON OF ALTERNATION OF 4. Reduces chances of extinction
GENERATION BETWEEN MOSSES AND 5. Gametes are formed by meiosis which brings about
FERNS genetic variations
Similarities
PHYLUM CONIFEROPHYTA
 Spore formation occurs in spore bearing sporangia
Members include Cedars, Horches and Christmas tress
 Sporophytes are diploid while gametophytes are
i.e. Firs and Spruce
haploid
 Spores form by meiosis whereas gametes are Characteristics
formed by mitosis
 They bear cones in which spore producing
 Sexual and asexual reproduction occurs sporangia and seeds develop
 Male gametes are motile while eggs are non-motile  They lack fruits and flowers
 In both there’s only one dominant stage  The seed is naked i.e. it is not enclosed by the
 The gametophyte bears the archegonia ovary wall.
 Sperms formed in the antheridia are brought into  Leaves are usually needle-like with a thick waxy
contact with the eggs by some mechanism cuticle
Differences Economic importance
Moss Ferns 1. A source of soft wood for timber

2. Pine nuts are used in cooking
The sperms are The sperms are ciliated 3. Spruce and firs are used as Christmas trees
biflagellate
Sporophytes grow on the Sporophyte is self- PHYLUM ANGIOSPERMOPHYTA

gametophyte supporting
This phylum includes all flowering plants
Spores germinate into a Spores germinate
Characteristics
protonema first and then directly into a
into a gametophyte gametophyte  They flowers in which sporangia, spores and
seeds develop
Gametophyte is a Saprophyte is a
dominant generation dominant generation  The seeds are enclosed in an ovary
 After fertilisation, the ovary develops into a
There is much There is less fruit
dependency on water for dependency on water,  There are two classes of Angiospermatophyta
growth, spore dispersal only being used for  Monocotyledon
and gamete transfer gamete transfer and
 Dicotyledon
spore germination
Monocots Dicots
Gametophytes may or Gametophytes always
Embryo sac has one Embryo sac has two seed
may not bear both sexual bears both sexual
seed leaf (cotyledon) leaves (cotyledons)
reproductive organs reproductive organs
Have scattered vascular Have a ring of vascular
bundles in the stem bundles in the stem
Page 21 of 447
Flower parts are usually Flower parts are usually 4.Obtaining gases for respiration
in 3’s or multiples of 3 in 4’s or 5’s or multiples 5.Movement of the reproductive gametes
of 4 or 5 6.Environmental variables such as light
intensity, temperature, pH e.t.c.
Calyx and corolla are Calyx and corolla are
Summary of adaptations of seed bearing
not usually easily easily distinguishable
plants to life on land
distinguishable
1. Leaves possess stomata for gaseous exchange
They are usually wind They are often insect
pollinated pollinated 2. Leaves and stems are covered by a waxy cuticle
which minimises water loss
Have narrow leaves Have broad leaves with 3. They possess true roots which enable water and
with parallel venation network venation dissolved mineral salts to be absorbed
4. They undergo secondary growth which enable seed
bearing plants to compete effectively for light and
Comparison between conifers and other resources
angiosperms. 5. The fertilised ovule (seed) is retained for sometime
on the parent plant (sporophyte) from which it
Similarities
obtains protection and food before dispersal.
1. Both bear seeds 6. Fertilisation is not dependent on water therefore
2. Sporophyte generation is dominant reduces necessity for water inside the sporophyte
3. which is well adapted for terrestrial life.
Differences The adaptations above may also be considered as the
advantages of seed bearing plants over mosses and ferns
Angiosperms Conifers
Have flowers and Do not have flowers
Produce seeds enclosed Seeds unprotected by an
within a carpel ovary or fruit
Leaves are flat Leaves are scale-like
Xylem contains vessels Xylem only contains
tracheids, but not vessels
Phloem contains sieve Phloem does not contain
tubes with companin sieve tubes with
cells companin cells
Both male and female Reproductive structures
reproductive structures occur in the cones
occur in the flower
Flowers can be unisexual Cones are always
or bisexual unisexual
Ovules are covered by Ovules are attached to Economic importance of plants in
the ovary the megasporophylls
Do not produce Have archegonia
the environment
archegonia 1. Unlike animals, plants synthesize their own food
Sperms do not contain Sperms have flagella via photosynthesis, which uses carbon dioxide and
flagella water to form carbohydrates with the help of
Under go double Do not under double sunlight and releases oxygen and energy into the
fertilisation fertilosation environment, thereby becoming the source of
Endosperm and plant Endosperm and plant oxygen that animals and humans breathe in to
body are triploid body are haploid sustain life.
Seeds are covered by a Seeds are naked • Humans and other living things on earth depend on
fruit plants for their food and energy.
• Plants along with the help of bacteria and other
organisms to fix the minerals and inorganic elements as
Challenges or problems faced by plants foods, hence known as primary producers.
• Primary herbivores that feed on plants incorporate the
1. Desiccation/ dry out
food into their body.
2. Support in air/ on land • Carnivores (secondary consumers) feed on these
3. Obtaining nutrients primary consumers.
Page 22 of 447
• Some carnivores feed on these secondary consumers This is the body which when cut, may produce halves
and become tertiary consumers. which are mirror (identical) images of each other.
2. Food Production/Agriculture Bilateral symmetrical body
Most of the foods that we consume are plant products
This is the body which can be divided into two identical
only: e.g. wheat, rice, corn, vegetables, nuts, oils,
beverages, and fruits. Agriculture also forms a way of halves along one plane only.
earning money for farmers. Radial symmetrical body
3. Plants in Industry This is the body which can be cut along more than one
Several industrial products are derivatives of plants: plane to produce halves that are identical to each other.
hemp, cotton, linen, rubber, furniture, paper pulp, and 6. Asymmetrical body
components to be used in other industries such as tannin This is a body which cannot produce halves that are
in leather industry; essential oils in soaps, perfumes,
mirror images of each other if cut along any plane.
and shampoos; and lubricants for automotive industry.
4. Plants as Medicines LEVELS OF ORGANISATION
Since ancient era, plants have been used in the
medicinal field to cure various diseases and conditions, Four levels of organisation are recognised;
and still, it is increasing. Herbs form the major sources 1. Unicellular level (single cell
of medicinal compounds in pharmaceutical industries.
organisation)
5. Plant Fossils as Fuel
Even the plant fossils are used by humans as source of Protists have all the functions which are carried out by
fuels: coal and petroleum. an organ system being performed by a single organelle
The economic importance of plants is almost found in in the cell. Such organisms include paramecium,
every aspect of the planet and other living things. amoeba plasmodium e.t.c.
Destruction of plants leads to ecological imbalance and 2. Tissue level of organisation
in turn survival of any organism on the planet. Hence,
These are primitive multicellular animals in which
saving the existing trees and planting more trees are
vital for the very existence of living organisms and physiological processes are carried out mainly by
protecting the environment. isolated cells and tissues. Apart from reproductive
organs, there are no structures that can be regarded as
KINGDOM ANIMALIA organs but most of the cells are integrated to form
tissues.
General characteristics
Such animals represent a stage in evolution preceding
 Their cells lack cell walls the development of organs and organisms which are the
 Most can move from one place to another i.e. they characteristics of higher forms.
are motile Tissue level is considered to be between the colonial
 They are multicellular eukaryotes and unicellular levels of organisation.
 They have a nervous common system except the Tissue level of organisation includes animals such as
sponges hydra.
Definition of terms 3. Colonial level of organisation

These organisms have different types of cells each
1. Tissue carrying out a different function. They are therefore
This is a group of cells, often similar in structure and regarded as colonies of single cells rather than
origin, operating together to perform a specific function multicellular individuals e.g. sponges
2. Tissue differentiation 4. Organ level of organisation
This is the specialisation of tissue for different functions Plants, mammals and the majority of animals have their
3. An organ functions carried out mostly by organ and organs
This is the structural distinct part of the body which systems
usually performs a particular function. Advantages and disadvantages of
An organ is made up of similar types of tissue which
unicellular level of organisation
are highly organised and have structural relationship
with each other. Advantages
4. Organism
1. Their small size enables living in a variety of
This is the interrelationship of different organ systems
habitats
which together perform a specific function
2. There is less food intake
5. Symmetrical body
Page 23 of 447
3. There is no need for the development of complex Transverse section through the body of Cnidarians
excretory organs since they take in less food.
4. No necessity for development of complex
circulatory and gaseous exchange structures since
simple diffusion combines with their large surface
area to volume ratio
5. There’s no need for development of complex
support systems like cartilage, bones, xylem e.t.c.
Disadvantages
1. Predators
Advantages and disadvantages of
multicellular level of organisation
 They have nematoblasts (stinging cells) which
Advantages when touched (stimulated) release a chemical
which can be used to capture prey or used to
1. Worn out cells are easily replaced by cell division
defend against predators
2. Multicellularity allows tissue specialisation which
Nematoblasts occur in the ectoderm and when touched,
increases efficiency in performing body functions
can inject toxins into the prey/ predator which results
3. They have complex physiological mechanisms into paralysis of the small animals.
which enable the maintenance of a relative constant
internal environment The structure of a body wall of hydra
4. They have a larger complex support system which
increase the chances of catching prey but also
reduces chances of predation
5. They have an efficient sensory system due to tissue
specialisation which enables animals to escape
from predators quickly.
Disadvantages
1. They require large quantities of food

2. They require specialised locomotory structures to
enable motion
3. They produce a large quantity of waste products
hence a necessity for development of complex
excretory systems  They are radially symmetrical i.e. the body can be
4. They have a small surface area to volume ratio that divided into equal halves by more than one straight
requires development of transport systems since line/plane which passes through the central body
simple diffusion cannot supply enough nutrients to  They exhibit polymorphism i.e. individual cells
the animal have specialised shapes with different functions
PHYLUM CNIDARIA (Cnidarians) Polymorphism is the existence of the cell organisms in
a number of morphologically distinct forms.
Members include Hydra, Obelia, sea anemone,
Portuguese man of war and Jelly fish.  They have tentacles which bear stinging cells
called nematoblasts
Characteristics
Hydra belongs to the tissue level of organisation which
enables cells to act together in a relatively coordinated
manner so as to carry out various functions effectively.
 They are diploblastic animals i.e. they have two
cell layers separated by the mesogloea (a Jelly-like
non-cellular layer)
The mesogloea may contain cells that have migrated
from other layers.
Page 24 of 447
Significance or importance of possessing a

mesoderm in the body
1. It allows triploblastic organisms to increase in size

and thus results into the considerable separation of
the alimentary canal from the body wall
2. Used in forming a variety of organs which may
combine together and contribute towards an organ
system of organisation
3. It enables the improvement of muscular activity by
triploblastic organisms. It’s necessary because of
their increased size which renders the use of
flagella or cilia inappropriate.
ADAPTATIONS OF PLATYHELMITHES TO A
PARASITIC MODE OF LIFE
1. They have a special way of gaining entry into the

body of the host but locomotory structures are
PHYLUM PLATYHELMINTHES generally reduced or absent.
2. They have structures which anchor them onto their
Members include tapeworm (Taenia solium and Taenia host. Liver flukes have suckers; tapeworms have
Saginata), blood fluke (Schistosoma), liver fluke both hooks and suckers.
(Fasciola hepatica) and planaria which causes
Schistosomiasis (Bilharzia) in tropical countries,
Characteristics
 They are Triploblastic i.e. the body is composed of

three layers, the outer ectoderm and the inner
endoderm and between these two is the mesoderm
3. They protect themselves against the internal

environment. Flatworms produce inhibitory
 They have bilateral symmetry i.e. The body can
substances to prevent their being digested by host
only give two equal and opposite halves if cut
enzymes.
along one plane.
4. They have complex lifecycles. Fasciola and Taenia
 They have an un-segmented body
have a secondary host which transfers one parasite
 Their bodies are dorsal ventrally flattened
from the primary host to another.
 They are hermaphrodites, often with elaborate
5. They have a very high reproductive output. Adults
precautions to minimise self fertilisation
devote much of their energy and body space to
 They have flame cells for excretion and sexual reproduction.
osmoregulation
Page 25 of 447
PHYLUM NEMATODA (round worms) 2. They burrow tunnels which improves aeration and
drainage of the soil
Members include; 3. They add organic matter to soil by excretion and
a. Ascaris Lumbricoides, which is an intestinal death
parasite 4. Secretions of the gut neutralise acidic soils
b. Wucheriria bancroft, which infects the human 5. Dead vegetation is pulled into the soil where decay
lymphatic system and causes elephantiasis takes place
c. Thread worms which are endoparasites of dogs and
cats plus humans, mainly children. THE COELOM
Characteristic features
This is the main (secondary) body cavity of many
 They are triploblastic triploblastic animals, in which the gut is suspended. The
 They have bilateral symmetry principal mode of origin is by separation of the
 They have an un-segmented cylindrical body mesoderm from the endoderm. It contains a fluid
(coelomic fluid) which receives excretory wastes and/
 Their alimentary canal is straight from the mouth to
gametes, which reach the exterior via ciliated funnels
the anus.
and ducts.
 Their sexes are separate
 They lack cilia
 A cuticle of protein is present
 Some are free living plant and animal parasites
 They are elongated and round in cross-section with
pointed ends
PHYLUM ANNELIDA
(segmented worms)
General characteristics
 They are coelomate and triploblastic

 They have no Chitineous cuticle
 They possess Chitineous bristles called chaetae
 They exhibit metameric segmentation i.e. their Biological significance of the coelom
segments are repeated and are of the same age and
size 1. It provides space in which internal organs can
 They have bilateral symmetry grow, develop and function independently of each
Examples include Lumbricus and Hirudo the medicinal other
leech 2. It contains coelomic fluid which bathes the organs
and can act as a hydrostatic skeleton
3. It allows the animal’s internal organs to move
independent of each other and move independent of
the whole body e.g. the gut can perform peristalsis
without causing the body wall to move into waves
of contraction
4. Coelomic fluid may be used to circulate food,
waste materials and respiratory gases although
these functions are mainly carried out by the body
vascular system.
5.
Problems caused by the coelom
i. It separates the body wall from the gut, causing

Biological importance of earthworms
difficulty in transporting digested food and
1. They mix soil layers respiratory gases resulting into the development of
transport system
Page 26 of 447
ii. Increased size and complexity requires a more PHYLUM ARTHROPODA

complex coordination system, therefore a more
elaborate nervous system Arthropods contain more species than any other phyla.
Insects in particular, account for more than half of all
known arthropods. Insects have been successful in
PHYLUM MOLLUSCA exploiting every type of habitat because they have
undergone adaptive radiation i.e. they suited for flying,
Characteristics burrowing, living in aquatic areas, parasitism e.t.c.
 These are triploblastic coelomate animals Diagnostic features of arthropods
 They have soft bodies which are covered by a
Possession of jointed appendages for feeding,
calcareous shell i.e. shell containing calcium. These
locomotion and sensory purposes
shells are produced by special epidermal tissue
called mantle Possession of an exoskeleton comprising of a chitineous
 They have an un-segmented body with a head, foot cuticle and sometimes calcareous matter which makes it
with a visceral hump is a central mass of internal rigid and stiff at the mouth parts but flexible at the
organs joints
 They have bilateral symmetry Other characteristic features
Members include slugs and Helix (the garden snail),
mussels, oysters, clams, squids, octopus and cat fish.  Triploblastic coelomate
 Bilateral symmetry
PHYLUM ECHINODERMATA
 Metameric segmentation
Examples include starfish, sea cucumbers, sea lilies,  The coelom is much reduced and the main body
brittle stars and sea urchins. cavity is a haemocoel i.e. the coelom is almost
completely displaced during devolvement by
Characteristics
another cavity called the haemocoel which is filled
 Their skin bears spines hence the name of the with blood
phylum
 Adults show penta-radiate symmetry (5-way
symmetry) but their larval forms show bilateral
symmetry
 The mouth generally occurs on the lower side (oral
side) while the anus occurs on the upper side
(literal side)
NOTE
 Arthropods are at a high danger of blood loss from

injury because they have the open blood system
 The high blood volume in arthropods enables them
to maintain a high metabolic rate allowing them to
be very active animals
 They lack a proper circulatory system Disadvantages associated with the
 They are exclusively marine inhabitants presence of an exoskeleton
 They have a calcareous skeleton
1. It’s weight to strength ratio decreases with the size
 They move slowly by the concerted action of of the animal making it less efficient as the animals
numerous suctorial tube feet becomes larger

Page 27 of 447
2. It resists growth and therefore periodical moulting Class Arachnida

(ecdysis) is required if the animal is to grow
Members include mites, ticks, scorpions, spiders
3. During moulting, the body of the arthropod is soft
(Epeira, the web spinning spiders) e.t.c.
and very vulnerable to attack by predators and
pathogens Characteristics
The groups of arthropods include;
 Mainly terrestrial
Class Crustacea/ crustaceans  Mainly carnivorous
Members in this phylum include; Lobsters, Barnacles,  Two major body divisions present i.e. a
Water fleas, Daphnia, and Astacus cephalothorax and abdomen
 No antennae
Barnacles are sessile and remain attached to rocks by  No true mouth parts but a pair of appendages are
the head. Wood lice are the only terrestrial crustacean. used for capturing prey and the second pair is used
Characteristics as sensory palps.
 Simple eyes present but no compound eyes
 Two pairs of antennae  Four pairs of walking legs
 A pair of compound eyes  No larval form
 Gaseous exchange occurs by gills  Gaseous exchange is by lungs or gill books or
 Three pairs of mouth parts (jaws) trachea
 They are mainly aquatic Class Insecta
 Head and thorax are not distinctively separate
i.e. they possess a cephalothorax Diagnostic features
Class Chilopoda a. Three main body divisions i.e. head, thorax and
This class has Lithobius, (the centipede) abdomen
b. Three pairs of legs on the thorax, one pair per
 Terrestrial and mainly carnivorous segment
 Have a clearly defined head, but all other body c. Three thoracic segments i.e. prothorax, mesothorax
parts are similar and metathorax.
 They possess one pair of antennae Other characteristic features
 They possess one pair of mouth parts (jaws)
 Mainly terrestrial
 Eyes, either simple or compound, are absent
 No gills in adults
 Numerous identical legs i.e. one pair per segment
 They have simple eyes
 No larval form
 Usually three pairs of mouth parts
 Gaseous exchange occurs by the trachea
 Gaseous exchange occurs by trachea
Class Diplopoda
 Lifecycles commonly involves metamorphosis
The only member of this class is the millipede. Subclass Apterygota, these are wingless insects and
they include Lepisma (silverfish) a common inhabitant
Characteristics of bathrooms and kitchens
 Mainly terrestrial Subclass Pterygota, these are winged insects which are
 Mainly herbivorous further divided into two;
 The head is distinct but all other body segments are
similar a. Exopterygota
This is whereby the wings develop externally.
 One pair of mouth parts
They undergo incomplete metarmorphosis i.e.
 One pair of antennae
Hemimetabolus
 Eyes, either simple or compound, are absent
Examples include;
 Numerous identical legs with two pairs per
 Locusta (the long horned grass hopper)
segment
 Periplaneta (cockroach)
 They lack a larval form
 Dragon flies
 Gaseous exchange is by the trachea
b. Endopterygota
The wings develop internally.
Page 28 of 447
They undergo complete metamorphosis i.e. It includes aphids and cicadas. Their characteristics
holometabolus include the following;
Egg larva pupa adult  They have piercing and sucking mouth parts
 Incomplete or complete metarmorphosis
The larval stage is specialised for eating and growing.
They are known by such names as caterpillars and grab  Some species can reproduce without mating
The adult is specialised for dispersal and reproduction  Some are wingless, others possess one or two pairs
Examples include; of membranous wings
 Pieris (butterfly) Order Hymenoptera
 Apis (honey bee) Members include ants, wasps, bees and sawflies. Their
 Musca (housefly) characteristics include;
Some orders of class insecta
 Chewing and lapping mouth parts
Order Orthoptera  Worker ants and few others are wingless
Examples include crickets, grasshoppers and walking  Two pairs of small stiff and membranous wings
sticks. that interlock during flight
 The front wings are larger than the hind wings
Characteristics  They undergo complete metamorphosis
 Chewing mouth parts Order Lepidoptera
 Straight wings Members include butterflies and moth.
 Complete metamorphosis
 Two pairs of wings with the front wings being  Long antennae
narrow and leathery. The hind wings are  Complete metamorphosis
broad, membranous and folded when at rest  Sucking mouth parts shaped like a coiled tube
Order Dictyoptera when at rest
 The front wings are usually larger than the hind
Examples include cockroaches and mantids and their wings
characteristics include;  Possess to pairs of usually broad wings which
 They are dorso ventrally flattened possess scales
 They undergo incomplete metamorphosis 
 Two pairs of wings with the front wings being Order Diptera
narrow and leathery. The hind wings are broad, Members include houseflies, mosquitoes and midges.
membranous and folded when at rest. Their adult characteristics include;
Order Isoptera

Two large compound eyes
Members include termites and their characteristics 
Piercing mouth parts
include;

Complete metarmorphosis
 Chewing mouth parts 
The two front wings are transparent and the
 Workers and soldiers are wingless two hind wings are reduced to halteres which
 They undergo incomplete metamorphosis serve as balancing organs during flight
 Reproductive termites possess two pairs of Order Siphnoptera
similar membranous wings which are held out This order includes the fleas and their characteristics
flat when at rest and the wings are shed off include;
after the mating
Order Hemiptera  They are wingless
 They lack eyes
It includes all the bugs, and their characteristics include;  They exhibit incomplete metarmorphosis
 Piercing and sucking mouthparts  They possess piercing mouthparts
 Two pairs of membranous wings Order Odonata
Order Homoptera Members include dragon flies and damsel flies. Their
adult characteristics include;
Page 29 of 447
 Chewing mouthparts These are chordates without a skull and the notochord
 Two pairs of equal sized transparent membranous remains i.e. it is not replaced by a vertebral column.
wings that cannot be folded. Acraniates are sub-divided into two;
 They have huge eyes Tunicata (urochordata)
 They possess very small antennae Members of this subphylum include the sea squid and
 Legs cannot walk but are used to capture prey in air its characteristics include;
 They mate in flight  The notochord is present
 They exhibit incomplete metarmorphosis  The adult tunicates are sessile filter feeders
which are enclosed in a tunic.
Cephalochordata
Members of this phylum include amphioxus and its
PHYLUM CHORDATA characteristics include;
During their lifetime, all chordates possess the  The larvae are free swimming
following structures;  The adults possess a pharynx which is
modified for filter feeding
1. Notochord
 The notochord persists
This is a rigid but flexible dorsal rod which consists of
b. Craniata (vertebrata)
vacuolated cells surrounded by a tough outer coat. In
These are chordates with a cranium (skull) enclosing
primitive chordates, a notochord prevents shortening of
the brain. The notochord is replaced by a vertebral
the body so that most of the force of muscle
column made of cartilage/bone.
contractions is transmitted into bending movements,
They have two pairs of limbs/fins.
which are useful for swimming.
They have a well-developed central nervous
2. Hollow dorsal nerve cord (central nervous
system
system)
Vertebrates are subdivided into the following
This is formed by invaginations from the outer wall
taxa.
layer (ectoderm) of the embryo and develops as a group
Subphylum Agnatha i.e. craniates without jaws or
of cells which is later closed off at the top.
jawless fishes
3. Pharyngeal gill slits (visceral clefts)
These are perforations on either side of the pharynx Class cyclostomata
which occurs in all chordate embryos.
Members include Hampreys and Hag fish. Their
In vertebrates, the number of slits is greatly reduced and
characteristics include;
may be modified for different purposes. For example, in
fish and larval amphibians, their walls are lined with  No paired fins
feathery gills which are used for gaseous exchange. In  Semi ectoparasites i.e. they attach onto the body of
fish and larval amphibians, their walls are lined with fish, sucking on the fish’s blood.
feathery gills which are used for gaseous exchange. In
 They have numerous gills
reptiles, birds and mammals, the only opening which
 They have round suctorial mouthparts and a
remains is the Eustachian tube in the ear. In primitive
chordates, visceral clefts are retained for straining food rasping tongue
particles from water.  They have a well-developed notochord in adults.
Subphylum Gnathosotomata i.e. craniates with jaws.
Other features possessed by my most but not all It includes all the following classes.
chordates include;
Class chondrichtyes
4. Post anal tail i.e. a post anal extension of the body
or a true tail Examples of members of this phylum include dog fish,
5. Segmented muscle blocks (myotomes) which are skates, rays and sharks. Their characteristics include;
considered as a secondary adaptation for  The skin bears placid scales (tooth-like scales)
swimming.
 The skin contains dermal dentricles i.e. tooth-like
6. Closed circulatory system in which blood flows
structures with a central pulp cavity surrounded by
forward ventrally and backwards dorsally
an outer covering of enamel
Phylum Chordata is divided into two main groups
 Pectoral and pelvic fins are paired
a. Acraniata
Page 30 of 447
 Visceral clefts are present as separate gill openings Class reptilia

(5 pairs)
Members of this class include alligators, crocodiles,
 The anus is ventrally positioned
snakes and reptiles.
 They are poikilothermic
 They are marine dwellers.  They exhibit internal fertilisation
 The tail is heterocercal i.e. the dorsal lobe of the  They have a bony endo skeleton
tail fin is usually larger than the ventral lobe and  They have a dry scaly skin with horny scales
this enables balancing since a swim bladder is  They are poikilothermic
lacking  They have soft shelled eggs
 They have a cartilaginous skeleton  They are mostly terrestrial
Class osteichthyes  Gaseous exchange occurs by lungs
 They lay a cleidoic (shelled egg)
Members include tilapia, perch and the herring. Their
characteristics include; Class aves
This class includes all birds and their characteristics

 Bony endo skeleton
include;
 Mouth is terminal
 Visceral clefts present i.e. separate gill openings (4  The skin bears feathers
pairs) but covered by a bony flap called operculum  Their legs bear scales
 The skin bears a cycloid and others ctenoid scales  Fore limbs modified into wings
 Fertilisation is external  They exhibit internal fertilisation
 The tail is hormocercal  They lay well developed cleidoic eggs
 They are poikilothermic  They are homeothermic
 The swim bladder is present  They possess lungs for gaseous exchange
 Some are marine while others are fresh water Class mammalia
dwellers
 The characteristics for the members of this class
include;
Class crossopterygota
 Having mammary glands
It includes the lung fish.
 Possession of a pinna (external ear)
 They have paired fins  They are endothermic or homeothermic
 They are mostly predators  Fertilisation is internal
 They live mostly in fresh water  The skin bears fur with two types of glands i.e. the
Class amphibia sebaceous glands and the mammary glands
Members include Bufo (toad), Rana (frog), newts and  They are mostly viviparous i.e. they give birth to
salamanders. Their characteristics include; active young ones rather than laying eggs
Subclass prototheria
 Partly aquatic and partly terrestrial
It includes all egg laying mammals e.g. the spiny
 Have simple sac-like lungs
anteater and the duck billed platypus. They lay large
 Have a soft moist skin used as a supplementary
yoked eggs but like other mammals, their young ones
gaseous exchange surface suckle.
 They have two pairs of pentadactyl limbs
 Breeding occurs in water i.e. fertilisation is external Subclass theria
 They are poikilothermic These are non-egg laying mammals which are divided
 Visceral clefts (gills) are present in aquatic larvae into groups;
and gills are present in adults
 Newts and salamanders possess tails in adults and a. Meta-theria/marsupial mammals
in the larva stage but frogs and toads possess the These are mammals which have porches in which
tail in the larva form only. the young ones are located and suckle for most of
their development, having been born in a very
immature state e.g. kangaroo
Page 31 of 447
b. Eutheria/placental mammals body to be lifted off the ground and propel the animal
These are mammals whose young ones develop to forward.
mature ones while in the womb or placenta before
5. A variation in environmental conditions, most
they are born. Examples include humans
especially temperature has been overcome
Some orders of class mammalia include;
completely only by birds and mammals by
1. Order insectivora which includes moles and shrews evolving homeothermy i.e. they generate heat
2. Order carnivora which includes cats and dogs. within their tissues physiologically and maintain a
3. Order cetacea which includes dolphins and whales constant body temperature independent or external
4. Order chiroptera which includes bats conditions. This provides optimum conditions for
5. Order rodentia which includes rats enzyme reaction and proper brain development. All
6. Order primate which includes chimpanzee, other remaining terrestrial animals are
humans, monkeys, apes and lemurs. poikilothermic and regulate their body temperature
7. Order proboscidea which includes the elephant by bathing in the sun e.g. reptiles
8. Order ungulate which includes cattle, sheep, horses Sample questions
and goats.
1. Figure 1 below shows three types of organisms
Problems faced by animals living on
(not drawn to the same scale )
land
i. Obtaining support
ii. Water loss
iii. Gaseous exchange
iv. Homeostasis
v. How to reproduce without water
Adaptations of animals to live on land a) Identify the three types of organism shown
1. Oxygen being less soluble and more plentiful in air above (03 marks)
than in water has led to the animals developing i. A.
moist gaseous exchange surface coupled with ii. B
breathing mechanisms e.g. lungs in invertebrates iii. C
2. To avoid desiccation, various animals have
b) List the organisms above in order of their
actual size, starting with the largest (01)
developed different mechanisms e.g. amphibians
c) State which of these organisms might
are restricted to damp habitats. Reptiles, birds,
bring about decay of organic matter (01)
mammals and insects have a water tight surface
d) On the figure above, label (02 marks)
layer which enables them to inhabit dry areas.
 A structure (N) that is always made
Reptiles and birds produce a semi-solid
mostly of DNA
nitrogenous waste containing uric acid which
 A structure (P) that is made of protein
requires less water.
2. (a) Explain why it may be incorrect to state
3. Internal fertilisation and production of shelled eggs that bacteria are unicellular (04 marks)
in reptiles and internal development in mammals (b) With the aid of examples, describe the
enables them to conserve water and become fully classification of bacteria as heterotrophic (06)
terrestrial. Amphibians have failed to overcome the 3. Table 3 shows the output of the contractile
problem of reproducing on land as they keep vacuoles of two protists of different sizes. The
reverting to water for egg laying to prevent them data are the mean value of numerous
from drying. estimations.
4. Air provides very little supply to terrestrial animals Species Rate of Cell Time to
because of its low density as compared with water out volume eliminate
which has a high density. These animals have put/µm3s /µm3 Equivalent of cell
developed skeletons for support in air and muscular ec-1 volume/hours
mechanisms for locomotion. Amoeba 80 1200x10 8.5
Amphibians, reptiles, birds and mammals have strong proteus 3
muscles and they are tetrapods (four limbed animals) Paramecium 105 305x103 0.5
with limbs built on the pentadactyl. This enables the
caudatum
Page 32 of 447
(a) i) Compare the rate of the output of the bryophytes and pteridophytes (02 Mar)
contractile vacuoles in both protists (01 m) (b) There is a wide range of algal types
(ii) State the relationship between output and ranging from unicellular
the time taken to eliminate equivalent of cell chlamydominas to large multicellular
sea weeds. State any;
volume (01 mark)
i. Advantages of multicellular
4. b i) Calculate the rate of discharge of the two organisms over unicellular organisms.
protists per hour (02 minutes) (03 Marks)
(ii) Calculate the volume of the fluid discharge ii. Disadvantages of multicellular
by the two protists (02 marks) organisms over unicellular organisms.
c) Explain the difference in the volume of fluid (02 Marks)
discharge by the two protists (04 marks) (c) What is the ecological role of algae in the
(d) Explain why contractile activity is biosphere? (03 Marks)
important in the life of protists.(04 marks) 10. Fungi were originally classified under the
5. (a) State two similarities and three differences plant kingdom.
(a) State the unique features of fungi that
in the structure of the Angiospermophyta and
necessitated them to have a kingdom of their
coniferophyta (05 marks) own. (05 marks)
6. Table 2 below shows the results an (b) What is the economic importance of fungi in
investigation in the morphological natural ecosystem (05 marks)
characteristics of different migratory East 11. Hydra is a diploblastic, radially symmetrical animal
African Grasshoppers, Homorocorypus (a) Explain how radial symmetry may be an
nitudulus. Study it carefully and answer the advantage to a sessile animal (02 s)
questions that follow (b) Hydra exhibits polymorphism
Colour Sex (percentage ) Parentage i) What is meant by the term
forms Female Male total polymorphism? (02 marks)
Green 42 14 56 ii) State the forms of polymorphism found in
hydra (01 marks)
Brown 14 28 42
iii) Of what significance is polymorphism to
Other 1 1 2 hydra (02 marks)
colours (c) Larger animals, such as members of phylum
Total 57 43 100 chordate, possess blood systems while smaller
a) What general conclusion can you draw animals, such as members of phyla Cnidaria
from the data? (04 marks) and Platyhelminthes, do not. Explain the link
b) Of what ecological advantage are the between body size and the possession of a
green and brown forms? (02 marks) transport system.
c) State reasons why class insecta is referred 12. Distinguish between classes of phylum
Angiospermophyta, using features of their stems
as the most successful class among the
(05 marks)
classes in phylum arthropoda. (04 marks) (b) (i) Describe the adaptations of
7. Certain animals have the following features in angiosperms to life on land (10 marks)
common (ii) State any five importances of plants in the
Segmentation, setae, bilateral symmetry and a environment (05 marks)
coelom 13. a) What is meant by the term alteration of
(a) (i) Name the phylum to which they generations (03 marks)
belong (01 mark) b) Outline the similarities and differences between
(ii) State any one class of the Bryophytes and Pteridophytes.
phylum mentioned in a) (i) above (01 c) What is the significance of alteration of
mark) generations? (07 marks)
(iii) Name one other phylum which has 14. a) What are the adaptations of the plasmodia to its
three of the features stated (01 mark) parasitic mode of life (03 marks)
(b) State three features that distinguish b) Describe the life cycle of plasmodia. (11)
members of the phyla stated in a) i) and c) Why is malaria still such an unrelenting
a) iii) other than those listed above disease in sub-Saharan Africa? (06 marks
(03 marks) 15. (a) What is meant by alternation of
8. (a).State four distinguishing features of generations. (04 marks)
angiosperms (04 marks) (b) Give an outline of the life cycle of a moss plant.
(b) Briefly describe any six features of seed plants (09 marks)
that have contributed to their success on land?
9. Give 2 main differences between
Page 33 of 447
(c) How does the lifecycle of a moss plant differ Figure 3

from that of a typical flowering plant?
(07 marks) CATTLE
Month Total Infected Infected %
16. (a) What is meant by the term alternation of slaughtered livers lungs infection
generations. (03 marks) Jan 1099 10 18 2.5
(b) Give an outline of the life cycle of a moss Feb 1014 1 18 1.8
plant. (09 marks) March 947 8 45 5.6
(c) How does the life cycle of a moss plant differ April 1201 0 71 5.9
from that of a typical flowering plant? May 940 4 59 6.7
17. (a) State the distinguishing features of the classes
June 1070 7 74 7.6
of the angiosperms seen in the structure of the
July 1050 8 112 11.4
stems (05 marks)
August 768 8 65 9.5
(b) Discuss the characteristic features that have
made angiosperms achieve success in the terrestrial September 1253 8 81 7.1
environment (15 marks) October 1091 5 55 5.5
18. (a) Describe the process of alternation of GOATS
generations in a named pteridophyte. Total Infecte Infecte %
Month
(b) How are pteridophytes better adapted to life on dry slaughtered d livers d lungs infectio
land than bryophytes? (08 marks) n
Jan 1074 5 15 1.9
19. Figure 1 below shows the body temperature Feb 1481 3 8 0.7
of a person suffering from a certain kind of March 1055 4 39 4.1
malaria. April 1053 0 0 0.0
May 60 0 29 4.8
June 1239 5 42 3.8
July 1209 12 89 8.4
August 1150 5 41 4.0
Septem 851 3 37 4.7
ber
October 649 2 22 3.7
From figure 1,
(a) (i) Comment on the data given.
(ii) Explain the changes in temperature of the patient
above
(b) Give an account of the main features of
Plasmodium life cycle.
In another experiment, rats were infected with different
Using the data of figure 2,
numbers of the tapeworm, Hymenolepis diminuta.
(c) Plot a suitable graph to reflect the results
After 16 days, the mature worms were measured.
(d) What is the effect of crowding of worms on their:
Figure 2 below shows the results of the investigation.
(i) growth
Figure 2
(ii) reproduction
Number of Mean mass Mean number of
(e) (i) Explain why there never exists more than a single
worms per per worm eggs per gram of
tapeworm, Taenia in the human gut.
rat (g) worm
(ii) Compare the mode of life of this endoparasite
1 2.2 3.1 x 106
within its host with that of mammalian foetus in the
5 1.35 1.7 x 106 uterus
10 0.98 1.05 x 106 (f) Using the figure 3 data,
30 0.33 0.18 x 106 (i) Comment on the results
Echnococcus granulosus is a tapeworm that inhabits (ii) Plot histograms of percentage infection for the
intermediate hosts, mainly domestic animals with man entire period, using the same axes
as the definitive host. The infection of man with this (ii) Account for the observed patterns of infection for
parasite results from consumption of meat infected with both goats and cattle.
larvae. Figure 3 below shows the results of an END
investigation of the incidence of this parasite at a
Kampala Capital City Authority abattoir.
Page 34 of 447
P530 By Nakapanka Jude Mayanja (2020 0704716641
REFERENCES
1. D. T. Taylor, N.P.O. Green, G.W. Stout and R. Soper. Biological Science, 3rd edition, Cambridge
University Press
2. M. B. V. Roberts, Biology a Functional approach, 4th edition, Nelson
3. C. J. Clegg with D. G. McKean, ADVANCED BIOLOGY PRICIPLES AND APPLICATIONS, 2nd
EDITION, HODDER EDUCATION
4. Glenn and Susan Toole, NEW UNDERSTANDING BIOLOGY for advanced level, 2nd edition, Nelson
thornes
5. Michael Kent, Advanced BIOLOGY, OXFORD UNIVERSITY PRESS
6. Michael Roberts, Michael Reiss and Grace Monger, ADVANCED BIOLOGY
7. J.SIMPKINS & J.I.WILLIAMS. ADVANCED BIOLOGY
END
Page 35 of 447
TOPIC 2: CHEMICALS OF LIFE

Syllabus extract
Content SPECIFIC OBJECTIVES
The learner should be able to:
Acids, bases, salts and vitamins.
• Describe properties of acids, bases and vitamins.

• Properties of acids, bases, and vitamins
• Explain the role of acids, bases, salts and vitamins
• Functions of acids, bases, mineral salts and vitamins in
in maintaining a stable internal environment for physiological
organisms.
processes.
• Test for presence of mineral salts in food Mineral salts, to organic

samples/extracts (refer to inorganic analysis in chemistry
• Identify salts using quantitative and qualitative analysis.
practical) • Test for Vitamin C.
• Testing for Vitamin C. • Demonstrate the effect of heat on vitamin C content in
• Effects of heat on vitamin C content in vegetables. vegetables.
• Effects of storage on quality of fresh foods.
• Demonstrate the effect of storage on quality of fresh foods.
Water & practical

• Molecular structure of water.
• Functions of water. • Describe the molecular structure of water.
• Water as a solvent. • State functions of water.
• Role of water in the life of organisms (Biological • Explain the importance of water as a solvent.
significance in relation to properties water.) • Relate the water properties to its role in the life of organisms
(biological significance)
• Testing for water
• Measuring water content in tissues • Test for water
• Field study on water habitats. • Curry out dry weight technique to determine water content in
tissues
(The natural relationship of water and organisms). • Explain the natural relationship of water and organisms in a
habitat (including humans)
Structure of Carbohydrates
• Structure and components of carbohydrates.
• Properties of carbohydrates. • Describe the structure and components of various
• Importance of carbohydrates: monosaccharide, carbohydrates
disaccharides, polysaccharides • Explain properties of carbohydrates.
• Condensation of carbohydrates. • Explain the functions of carbohydrates in organisms.
• Hydrolysis of carbohydrates. • Describe condensation of carbohydrates.
• Describe hydrolysis of carbohydrates.
Test for carbohydrates Practical
• Testing for carbohydrates • Carry out food test for carbohydrates on food samples /
• Hydrolysis of non-reducing sugars to reducing sugars extracts.
• Demonstrate hydrolysis of non reducing sugars.
• Structure and components of lipids molecules. Structure of Lipids.

• Properties of lipids.
• Importance of lipids • Describe the structure and components of lipid molecules.
• Steroid structure. • State properties of lipids.
• Effects of lipids and Steroids to organisms • Explain the functions of lipids in the organisms.
• Condensation of fatty acids and glycerol to form • Describe structure of steroid.
Page 36 of 447
lipids. • Explain effects of lipids and steroids to organisms
• Hydrolysis of lipids to fatty acids and glycerol. • Describe the condensation of fatty acids and glycerol.
• Comparison between waxes and lipids. • Describe the hydrolysis of lipids.
• Importance of cholesterol in organisms. • Compare waxes and lipids.
• State the importance of cholesterol in organisms.
Test for lipids practical

• Tests for Lipids
• Carry out food tests for lipids on food samples extracts/
• Food samples /extracts containing lipids extracts
• Identify food samples/extracts containing lipids.
 Structure and components of proteins

Structure of proteins
 Properties  Describe the structure and components of proteins.
 Describe the properties of proteins

 Importance of proteins
 Functions of proteins (buffer, enzymes/catalytic,  Explain the importance of proteins

growth, carriers e.t.c.)
 Explain the functions of proteins to organisms.
 Condensation of amino acids
 Describe condensation of amino acids.
 Hydrolysis of proteins
 Describe hydrolysis of proteins.
 Effects of heat on peptide bond linkages or formation
 Explain effects of heat/temperature changes on proteins.
in amino acids/ proteins
 Testing for proteins. Test for proteins practical

 Carry out food tests for proteins on food samples / extracts
 Criteria of naming enzymes. (Use of suffix –ase, Enzymes

intracellular and extracellular.)  Describe the criteria for naming enzymes
 Characteristics of enzymes: Protein in nature i.e. can  Explain characteristics of enzymes

be denatured.
 Explain the properties of enzymes
 Properties of enzymes relating to factors affecting
enzyme activities.  State factors that affect enzyme action
Catalytic/change rates of reactions.  Explain the lock and key mechanism of enzyme action
Work in small amounts.
Specific to reactions they catalyze.  Explain the role of enzymes in the organisms’ life
Reversible reactions.
Can be inhibited.
Affected by temperature, pH, concentration of
substrate and enzymes.
 Factors affecting enzyme action pH, temperature,
inhibitors, substrate concentration etc.
 The lock and key mechanism of enzyme action.
Induced fit
 Role of enzymes in living organisms including
inhibition, competitive/noncompetitive,
reversible/non reversible
Page 37 of 447
 Enzyme properties relating to factors (temperature Enzymes
and pH, concentration of substrate and enzyme)  Demonstrate properties of enzymes action in specific
affecting enzymes’ activities. temperature, pH range, substrate/enzyme concentration.
 Enzymes in the different parts of the gut based on  Identify enzymes in the different parts of the gut based on
their actions on different food substances. their actions on different food substances.
 Food tests using the animal gut contents and enzymes.  Carry out food test on gut contents.
Page 38 of 447
Introduction
All living organisms made up of chemicals which constitute the protoplasm of their cells. These are known as
the chemicals of life i.e. the chemicals which keep the cells alive.
The study of the chemicals of life and the chemical reactions in which they take place is known as bio-
chemistry. These chemicals of life are divided into two categories; organic and inorganic chemicals of life.
The organic chemicals of life are all derived from carbon and include; carbohydrates, proteins, lipids, nucleic
acids (DNA and RNA), waxes and steroids as well as vitamins. The inorganic chemicals of life include,
water, mineral salts, acids and bases. All inorganic and organic chemicals of life must be supplied in
appropriate quantities in the diet except nucleic acids and a few vitamins. Therefore there is need for a
balanced diet to keep the cells alive.
INORGANIC CHEMICALS OF LIFE

These are mainly acids, bases, water and inorganic mineral salts such as calcium, magnesium, potassium,
nitrates, chlorine, phosphates e.t.c.
ACIDS AND BASES

Acids Functions of acids
A compound which when dissolved in water ionizes to produce  They provide a suitable pH for
hydrogen ions as the only positive charged ions e.g. hydrochloric the proper functioning of
acid, nitric acid, Sulphuric acids e.t.c. enzymes e.g. pepsin
Note: The strength of the acid is determined by the extent to which it
dissociates .e.g. HCl is considered to be a strong acid because it  Acids like hydrochloric acids
completely dissociates in solution to give hydrogen ions. Whereas activate organic substances like
ethanoic acid is a weak acid because it partially dissociates in pepsinogen
solution
 Acids kill bacteria, which may
A PH of 7 represents neutrality while a pH below 7 represents acidity be ingested together with food
while that above 7 represents alkalinity or basis.
Bases
A base is a compound, which can react with acids to produce a salt Functions of bases
and water only. Some bases are alkalis.
 Provide an optimum pH range
An alkali on the other hand is a substance which when dissolved in a for enzyme activity e.g.in the
solvent produces hydroxyl ions as the only charged ions. This implies duodenum
that alkalis are bases but not all bases are alkaline. Strong alkalis
 They are buffers in the body
completely ionize e.g.  Na  ( aq)  OH 
NaOH (aq) 
Weak alkali don’t ionize completely e.g. ammonium hydroxide
Buffers
A buffer is a substance that minimizes changes in the concentrations of H+ and OH- in a solution when small
amounts of acids or bases are added.
The internal pH of most living cells is close to 7. Even a slight change in pH can be harmful because the
chemical processes of the cell are very sensitive to the concentrations of hydrogen and hydroxide ions. The
pH of human blood is very close to 7.4, which is slightly basic.
Page 39 of 447
Carbonic acid (H2CO3), which is formed when CO2 reacts with water in blood plasma dissociates to yield a
bicarbonate ion (HCO3-) and a hydrogen ion (H+):
The chemical equilibrium between carbonic acid and bicarbonate is a pH regulator, the reaction shifting left or
right as other processes in the solution add or remove hydrogen ions.
Thus, the carbonic acid–bicarbonate buffering system consists of an acid and a base in equilibrium with each
other. Most other buffers are also acid-base pairs.
Consider the reactions below;
HCO3-(aq) + H2O (l) H2CO3(aq) + OH-(aq) HCO3( aq )  OH  ( aq ) CO3(2aq )  H 2O( l )
from the above equations, it is clear that NaHCO3 removes ions from aqueous solutions thereby lowering the
aqueous solutions acidity in so doing it is working as buffer however, though sodium hydrogen carbonate
works as a buffer on its own, in most cases two or more compounds interact to form a buffer solution or
system.
In case of increased acidity, the NaHCO3 combines with free hydrogen ions as shown above if alkalinity is
increased, it reacts with free hydroxyl ions to form carbonate ions and water.
Salts e.g. K3 PO4 Na3 PO4 etc combine with hydrogen ions to form H 2 PO 4 (Di-hydrogen phosphate).
H  ( aq )  HPO22(aq ) 
 H 2 PO4( aq )
Certain organic compounds like proteins and haemoglobin can also accept H+ and are therefore important as
buffer. Since they occur in higher ions, than the phosphate salts they are even more important than the acids
and the bases. The biological importance of these buffers is that cells and tissues can only function properly at
a narrow range of pH, which is usually around neutrality.
Acids and bases also provide rightful pH ranges for certain chemical reactions to effectively proceed in the
body basicity.
NB: A number of acids are found in the body and these include
- Nucleic acid -amino acids

- succinic acid –lactic acid
- HCl –Uric acid
MINERAL ELEMENTS
A salt is a compound which is formed when the hydrogen ions in an acid are either partially or fully replaced
by a metal ion or NH4+ e.g.
HCl NaCl, KCl, NH4Cl
H2CO3 Na2CO3, NaHCO3
CH3COOH CH3COONa
Page 40 of 447
Functions of mineral salts
1. They form body structures e.g. the bones, the teeth, etc. Comprise calcium ions, phosphate ions etc. They
also form connective tissue and other structures a body.
2. They form body pigments e.g. Haemoglobin contains Iron, cytochromes contain copper and chlorophyll
contains magnesium.
3. They form chemicals in the body e.g., Sulphur and Nitrogen form proteins, nucleic acids, ATP etc.
4. They are metabolic activators. Certain ions activate enzymes e.g. magnesium activates enzymes that are
involved in phosphorylation of glucose.
5. They are constituents of enzymes e.g. nitrogen in proteins.
6. Constituents of various chemicals e.g. ATP contains phosphorous while thyroxin contains iodine.
7. They are determinants of osmotic pressure. Mineral salts and other solutes determine the osmotic pressure
of cells and body fluid. The osmotic pressure must not be allowed to fluctuate beyond narrow limits since
much of the physiology is directed to preventing this.
The mineral ions in the body can be grouped as major or minor ions depending of their need in the body.
Major/ macro ions are needed fairly in large amounts than minor ions.
Mineral major dietary Major functions in the body Symptoms of deficiency or excess
element sources for in animals
humans
MACRO ELEMENTS
Calcium Dairy products, bone and tooth formation , blood Retarded growth, possibly loss of
dark green clotting, nerve and muscle function bone mass
vegetables and
legumes Stunted growth
Phosphorous Dairy products, bone and tooth formation , acid- Weakness, loss of minerals from
meats and base balance, nucleotide synthesis bones, calcium loss
greens
Stunted growth particularly of roots
Sulphur Proteins from Proteins from many sources Symptoms of protein deficiency
many sources
Chlorosis
Potassium Meats, dairy Acid-base balance , water balance Muscular weakness, paralysis,
products, grains, and nerve function, cofactor in nausea , heart failure
many fruits and photosynthesis and respiration
vegetables, Yellow and brown leaf margins;
premature death;
Chloride Table salt Acid-base balance, formation of Muscle cramps, reduced appetite
gastric juice, nerve function,
osmotic balance
Sodium Table salt Acid-base balance, nerve function, Muscle cramps, reduced appetite
water balance
Magnesium Whole grains, Co-factor, ATP synthesis Nervous system disturbance

green leafy
vegetables Chlorosis
Nitrogen Stunted growth
Page 41 of 447
Lean meat, fish, Synthesis of proteins, nucleic Stunted growth and strong chlorosis
milk acids; formation of chlorophyll of old leaves
and a coenzyme
MICRO ELEMENTS
Iron Meats, eggs, legumes, Component of haemoglobin Iron-deficiency anaemia, weakness,

whole grains, green and of electron carriers in impaired immunity
leafy vegetables energy metabolism, enzyme
cofactor strong chlorosis of young leaves
Fluorine Drinking water, tea, Maintenance of tooth and Higher frequency of tooth decay
seafood bone structure
Zinc Meats, seafood, grains Components of certain Growth failure, skin abnormalities,
digestive enzymes and reproductive failure, impaired
other proteins immunity
Malformed leaves e.g. in cocoa
Copper Seafood, nuts, legumes, Enzyme cofactor in iron Anemia, cardiovascular

organ meats metabolism, melanin abnormalities
synthesis, electron transport
Die back of shots
Manganese Nuts, grains, Enzyme cofactor Abnormal bone and cartilage

vegetables, fruits, tea
Leaf flaking e.g. grey specks in oats
Iodine Seafood, dairy Components of thyroid Goiter

products, iodized salt hormones
Cobalt Meats and dairy Component of vitamin B12 None except as B12 deficiency
products
Selenium Seafood, meats, whole Enzyme cofactor; Muscle pain, possibly heart muscle
grains antioxidant functioning in deterioration
close association with
vitamin E
Chromium Brewer’s yeast, liver, Involved in glucose and Impaired glucose metabolism
seafood, meats, some energy metabolism
vegetables
Molybdenum Legumes, grains, some Enzyme cofactor Disorder in excretion of nitrogen

vegetables containing compounds
Page 42 of 447
WATER
a) Structure
Water is formed when two hydrogen atoms combine
with an oxygen atom by sharing electrons. The result is
a stable molecule, which is relatively unreactive. The
shape of the water molecule is triangular rather than
linear (figure 1) and the angle between the nuclei of the
atoms is approximately 105o. Overall the molecule is
electrically neutral, but in both of the oxygen-hydrogen
bonds, the oxygen draws electrons away from the
hydrogen nucleus. Thus there is a net negative charge
on the oxygen atom and a net positive charge on the
hydrogen atom. A molecule that carries an equal
distribution of electrical charge (figure 2) is called a Figure 1
polar molecule. Polarity is uneven charge distribution
within the molecule. In water one part or pole of the
molecule is slightly negatively charged and the other
slightly positive, this is known as dipole. This occurs
because the oxygen atom has a greater electron
attracting power (electronegativity) than the hydrogen
atoms. As a result, the oxygen atom pulls the bonding
electrons more towards itself than towards hydrogen.
These attractions are not as strong as normal ionic or
covalent bonds and are called Hydrogen bonds. Figure 2
They are constantly formed, broken and reformed in water although individually weak their collective effect is
responsible for the unusual properties of water.
Because of this charge separation, water is an overall neutral molecule. Water molecules form relatively weak
hydrogen bonds with other water molecules. Hydrogen bonds are also formed with any charged particles that
dissolve in water, and charged surfaces in contact with water. Hydrogen bonds account for the unique
properties of water.
b) Functions
Water is biologically important as shown by each of its properties.
1. Solvent properties
It is a universal solvent for polar substances (charged or ionisable substances) e.g. salt and it is also a solvent
for non-polar substances e.g. sugar. It is able to attract other polar substances, forming Hydrogen bonds with
them, thereby dissolving them. Polar molecules such as salts, sugars and amino acids dissolve readily in water
and so are called hydrophilic ("water loving"). Uncharged or non-polar molecules such as lipids do not
dissolve so well in water and are called hydrophobic ("water hating"). Most non-polar substances such as
lipids are immiscible in water and serve to separate aqueous solutions into compartments.
This property enables water to carry out the following functions;
i. It is a lubricant e.g. in the joints where it forms the synovial fluid which enables protection against
damages.
Page 43 of 447
ii. It acts as a transport medium as blood, lymph, in the expiratory system as well as in the alimentary
canal where it transports materials from one point to another.
iii. It is an important constituent of the excretory waste products, by which toxic materials are removed
from the body.
iv. It is the largest constituent of the protozoan protoplasm of all cells where it contributes up to 60%.
2. Water has a high specific heat capacity
Heat capacity refers to the amount of heat required to raise the temperature of 1 kg of water by 1OC. The high
heat capacity of water means that the large increase in heat energy around water results into a relatively small
rise in the temperature of water because much of the energy supplied to water is used in breaking the
hydrogen bonds which restricts the movement of molecules. The temperature changes within water are
therefore minimized as a result of its high heat capacity, this property is significant because;
i. It enables life processes such as temperature regulation and gaseous exchange to occur in organisms.
ii. Such a suitable temperature enables body enzymes to function well without denaturation and/or
inactivation.
iii. It provides a constant internal and external environment for many cells and organisms.
3. High heat of vaporization
A relatively large amount of energy is needed to vapourise water due to the hydrogen bonds within water and
as a result water has a high boiling point. The transition of water from a liquid to a gas requires the input of
energy to break its many hydrogen bonds, the evaporation of water from a surface causes cooling of that
surface. This is made use of as a cooling mechanism (evaporative cooling) in animals (sweating and panting)
and plants (transpiration). As water evaporates it extracts heat from around it, cooling the organism. This is
significant because;
a. It results into the cooling of the organisms so as to reduce body temperature.
b. It is an important heat sink where large bodies of water are responsible for modifying climate by
absorbing heat from the sun.
NOTE.
The energy transferred to water molecules to allow them vapourise results in loss of energy from their
surroundings so that cooling takes place.
4. High heat of fusion
Latent heat of fusion is the amount of heat energy required to melt a solid such as ice.
With its high heat capacity, water requires relatively large amounts of heat energy to melt from ice to liquid
water. Liquid water therefore must lose a relatively large amount of heat energy to freeze. This property is
important because it ensures that the cell contents and their environments are unable to freeze.
5. Density and freezing properties
The density of water decreases below 4OC and ice therefore floats on relatively warmer water below. Water
below 4 OC tends to rise which maintains the circulation in large water bodies therefore this property is
important because;
Page 44 of 447
i. It makes water an important factor in the cycling of nutrients needed by living things.
ii. It makes water a suitable habitat for many aquatic organisms, both plants and animals.
6. High surface tension and cohesion
Cohesion is the force of attraction between molecules of the same kind. At the surface of the liquid a force
called surface tension exists between the molecules due to the cohesive forces between the molecules. This
causes the water surface to occupy the least possible surface area. Water has a higher surface tension than any
other liquid.
This property is important as follows;
a. The high cohesion of water molecules enables the movement of
water through the xylem to the leaves.
b. Surface tension enables small organisms to settle on water or

skate over the water surface (figure 3)
c. It enables the water to participate in the absorption of mineral

salts from the soil.
Figure 3
7. Water as a reagent
As a reagent, water is an essential metabolite i.e. it participates in the chemical reactions of metabolism. This
property is significant in the following ways;
a. Water is a raw material of most bio-chemical reactions taking place such as photosynthesis,
respiration, and digestion.
b. Water is a medium in which most bio-chemical reactions take place.
c. Water is a pre-requisite for fertilization, where fertilization involves mobile gametes e.g. external
fertilization in lower plants, fish, amphibians, and internal fertilisation in higher vertebrates and
plants.
8. Incompressibility
This property enables water to carry out the following functions;
a. It forms the hydro-static skeleton of animals such as earthworms.
b. It provides support to the non woody plants e.g. herbaceous plants by maintaining turgidity of the
cells.
c. Water provides stomata movement, movement of leaves, opening and closing the flowers e.t.c. to take
place through changes in the turgidity of the cells.
9. High tensile strength
Water can be lifted by forces applied at the top as seen in movement of water to the xylem of tall trees due to
strong cohesive forces between water and the walls of the conducting vessels.
10. Water is transparent
It is important because it enables light to penetrate the water bodies to allow photosynthesis of aquatic plants
and also to allow vision to the aquatic animals.
Page 45 of 447
11. Water is denser than air
Water supports organisms as large as whales. It also supports and disperses reproductive structures such as
larvae and large fruits e.g. coconuts.
12. pH
 H   aq   OH  aq  so it is a source of protons (H+ ions), and

Water itself is partially ionized H 2O( l ) 
indeed many biochemical reactions are sensitive to pH (-log [H+]). Pure water cannot buffer changes in H+
concentration, so it is not a buffer and can easily be any pH, but the cytoplasm and tissue fluids of living
organisms are usually well buffered at about neutral pH (pH 7-8).
13. Water has a low viscosity

This is a measure of how resistant a liquid is to flowing. The lower the viscosity the easier the liquid flows.
Water has a viscosity that is lower than that of ethanol. The ease with which water flows is important in the
transport system of living organisms e.g. in blood as it flows through vessels.
- The significance of this property is that water can easily be pumped and moved in the small
tubes of the body.
- Water also forms a medium within which swimming is made easy.
- Water can flow freely through narrow vessels.
- Watery solutions can act as a lubricant
If too much water is lost from the body, then the viscosity of blood increases, flow slows and transport is less
efficient.
Plants rely on the flow of water in the xylem and phloem vessels to transport substances around their bodies.
Aquatic organisms too are able to swim in water because of the relatively low viscosity of water.
BIOLOGICAL IMPORTANCE OF WATER TO ALL ORGANISMS
Metabolic role of water
i. Hydrolysis
Water is used to hydrolyse many substances like proteins to amino acids, fats to fatty acids and
glycerol, starch to maltose,
ii. Medium for chemical reactions
All biochemical reactions take place in aqueous medium provided by water.
iii. Diffusion and Osmosis
It is essential for the diffusion of materials across surfaces such as the lungs or the alimentary
canal e.g. diffusion of food materials into the blood stream since such surfaces are moist to
facilitate diffusion and the moisture is provided by water.
iv. Photosynthetic substrate
Water is a raw material for photosynthesis
Water as a solvent
It dissolves other substances and is therefore used in the following ways;
i. Transport
The solvent properties of water mean that it is a transport medium, as it is in blood plasma, tissue
fluid, lymph, in mammals and Xylem and Phloem in plants. They are all made up of water and
dissolve a number of substances which can then be easily transported.
ii. Excretion
Metabolic wastes like ammonia, urea, excess salts require water to be removed from the body in
solution form.
iii. Secretion
Page 46 of 447
They are transported from their place of secretion in solution form (aqueous form) e.g. most
digestive juices have enzymes in solution, tears mainly consist of water, snake venoms have
toxins in suspension composed of water.
Water as a lubricant
Water’ properties especially its viscosity makes it a useful lubricant. Lubricating fluids that have a component
of water include;
 Mucus which externally facilitates movement in organisms like the snail and earthworm or internally
in the walls of the gut and vagina
 Synovial fluid which lubricates movements in the joints of vertebrates.
 Pleural fluid which lubricates movements of the lungs during breathing
 Pericardial fluid which lubricates movements of the heart
 Peri visceral fluid which lubricates movements of internal organs like peristaltic movement of the
alimentary canal
Supporting role of water
With its large cohesive forces, water molecules lie close together due to the hydrogen bonds between them
and therefore not easily compressed, making it a useful means of supporting organisms.
i. Hydrostatic skeleton
Animals like earthworms are supported by the pressure of the aqueous medium within them.
ii. Turgor pressure
Herbaceous plants and herbaceous parts of woody plants are supported by osmotic influx of water
into their cells.
iii. Humours of the eye
Aqueous and vitreous humours give the shape of the eye and they are mainly made up of water.
iv. Amniotic fluid
It supports and protects the mammalian foetus during development and is mainly made up of
water.
v. Erection of the penis
The pressure of blood which is mainly made up of water makes the penis erect for copulation to
take place.
vi. Habitat
Water supports organisms that live in it. Very large organisms like whales return to water as their
sizes make movement on land very difficult.
Other biological functions of water include
 Water enables dispersal of seeds and fruits such as coconut as well as dispersal of the gametes and
larval forms of aquatic organism. Medium of dispersal i.e. seed dispersal, gametes and larvae stages
of some aquatic organisms
 Seed germination
 Osmoregulation
 Migration of aquatic organisms
 Fertilization, by transporting gametes
 Hearing and balance. The watery endolymph and perilymph in the mammalian ear plays a significant
role in hearing and balancing
 It breaks the testa of seeds to allow embryo growth during germination.

THE ORGANIC CHEMICALS OF LIFE
These are the chemicals of life which always contain carbon, hydrogen and oxygen as the major elements. The
proteins and nucleic acids in addition to these elements also contain nitrogen. These organic chemicals of life
are important because of the following reasons;
- They are the structural components of the bodies of organisms.
Page 47 of 447
- They are regulators of chemical processes occurring in organisms.
These organic chemicals of life include the following; carbohydrates, proteins, lipids, vitamins and nucleic
acids
The building blocks of life
Monosaccharide Organic bases Fatty acids & glycerol Amino acids

s
Polysaccharide Nucleic acids Lipids Proteins
VITAMINS
Vitamins are organic molecules with diverse functions that are required in very small amounts. For humans,
13 essential vitamins have been identified and are classified as water-soluble or fat-soluble.
Vitamins Major dietary Major functions in Symptoms of deficiency

sources the body
Extreme excess
Water soluble vitamins
Vitamin B1 Pork, legumes, Coenzymes used in Beriberi (nerve disorders, emaciation

peanuts, whole grains removing carbon dioxide anemia)
Thiamine from organic compounds
Vitamin B2 Dairy products, meats, Component of coenzymes Skin lesions such as cracks at corners of
enriched grains, FAD and FMN the mouth
Riboflavin vegetables
Vitamin B3 Nuts, meats, grains Component of coenzymes Skin and gastrointestinal lesions,
NAD+ and NADP+ nervous disorders
Niacin
Liver damage
Vitamin B6 Meats, vegetables, Coenzyme used in amino Irritability, convulsions, muscular

whole grains acid metabolism twitching, anemia
Pyridoxine
Unstable gait, numb feet, poor
coordination
Vitamin B5 Most foods: meats, Component of coenzyme Fatigues, numbness, tingling of hands
dairy products, whole A and feet
Pantothenic grains e.t.c.
acid
Vitamin B9 Green vegetables, Co enzyme in nucleic Anemia, birth defects

oranges, nuts, legumes, acid and amino acid
Folic acid whole grains metabolism
(folanin) May mask deficiency of vitamin B12
Page 48 of 447
Vitamin B12 Meats, eggs, dairy Co enzyme in nucleic Anemia, nervous system disorders
products acid metabolism,
maturation of red blood
cells
Biotin Legumes, other Coenzyme in synthesis of Scaly skin inflammation,

vegetables, meats fat, glycogen, and amino neuromuscular disorders
acids
Vitamin C Fruits and vegetables Used in collagen Scurvy (degeneration of skin, teeth,
especially citrus fruits, synthesis (such as for blood vessels ), weakness, delayed
Ascorbic acid cabbage, tomatoes, bone, cartilage, gums); wound healing, impaired immunity
green pepper antioxidant; aids in
detoxification; improves Gastrointestinal upset
iron absorption
fat soluble vitamins
Vitamin A Beta-carotene (pro- Component of visual Blindness and increased death rate
vitamin A) in green pigments, maintenance of
Retinol and orange vegetables, epithelial tissues,
retinal in dairy antioxidant, helps prevent Headache, irritability, vomiting, hair
products damage to cell loss, blurred vision, liver and bone
membranes damage
Vitamin D Dairy products, egg Aids in absorption and Rickets (bone deformities) in children,
yolk; also made in use of calcium and bone softening in adults
human skin in presence phosphorous; promotes
of sunlight bone growth Brain, cardiovascular, and kidney
damage
Vitamin E Vegetable oils, nuts, Antioxidant; helps Desecration of the nervous system
seeds prevent damage to cell
Tocopherol membrane
Vitamin K Green vegetables, tea; Important in blood Defective blood clotting

also made by the colon clotting
phylloquinone bacteria Liver damage and anemia
CARBOHYDRATES
These are organic compounds made up of carbon, hydrogen and oxygen, in which the ratio of hydrogen to
oxygen is 2:1 as in water. The word carbohydrate suggests that these organic compounds are hydrates of
carbon. They have a general formula of Cn(H2O)m where m and n are either the same or different units (n =
number of carbon atoms). Most examples of carbohydrates do conform to the general formula e .g.
Glucose C6H12O6 C6(H2O)6 n =6 m =6
Sucrose C12H22O11 C12(H2O)11 n = 12 m = 11
Some few carbohydrates do not conform to the general formula e. g. Deoxyribose sugar, C5H10O4
Carbohydrates are mainly concerned with the storage and liberation of energy. A few carbohydrates such as
cellulose form important structures of organisms e.g. the plant cell walls.
Chemically carbohydrates have the following properties;
Page 49 of 447
i. They are either aldehydes or ketones.
ii. They contain hydroxyl groups.
There are 3 groups of carbohydrates namely;
 Monosaccharides (single sugars).
 Disaccharides (double sugars)
 Poly saccharides (Many sugars or complex sugars)
MONOSACCHARIDES
These are a group of sweet, soluble, crystalline molecules of

relatively low molecular mass made of a single sugar. They
may contain either an aldehyde group or a ketone within their
molecule. If they contain an aldehyde group (-CHO) they are
called aldoses or aldo-sugars (figure 4) such as
glyceraldehyde. If they contain a ketone group in their
molecules, they are called ketoses or keto-sugars (figure 5)
such as dihydroacetone.).
Note: The carbon atom with a double bond in Aldehydes is at Figure 4 Figure 5
the end of the chain while in Ketones it is on the second carbon
or on the carbon next to last
The general formula for Monosaccharides is (CH2O)n where n = number of carbon atoms. Where n=3, the
sugar is called a triose sugar (e.g. glyceraldehyde and dihydroxyacetone), where n=5, pentose sugar (e.g.
Ribulose and ribose) and when n=6 hexose sugar (e.g. mannose, fructose, galactose, glucose, sorbose).
The names of
monosaccharides end
with a suffix – ose.
Monosaccharides have
ringed structure (Figure
6) and they exhibit
isomerism. Isomers are
compounds with the
same molecular
formulas but different
structure formulae. For
example, the formula
C6H12O6 can be used
for glucose, fructose
and galactose. Figure 6
Monosaccharides can link together to form larger molecules i.e. they form building units used to form
complex sugars. Some monosaccharides act as a source of energy when oxidized in respiration e.g. glucose.
Page 50 of 447
Existence of α and rings gives a greater chemical The structures of the various isomers of monosaccharides include the
β
following;
variety and helps in building up the complex
carbohydrate atom on the 4th carbon atom to give a 5-
GLUCOSE (C6H12O6)
member ring called fructose ring. In the Hexoses, it is
a 6-member ring called pyranose. Consider the ribose β - Glucose differs from α-glucose in that at carbon 1 in β – glucose,
ring below. the -OH group faces upwards while α - glucose it faces downwards
β - glucose (beta glucose) α- glucose (Alpha glucose)
Figure 7
Fructose (C6H12O6)
Most of the monosaccharides are the reducing sugars
because they reduce Cu2+ in Benedict’s solution to Pyranose (β – fructose) Furanose (α- fructose)
Cu+ ions giving an orange precipitate of copper (1)
oxide (Cu2O). They have an aldehyde group or a free
ketone group. Ketoses first isomerise to aldoses
before they can act as reducing sugars.
Importance of monosaccharides
Trioses C3H10O5 e.g. glyceraldehydes, dihydroxyacetone are intermediates in respiration, photosynthesis and
other branches of carbohydrate metabolism.
Pentoses C5H10O5 e.g ribose, ribulose, deoxyribose
i. Synthesis of nucleic acid; Ribose is a constituent of RNA, deoxyribose of DNA.

ii. Synthesis of some co-enzymes e.g. Ribose is used in the synthesis NADP and NAD, FAD.
iii. Synthesis of (ATP), ADP AMP also requires ribose.
iv. Ribulose bisphosphate is the CO2 acceptor and is made from a 5C sugar ribulose.
Hexoses e.g. glucose, fructose, galactose
v. Source of energy when oxidised in respiration; glucose is the most common monosaccharide.
vi. Synthesis of disaccharides; two monosaccharide can link together to form a disaccharide.
vii. Synthesis of polysaccharides; glucose is particularly important in this role
DISACCHARIDES
A disaccharide is a sugar formed as a result of the combination of two monosaccharides sugars. Because of
this reason they are also known as double sugars.
Page 51 of 447
General formula C12H22O11 and not C12H24O12 as expected because these formations involve the loss of one
water molecule as shown in the equation below;
Such a reaction which involves the loss of a water molecule during the synthesis of a new compound, is
known as a condensation reaction. The two monosaccharide units in a disaccharide are held together by a
covalent bond known as a glycosidic bond through the loss of small molecules usually water. A condensation
reaction between the hydroxyl groups at carbon 1 of one monosaccharides and carbon 4 of the other results in
a bond called 1-4 glycosidic bond. If the reaction is between the hydroxyl groups at carbon 1 and carbon 4,
1-6 glycosidic bond.
The addition of water, under suitable conditions, is necessary if the disaccharide is to be split into its
constituent monosaccharides. This is called hydrolysis i.e. breakdown by water.
Most disaccharides are reducing sugars however there are some few which are non-reducing sugars e.g.
sucrose because they lack the reducing group in these molecules.
Like monosaccharides, disaccharides are also sweet, soluble in water and crystalline like monosaccharides
Formation of disaccharides
This is illustrated by the following example

α- glucose α- glucose α- maltose + water
+ C12H22O11 + H2O
C6H12O6 C6H12O6
α- glucose α- fructose α- sucrose + water

+ C12H22O11 + H2O
C6H12O6 C6H12O6
β- glucose β- glucose β- maltose + water

+ C12H22O11 + H2O
C6H12O6 C6H12O6
Page 52 of 447
Note. Monosaccharide
monomers may also combine
other types of molecules to
form conjugated molecules.
Chains of monosaccharide
units can combine with lipids
to form glycolipids, or with
proteins to form
glycoproteins. These
molecules are important in
the cell membrane.
Functions of disaccharides
i.
They are food reserves in organisms and when they are hydrolysed to monosaccharide and used in
cell metabolism.
ii. Storage materials in some plants like sugar canes.
iii. They are energy reserves.
iv. They are the main forms of transport of organic substances in the phloem. Sucrose is particularly
important as the main form of transport of organic solutes in the phloem. This is because sucrose is
soluble but metabolically inert hence does not cause an osmotic pressure in plant cells. Glucose is not
transported because it’s soluble and metabolically active hence causing an osmotic potential in plant
cells which can affect the movement of water in plant cells.
v. Lactose, also called milk sugar, is the nutritional source of energy for infants during nursing. Lactose
makes milk taste sweet and is an ingredient in many processed foods that contain dairy such as
breads, cookies, cakes, doughnuts, breakfast bars and ice cream.
NB: Starch is hydrolysed in plants to maltose so as; -
i. To be transported easily because it’s soluble in water.

ii. Maltose is less reactive hence won’t be used.
POLYSACCHARIDES
These are the sugars formed when many monosaccharides combine as a result of condensation reactions to
form chains.The chains in polysaccharides may be of;
a. Variable length although usually very long.

b. Branched or unbranched.
c. Folded in which case they are suitable for storage e.g. starch
d. Straight or coiled: in which case they are ideal for making meshes and for construction e.g. in cellulose
used in building cell wall.
Most polysaccharides are formed from hexose sugars and the general formula of (C6H10O5) n where n is a number
greater than 40.
Characteristically polysaccharides are un-sweet, insoluble in water and non-crystalline. Due to their
insolubility in water, they form good storage compounds in organisms because they cannot diffuse out of the
cell and they do not affect the osmotic potential of the cells.
Page 53 of 447
The most common polysaccharides are starch, cellulose and glycogen. Other polysaccharides include inulin
and chitin. All polysaccharides are non- reducing sugars.
Upon hydrolysis, polysaccharides can be converted into their constituent monosaccharides such as glucose,
ready for use as a respiratory substrate.
Polysaccharides are normally used for food storage because; -
i. They can be easily hydrolysed to sugars when required for production.

ii. They fold into compact shapes which cannot diffuse out of the cells.
iii. They exert no osmotic or chemical influence on the cell.
iv. They have large sizes that make them insoluble in water.
v. They are non-diffusible i.e. they don’t leave sites of storage
vi. Making structures compact e.g. cellulose.
STARCH
Starch is made up of two major components namely amylose (20%
of starch) and amylopectin (79% of starch). The 1% of starch is
made of other substances such as phosphates.
Starch is made up of many alpha glucose molecules which is found
in most parts of the plant. Starch is made from excess glucose
produced during photosynthesis and it is the reserve food in plants.
It is common in the seeds of most plants such as maize where it forms the food supply for germination.
Amylose consists of unbranched chains while amylopectin consists of branched chains. These chains are
coiled to form a helix in amylopectin where the -OH groups project into the interior and cannot therefore be
free to take part in hydrogen bonding.
For this reason amylopectin has no cross linkages as amylase whose -OH groups point outwards and can
therefore form hydrogen bonds. Therefore starch is not strong enough as a structural polysaccharide like
cellulose. Due to its branching and numerous ends, amylopectin can easily be broken down to maltose by
amylase enzyme at a higher rate as compared to amylose
AMYLOSE AMYLOPECTIN
It has only 1-4 glycosidic bonds It has both 1-4 and 1-6 glycosidic bonds
It stains deep blue with iodine. It stains red to purple with iodine.
Its related molecular mass is 50,000. Its relative molecular mass is 500,000.
It is made up of un branched helical chains. It is made up of branched helical chains.
It is made up of 300 glucose units. It is made up of 1300 glucose units.
Page 54 of 447
GLYCOGEN
This is the major polysaccharide storage material in animals and
fungi. It is stored mainly in the liver and muscles this is mainly It is more soluble in water than starch.
because it provides energy more readily than fat within the active
tissues of the muscles and the liver.
Besides glycogen can be used during anaerobic respiration to
provide energy in the muscles e.g. during heavy and physical
exercise.
Like starch, glycogen is made up of alpha glucose molecules
structure is similar to that of amylopectin except that it has highly
branched short tails of alpha glucose molecules as compared to
amylopectin. Because it is so highly branched, it can be broken
down to glucose very quickly by enzyme glycogen phosphorylase
to release energy.
CELLULOSE
This is a polysaccharide made of many beta glucose molecules that form long unbranched parallel chains.
It is mainly found in plants because it is the main structure material in plant cell walls and in cotton it makes
up to 90%.
Many chains run parallel to each other and have cross linkages between them. These cross linkages give
cellulose its considerable stability which makes it a valuable structural material. This stability also makes it
difficult for animals to digest cellulose and therefore it is not such a valuable food source to the animals.
The difference in the positions of the -OH and the H groups
between the alpha glucose and beta glucose on carbon one
affects the structural properties of cellulose, in that, the - OH
group on carbon 1 in beta glucose faces upwards while it
faces downwards in alpha glucose. This makes the -OH
groups in cellulose to project outwards from both sides at
alternate positions. Cellulose consist of straight chains of
molecules where the -OH groups project outwards on both
sides of the chain to alternate position which enable cellulose The above structure shows cross linkages
to form cross linkages therefore the free -OH groups are in which if combined the strengths of glycosidic
exposed positions for hydrogen bonding with neighbouring - bond and covalent bonds make cellulose such
OH groups of other chains which results in the formation of a very strong polysaccharide suitable for
bundles of cross linked parallel chains. causing strength in the cell walls of plant cells.
Page 55 of 447
Cellulose is commercially used in textiles in the

making of papers, cellophane, tyre cords and
celluloid that is used in making photographic
films because its chains are linked by hydrogen
bonds to make cellulose microfibrils which are
very strong and rigid and therefore give strength
to plant cells and young plants. It is also food for
some bacteria and protozoans
OTHER POLYSACCHARIDES
These include the following;
CHITIN
Chemically and structurally chitin resembles cellulose however it differs from cellulose in
possessing an acetyl group instead of one of the hydroxide groups in beta glucose.
Like cellulose, it has a structural function and it is the major component of the exo-
skeleton of insects and crustaceans. It is also found in fungal cell walls.
2. SUGAR DERIVATIVES
Some compounds contain sugar molecules linked with other non-sugar compounds, such compounds are
called sugar derivatives. Some of these are described below;
a. Mucopolysaccharides. These are formed from amino sugars e.g. glucose amine. An amino sugar is a
sugar containing nitrogen. Examples of mucopolysaccharides include the following;
i. Hyaluronic acid: this forms part of the vertebrate connective tissues. It is therefore found in
cartilage, bones, vitreous humor of the eye and in the synovial fluid. Hyaluronic acid is also
found in anti-coagulant called heparin.
ii. Other mucopolysaccharides are mainly found in the cell walls of prokaryotes such as bacteria.
b. Nucleotides. A nucleotide is where pentose sugars join with organic bases. Nucleotides are the basic
building blocks of nucleic acids such as DNA on which heredity depends and RNA on which protein
synthesis depends. Other nucleotides are mainly used in respiration and these include Adenosine Tri
Phosphate (ATP), Nicotinamide Adenine Dinucleotide (NAD), and Flavine Adenine Dinucleotide
(FAD).
Page 56 of 447
3. Inulin is an unbranched chain of fructose with 1-2 glycosidic bonds found as a storage carbohydrate
in some plants
4. Chitin is an unbranched chain of β-acetylglucosamine units with 1-4 glycosidic bonds
5. Lignin is chemically, it resembles mucopolysaccharides. It is a polymer formed from sugars and

amino acids. It is rigid involving chain molecules which are condensed and it binds cellulose chains to
form microfibrils.
Lignin impregnates the cell walls of water transporting tubes (xylem) to form an impermeable lining,
a process called lignification.
It also prevents rot, infections and decay.
Carbohydrates have a variety of structural features which account for the wide variety of polysaccharide
formed and these include:
 Both pentoses and hexoses can be used to make polysaccharides though normally one type of
monosaccharides is used in each polysaccharide type like hemicellulose, nucleic acid sugars may be
aldoses and ketoses.
 Capacity to form 1, 4 and 1, 6 glycosidic bonds are common between sugar units e.g in cellulose. This
accounts for the case of branching and hence formation of different types of polysaccharides.
 Capacity to form chains of various length and branching
 Existence of alpha and Beta forms of monosaccharide account for the variation of polysaccharides
e.g. starch, alpha glucose monosaccharide while cellulose made of beta glucose units.
 Sugars may be Ketoses or aldoses, these increase the polysaccharide variation like inulin is made of
Ketose monosaccharide units while starch and glycogen are made of aldose monosaccharide units.
 The high chemical reactivity of sugar and OH groups and their variation in exposure increases
polysaccharide variability.
Main functions of carbohydrates
 They are a primary source of energy being oxidized in the body to release energy.
 They are structural components of cells e.g. cellulose making up the cell wall.
 They are determinants of osmotic potential of body fluids therefore maintain blood pressure.
 They are recognition units on the surface of body cells i.e. they are component structures of the
surface cell membranes recognized by antibodies.
Page 57 of 447
 Energy stores/ food stores in form of starch and glycogen
Chemical tests for polysaccharides
Starch
The iodine test is the standard test for starch. Addition of Iodine to a starch containing substance results to a
blue-black colour and absence of starch is manifested by the colour of Iodine remaining unchanged.
Cellulose
The chemical test for cellulose is using the Schultz solution which when added to a cellulose containing
substance turns violet in colour. An alternative test would be conc. Sulphuric acid and Iodine solution and if
the substance contains cellulose, an intense blue colour is observed.
LIPIDS (fats and oils)

Lipids are natural fats and oils made up of carbon, hydrogen and oxygen but the ratio of hydrogen to oxygen
in not 2:1 as in carbohydrates instead the hydrogen atoms are far more than the oxygen atoms. All lipids have
a high proportion of hydro carbon group (CH2) in their molecules. They are insoluble in water but can
dissolve in organic solvents such as chloroform, benzene, acetone, alcohols e.t.c. The low solubility of lipids
is due to the low oxygen content and very many CH2 groups, the numbers of polar -OH groups that are present
in the molecule are very few thus preventing dissolving. It is these polar groups that normally confer solubility
in water (H2O) through ion interaction with water in the case of carbohydrates.
Fats are solids at room temperature whereas oils are liquids. Lipids also include waxes, steroids and
phospholipids.
CONSTITUENTS OF LIPIDS
Lipids are made up of esters called fatty acids and an alcohol of which glycerol is the most common.
Glycerol has three hydroxyl groups (-OH) and each of these may combine - with separate fatty acids forming
triglyceride. This combination occurs by condensation reaction in which three water molecules are formed
and therefore the hydrolysis of the triglyceride will again yield glycerol and 3 fatty acids.
Fatty acids have a general formula of CnH2nO2. Their structural formula can be summarized as below
R(CH2)nCOOH. Where n is any even number between 4 and 24. R can be CH3CH2, CH3CH2CH2 e.t.c.
Fatty acids can be classified as unsaturated if they contain one or more double bonds e.g. oleic acid. Fatty
acids lacking double bonds are said to be saturated e.g. steoric acid. Unsaturated fatty acids melt at a much
lower temperature than saturated fatty acids. Consequently, saturated fatty acids are normally found in fats
while unsaturated fatty acids are commonly found in oils. Lipids vary due to the presence of many fatty acids.
Fatty acids Formula Saturation Sources

Fats differ from oils in two
Linolenic acid C17H31COOH Unsaturated Vegetable oil fundamental ways;
Linoleic acid C17H31COOH Unsaturated Sunflower oil a) Fats are made from saturated
fatty acids while oils are
Oleic acid C17H33COOH Unsaturated Olive oil made from unsaturated fatty
acids.
Page 58 of 447
Palmitic acid C15H31COOH Saturated Palm oils b) Fatty acids in oils are smaller
than those in fats.
Stearic acid C17H35COOH Saturated Adipose fats
Arachidonic acid C19H31COOH Unsaturated Meat, eggs, fish
Lauric acid C11H23COOH Saturated Coconut oil
GLYCEROL (Propan-1, 2, 3-triol)
This is an alcohol with the molecular formula of C3H8O3.

There is only one type of glycerol that exists in both fats
and oils whose structure is shown on the right
FORMATION OF A TRIGLYCERIDE
During its formation, a condensation reaction occurs

in which 3 fatty acids of the same type or different
types, combine with one glycerol molecule. During this
reaction, the hydroxyl group of glycerol reacts with a
carboxyl group (COOH) of the fatty acids to form
water and triglyceride joined by ester bonds as
illustrated on the right;
a) Because fatty acids are synthesized from fragments

containing two carbon atoms, the number of carbon
atoms in the lipid chains is always an even number.
b) Lipids require too much oxygen to be oxidized in

respiration as compared to glycogen and are
therefore used in respiration.
ESSENTIAL AND NON-ASSENTIAL FATTY ACIDS
The essential fatty acids are the ones which cannot be synthesized by the body and must therefore be obtained
from the diet e.g. linoleic acid and linolenic acid. A common dietary source for these fatty acids which are
essential in our bodies is vegetables and seed oils. Deficiency of essential fatty acids results into retarded
growth or reduction in the growth rate, reproductive deficiency and even kidney failure.
STEROIDS AND WAXES
WAXES STEROIDS
These are similar to lipids in composition except that the fatty These are lipids whose molecules
acids are linked to long chained alcohols instead of glycerol. contain 4 rings of carbon and
These form a water proof layer on the surfaces of most terrestrial hydrogen atoms. Steroids are therefore
plants and animals. They may also be used as a form of storage bigger than the common lipids and they
in a few compounds such as castor oil and in fish. are saturated hydro carbons.
Page 59 of 447
The functions of some important steroids are given below;
STEROID FUNCTION
Cholesterol It is a major component of the cell membrane.
It is a raw material for many other steroids.
Bile acids (glycocholic acid and taurocholic acid) These are used in emulsification of fats during digestion.
sex hormones
a. Oestrogen and progesterone These are reproductive hormones in female mammals

which regulate the menstrual cycle and controlling
pregnancy
b. Testosterone This is a reproductive hormone in male mammals
controlling sexual behavior and sperm production.
Vitamin D (Calciferol) It promotes calcium and phosphate absorption and

metabolism
It is also important for the hardening of bones and teeth
Ecdysone (Moulting hormone) It causes moulting (shedding off the cuticle in arthropods)
PHOSPHOLIPIDS
A phospholipid differs from having a phosphate The phosphate group is electrically charged (PO43-) and therefore polar
group (PO43-) group attached to one of the hydroxyl and so unlike fatty acids dissolve in water. Phospholipids are therefore
groups of glycerol such that they have two fatty able to dissolve in both water and organic substances i.e. phospholipids
acids linked to glycerol by condensation reaction are both hydrophilic and hydrophobic. This property of phospholipids
instead of three fatty acids. is important in determining the structure and functioning of the cell
membrane. In water, phospholipid molecules collect together in a single
Other groups including nitrogenous bases could
layer (monolayer) with the hydrophilic head poking into the water. In
even be attached to this phosphate group to make
cells, both the intracellular environment and immediate external
the structure even more complex. environment are watery. This causes phospholipids to form a double
layer, with the hydrophobic tails pointing inwards, away from the
watery environment. The phospholipids bilayer gives cell membranes
their fluid properties and allows lipid soluble substances to pass easily
through them
1. Glycolipids
They are lipids with a carbohydrate attached by a glycosidic bond. Their role is to serve as markers for
cellular recognition. The carbohydrates are found on the outer surface of all eukaryotic cell membranes
2. Lipoproteins
This forms part of the cell membranes and it is the chemical form in which lipids are transported.
3. Steroids
Page 60 of 447
These are lipids whose molecules contain 4 rings of Carbon and Hydrogen atoms. Three of the rings are
six numbered and one of them is five numbered. All together there are 17 carbon atoms, six of which are
shared between the rings and they are saturated hydrocarbons. They cannot be hydrolysed. Some are
formed by the smooth ER of cell membranes
Functions of lipids
1. An energy source.
Lipids store more energy than similar quantities of carbohydrates. Upon hydrolysis lipids yield more
energy than carbohydrates i.e. lipids yield 38KJg-1 of energy compared to 17KJg-1 for the carbohydrates.
This is so because of many covalent bonds of carbon to carbon (C-C) and carbon to hydrogen (C-H) type
that are present in lipids due to many hydrogen atoms they contain. These bonds contain large quantities
of energy that can be released and used by the cell when required.
Therefore carbohydrates yield less energy for the cell but are readily hydrolysed than lipids.
2. Storage of materials
Lipids are good storage compounds in the body e.g. they store a lot of water and fat soluble vitamins e.g.
A, D, E and K. Lipids are good storage compounds because of the following reasons;
 They are insoluble in water and therefore cannot dissolve away and cannot affect the osmotic
potential of the cells
 They are much lighter than carbohydrates so as to keep the weight to the minimum
 They have a high calorific value i.e. they have a high energy content
 They are compact and therefore they take up very little space in the cells
 Lipids are poor conductors of heat in the body
3. Lipids insulate the body against heat loss as they are poor conductors of heat. This explains why the
major fat deposits of the body are found under the skin as subcutaneous fat layer, and around vital
organs such as the heart, kidneys, lungs, intestines e.t.c. whose temperatures should not vary much.
Aquatic mammals, e.g. whales, seals and manatees, have an extremely thick subcutaneous fat, called
blubber, which forms an effective insulator.
4. Fats are used as packing material around delicate organs of the body such as kidneys, heart, lungs and
intestines so as to protect them from physical damage by acting as shock absorbers.
5. Lipids speed up impulse transmission along nerves using the myelin sheath
6. Lipids are useful source of metabolic water for desert animals when broken down in respiration
7. Plant scents are fatty acids or their derivatives and so aid in the attraction of pollinators
8. Lipids form very important structures in organisms, the structures include;
 They form the phospholipid layer of the cell membrane by combining with phosphorous to form the
phospholipids
 They form the subcutaneous fat layer beneath the dermis of the skin
 They form the waxy cuticle of the insects and plants which prevent excessive water loss
 They form the adipose tissue usually around the delicate organs such as the heart
 They form suberin in plant cell walls especially in endoderm cells
 Bees use wax in constricting their honey combs
PROTEINS
Proteins are complex organic compounds with a large molecular mass made of small units called amino acids. Amino
acids consist of carbon, oxygen, hydrogen, nitrogen and in some cases sulphur. They are not truly soluble in
water, but form colloidal suspensions. Proteins are rarely stored by organisms except in eggs or seeds where
they are used to form the new tissue. The variety of proteins is unlimited because the sequence of amino acids
in each protein molecule which is genetically determined by DNA within cells during protein synthesis.
Page 61 of 447
Proteins are the most abundant molecules to be found in the cells and comprise over 50% of their total dry weight. They
are therefore an essential component of the diet of animals and may be converted to both fats and carbohydrates by the
cells. All proteins are composed of basic structural molecules known as amino acids.
AMINO ACIDS
There are 20 common naturally occurring amino acids whose different combinations result in a great variety
of the proteins since each amino acid has its own set of properties. The general formula of amino acids is
RCHNH2COOH whose structure is shown below;
The structure shows that amino acids are composed of four different
parts namely;
1. A hydrocarbon group (-CH)
2. A carboxyl group (COOH)
3. An amino group (NH2)
4. An R group. It is in this R group of the amino acid that lies the difference in the amino acid e.g. in
amino acids, glycerine which is the simplest amino acid, R is a hydrogen atom while it’s a methyl
group (CH3) in amino acid alanine.
Amino acids are soluble in water but insoluble in organic solvents. At neutral pH (found in most living organisms), the
groups are ionized as shown above, so there’s a positive charge at one end of the molecule and a negative
charge at the other end. The overall net charge on the molecule is therefore zero.
The presence of an amino group which is basic and a carboxyl group which is acidic in all amino acids
accounts for the name amino acids and also confer on the amino acids on amphoteric nature i.e. amino acids
have both acidic and basic properties. This implies that amino acids can donate hydrogen ion (protons) as
acids do and also can accept hydrogen ions (protons) as bases do. In amino acids, these abilities to donate or
receive protons are conferred by a carboxyl and amino groups respectively.
Their amphoteric nature is useful biologically as it means that they can act as buffers in solutions thereby
resisting changes in the pH of the solution. A buffer solution is the one which is able to resist changes in the
pH of the solution. Amino acids therefore can donate hydrogen ions as the pH increases so as to lower the pH
and also accept hydrogen ions from the solution as the pH decreases so as to raise the pH. Amino acids
therefore play an important role as buffer in the tissue fluid and in the cytoplasm of most cells thereby
maintaining the pH within the narrow limits needed for normal metabolism and efficient enzyme functioning.
This is because changes in pH denature enzyme which can be fatal to the living organism.
The charge on the amino acid changes with pH as shown below;
It’s these changes with change in pH, that explain the

effect of pH on enzymes. A solid, crystallised amino acid
has the uncharged structure, ,

but this form never exists in solution, and therefore
doesn’t exist in living things (although it is the form given
in most text books)
Page 62 of 447
AMINO ACIDS AND DIET
Amino acids are classified into two groups namely; essential and non-essential amino acids.
Essential amino acids The non-essential amino acids. These are amino acids
which the body can synthesise in such sufficient quantities
These are the amino acids which cannot be for them not to be required in the diet. The absence of one or
synthesized by the body and therefore must be more of these amino acids results in retarded growth and
obtained from the diet. particular symptoms, characteristics of the particular amino
These amino acids include the following acid lacking. Non-essential acids include the following
1. Isoleucine 4. Leucine 7. Lysine 1. Alanine 4. Aspartic acid 7. Glutamic acid

2. methionine 5. threonine 8. phenyalanine 2. Glycine 5. Proline 8. Serine 10.
3. tryptophan 6. valine 9. arginine 10. Tyrosine
histidine
Foods containing all the essential amino acids are 3. Cysteine 6. Asparagine 9. Glutamine
known as first class protein food and such foods
include all animal proteins and some few plant Non-essential amino acids are synthesised in the body
proteins e.g. the soya bean. Food lacking one or through a process known as transamination which involves
more essential amino acids is known as second the use of enzymes known as transaminases , the raw
class protein food and this includes most plant materials for this process are the essential amino acids
proteins and a few animal proteins. provided in the diet and carbon dioxide derivatives e.g.
pyruvic acid which is obtained from the breakdown of sugar
during respiration
FORMATION OF A POLYPEPTIDE
Initially two amino acids are united in a
condensation reaction to form a dipeptide with
the loss of a water molecule. Later several
dipeptides combine in several condensation
reactions to form polypeptides which consist of
up to 500 amino acids or more. The individual
\ amino acids within the polypeptide chin are
linked by peptide bonds to form a protein.
These polypeptides made are then folded and
twisted in an appropriate way as directed by a
particular gene (DNA) which also determines the
sequence of amino acids in the chain. This is
illustrated on the right
Amino acids are able to form other bonds with reactive groups apart from the peptide bond. Such bonds
include the following;
Page 63 of 447
1. the ionic bond
At a suitable pH an interaction may occur between ionised amino

groups and a carboxyl and this result into the formation of an ionic
bond between the two amino acids. This bond can be easily broken in
an aqueous medium by changing the pH of the medium.
2. the disulphide bond
This bond arises between sulphur containing groups of any two

oxidised cysteine molecules of amino acids. Disulphide bonds may be
formed between different parts of the same chain (hence folding the
3. theinto
chain hydrogen bond
a particular structure) or different chains of amino acids.
They are strong and not easily broken.
This occurs between certain hydrogen atoms and certain oxygen atoms that contain ion pairs of electrons. The
hydrogen bond is weak, but as it occurrence is more frequent, the total effect makes a considerable
contribution towards molecular stability, as in the structure of the α-helix.
4. Hydrophobic interaction
Within a polypeptide chain, hydrophobic interactions or bonds can be registered. They arise in situations
where the R-groups are non-polar and therefore hydrophobic. The polypeptide chain will tend to fold so that
the maximum number of hydrophobic groups come into close contact and exclude water. This is how many
globular proteins fold up. The hydrophobic groups tend to point inwards towards the centre while the
hydrophilic groups face outwards in the aqueous environment making protein soluble. They are also weak
bonds.
All the three types of bonds above are shown in the image above;
CLASSIFICATION OF PROTEINS
Proteins are classified according to their orders of organization, particularly of the amino acids within the
peptide chains. The proteins are also classified as primary structure proteins, secondary, tertiary and
quaternary structure proteins.
PRIMARY STRUCTURE
This refers to the sequence of amino acids found in the One major importance of the primary structure
polypeptide chains of the proteins. This sequence of the protein in relation to function is found in
determines the properties and shape of the proteins. The enzymes, in which the structural configuration
primary structure is specific for each protein and is of the active site of the enzymes determines
determined by the DNA of the cell from which it is made. whether a particular substrate will fit in the
active site of that enzyme.
A primary structure is held together by the covalent bonds
called peptide bonds between adjacent amino acids. All The primary structure is clearly shown by
other protein structures are modifications of these primary insulin hormone.
structures.
Page 64 of 447
SECONDARY STRUCTURE
This refers to the regular arrangement of the polypeptide chains of the proteins as a result of hydrogen
bonding which can be either alpha-helix or beta-pleated sheets. This is because after their formation, the
chain of amino acids in the polypeptide folds spontaneously to make complex configurations categorised
into alpha-helices or beta-pleated sheets held together by hydrogen bonds.
An alpha-helix is the one in which the polypeptide chain is loosely coiled into a regular spiral shape joined
by numerous hydrogen bonds. It is regular in that the repeating constituents of the polypeptide backbone in
the spirals are at a specific distances. The β-pleated sheets are chains of polypeptides arranged in a zigzag
format with antiparallel strands held together by hydrogen bonds.
α-helix β- helix
The hydrogen bonds stabilise the helix by joining together the amino group of one turn and a carboxyl group
of another turn. Therefore, the importance of the secondary structure is that it maintains a particular shape of a
protein keeping it stable by twisting it.
This secondary structure is of greater importance in the biological function of proteins particularly enzymes
and antibodies whose efficiency depends on maintaining a particular shape. It is also important in the
formation of fibrous proteins which are insoluble in water and are resistant to changes in temperature and pH.
The secondary structure of a protein is of particular importance in the formation of structural proteins such as
keratin, silk and collagen. Keratin is a fibrous protein found in the hair, nails, horns, feathers and wool.
Collagen is also a fibrous protein found in mammalian connective tissue such as bones, cartilage, tendons and
the skin. Both keratin and collagen contain a secondary structure in the form of an alpha –helix.
TERTIARY STRUCTURE
This is a structure resulting from other uniform coiling and
folding of the polypeptide helix in to a very compact structure.
For this to happen all the three types of bonds namely, ionic,
hydrogen and disulphide bonds must be present in the protein
so as to contribute to the maintenance of the structure
It is the structure which explains the complex molecular shape
of some proteins especially globular proteins, especially
enzymes, myoglobin and insulin.
This structure contains many cross linkages formed by many bonds within the polypeptide chains which make
the proteins strong molecules.
These are soluble in water because they consist of polar groups and amino acids which congregate outside and
interact with water. There hydrophobic chains contain non polar amino acids and are usually pushed inwards
into the centre of the molecules.
Page 65 of 447
QUATERNARY STRUCTURE
This is the structure which arises from the combination of a number
The structure of haemoglobin is
of different polypeptide chains and associated non protein groups
shown below;
into a large protein molecule. Such a structure is shown by
haemoglobin.
Structurally haemoglobin consists of 2 α-polypeptide chains and 2
β-polypeptide chains arranged around a complex ion containing
prosthetic groups called haem groups. Such polypeptide chains are
normally fitted together in such a way that they form larger and
more complex protein structure.
The 4 polypeptide chains in heamoglobin are called globin. Each
chain in haemoglobin carries a haem group to which one molecule
of oxygen bonds.
Summary
Proteins are classified into two main groups on the basis of their tertiary structure
a) Fibrous proteins. These have a primary structure of regular repetitive sequences. They form long chains
ture;
which may run parallel to one another, being linked by cross bridges. They are very stable molecules and
have structural roles with organisms e.g. collagen.
b) Globular proteins. They have irregular sequences of amino acids in their polypeptide chains. They are
compact and are far less stable and have metabolic roles within organisms. All enzymes are globular
proteins.
c) Conjugated proteins. These are proteins with other chemicals incorporated within their structure and the
non-protein part is referred to as the prosthetic group. If the prosthetic group in a protein is organic in
nature then such a group is called a co-enzyme. If the prosthetic group is inorganic in nature then such a
Examples of conjugated proteins
group
Nameis ofcalled a co-factor.
protein Location Prosthetic group
Haemoglobin Blood Haem (containing iron)
Mucin Saliva Carbohydrate
Casein Milk Phosphoric acid
Cytochrome oxidase Electron carrier pathway of Copper
cells
Nucleoprotein Ribosomes Nucleic acid
Page 66 of 447
Comparison of globular proteins and fibrous proteins
Fibrous proteins Globular proteins
Repetitive regular sequence of amino acids Irregular amino acid sequence
Actual sequence may vary slightly between two Sequence highly specific and never varies between
examples of the same protein two examples of the same protein
Polypeptide chains form long parallel strands Polypeptide chains fold into a spherical shape
Length of chain may vary between two examples of Length always identical in two examples of the
the same protein same protein
Stable structure Relative unstable
Insoluble Soluble
Support and structural functions Metabolic functions
e.g. collagen and keratin e.g. enzymes, hormones and haemoglobin
CHARACTERISTICS OF PROTEINS
1. They are colloidal in nature

In solution, proteins form colloids since they have large sizes, they do not go into true solutions but form
colloidal suspensions. A colloidal is a particle which remains suspended in solution rather than dissolving,
settling down or floating. I.e. too small to settle out under gravity but also too large to dissolve.
The importance of colloids being dispersed in solution is that it gives them a large surface area which makes
them very reactive. This is important in enzymes.
2. They have amphoteric properties

Proteins are amphoteric i.e. they have basic and acidic properties. The basic and acidic properties.
3. They are made of large molecules

4. They show specificity e.g. in enzymes which are specific in nature
5. On hydrolysis, they yield a mixture of amino acids.
6. They are insoluble in organic solvents.
Protein denaturation
The three-dimensional structure of a protein is, in part at least, due to fairly weak ionic and hydrogen bonds.
Any agent which breaks these bonds will cause the three-dimensional shape to be changed. In many cases, the
globular proteins revert to a more fibrous form. This process is called denaturation. The actual sequence of
amino acids is unaltered; only the overall shape of the molecule is changed. This is still sufficient to prevent
the molecule from carrying out its usual functions within an organism. Denaturation may be temporary or
permanent and is due to a variety of factors as shown in the table below;
Factor Example Explanation
Heat Coagulation of albumen (boiling eggs Causes the atoms of the protein to vibrate more
makes the white more fibrous and less due to increased kinetic energy, thus breaking the
soluble) hydrogen and ionic bonds
Page 67 of 447
Acids The souring of milk by acid e.g Additional H+ ions in acids combine with
Lactobacillus bacterium produces lactic COO- groups on amino acids and form COOH,
acid, lowering the pH and causing it to ionic bonds are hence broken.
denature the casein, making it insoluble
Alkalis and thus forming curds Reduced number of H+ ions causes NH3+ to lose
H+ ions and form HN2, hence ionic bonds are
broken.
Inorganic Many enzymes are inhibited by being The ions of heavy metals such as mercury and
chemicals denatured in the presence of certain silver are highly electropositive. They combine
ions, e.g. cytochrome oxidase with COO- groups and disrupt ionic bonds.
(respiratory enzyme) is inhibited by Similarly, highly electronegative ions e.g. cyanide
cyanide. (CN-), combine with NH3+ groups and disrupt ionic
bonds.
Organic Alcohol denatures certain bacterial Organic solvents alter hydrogen bonding within
chemicals proteins. This is what makes it useful proteins.
for sterilization.
Mechanical Stretching hair breaks the hydrogen Physical movement my break hydrogen bonds.
force. bonds in the keratin helix. The helix is
extended and hair stretches. If released,
the hair returns to its normal length. If,
however, it is wetted and then dried
under tension, it keeps its new length-
the basis of hair styling.
Renaturation
This is the reconstruction of a protein that has been denatured to a small extent such that its molecules regain
the original 3-dimensional configuration and function by providing them with the ideal conditions of mainly
the pH, and temperature. If the degree of denaturation is great, renaturation cannot take place even if the ideal
conditions are provided.
Functions of proteins
Vital activity Protein function Function
Nutrition Digestive enzymes, e.g. trypsin Catalyses the hydrolysis of protein to polypeptides
Amylase Catalyses the hydrolysis of starch maltose
Lipase Catalyses the hydrolysis of fats to fatty acids and glycerol
Fibrous proteins in granal Help to arrange chlorophyll molecules in a position to receive

lamellae maximum amount of light for photosynthesis
Mucin (1) Assists trapping of food in filter feeders.

(2) Prevents autolysis. (3) Lubricates guts wall.
Casein Storage protein in milk
Page 68 of 447
Ovalbumin Storage protein in egg white
Respiration Haemoglobin/ haemoerythin/ Transport of oxygen

and transport haemocyanin/ chlorocruorin
Myoglobin Stores oxygen in muscle
Prothrombin/fibrinogen Required for the clotting of blood
Mucin Keeps respiratory surface moist
Antibodies Essential to the defence of the body, e.g. against bacterial

invasion
Growth Hormones, e.g. thyroxine Controls growth and development
Excretion Enzymes, e.g. urease, arginase. Catalyses reactions in the ornithine cycle thus useful in protein
breakdown and urea formation
Support and Actin/myosin Needed for muscle contraction

movement
Ossein Structural support in bones
Collagen Gives strength with flexibility in tendons and cartilage
Elastin Gives strength and elasticity in ligaments
Keratin Tough for protection, e.g. in scales, claws, nails, hooves, skin.
Sclerotin Provides strength in insects exoskeleton
Lipoproteins Structural components of all cell membranes
Sensitivity Hormones, e.g. insulin/glucagon, Controls blood sugar level

and adrenocorticotrophic hormone, Controls the activity of the adrenal cortex
coordination vasopressin Controls blood pressure
Rhodopsin/opsin Visual pigments in the retina, sensitive to light
Phytochromes Plant pigments important in control of flowering, germination,

e.t.c.
Reproduction Hormones e.g. prolactin Induces milk production in mammals
Chromatin Gives structural support in chromosomes
Gluten Storage protein in seeds, nourishes the embryo
Keratin Forms horny and antlers which may be used for sexual display
CLASSIFICATION OF PROTEINS ACCORDING TO FUNCTIONS
1. Enzymes. These are biological catalysts which control chemical reactions in organisms e.g. amylase
2. Structural proteins. These form part of the body of organisms e.g. collagen which makes up tendons and
ligaments. Keratin is a major component of hair and nails
3. Signal proteins. These carry messages around the body e.g. insulin hormone and glucagon involved in
controlling glucose levels in blood
4. Contractile proteins. These are involved in movement after contraction e.g. actin and myosin which are
proteins that aid muscle contraction
Page 69 of 447
5. Storage proteins. These keep materials e.g. albumen in the egg which nourishes the chick while it is still
inside the egg.
6. Defensive proteins such as antibodies, thrombin, fibrinogen which are important for fighting infections.
7. Transport proteins e.g. haemoglobin which carries oxygen around the body
ENZYMES
An enzyme is an organic catalyst protein in nature which speeds up the rate of metabolic reactions in an
organism without itself undergoing a permanent change.
Without enzymes the reactions that occur in living organisms would proceed so slowly, if at all, to cope up
with the rates required for maintenance in life. Also increasing the rate of a body reaction would be by
increasing the temperature of the body. This would denature proteins, disrupt membranes and be very
expensive in terms of energy expenditure. Enzymes therefore enable metabolic reactions to proceed rapidly
and at low temperatures.
Enzyme reactions may be described as either catabolic, if they are involved in the breakdown of compounds
or anabolic, if they are involved in the synthesis of compounds. The total of all catabolic and anabolic
reactions in a living cell or organism is what is called metabolism of the cell or organism.
THE CONTROL OF METABOLIC PATHWAYS
Commonly a number of enzymes are used in sequence to convert one substance into one or several products
via a series of intermediate compounds. The chain of reactions involved in converting the substrates to their
products through a series of intermediate compounds i.e. known as the metabolic pathway.
Enzyme C
Enzyme A Intermediate Enzyme B Intermediate
Substrate A Product D
compound B compound C
Negative feed back
Many such pathways can proceed simultaneously in a single cell. The reactions proceed in an integrated and
controlled way and this can be attributed to the specific nature of enzymes.
A single enzyme will catalyse only a single reaction, therefore enzymes serve to control the chemical
reactions that occur within the cells and ensure that these reactions proceed at an efficient rate. The cells also
make use of the properties of enzymes to exercise control over metabolic pathways as illustrated in the
example above. The high concentration of the end product of the pathway may inhibit the enzyme at the start
of the pathway this is called end product inhibition.
In the example illustrated above, end product D acts as an inhibitor to enzyme A. If the level of product D
falls, this inhibition is greatly reduced and so more of substrate A is converted to B, more of B is converted to
C, and finally more of C is converted to D. If the level of end product D rises above normal, inhibition of
enzyme A increases greatly and so the level of D is reduced. This is because substrate A will no longer be
converted to intermediate compound B. In this way homeostatic control of D is achieved. The mechanism is
termed as negative feedback because the information from the end of the pathway which is feedback to the
start of the pathway has a negative effect i.e. a high concentration of product D reduces its own production
rate.
Control of the metabolic pathways has the following advantages;
Page 70 of 447
a. It allows energy to be derived in usable form from many small catabolic reactions than it would be in a
single large reaction.
b. It allows substrates to be partially broken down so as to provide raw materials for other reactions in the
cell. Some of the intermediate compounds formed in the pathway have increased functions to perform
within the cell.
c. It allows the synthesis of complex organic compounds from simple raw materials using the genetic
conditions prevailing in the cells which would not be synthesized in one step pathway.
d. It increases the ability of the cell to control the products made in anabolic pathways when the reactions in
them proceed in small steps.
CLASSIFICATION OF ENZYMES
TYPES OF ENZYMES
An enzyme name is based on two criteria;
a. The name of the substrate acted upon b. The type of the reaction it catalyses e.g.
by the enzyme e.g. succinate dehydrogenation, hydrolysis, polymerization,
dehydrogenase acts on succinic acid. decarboxylation e.t.c.
In most cases an enzyme is named by (1) DNA polymerase which catalyze the formation of
attaching the suffix “ase” to the name of DNA by polymerization of DNA nucleotides
the substrate on which it acts for (2) RNA polymerase which catalyses the formation of
example; (1) Proteins to protease (2) RNA by polymerization of RNA nucleotides.
Lipids to lipase (3) Maltose to maltase (3) Cytochrome oxidase catalyses oxidation reactions of
(4) Sucrose to sucrase cytochrome proteins
However, enzymes like Pepsin and Trypsin do not follow this naming convention
THE STRUCTURE AND MECHANISM OF ACTION OF ENZYMES
ENZYME STRUCTURE
Structurally an enzyme is a complex three dimensional globular protein some of which have other associated
molecules.
Even though the enzyme molecule is normally larger than the substrate molecule it acts upon, only a small
part of the enzyme molecule actually comes into contact with the substrate. This region of the enzyme
molecule which comes into contact with the substrate is called the active site.
Only a few of the amino acids of the enzyme molecule actually make up the specific sequence of amino acids
that make up the active site. The rest of the amino acids in the enzyme molecule are used to maintain the
globular structure of the enzymes.
Page 71 of 447
ENZYME GROUP TYPE OF REACTION CATALYSED EXAMPLES
Oxido reductase These catalyse the transfer of oxygen and hydrogen Oxidase
atoms between substances i.e. they catalyse redox
reactions Reductase
Transferases These catalyse the transfer of one chemical group Transaminases

from one substance to another.
Phosphorylase
Hydrolases These catalyse hydrolysis reactions Lipases
Peptidases
Phosphatases
Lyases These catalyse the addition or removal of a chemical Decarboxylases

group other than hydrolysis.
Isomesales These catalyse the re-arrangement of groups within a Isomesales

molecule. In other words it converts one isomer into
another Mulales
Ligases This catalyses the formation of bonds between two Synthetases

molecules using energy derived from the breakdown
of ATP
The specific sequence of amino acids in the active site gives the active site of a specific configuration. It is
the active site configuration which controls enzyme functioning and properties. It is at the active site that
bonding of substrates occurs.
THE MECHANISM OF ENZYME ACTION

Enzymes generally work by lowering the activation
energy. Enzymes therefore make it easier for a reaction
to take place than it would without them.
How an enzyme lowers activation energy of the
reaction is explained by a number of mechanisms
described below;
THE LOCK AND KEY HYPOTHESIS
According to this hypothesis, enzymes have active
sites into which specific substrate molecules fit
exactly. The substrate molecule is the key whose shape
is complementary to that of the enzyme active site
The enzyme is the lock where the substrate fits therefore both the enzyme and the substrate have the
complementary structures.
The substrate molecules combine with an enzyme molecule to form a compound called enzyme substrate
complex.
Page 72 of 447
When the substrate binds with the enzyme molecule, the substrate molecules become slightly distorted putting
a strain on the bonds of the substrate molecules which results into breaking of these bonds and rejoining them
using less energy.
The enzyme-substrate molecule forms an enzyme-end product complex which splits into the enzyme and the
end products. The enzyme remains unchanged while the products are released from the active sites since they
have a different shape from the substrate.
The lock and key hypothesis is important in that it explains the various properties of enzymes in the following
ways;
a) It explains the specificity of enzymes because it shows that only substrates with complementary
shapes to the active sites can actually fit into the active sites to form products.
b) It explains how enzymes can be used over and over again. In other words, it shows that once the
active site is set free at the end of the reaction, another substrate can combine with it to form an
enzyme substrate complex.
c) It explains why to some extent the rate, of an enzyme controlled reaction is limited by increasing the
substrate concentration. This is so because the reaction is inhibited when all the active sites of an
enzyme have been bonded to.
d) It explains why and how enzymes can be inhibited this is because inhibitors having a similar shape to
that of the active site of the enzyme may occupy the active site before the substrate and prevent the
substrate from occupying the active site hence inhibiting the reaction.
e) It further explains how heating lowers the rate of a controlled reaction. This is because heating
denatures the enzyme their by changing its shape which prevents the substrate from fitting into the
active site.
f) Also changes in PH break the bonds which maintain the three dimensional shape of the enzyme and as
a result change the active site configuration. This makes the substrate fail to fit through the active site.
g) It explains why enzymes are protein in nature because the structure of proteins is based on a sequence
of amino acids in their primary structures which sequence also exists in the active sites of enzymes
thereby determining the properties of enzymes.
h) It explains how enzymes reduce the activation energy of a chemical reaction by showing that when a
substrate binds to the enzyme, substrate molecule becomes slightly distorted which strains the bonds
in it and as a result less energy is needed to break the bond.
i)
THE INDUCED FIT HYPOTHEISIS
This alternative hypothesis is proposed in line with more recent evidence that the lock and key are not actually
static but are able to change their shapes during combination so that the two fit each other properly .In the
presence of the substrate, the active site of an enzyme may change in order to suit the shape of the substrate.
Page 73 of 447
The enzyme in this hypothesis has a binding site configuration which attracts the substrate. On binding to the
enzyme the substrate disturbs the shape of the active site and causes it to assume a new configuration. It is this
new configuration which allows the substrate to suit properly in the active site and this enables the formation
of an enzyme substrate complex in which the substrate molecules become slightly distorted. This strains the
bonds in a substrate and as a result less energy is needed to break these bonds to form an enzyme product
complex.
PROPERTIES OF ENZYMES
The properties of enzymes can be explained in relation to the lock and key hypothesis and the induced fit
hypothesis. These properties include the following;
a) They are protein in nature.

b) They are all produced in living cells.
c) They are soluble in water like any other globular proteins.
d) They are not used up in the reactions they catalyse and therefore can be used over and over again.
e) They work in very small quantities.
f) They remain chemically unchanged by the reactions they catalyse.
g) They are usually specific in their actions.
h) They are denatured at higher temperatures beyond the optimum temperature and inactivated by lower
temperatures.
i) They are sensitive to change in pH. PH ranges out of the range in which enzymes work best denature
enzyme and make them unable to catalyse reactions.
j) They can work in either direction and this means that their reactions are reversible.
k) Their reactions can be inhibited.
l) They generally work very rapidly in their reactions. Their speed of action is known as the turn over
number i.e. defined as the number of substrate molecules which molecules of an enzyme turn into
products per minute. Some of the fastest enzymes are catalase (turn over number is 6 million) and
carbonic anhydrase (turn over 36 million).
m)
THE RATE OF ENZYME CONTROLLED REACTIONS
The rate of an enzyme controlled reaction is measured by the amount of substrate changed into products or
The factors affecting the rate of reactions include the following;
Page 74 of 447
1. The concentration of an enzyme
Provided that the substrate concentration is maintained at a high level

and other conditions such as pH and temperature are maintained
constant, the rate of a reaction increases with increase in enzyme
concentration until when the rate remains constant. Usually the
enzyme concentration is much lower than the substrate concentration.
Therefore as the enzyme concentration increases, the rate of substrate
is either being exhausted in the reaction or greatly reduced thereby
limiting the reaction.
2. Substrate concentration
The rate of enzyme controlled reaction increases with increase
in the substrate concentration for a given quantity of an enzyme
until such a concentration when all the active sites of an enzyme
are saturated. At such concentration the rate of reaction becomes
constant or levels. After leveling of the rate of the reaction, the
rate can only be increased by increasing enzyme concentration
which would provide new active sites for the substrate.
The increase in substrate concentration increases the interaction
between the enzyme molecules and the substrate molecules
which increases the rate of collision between the enzyme and
the substrate so as to form the products.
3. Temperature
An increase in temperature affects the rate of an enzyme controlled reaction in two ways;
a. As the temperature increases the kinetic energy of the substrate and enzyme molecules also increases and so
they move fast. The faster these molecules move, the more they collide with one another and therefore the
greater the rate of reaction.
b. Secondly as temperature increases more atoms which make up the enzyme molecules vibrate. These vibrations
break the hydrogen bonds and other forces which hold the molecules in there precise shape hence changing
enzyme active sites. The three dimensional shape of the enzyme molecules is therefore changed by these
vibrations as the bonds, hydrogen bonds and hydrophobic interactions, which were holding it get broken to
such an extent that the active site no longer allows the substrate to fit. Under these conditions the enzyme is
said to be denatured by the increasing temperature and therefore loses its catalytic properties. Therefore
increasing the temperature beyond the optimum temperature rapidly denatures enzymes and very low
temperatures inactivate enzymes. At the optimum temperature enzymes attain there maximum activity thereby
providing the maximum rate of the reaction. Inactivated enzymes are not denatured and therefore they can
regain their catalytic properties when higher temperatures are provided.
Note. The optimum temperature for an enzyme varies considerably. Many arctic and alpine plants have
enzymes which function at a temperature 100C, whereas those in algae inhabiting some hot springs continue to
function at temperatures around 800C. For many enzymes, the optimum temperature lies around 40 0C and
denaturation occurs at about 600C.
Page 75 of 447
4. PH
The hydrogen bonds which make up the three dimensional

molecular shape of the enzyme may be broken by the
concentration of hydrogen ions present. PH is the measure
of the hydrogen ion concentration. By breaking the
hydrogen bonds which give enzyme molecules their shape,
any change in the pH can effectively denature enzymes.
Each enzyme works best at a particular pH and deviations
from this optimum pH may result into denaturing of these
enzymes.
Inhibition
The rate of enzyme controlled reaction may be decreased by the presence of inhibitors. There are two types of inhibition
namely;
I. Competitive inhibition.
II. Non- competitive inhibition.
Competitive inhibition
This is where inhibitors are structurally similar to the substrate molecules and as a result compete with the substrate for
the active site on the enzyme molecule.
The degree of inhibition depends on the relative concentration of a substrate and inhibitor. This inhibition is therefore
always reversible i.e. the inhibition effect can be removed by increasing the concentration of the substrate. This inhibition
occurs when the inhibitor is of a higher concentration than the substrate. This inhibition is therefore temporary and
therefore does not cause permanent change to the enzyme
Once the inhibitor combines with the enzyme active site it prevents the substrate molecules from occupying the active
site and so reduces the rate of the reaction. Melanic acid is an example of a competitive inhibitor.
Page 76 of 447
Non- competitive inhibition
This is where inhibitors are structurally different from the substrate and as a result do not compete with the substrate for
active site on the enzyme molecules but its attachment elsewhere on the enzyme changes the structure of the active site
so that the substrate cannot fit. These inhibitors show no structural resemblance to the substrate
These inhibitors attach themselves on the surface of the
enzyme other than the active site thereby changing the shape
of the active site which is at another location of the enzyme
molecule. This change of the active site is achieved by an
allosteric change and these inhibitors prevent the enzyme
from carrying out it activities.
The degree of inhibition depends on the concentration of the
inhibitor alone and cannot be varied by changing the amount
of the substrate. This inhibition may be reversible to some
extent or irreversible in most cases it is irreversible, this is
because it depends mainly on the concentration of the
inhibitor alone because the substrate does not compete with
the inhibitor. In this inhibition the enzyme active site is
changed in such a way that it can no longer accommodate the
substrate.
Irreversible non-competitive inhibitors leave the enzymes permanently damaged and so unable to carry out its
catalytic function. Example of inhibitors include potassium cyanide which attaches its self to the copper
prosthetic groups of an enzyme called cytochrome oxidase thereby inhibiting respiration hence causing death.
Others include heavy metal ions such as mercury ions Hg, Pb and Ag which cause disulphide bonds in
proteins to break whereby denaturing all the proteins. Disulphide bonds maintain the shape of the enzyme
molecule and once broken the structure of the enzyme molecules becomes irreversibly altered with a
permanent loss of its catalytic property
Page 77 of 447
Importances of enzyme inhibitors
i. They provide important information about the shapes and properties of the active site of an enzyme.
ii. They can be used to block particular reactions thereby enabling bio-chemists to re-construct metabolic
pathways
iii. They can be used in medicine and agriculture e.g. as drugs and pesticides respectively.
iv. Enzyme inhibition is also used to control the metabolic pathways by regulating the steps in them. This
usually occurs during end product inhibition.
NOTE: allosteric enzymes are the ones which can change the shape of the active site due to the presence of a
non-competitive inhibitor at a second site where the inhibitor binds known as allosteric sites.
An allosteric effect is the one where a chemical reaction involving one region of a protein molecule changes
the shape and property of the second region of the protein molecule known as an active site.
ENZYME CO-FACTORS
A co-factor is a non-protein substance which is essential for some enzymes to function efficiently. There are
three types of co-factors i.e. activators, co-enzymes and prosthetic groups.
Activators
These are inorganic substances, usually metal ions, which are necessary for the functioning of certain
enzymes. The enzyme thrombokinase which converts prothrombin protein in blood plasma to thrombin during
clotting is activated by calcium ions (Ca2+).
Thrombokinase Thrombin
Prothrombin
Ca2+
Co-enzymes
These are non-protein organic substances which are essential for the efficient functioning of some enzymes
but are not themselves bound to the enzyme i.e. acetyl co-enzyme A.
Page 78 of 447
Prosthetic group
This is a non-protein organic or inorganic substance which is essential for the efficient functioning of some
enzymes and it bound to the enzyme.
Control of enzyme activities
a) Secretion in inactive form like pepsinogen which is only activated at the site of action
b) Precursor activation where accumulation of a potential substrate causes particular reaction pathways to be
opened up
c) Enzymes are contained in membranes like lysosome being released only when there is work to be done
d) Dynamic regulation like negative feedback where the end-products inhibit the initial reactions
e) Through genetic control where the information stored in the nucleus is used to determine which enzymes
are synthesised which in turn determines the limits of cell metabolism
INDUSTRIAL APPLICATIONS OF ENZYMES
i. They are used in making biological detergents which are usually made using proteases produced in an
extra-cellular form from bacteria
ii. They are used in baking industry in which fungal α-amylase enzymes which catalyses the breakdown
of starch in the flour to be used.
iii. They are used in making baby foods which contain trypsin used to pre-digest the baby foods.
iv. They are used in the brewing industry which uses enzymes produced from cereals during beer
production to produce simple sugars from starch which is used by the yeasts during fermentation to
enhance alcohol production.
v. They are used in the dairy industry where an enzyme rennin derived from the stomach of young
ruminant animals is used to manufacture cheese. In addition lactose breaks down lactose glucose and
galactose.
vi. The rubber industry uses catalase enzyme to generate oxygen from peroxides so as to convert latex to
form rubber.
vii. They are used in the paper industry which uses amylase to degrade starch to a lower viscosity product
needed for sizing and coating paper.
viii. They are used in the photographic industry which uses protease to dissolve gelatin away from the
scrop films thereby allowing the recovery of the silver present.
REVISION QUESTIONS
1. Fat and glycogen are energy storage compounds in animals.
a. State the properties of both compounds as energy storage compounds. (4 marks)
b. State the advantages of storing fat over glycogen. (3 marks)
c. Why is glycogen a more suitable energy compound than fat? (3 marks)
2. (a) Using the structural formula below and CH3(CH2)nCOOH, show how a triglyceride is formed. (03 marks)
(b) What properties do lipids posses as storage food substances? (03 marks)
(c) Give the adaptations of the following to their functions
i. Cellulose (0 2 marks)
ii. Starch (02 marks)
3. The diagram represents a phospholipid molecule.
A B C
a) i) Name the parts of the molecule A,B and C (03 marks)
Page 79 of 447
ii) Explain how the phospholipid molecules form a double layer in a cell membrane. (06 marks
b) Give two functions of the protein molecules in the cell membranes. (02 marks)
4. (a) Giving an example in each case; explain what is meant by;
(i) Aldose sugar. (1½ marks)
(ii) Ketose sugar. (1½ marks)
(b) Explain how the storage property of starch is related to its molecular structure. (04 marks)
(c) Although chitin and cellulose are both tough structural polysaccharides, chitin is a more suitable
component of insects’ exoskeleton than cellulose. Explain this statement. (03 marks)
5. a) describe how polypeptide chains may be arranged to form protein molecules 4marks
b) Explain how inhibitors can alter the rate of reaction acting indirectly 3mar
c) Suggest why amylase breaks down starch but it does not break down cellulose 3marks
6. Figure 7 below shows the effect of varying substrate concentration on an enzyme catalysed reaction, in absence and
presence of compound A.
(a) Explain the relationship between rate and substrate concentration

(i) in absence of compound A (03 marks)
(ii) in presence of compound A (04 marks)
(b) state two factors which would have to be kept constant in this experiment (01 )
(c) What would be the effect of increasing the concentration of compound A in the experiment? (02
marks)
7. Figure 7 shows the effect of substrate concentration on enzyme activity
Rate of Product Formation
a) Compare the differences in the rate of reaction in catalyzed and uncatalysed reactions (05 marks)
b) Explain why the enzyme catalyzed reaction finally levels off ( 02 marks)
c) Describe any three substances that help enzymes to perform their catalytic activity (03 marks)
Page 80 of 447
8. Figure 3 shows the activity of bacterial enzymes at different PH and temperature
(a) Which graph represents the bacteria that live in
(I) cool and neutral conditions……………………………………………………. (1/2 mark)
(ii) hot and acidic conditions………………………………………………………… (1/2 mark)
(b) Compare the changes in enzyme activity with temperature and PH for organisms that live in hot and
acidic environment to those that live in cool and neutral environment(04 marks)
(c) With reference to enzyme structure explain how the following factors affect enzyme activity
(i) PH (02 marks)
(ii) temperature(01 mark)
(d) Explain why the same enzyme may be able to work at different optimum PH and temperature
conditions in similar organisms living in different environments (02 marks)
9. Figure 3 is a graph that shows the comparative effects of non-competitive and competitive inhibitor on the rate of an
enzyme-catalysed reaction
Figure 3
Page 81 of 447
a) Identify the curve that shows the effect of (02 marks)
i. A competitive inhibitor
ii. A non-competitive inhibitor
b) Explain the changes in relative rate of reaction in;
i. Curve A (03 marks)
ii. Curve B (03 marks)
c) State any two applications of enzyme inhibitors (02 marks)
10. (a) Clearly distinguish between the following

(i) The primary structure and the secondary structure of proteins. (02marks)
(ii) Co-enzyme and a prosthetic group. (02marks)
(b) How is the structure of a protein used in regulation of blood pH? (03marks)
(c) Explain how excess proteins lead to kidney stones. (03marks)
11. Locusts are insects that are capable of flying for relatively long periods of time. When flying, locusts use
carbohydrates and lipids as energy sources. An experiment was carried out to investigate changes in the
concentration of monosaccharides and lipids in the blood of a locust during flight. Measurements were made of the
concentrations of monosaccharide and lipid at the beginning of the flight and at 60 minutes intervals during the
flight. The results are shown in table 4 below.
Time during Concentration in gmm-3 of
flight in minutes. monosaccharide Lipid
0 30 3.0
60 13 10.0
120 12 19.0
180 11.5 20.0
240 11.0 20.0
300 11.0 20.0
(a) Compare the changes in the concentration of monosaccharide with the changes in the concentration of lipid
during flight. (04 marks)
(b) Suggest an explanation for the changes in the concentrations of both of these compounds during flight (05
marks)
(c) In this investigation, the mass of stored glycogen in the locust was also measured and was found to decrease by
390 during flight. Suggest an explanation for this change in the mass of glycogen. [01 mark]
12. a) Describe the induced fit hypothesis of enzyme action. (08 marks)
b) Explain how the following affect enzyme activity
i. temperature (06 marks)
ii. competitive inhibition (06 marks)
13. (a) Explain how temperature affects enzyme activity in a metabolic reaction. (12 marks)
(b) Describe the induced fit hypothesis of enzyme action. (08 marks
14. (a) What are the ways in which lipids differ from carbohydrates? (05 marks)
(b) With examples describe the functions of lipids in organisms. (10 marks)
(c) Why do animals store lipids instead of carbohydrates? (05 marks)
15. (a) Compare the suitability of lipids and carbohydrates as storage compounds in organisms. (06 marks)
(b) With examples, describe the functions of lipids in organisms. (14 marks)
16. (a) Distinguish between the lock and key and induced fit hypothesis of enzyme action. (05 mark
(b) Explain how temperature affects the activity of an enzyme. (10 marks)
(c) How are enzymes activities controlled? (05 marks)
17. A group of students carried out an experiment to compare the properties of two enzymes. Catalase and carbonic
anhydrase. The concentrations of the substrate and enzyme were the same at the beginning of the experiment and
temperature was maintained at 37oC. Catalase hydrolysed substrate A while carbonic anhydrase hydrolysed
Page 82 of 447
substance B. The students determined the mass of substrates A and B every 10 minutes intervals to establish the rate
of reactions A and B. The results are shown in the table below.
Time in Mass of substrate in g Rate of reaction
minutes A B A(g min -1) B(gmin -1)
0 200 200 0
10 192 182 19.2
20 184 176 18.4
30 170 165 17.0
40 162 150 16.2
50 104 98 10.4
60 80 30 8.0
70 30 10 3.0
80 10 5 1.0
a) Copy and complete the table by calculating the rate of enzyme controlled reaction B at every 10 minutes
intervals (04 marks)
b) Plot a suitable graph to compare the rate of enzyme controlled reactions A and B. (10 marks)
c) Which of the enzymes has a higher turnover number? Give reasons for your answer. (02 marks
d) (i) suggest the names of the substances used in reactions a and B. (02 marks)
(ii) Explain the changes in the rates of reactions A and B shown by your graph. Illustrate your explanation with
equations. (05 marks)
e) Explain what would happen
(i) If mercury was added to reaction B (08 marks)
(ii) Malonic acid was added to reaction A (08 marks)
f) Why was temperature kept constant? (01 mark
18. (a). Describe the structure of the protein molecule. [7mks]

(b). Compare the structure of proteins to those of carbohydrates [9mks]
(c). What structural features of carbohydrates account for the wide Variety of polysaccharides? [4mks]
19. a) Give an account of the chemical nature and variety of carbohydrates (10 marks)
b) Outline the role of carbohydrates in the life of a plant. (10 marks
20. a) What is meant by the term protein? (03 mark)
b) Discuss with suitable examples the variety of functions of proteins. (12 marks)
c) Explain how their structure permits this wide variety of functions. (6 marks)
21. a) Describe the structure of the following. (10mks)
i) Starch
ii) Cellulose
b) Show how a triglyceride is formed. (4mks)

c) Outline the importance of triglycerides in living organisms. (11mks
22. a) Why is calcium ion important in the human body? (5mks)
b) Water is essential for life. Explain in what ways this statement is true for plants and animals. (15 mks)
An experiment was carried out to investigate the effect of alcohol on the activity of pancreatic amylase. Dilutions of
ethanol in water were prepared in order to give a range of percentage concentrations from 0% to 70%.
To each of the 8 test tubes was added 1cm3 of the appropriate concentration of ethanol, followed by 2cm3 of a 1%starch
solution. 2% of pancreatic extract were added to each test tube and the time, in seconds, required for starch hydrolysis
was recorded. The table below shows the results:
Test tube % concentration of ethanol added Time required for complete starch
to each test tube. hydrolysis/seconds.
1 0 100
2 10 80
3 20 90
4 30 100
5 40 130
Page 83 of 447
P530 [2020] By Nakapanka J Mayanja & Mugenyi S Paul 0704716641
6 50 190
7 60 240
8 70 300
a) Calculate the initial mass, in mg, of starch per test tube.

b) Calculate the ;
i) % of ethanol in the reaction mixture for each test tube.
ii) rate of starch hydrolysis in each test tube, in milligrams per minute.
c) Draw a graph to show the relationship between ethanol concentration and the rate of starch hydrolysis.
d) Give reasons for the change in the rate of starch hydrolysis with increasing ethanol concentration
e) Organic solvents like ethanol are known to alter the three dimensional structure of globular proteins.
Explain, in terms of enzyme structure and function, the effect of concentrations of ethanol greater than 6% in the
reaction mixture.
23. The browning which occurs when many types of vegetables and fruits are peeled is caused by enzymes called phenol
oxidases. These catalyse the relatively slow conversion of naturally phenolic compounds into dark brown melanins.
Phenols Quinones Melanins

(Colourless) (Yellow) (Dark brown)
The results in the table below were obtained from investigation into the browning of cubes of apples. Study
the information and use it to answer the questions that follow
Cube Contents of cube (cm3) Appearance of cube contents
number Catechol Apple Buffer Dilute Dilute after 10 minutes at room
extract (pH = 7) acid base temperature
1 2 - 5 - - Colour less
2 - 2 5 - - Light brown
3 2 2 3 - - Dark brown
4 2 2 - 3 - Colour less
5 2 2 - - 3 Light brown
6 2 2 (boiled) 3 - - Colour less
a. From the information given in the table, what type of substance do you think catechol is, and what
purpose it serves in this investigation? (2 marks)
b. Use the results above to;
i) Suggest two ways in which apples, once peeled, can be prevented from turning brown?
(2 marks)
ii) State what the apple extract contains? (2 marks)
c. Explain your answer in b (i) above (10 marks)
24. The rate of hydrolysis of starch by amylase enzyme was used to investigate the effect of a competitive inhibitor on
enzyme action. A fixed amount of the enzyme and inhibitor was used at varying concentrations of the substrate. The
data in the table below was obtained from the investigation. Use it to answer the questions that follow.
Substrate concentration (mol) 0.0 0.1 0.25 0.5 0.75 1.0 1.25 1.50
Rate of reaction No inhibitor present 0.0 0.20 0.40 0.63 0.78 0.93 0.93 0.93
(arbitrary units) Inhibitor present 0.0 0.15 0.30 0.45 0.60 0.73 0.80 0.92
(a) Represent the data on a suitable graph. (10 marks)

(b) Explain the shape of the graph obtained when,
(i) Only the enzyme was used? (4 marks)
(ii) The inhibitor was present? (4 marks)
Page 84 of 447
(c) (i) Indicate on your graph, the results that could have been obtained if a non- competitive inhibitor was used
instead of a competitive inhibitor? (2 marks)
(ii) Explain your answer in c (i) above? (5 marks)
(d) Use the lock and key hypothesis to explain the mode of action of amylase enzyme (8 marks)
(e) Explain how gastric juice affects the action of amylase enzyme? (7 marks)
25. Give an account of the diversity of polysaccharides? (20 marks)
26. (a) Give an account of the structure of starch, and explain how structure is related to functioning?.
(b) Explain why:
(i) animal cells store glycogen and not starch as an energy source.
(ii) many organisms store fats rather than carbohydrates in their bodies
27. (a) Compare the suitability of lipids and carbohydrates as storage compounds in organisms. (06 marks)
(b) With examples, describe the functions of lipids in organisms (14 marks)
28. Briefly describe how starch and cellulose molecules form from their monomer subunits. (10 marks)
b) Explain the role of carbohydrate molecules in plant life. (10 marks)
29. a) Outline the functions of carbohydrates in animals (05 marks)
b) Starch is the major storage form of carbohydrates in plants. Describe;
i) the structure of starch and
ii) how the structure is related to function (15 marks)
30. a)Describe how starch and cellulose are formed from their monomer units (10 marks)
b) Explain the importance of carbohydrates in plants (5 marks
c) Explain why certain organisms store lipids as the main storage form of energy instead of starch. (5 marks)
31. a)Distinguish between enzymes and inorganic catalysts. (05 marks)
b) Give an account of how substrate concentration, pH and temperature can affect rate of enzyme catalyzed
reactions. (15 marks)
32. (a). Describe the biological function of amino acids. (05marks)
(b). Describe how amino acids form a polypeptide. (09marks
(c). How do inhibitors change the rate of enzyme controlled reactions? (06marks)
33. Describe the various characteristics of the carbon atom that makes possible the building of a variety of biological
molecules. (06 marks)
(b) What structural features of carbohydrates account for the wide variety of polysaccharides? (07 marks)
(c) How is cellulose different from glycogen? (07 marks)
REFERENCES
1. D.T.Taylor, N.P.O. Green, G.W. Stout and R. Soper. Biological Science, 3rd edition, Cambridge University
Press
2. M.B.V.Roberts, Biology a Functional approach, 4th edition, Nelson
3. C.J.Clegg with D.G.Mackean, ADVANCED BIOLOGY PRICIPLES AND APPLICATIONS, 2 nd EDITION,
HODDER EDUCATION
4. Glenn and Susan Toole, NEW UNDERSTANDING BIOLOGY for advanced level, 2 nd edition, Nelson
thornes
Page 85 of 447
TOPIC 3: INHERITANCE
SYLLABUS EXTRACT
Content & Subtopic Specific objectives : The learner should be able to:
Chromosomes  Describe the composition of chromosomes and
 Composition of the chromosomes and structure structure of nucleotides.
of nucleotides.  Describe the structure of the DNA and RNA.
 Structure of the nucleic acids the DNA and  Differentiate the DNA and RNA
RNA  Explain the Watson Crick hypothesis of the nature
 differences between the DNA and RNA of DNA.
 The Watson- Crick hypothesis and DNA.  Explain the process of DNA replication.
 The DNA replication.  Describe the nature of genes.
 Nature of genes.  Describe the structure of the genetic code.
 Structure of genetic code.
Cell division
 Mitosis and Meiosis
 Describe the mitosis and meiosis.
 Comparison of mitosis and meiosis.
 Compare mitosis and meiosis
 Role of mitosis and meiosis in living organisms
 State the significance of mitosis and meiosis to
 Significance of the cell division events e.g.
living organisms.
formation of the spindle fibres, chiasmata,
 Explain the significance of changes in the nucleus
synapsis, bivalents, and movement of
during cell division
chromosomes, e.t.c.
Protein synthesis  Describe the formation of RNA (tRNA, mRNA).
 Formation of RNA (tRNA, mRNA).  Describe the process of protein synthesis
 Process of protein synthesis  State the role of DNA and RNA in protein
 Role of DNA and RNA in protein synthesis synthesis
GENETICS  Explain the concept of inheritance.
 Concept of inheritance  Define genetics terms
 Definition of genetics term e.g. Inheritance,  Describe Mendel’s investigations on heredity
gene, allele, chromosome, DNA, trait e.t.c.  Explain the two Mendel’s laws of inheritance
 Mendel’s work on heredity  Explain inheritance of traits using the monohybrid
 Monohybrid inheritance and dihybrid and dihybrid crosses.
inheritance.  Discuss the challenges of disorders
 Mendel’s laws of inheritance: Law of
independent assortment and law of segregation.
 Challenges of heritage disorders,
 Chromosomes and genes  Explain the terms: gene interactions, sex linkage,
 Terms sex determination, sex limitation, lethal genes and
- Gene interactions definition and examples polygenes.
linkage, multiple alleles , codominance,  Explain gene and chromosome mapping
incomplete dominance, dominant and
recessive traits, epistasis, complementary
gene.
- Sex linkage; definition, examples and
inheritance
- Sex determination; definition example in
humans
- Sex limitation definition and examples
- Lethal genes definition and examples:
phenylketonuria neurospora e.t.c.
- Polygene definition and examples
Page 86 of 447
 Gene and chromosome mapping.

VARIATION
 Population traits and types of variations.
Continuous (quantitative) and discontinuous
(qualitative).  Explain population traits and types of variation.
 Causes of variation: genetic and environmental  Describe the causes of variation
factors.  Define mutation
- Definition of mutation.  Described types and causes of mutations
- Types of mutations gene and chromosomal  Distinguish between chromosomal and gene
mutation. mutations.
- Causes of mutation: chance, radiation,  Explain the significance of mutations
chemicals.  Identify variations in organisms.
- Differences between the chromosomal and  Collect data on variations among themselves
gene mutations.
- Variation among organisms
- Data on variations among organisms (e.g.
sex, height, tongue rolling
Page 87 of 447
NUCLEIC ACIDS
These are nitrogen containing organic acids important for making the genetic material and proteins of all
organisms.
Nucleic acids are made of short chains called nucleotides, made up of CHONP. Nucleic acids include
Deoxyribose Nucleic Acid (DNA) and ribonucleic acid (RNA).
THE STRUCTURE OF NUCLEOTIDES
A nucleotide is made up of 3 components namely, pentose sugar, a nitrogenous base and a phosphate derived
from phosphoric acid i.e. all nucleotides contain phosphoric acid.
A. PENTOSE SUGAR
The pentose sugars in
nucleic acids are of 2 types
namely; ribose sugar in
RNA and deoxyribose sugar
in DNA. The only difference
between these two sugars is
that deoxyribose lacks an
oxygen atom on the second Roberts
carbon atom in the ring; page 481
hence the name deoxyribose.
B. NITROGENOUS BASES
Each nucleic acid contains four different bases of which two are derived from purines and another two are derived from
pyrimidines. The nitrogen in the rings gives the molecules their basic structure. These bases are;
Pyrimidines Cytosine (C) Thymine (T) Uracil (U)

Purines have two rings in their
DNA contains C and T while RNA contains C and U structure while the pyrimidines have
one ring in their structure i.e. purines
Purines Adenine (A) Guanine (G) are larger than pyrimidines. These
The purines A and G are found in both DNA and bases are commonly represented by
their initial letters A, G, C, T and U.
RNA
Soper pg 108 fig 3.42 OR Roberts pg 486 fig 30.6

C. PHOSPHORIC ACID
This acid gives the acidic character to nucleic acids and

its structure is shown below;
Roberts pg 482
Page 88 of 447
The three components of nucleotides are combined together by two condensation reactions to give a
nucleotide whose structure is shown below
Fig 1 pg 36 Kent OR Fig 3.38 (diagrammatically) 106 Soper
Simply →
During this combination, the pentose sugar and organic base join together by a condensation reaction to form
a nucleoside. Another condensation reaction joins the nucleoside to form the nucleotide joined together by a
phosphoester bond located between carbon 5 of the sugar and the phosphate.
By similar condensation reactions between the sugar and a phosphate group The main function of nucleotides
of two nucleotides a di-nucleotide is formed linked together by is the formation of nucleic acids,
phosphodiester bonds between carbon-3 of sugar and the OH group of the RNA and DNA which play vital
phosphate. Continued condensation reactions lead to formation of a roles in protein synthesis and
polynucleotide as shown below; heredity. In addition, nucleotides
form part of other metabolically
Fig 30.1 B pg 482 Roberts
important molecules; such
molecules include Adenosine Tri
Phosphate (ATP), Adenosine
Mono Phosphate (AMP),
Nicotinamide Adenine
Dinucleotide (NAD), Flavine
Adenine Dinucleotide (FAD),
Nicotinamide Adenine
Dinucleotide Phosphate (NADP)
and co-enzyme A.
Note
Nucleotides polymerise by forming phosphodiester bonds between carbon 3' of the sugar and an oxygen atom
of the phosphate. This is a condensation reaction. The bases do not take part in the polymerisation, so there is
a sugar-phosphate backbone with the bases extending off it (projecting outwards). This means that the
nucleotides can join together in any order along the chain. Two nucleotides form a dinucleotide, three form a
trinucleotide, a few form an oligonucleotide, and many form a polynucleotide. A polynucleotide has a free
phosphate group at one end, called the 5' end because the phosphate is attached to carbon 5' of the sugar, and a
free OH group at the other end, called the 3' end because it's on carbon 3' of the sugar. The terms 3' and 5' are
often used to denote the different ends of a DNA molecule.
Page 89 of 447
RNA (ribonucleic acid)

RNA is a single stranded polymer of nucleotides where the pentose sugar is always ribose and the organic
bases are adenine, cytosine, guanine and thiamine. There are many types of RNA found in cells, 3 of which
are involved in protein synthesis. These include the following;
a) Ribosomal RNA (rRNA) (80%)
b) Transfer RNA (tRNA) (15%)
c) Messenger RNA (mRNA) (3-5%)
RIBOSOMAL RNA (rRNA)

This is a large complex molecule made up of both double and single helices.
Although it is manufactured by the DNA of the nucleus it is mainly found in cytoplasm where it makes up
more than half of the mass of the ribosomes. It comprises of more than a half of the mass of the total RNA of
the cell and its sequence is similar in all organisms.
Ribosomes are the site of protein synthesis, at the ribosomes the mRNA code is translated into a sequence of
amino acids in a growing polypeptide chain. This is possible because ribosomes are often found in clusters
linked together by strands of mRNA. This cluster of ribosomes is known as poly-ribosome or polysome and
this enables several molecules of the same polypeptide chain to be produced simultaneously.
RNA (ribonucleic acid) RIBOSOMAL RNA (rRNA)
RNA is a single stranded This is a large complex molecule made up of both double and single helices.
polymer of nucleotides where Although it is manufactured by the DNA of the nucleus it is mainly found in
the pentose sugar is always cytoplasm where it makes up more than half of the mass of the ribosomes. It
ribose and the organic bases comprises of more than a half of the mass of the total RNA of the cell and its
are adenine, cytosine, guanine sequence is similar in all organisms.
and thiamine.
Ribosomes are the site of protein synthesis, at the ribosomes the mRNA
There are many types of RNA code is translated into a sequence of amino acids in a growing polypeptide
found in cells, 3 of which are chain. This is possible because ribosomes are often found in clusters linked
involved in protein synthesis. together by strands of mRNA. This cluster of ribosomes is known as poly-
These include the following; ribosome or polysome and this enables several molecules of the same
polypeptide chain to be produced simultaneously.
TRANSFER RNA (tRNA) The structure of t RNA is shown below;

This is a small molecule with about 80 nucleotides made up of Fig 30.11 pg 493 Roberts OR Fig 23.25
a single strand. It comprises of 10 to 15% of the total RNA pg 801 Soper
within the cell and all types of tRNA are fundamentally
similar. There are at least 20 types of tRNA each one carrying
a different amino acid.
It forms a clover leaf shape with one end of a chain ending in
a cytosine-cytosine-adenine(C-C-A) base sequence. It is at
this base sequence that an amino acid attaches itself.
At an intermediate point along the tRNA point is an important
sequence of 3 bases called anticodon. These bases line up
alongside the appropriate codon on the mRNA during protein
synthesis.
This implies that each amino acid has its own tRNA molecule
which transfers it from the cytoplasm to the ribosome to join
the polypeptide chain being made.
Page 90 of 447
Consequently tRNA acts as an intermediate molecule between the codon of mRNA and the amino acid
sequence of the polypeptide chain on the ribosomes on the ribosomes. A codon is sequence of three organic
bases which together form a unit of genetic code in a DNA or RNA molecule to specify an amino acid that
joins a polypeptide
MESSENGER RNA (mRNA)
This is also a single stranded molecule containing triplets of bases known as codons. It is formed from a single
strand of DNA during protein synthesis by a process known as transcription. During transcription, the DNA
genetic information for protein synt
hesis is copied from the DNA strand to form codons of mRNA. Thereafter, mRNA attaches itself on a group
of many ribosomes thereby forming a structure called polysome which is the site of protein synthesis.
(DEOXYRIBO NULCEIC ACID) DNA
This is a double stranded molecule containing repeated combination of many nucleotides which is transmitted
from generation to generation in organisms. DNA is perhaps the most important molecule in biology. It
contains the instructions that make every single living organisms. DNA is a polymer, composed of monomers
called nucleotides.
Structure of DNA
According to Watson and Click, DNA consists of
two strands each made up of very many nucleotides
that repetitively combine to form very long
polynucleotides. The strands are anti-parallel i.e. run
in opposite directions. Each polynucleotide
chain/strand forms a right handed helical spiral and
consequently the two chains coil around each other to
form a double helix.
The sugar-phosphate back bone is made of alternating
deoxyribose sugar and phosphates. The two chains run
in opposite directions. The double strands are held
together by complementary base pairs between them.
Each of these base pairs is in turn held by hydrogen
bonds. During the complementary base pairing,
Adenine must combine with Thymine while Guanine
combines with Cytosine.
DNA is like a ladder where the alternating deoxyribose and phosphate units form the uprights and the organic
base pairing to form the rungs. However, the strands are twisted instead of being like a ladder into a double
helix so that each upright winds around the other. Such a double helix structure is shown on the right;
NOTE: the width between the two strands is constant and equal to the width of the base pair i.e. the width is
equal to the purine plus the pyrimidine. Two purines would be too large and two pyrimidines would be too
small to span the gap between the two chains of DNA. Therefore, adenine must combine with thymine while
guanine must combine with cytosine.
The sequence of bases in one chain of DNA determines that in the other and consequently the two DNA
chains are said to be complementary. Each of the polynucleotide chains in DNA is extremely long and may
contain many million nucleotide units.
Page 91 of 447
The amount of guanine is equal to the amount of cytosine in DNA and similarly, the amount of adenine is
equal to that of thymine e.g. if a DNA molecule contains 40% of its bases as adenine and thymine, how many
bases will be guanine in a DNA molecule?
A and T=40%
G and C=100-40 = 60%
Since the amount of C = the amount of G.
60
Then the number of guanine bases= =30%
2
COMPARISON OF DNA AND RNA
RNA DNA`
It contains fewer nucleotides It contains very many nucleotides i.e. it is longer than
RNA
It is single stranded It is double stranded
It may be a single or a double helix. It is always a double helix
The pentose sugar is ribose The pentose sugar is deoxyribose
It contains uracil It contains thymine
The ratio of adenine to uracil and cytosine to The ratio of adenine and thymine to cytosine guanine is
guanine varies. constant
It is manufactured in the nucleus but found It is found almost entirely in the nucleus
throughout the cell
The amount varies from cell to cell and The amount is constant for all cells of the species
within the cell according to metabolic needs. except for the gametes where it is half
It exists in three basic forms; tRNA, mRNA It exists in only one basic form but with an almost
and rRNA. infinite variety within that form
It is chemically less stable. It is chemically more stable.
It may be temporary for short periods It is permanent.
Similarities Both:
(1) are polymers of nucleotides (2) occur in the cytoplasm
(3) carry genetic information (4) originate from the nucleus
(4) have same purine bases adenine and guanine plus pyrimidine bases cytosine
The following is the evidence to show that DNA is a genetic material

i. The chromosomes which play a role in cell division are made of DNA and histone proteins only. This
implies that when a cell divides DNA is carried to the daughter cells formed.
ii. DNA is constant in amount in all cells within the species except in gametes where it is a half.
Page 92 of 447
iii. It undergoes mutations which are inherited and without which it remains very stable so that the code
instructions it contains remains unchanged from generation to generation.
iv. DNA controls the activities of a cell by directing the synthesis of proteins. This is shown by the
various transduction experiments in which the Bacteriophage (virus that attacks bacteria) transfers
DNA to a bacterium called Escherichia coli (E. Coli). The virus DNA instructs E. coli to make many
new bacteriophage viruses.
v. The bases are protected on the inside of the molecule and the two strands are held together by
numerous hydrogen bonds, so DNA a very stable molecule and is not easily damaged.
vi. There are four different bases, which can appear in any order, so their sequence can encode
information, like writing with a 4-letter alphabet.
vii. DNA is a very long molecule, so it store a great deal of information (human DNA has 3 billion base-
pairs).
viii. The two complementary strands means there are two copies of the information, which is useful for
repair, copying and error checking.
DNA REPLICATION
DNA
This isSTRNAD
the process by which two DNA molecules make
exact copies of its self.
This enables the transmission of the same genetic
information from cell to cell and generation to generation.
Replication is controlled by an enzyme DNA polymerase
which links the DNA nucleotides to form long strands of
DNA and helicase enzyme which causes the unwinding
(opening up) of the DNA double strands into separate DNA
strands by breaking hydrogen bonds between base pairs.
How DNA replication occurs
DNA starts when Helicase enzyme attaches on one of the
DNA double helix strands and starts moving in the 5l to 3l
direction along the strand. Helicase unzips the DNA double
helix by catalyzing the breakdown of hydrogen bonds
between the complementary base pairs of DNA.
DNA polymerase enzyme then binds to the unzipped DNA
strand and also moves in the 5l to 3l direction following
helicase. Many free nucleotides align alongside the DNA
strand where DNA polymerase is attached. Each time DNA
polymerase meets the next base on the strand; free
nucleotides with the correct complementary bas is inserted
into the new growing DNA strand. The free nucleotide is
held in place by DNA polymerase until it binds to the
preceding nucleotide on the new growing DNA strand, thus
extending the new strand of DNA. For example, if
DNA polymerase meets thymine, a nucleotide carrying adenine is inherited into the new DNA strand. DNA
polymerase continues to move in the 5l to 3l direction along one strand meeting one base at a time and
instructing a complementary base to be added to the new DNA strand growing as this enzyme moves. This is
called continuous replication because both DNA polymerase and helicase are moving in the same 5l to 3l
Page 93 of 447
direction without leaving gaps in the new strand being synthesised. The strand formed is called the leading
strand.
Discontinuous replication occurs when DNA polymerase attaches on another DNA strand and starts moving
discontinuously in the 3l to 5l direction as helicase moves in the 5l to 3l direction. In this case, the copying of
the parent DNA strand to form a new strand keeps on being started again because it has to move away from
the unwinding enzyme in the 5l to 3l direction. This results in small gaps being left as many short segments of
DNA are made. These gaps are closed by DNA ligase enzyme to form the second DNA strand, which joins
the 5l end of DNA to the 3l end. The strand formed is referred to as the lagging strand.
The action of DNA ligase brings about three types of replication as it tries to complete the work of DNA
replication. These types are;
1. Conservative (2) Semi-conservative (3) Dispersive
In semi-conservative replication the DNA strands unzips (separates) under

the influence of helicase and then forms a new DNA strand for each of the
old DNA strands using DNA polymerase. The new DNA molecule is
composed of one old strand and one new strand.
In conservative replication the DNA strand unzips using helicase enzyme
and forms two new DNA strands which zip together to become a DNA
double helix made of new strands only. The old strands also zip together
again after replication so that they form another new DNA molecule made of
old strands only.
In dispersive replication, two new strands are formed each containing
alternating old and new bases of nucleotides;
THE GENETIC CODE DETERMINING THE FIXING OF ONE

The genetic code is the sequence of bases in DNA AMINO ACID IN A POLYPEPTIDE
which codes for the sequence of amino acids in protein There are 20 amino acids which regularly occur in
molecules. DNA provides the code (Genetic message) proteins and each of these must have its own code
for the formation of proteins in an organism which of bases on DNA i.e. the base pairs arrange
may in turn determine the characteristics of that themselves in form of triplets. This code is triplet
organism. because with only four different bases (A, G, C
Every species possesses different DNA and therefore and T) present in the DNA, if each of them was
produces different enzymes. The DNA of the different coded for one amino acid, only four different
species therefore differs not in the chemicals which it amino acids could be coded for during protein
comprises but in the sequence of base pairs along its synthesis which would be insufficient for protein
length. This sequence of triplet base pairs in DNA and formation. Using a pair of bases to specify an
mRNA is the code that determines which proteins are amino acid that should be picked by tRNA during
manufactured. protein synthesis gives 16 different codes that are
possible for this picking. Therefore, a triplet code
A G T A A T G C T T T A of bases has to be used to specify one amino acid
that can be picked by tRNA during protein
1 2 3 4 synthesis as this will produce 64 possible codes
more than enough to specify the requirement of
TRIPLETS
the 20 amino acids.
Page 94 of 447
Table 23.4 pg 799 Soper OR Table 30.1 pg 498 Roberts
The genetic code is therefore a triplet code of word. Each word specifies the position of an amino acid in the
corresponding protein chain. The triplet code constitutes the codons of mRNA as these codons are directly
made from DNA. For example;
1. It is a triplet code
2. This triplet code is also called a degenerate code since there is more than one triplet for most amino
acids i.e. it is a degenerate code because a given amino acid may be coded for by more than one code
3. It is punctuated i.e. it has a start codon usually AUG and three stop codons namely UAA, UAG and
UGA.
4. The genetic code is also described as universal because the same triplets of bases code for the same
amino acid in all organisms. In other words all codons are precisely the same for all organisms.
5. In addition, the genetic code is non-over lapping e.g. each triplet of bases is read separately
UACACCAUGGGC is read as UAC-ACC-AUG-GGC.
6. The genetic code also leads to the formation of 3 codons namely, UAA, UAG and UGA which are
called nonsense codons. These nonsense codons stop the process of protein synthesis by not coding
for a specific amino acid at all.
Page 95 of 447
P530 (2020) By Nakapanaka Jude Mayanja & Mugeneyi S Paul 0704716641
THE CENTRAL DOGMA OF MOLECULAR TRANSCRIPTION

BIOLOGY This is the mechanism by which the base sequence
It states that DNA makes RNA makes protein of a cistron of DNA strand is converted into the
complementary base sequence of mRNA.
During transcription which occurs after replication
PROTEIN SYNTHESIS
of DNA, an enzyme RNA polymerase first
This is the process by which the coded information is recognises the start sequence in the DNA coding
transferred from the chromosomes in the nucleus to strand and becomes attached to the DNA at this
the ribosomes in the cytoplasm to make the proteins. point. This DNA coding strand has a specific
There are three main stages in the formation of a region called cistron which is used for making
protein namely; mRNA. It is the cistron which is referred to as a
1. Transcription gene.
2. Amino acid activation The cistron comes into existence during replication
3. Translation when it unwinds. This unzipping is due to the
The process of protein synthesis is summarized in the breaking down of hydrogen bonds between the base
diagram below; pairs in the DNA double helix by helicase enzyme.
Fig 23.26 pg 801 Soper The unzipping exposes the bases along the cistron.
RNA polymerase then travels along the DNA
cistron and new nucleotides complementary to
those in the DNA strand are inserted into the
growing mRNA strand. When this enzyme
encounters thymine, adenine is inserted into mRNA
and when it encounters cytosine, guanine is inserted
into mRNA.
Therefore DNA acts as a template against which
mRNA is constructed. A single molecule of DNA
in each chromosome contains numerous shorter
sections called genes (cistrons) each of which
Fig 30.12 (1) pg 494 Roberts contains the instructions for making one protein.
The coded instructions in each gene must specify
the overall length of the protein chain and the exact
position of the amino acids within the chain.
At the end of transcription RNA polymerase
recognizes the stop sequence on the cistron and
becomes detached from the cistron at this point.
Being too large to diffuse across the nuclear
membrane, mRNA instead diffuses through the
nuclear pores to the cytoplasm where it attaches
itself on the ribosomes. In this way the instructions
needed for protein synthesis is transferred into the
AMINO ACID ACTIVATION cytoplasm inform of mRNA codon.
Activation is the process by which amino acids combine with tRNA using energy from ATP under the
influence of an enzyme amino acyl tRNA synthetase. This produces an amino acid tRNA complex with
sufficient energy to form a bond with the neighbouring amino acid. The tRNA molecule with attached
amino acid now moves to the ribosomes in order to form the polypeptide chain.
TRANSLATION
This is the mechanism by
which the codons of mRNA
are converted into a specific
sequence of amino acids in
a polypeptide chain on the
ribosomes.
During this process mRNA
attaches itself on a group of
ribosomes (like beads on a
string) to form a structure
called polysome. Within the
ribosomes there are two
tRNA sites where the
mRNA codon can become
attached by complementary
base pairing to a molecule
of tRNA baring the anti-
codon.
Therefore the
complementary anti-codon
of the tRNA-amino acid
complex is attracted to the
first codon on the mRNA
strand enclosed by the
ribosomes. The second
mRNA codon likewise
attracts its complementary
anti-codon of the second
tRNA amino acid complex.
The ribosome acts as a
framework which holds the
mRNA and the tRNA
amino acid complexes
together until the two
amino acids form a peptide
bond by a condensation
reaction there by forming a
dipeptide.
Roberts pg 494 Fig 30.12 (2) Once the two amino acids
have combined into a
dipeptide, the first tRNA is
disconnected from its
amino acid and therefore
leaves the ribosome which
moves one step along the
mRNA strand so as to hold
the next codon-anti codon
complex together until the
third amino acid is linked
with the second by a

Roberts pg 494 Fig 30.12 (3) condensation reaction. In
this way, a polypeptide
chain is assembled by the
addition of one amino acid
at a time along the
polysome (group of many
ribosomes).
Once each amino acid is

linked to the growing
polypeptide chain, the
tRNA which carried it to
the mRNA codon is
Roberts pg 494 Fig 30.12 (2) released back into the
cytoplasm. This tRNA is
again free to combine with
its specific amino acid in
the cytoplasm. This
sequence of the ribosome,
steadily reading the mRNA
code and translating it,
continues until the
ribosome comes into
The polypeptides formed in this way must now be assembled into proteins. Many polypeptides are made in
this way because the second and subsequent ribosomes usually pass along the mRNA immediately behind
the first ribosome, in this way many identical polypeptides are formed simultaneously.
The main steps involved in translation may be summarized as (1) binding of mRNA contact with one of the
nonsense codes (terminating codes) UAA, UAG and UGA at which point the polypeptide is cast off or
peeled off from the ribosome and dropped into the cytoplasm.to ribosome, (2) amino acid activation and
attachment to tRNA, (3) polypeptide chain initiation, (4) chain elongation, (5) chain termination, (6) fate of
mRNA
The genetic is of great importance during protein synthesis because;
1. The genetic code controls the formation of polypeptide chains in proteins by determining the
length/number of codons of mRNA (by transcription) that carries this code to the ribosomes for
polypeptide synthesis to proceed at the ribosomes hence indirectly determining the number of amino acids
to be used in assembling a specific polypeptide
2. The genetic code determines the inheritance of characteristics from parents by determining the sequence
of amino acids in a polypeptide chain since its triplet thereby determining the type of protein made
3. The genetic code directs the development of biochemical anatomical physiological and to some extent
behavioural traits of an organism this is achieved through its control of protein synthesis by instructing
the cell polypeptides to make
4. The genetic code has terminating codons (in mRNA) that stops the formation of a given polypeptide
NOTE:
1. The function of the ribosome in protein synthesis is to hold in position the mRNA, t RNA-amino acid
complex and the asserted enzymes controlling the process until a peptide bond forms between adjacent
amino acids.
2. DNA controls polypeptide chain synthesis
i. by instructing the cell which/what peptides to make.
ii. by forming mRNA by transcription, mRNA carries coded genetic information to the ribosomes
for polypeptide synthesis to proceed at the ribosomes.
iii. DNA’s cistron length determines the number of codons mRNA should have hence indirectly
determining the number of amino acids to be used in assembling a specific polypeptide
iv. through DNA’s triplet code system it determines the sequence of amino acids to be built in a
polypeptide thereby determining the type of polypeptide
v. during DNA’s transpiration to mRNA it forms nonsense codons that terminate the formation of a
given polypeptide
CELL DIVISION
Cells undergo a series of changes in their life time during which they produce new daughter cells. Indeed,
every cell is formed from an already existing cell by cell division i.e. where a cell exists, there must have been
a pre-existing cells. The continuity of life is based on the reproduction of cells or cell division. When a
unicellular organism such as amoeba divides and forms two offspring amoeba cells, the division of one cell
reproduces an entire organism. There are two forms of cell division namely mitosis and meiosis. Mitosis
promotes the multiplication of cells to bring about growth whereas meiosis promotes the multiplication of the
species by promoting gamete formation in sexual reproduction.
Cell division occurs due to the presence of chromosomes in the nucleus of the cell. Chromosomes are thread
like structures in the nucleus of the cell made of DNA molecules and histone protein. Structurally a
chromosome contains a pair of elongated structures called chromatids which are joined together by the
structure in the middle of the chromosomes called centromere.
Each chromatid contains many bead like structures made of DNA called genes which determine the
characteristics of organisms. Chromosomes occur in pairs within the nucleus of the cell and the chromosome
number varies from species to species e.g. in human beings there are 23 pairs of chromosomes in the nuclear
cell i.e. 46 chromosomes. Therefore human beings are described as diploid organisms because they have
diploid cells. A diploid cell (2n) is the one in which there 2 sets of chromosomes of which one set is inherited
from the mother and another set from the mother. This implies that human beings have a chromosome number
of 46 in their somatic cells or body cells and 23 chromosomes in their gamete cells. A haploid cell is one
having only one set of chromosomes in the nucleus.
Before replication, each single chromosome contains at least one
long linear DNA molecule that carries many genes which control
the characteristics of an organism. The associated histone protein
molecules maintain the structure of the chromosome and control the
activity of genes.
NOTE;
a. The sequence of events which occur between one cell division

and the next is called the cell cycle.
b. Sister chromatids are pairs of chromatids located on the same
chromosome while non-sister chromatids are pairs of
chromatids located on different chromosomes.
c. Homologous chromosomes refer to structurally similar chromosomes one obtained from the mother and
another from the father during fertilization which exists in the nucleus of a somatic cell of an organism.
THE CELL CYCLE

This is the sequence of events which occur between one cell division and the next. It can also be defined as
the life of the cell from the time it is first formed from a dividing parent cell until its own division into two
or four cells.
A dividing cell duplicates its DNA and allocates the two copies of DNA to opposite ends of the cell and then
splits into daughter cells, thereby making the daughter cells identical.
The cell cycle shows that the life cycle of the cell mainly involves interphase i.e. a period in which the cell
prepares for the next cell division which is followed by nuclear division during mitosis and finally
cytokinesis during telophase of mitosis. Mitosis therefore Diagram showing the cell cycle
involves both nuclear division and cytokinesis which alternates Soper pg 779 fig 23.5
with a much longer stage called interphase. INTERPHASE
A dividing cell spends about 90-95 % of time in interphase.
Prior to cell division, either mitosis or meiosis, the mother cell
undergoes a preparation stage known as interphase. During
interphase the chromosomes are usually seen as tiny coiled
threads known as chromatids and are therefore described as
invisible chromosomes because details of the chromosome
structure cannot be see. During interphase four important
changes take place in the cells.
a. There is duplication of DNA and chromosomes so as to
double their amounts i.e. there is replication of DNA and
chromosomes.
b. In addition replication of the centrioles occurs in animal cells.

c. There is synthesis of a lot of ATP so that there is sufficient energy for the next cell division.
d. There is replication of cell organelles like mitochondria, endoplasmic reticulum, and Golgi body e.t.c.
e. There is synthesis of histone proteins, RNA and other types of proteins occurs
f. The chromosomes are seen as tiny thread like structures that are highly coiled and therefore described as
invisible (as their details cannot be clearly seen)
g. The nucleus becomes enlarged and thin
Interphase is divided into three stages, namely first growth (G1) phase, synthesis (S) phase and second growth
(G2) phase. The following occur at each stage of interphase
G1 phase S phase G2 phase
- Intensive cellular synthesis occurs in -DNA & chromosome - intensive cellular synthesis
which many new cell organelles are replication occurs -histone - mitochondria replicate
made proteins are synthesised - mitotic spindle begins to
- metabolic rate increases - Chromatids (sister form
-the cell grows chromosomes) are formed from - Completion of centriole
- All chromosomes exist in single a single chromosome. replication
-chromatid form as they are uncoiled.
- Centriole replication starts
Note: There are two forms of cell division which occur in both mitosis and meiosis which are;
a. Nuclear division. This is where the contents of the nucleus divide and is distributed in the daughter
cells.
b. Cytokinesis. This is where the cytoplasm content divides and is distributed in the daughter cells.
MITOSIS
This is a type of cell division in which the mother cell divides into two identical daughter cells which are
similar to the mother cell with the same number of chromosomes as the mother cells. This implies that mitosis
maintains the chromosome number. Mitosis occurs in somatic cells and also can occur in haploid, diploid and
polyploidy cells.
Importance of mitosis
a. It maintains the chromosome number of the daughter cells similar to that of the parent cell i.e. it creates
genetic stability. The chromosomes in the daughter cell carry the same genetic information in their genes
similar to that of the parental chromosomes from where they were formed by replication. The daughter
cells are therefore genetically identical to the parent cell and no variation in genetic information can be
introduced in mitosis. This result in genetic stability within populations derived from cells made by
mitosis.
b. It promotes growth and repair of the body as it increases the number of cells within an organism to
cause growth. In addition cells are constantly dying and being replaced by mitosis to form new cells.
c. It is a basis for asexual reproduction.
d. It promotes formation of gametes in organisms that reproduce by parthenogenesis (in animals) e.g. male
bees called drones, aphids i.e. the development of an organism from unfertilized eggs e.g. bees, aphids
and parthenocarpy in plants e.g. pineapples.
e. Mitosis enables regeneration to occur. During regeneration, some animals are able to regenerate (re-
develop) whole parts of their bodies such as legs in crustacean and arms in starfish.
STAGES OF MITOSIS
The stages of mitosis include the following;
1. Prophase 3. Anaphase
2. Metaphase 4. Telophase
PROPHASE Early prophase (Roberts pg 368)
This is the longest stage of cell division. It is sub-divided into two sub
stages, early prophase and late prophase. During early prophase the
following changes occur in the cell;
i. Establishment of the poles and migration of the centrioles to
opposite poles of the cell. In case of animal cells.
ii. The centrioles begin to synthesis spindle fibers that grow towards
the nuclear membrane.
iii. The chromosomes coil and condense (shorten & fatten) and Late prophase (Roberts pg 368)
become visible as single threads with bead like structures in the
middle known as centromere.
iv. The nucleus starts shrinking.
By the late phase the following changes will have taken place in the cell;
By the late phase the following changes will have taken place in the cell;
Further condensation of chromosomes takes place and each chromosome is seen to consist of a pair of
chromatids joined at the centromere i.e. the chromosomes become visible.
i. The spindle fiber development is completed and these meet at the centre of the cell a point known as
the equator of the spindle.
ii. The nucleus completely disappears.
iii. The nuclear membrane completely breaks down.
Note:
 An aster refers to a radial array of short microtubules that extend from a centromere to the cell surface.
 Spindle fibres originate from Golgi apparatus in plant cell
 A centrosome is a non-membranous region at the pole of the cell containing centrioles which organizes the
microtubules of the cell
Drawing (Soper pg 780)

METAPHASE
This is the second stage of mitosis also having early metaphase and
late metaphase. During this stage the chromosomes line up at the
equator of the spindle independently attached by their centromere to
the spindle fibers i.e. homologous chromosomes do not associate
together.
At late metaphase the sister chromatids slightly repel each other at the
centromere due the contraction of the spindle fibres which also occurs
slightly, thereby orienting the chromatids towards opposite poles.
ANAPHASE Drawing (Soper pg 780)
This is the third and shortest stage of mitosis. It is divided into

early anaphase and late anaphase. During early anaphase, the
centromere split and the spindle fibers contract and start pulling
the daughter centromeres formed together with the sister
chromatids attached to opposite poles of the cell, the fibres
continue coiling thereby becoming shorter and this process uses a
lot of energy in form of ATP.
By late anaphase the chromatids will have reached the poles of the
cell.
TELOPHASE
Drawing (Soper pg 780)
This is the last stage of mitosis and it involves the following changes;
i. The chromatids at the pole uncoil and lengthen to form chromatin and
become invisible again i.e. the chromatids become chromosomes which
uncoil and gain their thread like nature
ii. The nucleolus and nuclear membrane reappears.
iii. The spindle fibers breakdown
iv. A nuclear membrane reforms around the chromosomes at each pole.
v. The cell constricts in the middle which separates the mother cell into two
daughter cells each having the same number of chromosomes as the mother
cell.
Cytokinesis
Separation of the mother cell into two daughter cells including the division of the cytoplasm is described as
cytokinesis. Cytokinesis in animal cells is brought about by the alignment of the micro filament in the middle
of the cell. When the microfilament contract, a furrow is formed from either side of the cell and when these
furrows become big enough, the mother cell divides into two daughter cells.
Cytokinesis in plant cells

Mitosis in plant cells is similar to that in animal cells except that,
i. Plants do not have centrioles and their spindle fibers are produced by the Golgi body
ii. Cytokinesis in plants does not involve formation of furrows but instead a primary cell wall develops in
the middle of the mother cell to separate it into two daughter cells.
iii. The development of the primary cell wall begins with small vesicles that line up across the mother cell
and eventually fuse together to form a cell plate which later becomes the primary cell wall
During telophase in plant cells, the vesicles derived from the Golgi apparatus move along microtubules to the
middle of the cell where they fuse together to produce a cell plate. The cell wall materials carried in the
vesicles collect in the cell plate as it grows. The cell plate then enlarges due to these materials until its
surrounding membrane fuses with the plasma membrane along the perimeter of the cell. Two daughter cells
result each with its own cell membrane and a new cell wall arising from the contents of the cell plate separates
the two daughter cells.
Page 102 of 447

COMPARISON OF MITOSIS IN PLANTS AND ANIMALS

Similarities Differences
In both: Mitosis in animal cells Mitosis in plant cells
i) Spindle fibres form Occurs almost all over the body Occurs at apical, lateral and
ii) During Prophase, intercalary meristems only
chromosomes condense Centrioles present Centrioles absent
iii) Before metaphase, the At telophase a contractile ring of At telophase a phragmoplast of
nuclear envelope breaks actin and myosin forms halfway actin, myosin, and microtubules,
down. between the two nuclei. forms at the future site of cell
iv) Spindle attaches to wall.
chromosomes at Cytokinesis occurs by cleavage Cytokinesis occurs by cell plate
centromeres method
v) At metaphase, the Cell becomes rounded before Cell shape does not change
chromosomes align at the division before division
equator A furrow is formed between two A solid middle lamella forms
vi) At anaphase, chromosomes daughter cells between two daughter cells
move towards opposite Mitotic apparatus contains asters Mitotic apparatus lacks asters
poles Spindle degenerates at cytokinesis Spindle in form of phragmoplast
vii) At telophase, the nuclear persists at cytokinesis
envelope appears again, Several hormones induce cell It is induced by a specific
chromosomes de-condense, division, not one specifically hormone called cytokinin
and the spindle breaks down
MEIOSIS
This is the form of cell division in which the diploid Soper pg 783 fig 23.10
mother cell undergoes two successive nuclear
divisions to form four haploid daughter cells which
are genetically different from each other and also
have half the number of chromosomes of the mother
cell. Meiosis occurs in gonads (gamete producing
cells called germ cells) such as ovaries in females
and testes in males where the diploid germ cells
produce gametes which are haploid. Therefore
meiosis occurs during gametogenesis in animals and
also during spore formation in plants as well as
formation of gametes in flowers (pollen grains and
the ovules). A gamete is sexually reproducing cell
which cannot develop further unless it fuses with
another gamete cell
Importance of meiosis
1. It leads to the production haploid gametes in
sexual reproduction.
2. It brings about genetic variation among
organisms which is a raw material for evolution
of new species.
3. It maintains the diploid chromosomes number of organisms by ensuring that doubling of chromosomes at
each succeeding generation does not occurs. When gametes with haploid number of chromosomes fuse
together at fertilization to form the zygote, the diploid number is restored in the offspring form
STAGES OF MEIOSIS
Meiosis is sub divided into two phases i.e. meiosis I (first meiotic division) and meiosis II (second meiotic
division), each of which is subdivided into four stages namely: prophase, metaphase, anaphase and telophase.
Page 103 of 447

MEIOSIS I b. Zygotene
1. Prophase I  In this stage, further condensation of the
This is the longest part of meiosis 1 and it is subdivided chromosomes occurs and each
into five sub stages namely; chromosome is seen to consist of a pair of
a. Leptotene (b) Zygotene (c) Paehytene (d) Diptotene sister chromatids joined at the centromere.
(e) Diokinese  The homologous chromosomes move close
a. Leptotene to each other, one from the male parent
This is the first step of prophase I and it involves the and the other pair from the female parent.
following changes  The process by which the homologous
 Establishment of the poles of the cell chromosomes come together in prophase I
 The centrioles migrate to opposite poles of the cell of meiosis to form a pair of bivalent is
 The centrioles begin to synthesize spindle fibres known as synapsis.
 The chromosome begin to condense and are seen as  The homologous chromosomes move close
single threads with beadlike structures in the middle together to form a pair called bivalent of
called centromeres which one pair comes from the male parent
 The nucleolus and the nuclear membrane begin to and the other pair from the female parent.
break down and eventually they disappear (Soper pg 784 fig 23.11) (a)
completely
c. Pachytene (Soper pg 784 fig 23.11) (b)

 At this sub stage the homologous chromosomes repel each
other and are partially separate but remain joined together at a
point called chiasma.
 In this stage the non-sister chromatids of the homologous
chromosomes overlap and join together at points known as
chiasmata. At the chiasmata the non-sister chromatids break
as the homologous chromosomes continue repelling each
other and then the broken segments portions which contain
genes are exchanged between the non-sisters chromatids to (Soper pg 784 fig 23.11) (c)
form new chromosomes.
This process by which the non-sister chromatids break and

exchange their genetic material is known as crossing over. This
is the basis for genetic variation among the gametes and among
the offsprings that are formed later. During crossing over, the
genes from one chromosome are exchanged with the genes from
the other chromosome in a pair, leading to a new combination of
genes in the resulting chromatids.
d. Diplotene e. Diakinesis (Terminalisation) After terminalisation, the

In this stage, the chromatids During diakinesis continued repulsion chromosomes completely
of homologous of homologous chromosomes occurs separate and this marks the end
chromosomes continue to between the homologous chromosomes of prophase I.
repel each other and this that are still fixed by chiasmata, this
makes bivalents to assume pushes the chiasmata towards the ends
Page 104 of 447

particular shapes depending of the chromatids a process known as Diagram to illustrate crossing
upon the number of terminalisation. However the over
chiasmata. chiasmata remain holding the non-sister
chromatids towards the end of the
chromatids.
1. Metaphase I 2. Anaphase I 3. Telophase I

During this stage, the The spindle fibers undergo This stage occurs when homologous
homologous chromosomes live spiral coiling thereby chromosomes arrive at the poles. The
up together at the equator of the pulling the homologous following events take place;
spindle inform of bivalents. The chromosomes on them to  The mother cell constricts to divide
chromosomes occupy one opposite poles due to into two daughter cells.
spindle fibre at the equator of the contraction of the proteins  Homologous chromosomes regain
spindle and so the chromosomes that make up these fibres. their thread-like nature at the poles
are said to have associated. The homologous and become invisible again
Drawing chromosomes part  The nucleolus and the nuclear
(Soper pg 785 fig 23.11) (e) company and move membrane reappear so as to enclose
towards the pole of the cell. the chromosome.
By late anaphase1, the  The spindle fibers breakdown and
chromosomes will have cytokinesis then occurs as in mitosis.
reached the poles. This halves the diploid chromosome
Drawing number into the haploid number into the
(Soper pg 785 fig 23.11) (f) two daughter cells. The chromosomes
arrange themselves across the middle of
the two daughter cells and each daughter
cell undergoes a second meiotic division
During this stage chromosomes to form two more daughter cells.
are distributed randomly at the Diagram (Soper pg 785 fig 23.11) (g)
equator of the cell and segregate
(separate) independently which
leads to the mixing of genes in
the daughter cells formed at the
end of meiosis. This results into
genetic variation.
MEIOSIS II
After meiosis II, each of the daughter cells formed enters a short interphase period. During this period, the
cells synthesize more ATP and replication of cell organelles such centrioles occur. However, during this
interphase period replication of DNA chromosomes does not occur. Meiosis II is also sub divided into four
stages namely; prophase II, metaphase II, anaphase II, and telophase II.
The events which occur during meiosis II are similar to those of mitosis as summarized in the diagrams
below;
Page 105 of 447

a. Metaphase II b. Anaphase II
The chromosomes line up individually on the The centromeres split and chromatids of the
equator of the spindle as in meiosis. two chromosomes in each cell separate and
move to opposite poles due to spiral coiling of
spindle fibers.
c. Telophase II
Each cell divides by constricting across in the
middle. The chromatids unwind and become
indistinct so as to become chromosomes. Four
new cells are formed each having half the
number of chromosomes compared to the
original parent cell. The genetic composition of a
chromosome is altered by the crossing over of
prophase I and events of metaphase I.
As in mitosis the spindle fibers disappear and the
nucleus, nucleolus as well as the nuclear
membrane reform such that the cells enter
interphase.
As shown in the diagrams above, the two haploid daughter cells formed in meiosis I immediately undergo
metaphase II in most cases, prophase II is very rare BUT when it occurs the following events occur;
- The centrioles move to opposite poles
- The nucleolus and nuclear membrane break down
- New spindle fibres are formed in each of the two daughter cells of meiosis I
Meiosis brings about genetic variation in the following ways
a. By crossing over between homologous chromosomes during the pachytene stage of prophase I which
separates linked genes on the chromosomes and rearranges these genes which were originally located on
the same chromosome. This leads to a variety of new gene recombinations on the chromosome in the
daughter cells which leads to genetic variation.
b. During metaphase I, homologous chromosomes are distributed randomly at the equator of the cell and
aggregate independently leading to the mixing of genes in the daughter cells formed.
c. It results into the formation of haploid cells (gametes) which when fused randomly at fertilization results
into offsprings with different genetic constitution due to the recombination of the parental genes.
COMPARISON BETWEEN MITOSIS AND MEIOSIS

DIFFERENCES
MITOSIS MEIOSIS
It results into formation of two daughter cells It results into formation of four daughter cells
Daughter cells which identical to the mother Daughter cells are different to the mother cell
cell
It occurs in somatic cells during growth and It occurs during the formation of gametes in germ
developing and in asexual reproduction cells
No crossing over occurs Crossing over occurs.
It occur in haploid, diploid and polyploidy cells It occurs in diploid cells only
Page 106 of 447

Prophase is sub divided into early and late Prophase I is sub divided into five stages namely;
stages leptotene, zygotene, pachytene, diplotene and
diakinesis.
Chiasmata are not formed Chiasmata are formed
Homologous chromosomes do not associate Homologous chromosomes associate
There is no formation of bivalents Bivalents are formed in prophase I.
It involves only one nuclear division It involves two successful nuclear divisions
It maintains the chromosomes number between It halves the chromosome number of the mother cell
the daughter cell and mother cell. within the daughter cells formed.
It takes a shorter time It takes a longer time
Chromosomes form a single raw at the equator Chromosomes form a double raw at the equator of the
of the spindle during metaphase I spindle during metaphase I
Chromatids move to the opposite poles. Chromosomes move to the opposite poles during
meiosis I
SIMILARITIES
Both;
 involve four stages of cell division namely; prophase,

metaphase, anaphase and telophase.
 require the interphase period before they occur.
 are energy consuming processes i.e. they require ATP.
 can lead to the formation of gametes.
 involve condensation of the chromosomes.
 involve nuclear division and cytokinesis.
Sample questions
1. The graph below shows how the position of (c) Explain the trend in distance represented by
centromeres change during mitosis. Line X is (i) curve X (09 marks)
the distance between the centromeres and the From 0 to about 15 minutes the distance between
ends of the spindle. Line Y is the distance centromeres of chromatids and poles of the cell
between the centromeres of pairs of remains constant; and relatively long; because the
chromatids. Measurements started at the cell is in metaphase stage chromosomes are at
beginning of metaphase. (Adopted from metaphase plate (half-way between the poles); with
Advanced Molecular sciences By Mike sister chromatids still held at centromeres;
nd
Bailey and Keith Hirst, 2 edition ) From about 15 minutes to about 23 minutes the
distance between centromeres of chromatids and
poles of the cell decreases rapidly; to 0 µm;
because after splitting during anaphase stage;sister
chromatids are pulled rapidly towards poles by
microtubules (spindle) ; and eventually arrive at the
poles during telophase stage;
(ii) curve Y (06 marks)
15 minutes to about 23 minutes the distance between
the centromeres of pairs of chromatids increases
rapidly; because during anaphase; the chromatids
are pulled apart by spindles;
From about 23 minutes to 25 minutes the distance
between the centromeres of pairs of chromatids
remains constant; because during telophase; the
centromeres remain the same distance away from
each other at the poles;
Page 107 of 447

(d) Explain the variation in the maximum distance

(a) How long was; (02 marks) achieved in X and Y (04 marks)
(i) Metaphase? The maximum distance for Y (between centromeres of
15 minutes; sister chromatids) is almost (twice); longer than for
(ii) Anaphase? X (distance between centromeres of chromatids and
8 minutes; poles) ;
(b) What was the distance between the poles of During metaphase, chromosomes are at metaphase
the spindle in this cell? (01 mark) plate which is equidistant from either pole of the cell
44μm; therefore maximum for X is shorter; Maximum for Y
is longer since spindles pull chromatids to the extremes
of the cell (poles) which are very distant apart;
2. The figure below shows changes in the quantities (c) For one cell cycle only, explain the trend
of nuclear DNA and cell mass during repeated cell in the:
cycle. (Adapted from BIOLOGY IN CONTEXT (i) Mass of DNA with time (12 marks)
For Cambridge International A Level By Glen and From 0 hour to 12 hours, the DNA mass remains
Susan Toole) constant; This the first growth (G1) phase;cell
contents replicate except DNA;
From 12 hours to about 18 hours the mass of DNA
increases rapidly;This is the synthesis (S)
phase; DNA replicates to double original
mass;
From 18 hours to about 23 hours the mass of
DNA remains constant; This is the second
growth (G2) phase;and mitosis; no DNA
synthesis;
During the 23 hour the mass of DNA decreases
very rapidly;This is because cytokinesis
occurs; halving the DNA mass in each new cell
to the original mass;
(a) For one cell cycle only, describe the: (ii) Mass of the cell with time (10 marks)
(i) Mass of DNA with time (09 marks) From 0 to about 23 hours the cell mass increases
One cell cycle lasts from 0 hour to about 23 hours; rapidly; this marks the period of interphase;
From 0 hour to 12 hours;DNA mass remains and mitosis; during which organelles like
constant; From 12 hours to about 18 hours;DNA mitochondria, cytoskeletal elements, endoplasmic
mass increases; reticula, ribosomes, Golgi apparatus, centriole,
From about 18 hours to about 23 hours;the DNA e.t.c. replicate and increase in number; and the
mass remains constant; cell grows (G1 phase);DNA replicates; and
During the 23 hour; the mass of DNA decreases; the chromosome content doubles; histones and
(ii) Mass of the cell with time (06 marks) other nuclear proteins are synthesised (S phase)
;Synthesis of additional proteins that support
From 0 to about 23 hours; the cell mass increases;
cell metabolism occurs (G2 phase);
to a peak; During the 23 hour; the mass of the cell
decreases; During the 23 hour, the mass of the cell decreases
very rapidly;cytokinesis divides the parent cell
(b) For once cell cycle only, compare the mass
into equal sized daughter cells;
of the cell and the mass of DNA, with time.
(04 marks)
Page 108 of 447

GENETICS
This is the study of the mechanism by which characteristics (traits) are transmitted from parents to the
offsprings. This transmission occurs via gametes during fertilization in sexually reproducing organisms.
Therefore genetics can also be referred to as the study of inheritance characteristics of the parents by the
offsprings. The characteristics of organisms are controlled by internal factors called genes located on
chromosomes. A gene is a section of DNA that determines a particular characteristic in an organization or a
section of DNA that controls the production of a polypeptide chain in an organism.
The importance of genetics
a. It is used in genetic engineering where better breeds and varieties of plants and animals are produced. This
is intended to increase production and improve resistance of diseases and pests. This can be done locally
through cross breeding.
b. It is used in the legal profession to determine the paternity of the child i.e. genetics is used to settle
paternal disputes by confirming who the father of the child is. This can be proved through use of blood
groups as these groups are genetically inherited and can therefore be used to prove the rightful father of
the child. If the blood groups fail to prove then DNA analysis can be used.
c. They are used in blood transfusion. Genetic principals are used during blood transfusion so that blood
being transfused is compatible to avoid blood clotting (Agglutination) in the recipient.
d. It is used in the control of the transmission of genetic diseases. These diseases are genetically engineered
e.g. hemophilia, colorblindness, e.t.c. can be eliminated from the human population by following the
principles of genetics as these diseases are genetically inherited.
e. It can be used in crime investigation i.e. use of the DNA finger prints to identify criminals
f. It is used in molecular biology to manufacture artificial enzymes, hormones and vaccines.
g. It enables humans to choose the right partners during marriage by choosing those with characteristics for
reproduction.
TERMINOLOGIES INVOLVED IN GENETICS
Alleles: These are alternative forms in which the gene can exist but control contrasting features of
characteristics. Alleles exist in pairs e.g. consider a gene for height. This gene can be expressed inform of
allele as T (for tallness) and t (for shortness). Therefore these two alleles can exist as TT and tt.
Locus (plural loci). This is the position on the chromosome where the genes are located.
Dominant allele. A dominant allele is the one that can express its self phenotypically in both homozygous
and heterozygous forms.
Recessive allele. This is an allele that can only express itself phenotypically in the homozygous form as it is
suppressed by the dominant allele in the heterozygous form.
Note: Recessive alleles are presented by small letters (lower case) while dominant alleles are represented by
capital letters (upper case)
Phenotype. This is the physical or outward appearance of an organism.
Pure breeding (breeding true). This is where the individuals crossed are homozygous and therefore
produce consistently the same characteristic, generation after generation. A pure breed should therefore be a
homozygous individual when considered for a particular characteristic
Crossing(X). This refers to the mating of the male and female organisms under a consideration.
Homozygous. This is a condition where an individual possess identical alleles for a particular gene e.g.
homozygous dominant (YY, TT, AA) or homozygous recessive (yy, tt, aa)
Heterozygous. This is a condition where an individual possess non-identical alleles for a particular gene e.g.
Tt, Bb. Heterozygous individuals are genetically called carriers of the recessive characteristic Recessive
characteristics can only be expressed when two carriers make an organism which is phenotypically recessive
e.g. the sickle cell anemia individuals, albinos, hemophiliac e.t.c
Hybrid. This is heterozygous individual obtained from crossing two parents with contrasting characteristics
but when these parents are pure breeding e.g. tt X TT
First Selfing. This refers to the crossing of offsprings of the same parents.
Filial generation (F1). This refers to the set of offsprings obtained from crossing two pure breeding parents
with contrasting characteristics. These individuals are therefore heterozygous or hybrids.
Trait. Each variant for a characteristic e.g. short stem or tall stem for pea plant. Height is the trait.
Page 109 of 447

Second filial generation (F2). This refers to the set of offsprings that are obtained from crossing mature
hybrid of parents of the first filial generation.
Test cross. This is the mating of a phenotypically dominant individual with a recessive individual so as to
determine the genotype of the phenotypically dominant individual. This is due to the fact that a
phenotypically dominant individual can either be heterozygous or homozygous. If the homozygous
offsprings resemble the dominant parent then the dominant parent is said to be homozygous and if the
offsprings formed from the test cross shows a phenotypic ratio of 1:1 (test cross ratio), then the parent
with an unknown genotype is heterozygous.
Back cross. This is the mating of an offspring with one of its parent so as to prove the genotype of the
parents.
Reciprocal cross. This is a cross in which the phenotypes of the same characteristics are interchanged
among the parents during a genetic experiment
Cistron. A length of DNA containing a specific sequence of bases that encodes a mRNA molecule
controlling formation of a specific polypeptide chain or protein
MENDEL’S GENETIC EXPERIMENTS AND MONOHYBRID INHERITANCE

This is the inheritance of a single pair of characteristics from the parent to offsprings. Examples include,
height, blood groups, albinism, sickle cell anemia, and sex linked characteristics e.t.c.
This mechanism of inheritance was discovered by a scientist called Gregory Mendel who carried out a number
of genetic experiments using the garden pea plants. He also observed many sexually reproducing organisms
and found out that they had variations among themselves despite being of the same species.
In these experiments, Mendel carried out cross pollination between tall pea plants and short pea plants he had
grown in his garden. In order to carry out a proper cross, Mendel covered the stigma of all flowers of one
group of pea plants in order to have male pea plants. He also removed all the antlers from the flowers of
another group of pea plants in order to have female pea plants. Using a brush he transferred pollen to tall pea
plants from short pea plants. He observed the F1 offsprings were all tall. He then selfed the F1 pea plants to
get F2 which was found to be a mixture of tall pea plants and short pea plants.
Conclusions from Mendel’s experiments
About the actual mechanism of inheritance
1. The phenotypic characteristics are under the control of internal factors (these factors were later named
genes).
2. It is these factor that are transmitted from the parents to the offsprings i.e. (from one generation to the
next).
3. For each character, an organism inherits from the parents two alleles (internal factors), one from each
parent and it is these factors which account for variations in inherited characters.
4. The factor which phenotypically appears in F1 generation is dominant to the one which fails to
phenotypically in F1 but instead appears in the F2generation.
5. During sexual reproduction the egg cell and the sperm makes equal contribution to each of the
characteristics of the offspring such that the offspring has both male and female parental characteristics.
This is because the two alleles for a heritable character separate or segregate during formation of gametes
in meiosis and end up in different gametes.
6. Always in F2 generation, the dominant and recessive offsprings appear in a phenotypic ratio of 3:1. The
results, using proportions only, are summarised in the table below;
Character Type of cross F1 generation F2 generation Ratio
Stem length Tall X Short All tall 787tall, 277short 2.84:1
Cotyledon colour Green X Yellow All yellow 6022yellow, 2001green 3.01:1
Seed type Smooth X Wrinkled All smooth 5474 smooth, 1850wrinkled 2.96:1
Seed coat Coloured X White All coloured 705 coloured, 224 white 3.15:1
Page 110 of 447

Pod colour Green X Yellow All green 428 green, 152 yellow 2.82:1
Pod shape Inflated X Constricted All inflated 882 inflated, 299 constricted 3:1
Flower position Terminal X Axial All axial 651 axial, 207 terminal 3.14:1
Flower colour Purple X white All purple 705 purple, 224 white 3:1
Mendel was successful in his genetic experiment because;

 He had a systematic approach to his work. This is because he dealt with a single characteristic
(monohybrid inheritance) and then later long he considered two characteristics simultaneously (di-hybrid
inheritance).
 He was very patient during his experiments so that he was able to reproduce the garden peas for several
generations.
 He used a very a good experimental organism, the garden peas (Pisum Sativum).
In order to perform good genetic experiments, Mendel used a garden pea plant because such plants have good
characteristics for genetic experimentation which included the following;
 They have many distinct contrasting characteristics without any intermediates such as tall and short
stems, smooth and wrinkled seeds, yellow and white flowers i.e. a good genetic organism must show
many discontinuous variation characteristics
 They produce large numbers of offsprings which provide a large sample for experimentation so as to
get reliable results.
 It is possible for them to undergo controlled pollination.
 They are so small that they can be conveniently handled.
 They have a short life span and they can be reproduced very quickly before the end of the
investigator’s life span.
 Pure breeds were easily obtained
Page 111 of 447

Currently there are two organisms which are also frequently used for genetic experiments. These are
Drosophila melanogaster and Neurospora crassa.
1. Drosophila melanogaster (fruit flies)

 The diploid nucleus contains only four pairs of chromosomes
 The larvae have giant chromosomes in salivary glands. These chromosomes have numerous dark
transverse bands which are useful in the study of chromosomal mutations.
 The flies are easily cultured in small bottles containing simple growth medium. They have a short
life cycle, each cycle is completed in about 10 days. They can produce a large number of
offsprings.
 The flies have many distinct characteristics which are easily mutated and thus can be used for genetic
studies. For example, body colour, eye shape and wing length.
 Males and females are easily distinguished. Controlled mating experiments and counting of flies can
also be carried out easily.
2. Neurospora crassa (Bread fungus)
 The fungus can be grown in a minimal medium containing sucrose, inorganic salts and the growth
factor biotin.
 It has a relatively short life cycle.
 The diploid nucleus contains only seven pairs of chromosomes. Gene positions on the chromosomes
can be mapped easily.
 The haploid ascospores occur in linear series in the narrow tubular ascus and can be dissected out and
grown individually.
 Most of the lifecycle occurs in the haploid stage and recessive gene are easily detected
Note. The choice of such experiments depends on a number of factors;
o easy to breed – must readily produce offsprings and not be particular with whom they breed
o readily grown/cultured/reared – the organisms should be convenient and easy to keep
o cheap and easy to breed – they should not have highly specific nutritional requirements
o small size – it follows that the smaller the organism the more likely the previous conditions are to be
met
o short life cycle – this allows many generations to be investigated in a short period
o production of many offspring – to give statistically accurate results large numbers of offsprings need
to be produced from each mating
o early sexual maturity – this allows more rapid production of subsequent generations
o obviously recognizable feature – genetic differences should be easy to observe
o sexual dimorphism – it is helpful if the male and female of the species are quickly and easily
distinguished
MENDEL’S FIRST LAW OF INHERITANCE Diagram Roberts page 453 fig 28.4
From this experiment about monohybrid inheritance he
suggests the law of genetics which is known as the law of
segregation.
This law states that in diploid organisms each
characteristic is controlled by a pair of alleles but during
gamete formation the alleles separate so that each gamete
possesses a single allele.
Explanation of Mendel’s first law of inheritance
This law is explained by meiosis which halves the
chromosome number in that each characteristic of an
organism is determined by a pair of alleles located on the pair
of homologous chromosomes in the nucleus of the cell of an
organism. Page 112 of 447
Each allele of the pair for a characteristic is therefore carried by a single chromosome of the homologous pair
when homologous chromosomes segregate and move towards opposite pole of the cell during anaphase I of
meiosis. This results into each gamete carrying one allele of the gene pair due to the separation of a pair of
chromatids during anaphase I.
WORKED EXAMPLES
1. In a garden pea plant there are two forms of heights i.e. tall and short. When a pure breeding tall pea plant
was crossed with a short pea plant all the offsprings obtained where tall when the offsprings were selfed a
phenotype ration was obtained in F2.
a. Using suitable genetic symbols, workout the genotypes and phenotypes of the F2 generation
b. What are the phenotypic and genotypic ratios of the F2 generation
c. Explain how you would determine the genotype of F1 tall pea plants formed
d. Suppose 300 pea plants where produced in the F2 generation
i. How many were tall?
ii. How many were short?
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………..……………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
………..…………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
Page 113 of 447

………………………………………………………………………..…………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
…………………..………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
…………………………………………………………………………………..………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………..……………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………..……………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
Page 114 of 447

Solution
2. Suppose a man who is a tongue roller marries a woman who is a non-tongue roller and all the children
obtained in F1 are tongue rollers.
(a) Represent the above information as a genetic cross
(b) One of the children married a non-tongue roller. And they had three children. What is the probability
that their 4th born is a tongue roller?
…………………………………...………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………….
……………………………………………………………………………………………….................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
........................................................................................................................
Page 115 of 447

Carriers and genetic diseases

A genetic disease is an illness that is caused by a gene. Most of the 400 genetic disease are caused by a
recessive allele of a gene. The diseases only develops in homozygous recessive individuals. Heterozygous
individuals do not show any symptoms of the diseases but can pass on the recessive allele to their offsprings.
These individuals are called carriers. Genetic diseases due to recessive alleles usually appear unexpectedly
since both parents must be carriers (they do not show symptoms of the disease) they are unaware of this. The
probability of these parents having a child with this disease is 25%. Dominant alleles cause very few genetic
diseases. Carriers of such genes also suffer from the diseases. If one parent has the disease, the chance of
inheriting it is 50 percent. Carriers are individuals who are heterozygous for an undesirable allele.
Aa X Aa Aa X aa
A a A a A a a
AA Aa aA aa AA aa
Not develops the disease doesn’t develop
Carrier carrier the disease

Genetic disease caused by a dominant allele
Note:
Does not develop the disease
a) Albinism is a monohybrid condition due to lack of melanin pigment in the skin. It arises due to a mutation
which alters the gene responsible for the synthesis of melanin. This makes an albino to have white hair,
very light coloured skin and pink eyes.
b) Most genetic diseases reduce the chances of survival and reproduction, so the alleles causing them are not
usually passed on to offsprings and remain very rare. There is a small number of genetic diseases where
the frequency of the allele causing them is much higher. In these cases the allele must confer an
advantage, causing its frequency to increase by natural selection. Sickle cell is an example of this
Worked example
3. A man with normal skin marries a carrier for albino skin.
(i) What is the probability that some of their children will be albinos?
(ii) What is the probability that the second born child will be a carrier?
Page 116 of 447

[Type text] [Type text] [Type text]
…………………………………...………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………….
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
FACTORS WHICH MODIFY OR AFFECT MENDEL’S MONOHYBRID 3:1 AND 1:2:1 RATIOS
1. Lethal genes: These are genes that lead to the death of the bearer. The gene can either be dominant or
recessive. Most of the lethal genes usually occur in homozygous recessive forms. An example of a lethal
gene with dominant alleles is the inheritance of coat colour in wild mice. Lethal genes are divided into 3
major categories;
a. Gametic lethal genes. These are genes which kill the gametes and therefore prevent fertilization.
b. Zygotic lethal genes. These are genes which kill the zygotes and embryos before birth e.g. the gene
that determine coat color in mice.
c. Infantic lethal genes. These are genes which kill individuals between birth and reproductive stages
e.g. the gene that determines chlorophyll formation in maize, sickle cell anemia in man e.t.c.
Page 117 of 447

Lethal genes in mice

The gene that determines coat color in mice is a zygotic lethal gene. In mice, there are two colours determined
by these genes i.e. yellow and grey (agouti). If two yellow mice are crossed they produce both yellow and
grey offspring however these offspring appear in a phenotypic ration of 2 yellow: 1 grey instead of 3:1.
This is because the homozygous dominant yellow mice die in the uterus which reduces the phenotypic ratio.
The yellow mice produced are always heterozygous and this changes the monohybrid genotypic ratio from
1:2:1 to 2:1. This is shown using the genetic symbols below;
……………………………………………………………………………………………………………………
……………………………………………………………….……………………………………………………
………………………………………….................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
.................................................................................................................................................................................
Note
a) Dominant lethal genes are very rare in a population because they are usually manifested easily in
growth and development of the offspring at an early age and hence easily eliminated.
b) A pleiotropic gene is the one which controls more than one aspect or characteristic in the metabolism
of an organism e.g. the Y gene in mice is controlling both viability and coat colour, for viability the Y
gene acts as a recessive gene since homozygous YY mice dies in the uterus and since Yy mice are
yellow this phenomenon is called pleitrophy.
Page 118 of 447

CO-DOMINANCE
This is a phenomenon whereby the alleles controlling a particular characteristic have equal powers of
expressing themselves in the phenotype in the heterozygote. Therefore the offspring produced will have a
mixture of the two parental characteristics in the phenotype. Codominance is found in both plants and
animals.
Co-dominance is taken to be a form of incomplete dominance since no allele suppresses the phenotypic
expression of another. In co-dominance we use capital letter to represent all the two alleles each letter
corresponding to each of the two characteristics.
Examples of co-dominance include the following;
a. The gene that determines coat color in cattle
b. Inheritance of blood group AB in man
c. Inheritance of sickle cell trait
d. Human MN blood group
Inheritance of coat colour in cattle

Remember that we cannot use upper and lower case letters for the alleles, as this would imply that one (the
upper case) was dominant over to the other (the lower case). We therefore use different letters R for red and
W for white –and use these as superscripts on a letter that represents the gene, in this case C for colour i.e. CR
and CW
Consider a cross between a red bull and a white cow whose F1 (spotted) offsprings are selfed. Workout the
genotypes and phenotypes in F1 and F2 generation stating in each case the ratios
Let CR represents the allele for red colour production in cattle
Let CW represent the allele for no colour production in cattle
……………………………………………………………………………………………………………………
……………………………………………………………….……………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
Page 119 of 447

……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
………………..…………………………………………………………………………………………………
……………………………………………………………………………………………………………………
………………………………..…………………………………………………………………………………
……………………………………………………………………………………………………………………
………………………………………………..…………………………………………………………………
……………………………………………………………………………………………………………………
………………………………………………………………..…………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………..
Inheritance of sickle cell anaemia

This is an abnormal condition in which the red Sickle cell anaemia is caused by a substitution
blood cells collapse into a sickle shape under mutation on DNA cistron, in the gene that codes for the
low oxygen concentration due to the presence of β-globin in the polypeptide in haemoglobin. A single
abnormal haemoglobin (HbS) in the red blood nucleotide base, Adenine is substituted by the nucleotide
cells. The normal haemoglobin is found in red base, Thymine.
blood cells with a bi concave disc shape During substitution gene mutation on DNA cistron,
Drawing adenine replaces thymine in beta polypeptide chain
cistron leading to formation of CAT code on the DNA
cistron instead of CTT (that codes for glutamic acid).
The mRNA formed therefore has a GUA codon which
codes for valine instead of GAA codon which codes
glutamic acid (responsible for normal haemoglobin in
red blood cells). valine leads to the formation of
abnormal haemoglobin.
The substitution mutation occurs at the sixth amino acid
in the β-chain, this results in wrong amino acid, valine,
being incorporated into two of the β-polypeptide chains.
Valine is non-polar and hydrophobic which makes its
presence in the haemoglobin (HbS) less soluble when
deoxygenated. Therefore when HbS loses its oxygen, the
molecules come out of solution and crystallise (solidify)
into rigid rod-like fibres. The chains of haemoglobin
join together into bundles that are rigid enough to distort
the red blood cells into a sickle shape.
Effects of sickling red blood cells
a. Anaemia this is occurs because the sickle cells are destroyed which lowers the amount of oxygen to be
carried leading to acute anemia. This leads to;
Page 120 of 447

 Fatigue (weakness)
 Poor physical development
 Dilation of the heart which may lead to heart failure
 Infections which lead to frequent illness
b. Interference with circulation of blood because the cells get jammed in capillaries and small arteries. This
leads to;
 Heart damage which leads to heart failure
 Lung damage which leads to pneumonia
 Muscle and joint damage which leads to rheumatism and pain
 Gut damage which leads to abdominal pain
 Kidney damage which leads to kidney failure
 Liver damage
c. Enlargement of the spleen because the sickle cells collect in the spleen for destruction
The effects above make the homozygous sufferers to often die before reproductive age.
Note: When sickle cells return high in oxygen conditions in the lung, the haemoglobin chains break up and
the cells return to their normal shape. These changes occur time after time, as the red blood cells circulate.
Both the haemoglobin and the plasma membrane are damaged and the life cycle of a red blood cell can be
shortened to as little as 4 days. The body cannot replace red blood cells at a rapid enough rate and anemia
therefore develops. This gene can also be described as pleiotropic since it has more than one effect in an
organism.
In heterozygous individuals, almost half the molecules made are HbS and BbA i.e. the alleles HbA and HbS are
co-dominant and the faulty HbSgene is not recessive in heterozygous but behaves as recessive in homozygous
state. Heterozygous people are not affected except at unusually low oxygen concentrations, such as when
flying in an unpressurised aircraft or climbing at high altitude. There some of the cells sickle due to
crystallization of their haemoglobins. The heterozygous condition is known as sickle cell trait. These
individuals have a selective advantage over non carriers because they are far less susceptible to malaria (the
malaria parasite multiplies inside normal red blood cells) so are more likely to survive in malaria infested
areas, and pass on their genes to the next generation. A single copy of the sickle-cell allele increases resistance
to severe malaria. The final frequency of the gene in the population varies according to the amount of malaria.
Both homozygous recessive (sickle cell anaemia) and heterozygotes (sickle cell trait) individuals suffer from
severe and mild malaria attacks respectively but sickle cell traits are more resistant to sever attacks of malaria
but suffer the resulting mild anaemia.
Using genetic symbols show the offsprings obtained if;
a) a normal man marries a sickle cell anaemic woman.
b) another man who is a carrier of sickle cell anaemia of the same disease maries the same
woman.
Work out the phenotypic and genotypic ratios arising from these two marriages.
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
Page 121 of 447

……………………………………………………………………………………………………………………
………………………………………………………………………………….…………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
Example
Page 122 of 447

Consider a normal man mating with a woman with sickle cell anemia to obtain F1 offsprings which will be
phenotypically normal but carriers, if the two carriers mate to form F2 the phenotypic ratio will be 1:2:1. Use
genetic symbols to represent the information above
Solution
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
Page 123 of 447

Carriers (heteozygotes) of sickle cell anemia show the sickle cell trait, a co-dominant condition, in which most
of the red blood cells have normal hemoglobin and only about 40% of the red blood cells have abnormal
hemoglobin S. This produces mild anemia and prevents carriers of the sickle cell trait from contracting severe
malaria. This is because when the plasmodium that causes malaria enters a red blood cell with hameoglobin S,
it causes extremely low oxygen tension in the cell through its aerobic respiration which leads to the cell
sickling in heterozygotes. These sickled cells are quickly filtered out of the blood stream by the spleen, thus
eliminating the parasites but resulting into mild anaemia.
In humans MN blood group, the blood group is determined by the antigen types on the membrane of red blood
cells.
Genotype Phenotype (antigen on RBC)
IMIM Blood type MM (antigen M only)
IMIN Blood type MN (antigen M and N)
ININ Blood type NN (antigen N only)
Antigens M and N are found on the surface of red blood cells. These antigens can stimulate production of
antibodies when injected into rabbits or guinea pigs. However, humans do not produce antibodies for antigens
M and N. The MN blood type is not medically important during blood transfusion.
INCOMPLETE DOMINANCE
This is a condition whereby the characteristics of the alleles blend together to form an F1 offspring
(heterozygous) phenotype which is intermediate between the two parental phenotypes. Therefore the F1
individuals do not resemble any of the parents. They are as a result of partial expression of both the alleles.
It can also be defined as a situation with by the heterozygote shows a phenotype intermediate between the
parental phenotypes.
In incomplete dominance no gene dominates the
other in the phenotype but instead forms
intermediate phenotypes and are therefore
represented using capital letters. Incomplete
dominance is found in both plants and
animals.
Examples of incomplete dominance are;
(a) flower colour of Antirrhinum (snapdragon)
(b) flower colour of Mirabilis jalapa (4 o’clock
flower)
Example
In a snap dragon plant, when a red flowered is crossed with a white flowered plant, all the F1 plants obtained
are pink flowered. When the F1 are selfed, the F2 phenotypic ratio is 1:2:1 instead of 3:1. Using suitable
genetic diagrams, explain the above results.
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
Page 124 of 447

……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………….
Note. The allele for red flower colouration controls the production of pigments to make the flowers appear
pink but not red
MULTIPLE ALLELES
This is another form of co-dominance. Multiple alleles refer to more than two possible alleles of which can
occupy the same gene locus on a pair of homologous chromosomes. However, only two of these alleles can
occupy a locus on a pair of homologous chromosomes in a single diploid organism.
Examples of characteristics controlled by multiple alleles include;
a. Blood groups in humans
b. Coat color in rabbits
c. Eye color in rabbits and mice
Page 125 of 447

Inheritance of ABO blood system

The ABO blood group system is controlled
by three alleles of an autosomal gene I GENOTYPE PHENOTYPE
A A
(isohaemogglutinogen) occur at a single I I (AA) Blood group A (homozygous)
A O
A
locus any time. These alleles are A or I , B I I (AO) Blood group A (heterozygous)
B B
B O A B
or I and O or I . These alleles I and I are I I (BB) Blood group B
B O
O
equally dominant while the allele. I is I I (BO) Blood group B
A B
recessive to both. The transmission of thee I I (AB) Blood group AB (co-dominant)
O O
alleles occurs in a normal Mendelian I I (OO) Blood group O
fashion.
The table summarizes the possible
phenotype and blood group.
Physiology of the blood groups in humans
Human blood contains blood group antigens and blood group antibodies. Some of these specifically determine
blood groups e.g. allele A determines the production of antigen A, allele B determines the production of
antigen B and allele O does not code for the production of any antigens. Antigens A and B occur on the
plasma membranes of red blood cells. These antigens have corresponding protein molecules known as blood
group antibodies (agglutinins) in blood plasma. These antibodies can react with the antigens under the lock
and key hypothesis should they be similar to the antigens brought into the recipient’s blood, leading to the
formation of a precipitate or an agglutinate in blood. Therefore an individual should not have blood group
antibodies corresponding or similar to his blood group antigens in order to avoid agglutination.
Consequently, individuals should have the following antibodies not corresponding to their antigen to avoid
blood clotting.
Blood A A B B AB O
group
Antigen A A B B AB None
Antibody a a b b None a and b
Example
1. A man having blood A marries a woman having blood group AB. What are the possible genotypes and
phenotypes of their offsprings if the man is heterozygous for blood group A?
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
Page 126 of 447

2. A boy has blood group A and his sister has blood group O. which combination of genotypes and
phenotypes do you think their parents have. Show your working.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
The importance of blood groups

a. They are important during blood transfusion where they are used to prevent agglutination (precipitation)
of blood of the recipient. To avoid agglutination, the donors blood group should be compatible (matching
with) to that of the recipient by having the donors blood group antigen that is different from the blood
group antibody of the recipient.
When the recipient’s gets antibodies from the donor, such antibodies become diluted in the recipient’s
blood and so cause either minor clotting of blood or no blood clotting at all and so cannot lead to death of
the recipient. However, in case the donor introduces an antigen that is similar to the antibody of the
recipient, it stimulates the recipient’s blood to produce more antibodies which attack and react with the
donor’s antigen to cause severe blood clotting. Therefore an individual with a specific antigen on the red
blood cell membrane does not possess its corresponding antibody in the blood plasma to avoid
agglutination.
Blood plasma permanently contains two blood group antibodies a and b which do not correspond with a
specific antigen in blood to avoid agglutination e.g. a person with blood group A has antigen A and
antibody to avoid agglutination. A person with blood group B cannot donate blood to a person of blood O
because antigen B in the donor’s blood will be attacked by antibody b in the recipient’s blood leading to
Page 127 of 447

agglutination. The same applies to blood group A and blood group AB donors to blood group O
recipients.
It is possible for blood group A to donate blood to blood AB, because the donors blood, blood group A,
has antigen A which cannot stimulate the recipient’s blood group AB to attack antigen A since blood
group AB individuals lack antibodies that can attack antigen A to cause an agglutination.
A person of blood group AB cannot donate blood to a person of blood group O. This is because the
donor’s blood has antigen A and antigen B, which stimulate the recipient’s blood to produce
corresponding antibodies a and b, which then attack and react with antigen A and B in the recipient’s
blood.
Blood group AB individuals can receive blood from all other individuals having other blood groups.
Therefore individuals with blood group AB are called universal recipients. This is because such
individuals have no antibodies in their blood plasma that can react with antigens A and B in the donor’s
blood.
Individuals with blood group O can donate blood to all other blood groups and are therefore called
universal donors. This is because blood group O individuals do not have any antigens in their red blood
cells that can react with antibodies in the blood plasma of the recipient to cause agglutination.
The table below summarises the possible and impossible blood transfusions.
Recipient Donor’s blood group
Blood group Antibody in A B AB O
plasma
A B  X X 
B A X  X 
AB None    
O a and b X X X 
 = compatible with recipients blood
X = Incompatible with recipient i.e. agglutination occurs

b. They are used in settling court cases about who the father of the child is (i.e. paternity suits). Although
blood groups cannot prove beyond reasonable doubt who the father of the child is it is possible to use their
inheritance to show that an individual could possibly by the father of the child.
Consider a mother who is of blood group O having child of blood group O and the child produced also
with blood group O. she claims that the father is a man whose blood group is AB. Since the child is blood
group O its only possible genotype is IOIO and it must therefore have inherited one IO allele from each
parents. Since the man is of blood group AB he cannot donate the IO to the child and therefore he cannot
be the father of the child. Even if the father was found to be of another blood group such as blood group A
still the evidence will be insufficient because any other man can possess such a blood group and donate
the IO allele to the child. Therefore a DNA test should be carried out to confirm who the father of the child
is.
c. Blood groups can also be used as an evidence of evolution. This is because organisms of different species
having similar blood group systems such as the ABO system are believed to have originated from the
same ancestor in the course of evolution for example humans, chimpanzees, gorillas, Baboons e.t.c.
THE RHESUS BLOOD GROUP SYSTEM

The rhesus blood group system is also inherited in a similar way to the ABO blood group system. Individuals
with red blood cells with the D-antigens (Rhesus factor) are said to be rhesus positive (Rh+) however Rh+
allele is taken to be dominant over the rhesus negative (Rh-) allele. The Rhesus factor is controlled by three
alleles C, D and E which determine the production of D-antigens on the surface of the red blood cells. Allele
mainly determines the production of D-antigens and it is this antigen which is the fundamental determinant of
blood grouping under the rhesus blood group system.
Marriage complications of the Rhesus system
If an Rh+ man marries an Rh- woman, most of their children are likely to die immediately after birth or before
birth. The first child usually survives because the time is too short for the mother to produce enough
Page 128 of 447

antibodies known as anti D agglutinins which can pass to the feotus to cause death. If a mother becomes
pregnant after the first child, the Rh+ feotus formed can die due to antibodies of the mother entering the foetal
circulation. This is because during the first pregnancy, especially the time of giving birth, the blood of the
child which is Rh+ may mix with that of the mother which is Rh-, thereby introducing D-antigens in the
mother’s blood. Also, some of foetal erythrocytes of the first child with D-antigens in them may cross the
placenta and enter the body of the Rh- mother towards the end of the gestation period. D-antigens will then
stimulate the mother’s blood to produce many antibodies called anti D-agglutinins which attack and react with
the D-antigens introduced in the mother’s blood if the mother becomes pregnant again and the child is Rh+.
These antibodies in the mother’s blood will pass via the placenta and enter the foetal blood circulation, where
they will attack and react with D-antigens in the child’s blood causing the red blood cells of the child to clamp
together, this disease is known as heamoltyic disease of the new born (erythroblastosis foetalis). This results
into acute anaemia of the foetus which can lead to death of the foetus. The problem may be solved in two
major ways;
a. The mother may be injected with anti-D-agglutinins in the first 72 hours after her first born so as to
make her immune system insensitive towards D-antigens.
b. By carrying out proper intermarriages where by Rh+ man marries Rh+ woman and Rh- woman gets
married to Rh- woman.
Another blood group system in humans called the MN blood group system is controlled by 2 alleles M and N
which are co-dominant. M and N alleles also determine the production of antigens respectively. Individuals
therefore have the following genotypes if this blood group system MM, NN, MN.
ASSIGNMENT
1. Suppose a man having blood group A marries a woman who is heterozygous for blood group B what are
the possible genotype and phenotypes.
2. A boy has blood group A and his sister has blood group B. what are the possible phenotypes and
genotypes of their parents.
3. If a father has blood group A and the mother blood group AB what are the possible genotypes and
phenotypes of the offspring.
DIHYBRID INHERITANCE
This type of inheritance whereby two characteristics are transmitted from the parents to the offsprings at the
same time
When Mendel considered the inheritance of two characteristics simultaneously, he concluded that these
characteristics are inherited independently and each pair of alleles separates during meiosis and during
fertilization each of the alleles combines randomly with either alleles of another pair. From this conclusion
Mendel made his second law of inheritance which states that; “each characteristic in diploid organisms is
controlled by a pair of alleles which separate so that each allele randomly combines with any other
allele of another pair.”
Mendel also described it as the law of independent assortment.
This law is explained by meiosis as follows. During gamete formation, during meiosis, the distribution of each
allele from a pair of homologous chromosome is entirely independent of the distribution of alleles of other
pairs. During metaphase I of meiosis homologous chromosomes lineup on the equator of the spindle and
subsequently separate (segregate) independently during metaphase I and move to opposite poles
independently during anaphase I which leads to a variety of allele recombination in the gametes formed, as
long as each gamete has one allele for each gene.
Example
In the garden pea plant, the gene controlling flower color is located on the same chromosome with that
controlling height. Suppose a pure bleeding tall red flowered plant is crossed with a white short flowered
plant, the F1 offsprings obtained are tall red flowered plants. If the F1 offsprings are selfed,
Page 129 of 447

a) What would be the phenotypic ratio in the F2 generation

b) If 700 pea plants are formed in F2 generation, what would be number of pea plants in each phenotypic
class
c) How would you experimentally determine the genotypes of the F1 plants
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
Page 130 of 447

Note: in dihybrid inheritance, some of the offsprings formed in the F2 have a mixture of the two parental
phenotypes that gave rise to F1 and such offsprings are known as recombinants while other offsprings in F2
resemble one of the two parental phenotypes that gave rise to F1 and such offsprings are known as parentals.
Recombinants arise when crossing over takes place during the formation of gametes in meiosis which leads to
the mixing of the two parental characteristics. The number of recombinants in F2 is usually smaller than that
of the offsprings which resembles the parental phenotypes (parental offsprings). This is because crossing over
occurs by chance which reduces the number of recombinants formed.
Example 2
In Drosophila melanogaster flies, the gene determining the size of the abdomen occurs on the same
chromosome with that determining the length of the wings. When a pure breeding broad and long winged
female fly was crossed with a narrow and vestigial winged male fly all the F1 offsprings obtained head broad
abdomen and long wings. If the F1 offsprings were selfed to obtain F2.
a. Using suitable genetic symbols work out the phenotypes and genotypes that were obtained in F2
generation.
b. Suppose 480 flies were obtained in F2 work out the numbers of the flies for each phenotype class.
c. How many of these flies were recombinants.
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
Page 131 of 447

……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………..
INHERITANCE OF COMPLEMENTARY GENES

These are two or more genes which interact together in order to control a single characteristic in an organism.
Inheritance of these genes therefore does not agree with Mendel’s laws of inheritance. Although these genes
control a single characteristic, they show independence assortment. Therefore these genes are passed on from
the parents to the offspring in a normal Mendelian fashion. The best example of complementary genes are
genes which control the shape of combs in chicken.
In chicken there are four types of combs namely;
i. walnut comb
ii. single comb
iii. pea comb
iv. rose comb
These four types of combs are controlled by the two genes located at two loci situated on different
chromosomes and which interact together to give rise to the four comb types. The shape of the combs is
controlled by two genes which are represented by two alleles shown below;
Let P represent the allele for pea comb
Let R represent the allele for rose comb
The pea comb develops in the presence of the P-allele and in the absence of the R-allele while the rose comb
develops phenotypically in presence of R-allele and in the absence of the P-allele. When both alleles, P and R,
are present together a walnut comb develops. A single comb appears only in the homozygous double recessive
condition
Consider a cross between a pea comb shaped crock with a rose combed hen whose F1 offspring are then
selfed. What is the phenotypic ratio obtained in F2?
Page 132 of 447

……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………….……………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
………………………………………………..……………………………….…………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………….
……………………………………………………………………………………………………………………
In this inheritance, the genes are usually situated at different loci at different chromosomes from where they
interact together and give rise to four distinct phenotypes for a single characteristic.
The walnut comb results from a modified form of co-dominance in which atleast one dominant allele of either
pea comb or rose comb is present.
Page 133 of 447

This is an incidence where by a 9:3:3:1 phenotypic ratio is obtained for a single characteristic. Although this
ratio is this pattern of inheritance differs from the hybrid inheritance because;
a. The F1 progeny (offsprings) resembles neither parents i.e. they are all walnut comb shaped unlike
their parents.
b. The F2 progeny also contains two new phenotypes which do not exist in the F1 parents namely walnut
and a single comb shaped and these appear in a higher ratio as compared to the rose and the pea comb
MODIFICATION OF 9:3:3:1 PHENOTYPIC RATIO
This ratio is mainly modified by the inheritance of lethal genes, linkage of genes and epistasis
EPISTASIS Dominant epistasis. This is the type of epistasis where the

This is an example of gene interaction. epistatic allele is dominant such that is presence
Epistasis refers to a condition in which non- suppresses the phenotypic expression of the recessive
allelic genes interact during which the allele on another locus. This type of epistasis changes the
epistatic allele of the epistatic gene on one phenotypic ratio from 9:3:3:1 to 12:3:1.
locus suppress the phenotypic expression of In Leghorn fowl, there are white and coloured birds.
the hypostatic allele of the hypostatic gene on Colour is due to a coloured pigment produced by a
another locus and are independently inherited. dominant allele C. Normally birds are only white when
For example, when genes act in a sequence, as there are two recessive alleles cc for the gene. However,
part of a biochemical pathway, an allele that another dominant allele I of a gene on a different
produces a defective enzyme early in the chromosome prevents the action of allele C. When the
pathway interferes with the expression of dominant allele C is present with dominant allele I, no
another gene hence epistasis. pigment is produced and the bird is white. The result of a
An epistatic allele is the one which dihybrid cross between the fowl with the genotype CCII
suppresses another allele in the phenotype and another with the genotype ccii produces an F2
though they are not located on the same locus generation with the following possible genotypes; CCII,
and the suppressed gene or allele at another CCIi, CcII, CcIi, ccII, ccIi, ccii, which are white and CCii
locus is the hypostatic one. and Ccii which are coloured. From the genotypes above, it
They are 3 types of epistasis which include can be concluded that a bird possessing both dominant C
the following; and dominant I allele will be white as well as genotype
i. Dominant epistasis ccii. Without epistasis only the genotypes ccII, ccIi and
ii. Recessive epistasis ccii would be white.
iii. Isoepistasis Recessive epistasis. This is the type of epistasis where the
epistatic allele is recessive, such that its presence in
homozygous condition, suppresses the phenotypic
expression of the dominant allele located on another locus.
This type of epistasis changes the dihybrid phenotypic
ratio from 9:3:3:1 to 9:3:4.
Isoepistasis. This is the type of epistasis in which both
alleles and the non-allelic genes have equal powers of
suppressing each other in the phenotype. This modifies the
dihybrid phenotypic ratio to 15:1
Examples
In oats the inheritance of color is controlled by the epistatic gene which has two alleles, one allele being
dominant for color appearance while the other allele is for no color formation (white or albino) i.e. the
hypostastic gene is responsible for color deposition or type of color. Where by black is dominant over white
Consider a cross between homozygous black oat plant with a homozygous white oat plant and then the F1
plants are selfed to get F2.
a. Work out the phenotypic ratio of the F2 generation
b. How many individuals are found in each of the phenotypic classes obtained in F2 if 130 individuals were
found in F2?
Page 134 of 447

……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
Inheritance of coat colour in mice is another example of epistasis. Three phenotypes can occur. Most wild
mice have agouti (grey) coat colour. However there are some mice with black fur and others have white fur.
Fur colour is controlled by a pair of genes present at different loci. The epistatic gene controls the presence of
coat colour and has two alleles. The allele for agouti coat colour (A) is dominant to the allele for black (a).
Page 135 of 447

White fur is caused by a recessive allele (w) on a different locus and presence of (W) leads to deposition of
colour. Homozygous recessive (ww) mice are white/albinos even if the alleles for coloured fur (A or a) are
also present. The colourless precursor molecules are not converted into melanin pigments. Example,
determine the probability of obtaining albino mice if black coat coloured (aaWW) mouse was crossed with an
albino (AAww) mouse
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……..……………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
1. LINKAGE OF GENES Although these genes occur on the same

Linked genes are more than two genes located on the same chromosome, each one controls the specific
chromosome but controlling different characteristics and characteristic irrespective of the other.
are inherited together as a single black. However, linked genes do not show
Linkage is the occurrence of more than one gene on the independent assortment during gamete
same chromosome which are inherited together along with formation and therefore the phenotypic ratio
the chromosome as a single block. There are two types of obtained in F2 is 3:1 instead of the expected
linkage; autosomal gene linkage, when the genes are on 9:3:3:1 for the two linked characteristics.
the same autosome, and sex-linkage, when the genes are Sometimes crossing over occurs, thereby
located on the sex chromosomes, mainly the X separating the linked genes on the
chromosome. chromosomes leading to the formation of
Linked characteristics are the ones controlled by genes recombinant gametes during meiosis and
located on the same chromosome and so are transmitted this gives an F2 phenotypic ratio of 9:3:3:1
together with the chromosome from generation to for the linked characteristics.
generation.
Page 136 of 447

Example
In Drosophila flies the genes controlling body color and the length of wings occur on the same autosomal
chromosomes and are linked together. Consider a cross between a pure breeding grey bodied long winged fly
with a black bodied vestigial winged fly whereby the grey bodied is female while the black bodied is male. If
all the F1 flies obtained have grey bodied and long winged what are the phenotypic and genotypic ratios of the
F2 flies.
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
Page 137 of 447

The above results are correct if there’s no crossing over during gamete formation.
In case the genes are not completely linked together in the chromosome crossing over can occur between the
non-sister chromatids so as to produce recombinant gametes and this gives a phenotypic ratio in F2 of the
9:3:3:1 as shown below.
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
Page 138 of 447

CROSSOVER VALUE AND CHROMOSOME MAPS

The recombinant gametes are formed by crossing over The distance between genes on the
between non-sister chromatids in meiosis I. Such chromosome is measured in arbitrary units
recombinant gametes lead to recombinant offsprings in the known as map units. 1 map unit = 1 cross
phenotypes. Recombinant gametes and recombinant over value (C.O.V)
offsprings occur in lower numbers compared to the parental The larger the C.O.V, the more separated the
gametes and parental offsprings because crossing over two genes are on the chromosomes and the
occurs by chance. The percentage of recombinant higher the chances of crossing over taking
offsprings in the progeny (total number of offsprings) gives place. The illustration of the distance
a cross over value which indicates the relative distance between the genes on the chromosome gives
between the genes on the chromosomes and the likely hood the chromosome map i.e. a figure that shows
of undergoing crossing over. a relative distance between the genes on the
Cross over value (C.O.V) = same chromosomes. The illustration of
number of recombinants distance between the genes on the
total number of offsprings (total progeny)
x 100%
chromosome is the chromosome map.
Example
In Drosophila flies the genes controlling body color and eye color occur on the same chromosome and are
linked together. In an experiment, a heterozygous female fly for grey body and normal eyes was crossed with
a black body and purple eyed fly. In these flies, grey body is dominant over black while normal eyes are
dominant over purple flies. If 1000 offsprings were obtained from this cross as shown in the table below;
Expected Phenotype Genotype Number
number obtained
250 Grey, normal eyes GgNn 480
250 Grey, purple eyes Ggnn 18
250 Black, normal ggNn 17
eyes
250 Black, purple eyes Ggnn 485
a) Parental phenotype: grey body normal eyed fly x black body purple eyed fly. Show the results of this
cross
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
Page 139 of 447

The obtained results in the test cross differ from the expected ones because the genes are linked together on
the chromosomes and were separated by crossing over which occurs by chance hence resulting into formation
of fewer recombinants compared to the parents.
number of recombinants
b) Cross over value = total number of offsprings (total progeny) x 100%
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
c)
Example 2
Further experiment on these flies indicated that the Expected Phenotype Genotype Obtained
genes for body color, length of wings and eye color 250 Grey, long GgLl 400
250 Grey, Ggll 95
are on the same chromosomes. Using the
vestigial
information in the table below calculate the cross
250 Black, long ggLl 105
over value and illustrate the distance between the 250 Black, Ggll 40o
genes. vestigial
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
Page 140 of 447

NOTE: Drawing the chromosome map is also called gene mapping where the position of genes are shown on
the chromosomes as well as the distance separating with them. Sometimes it is possible to indicate many
genes on chromosome and their distances of separation.
Consider the cross over values involving for different genes P, Q, R and S.
The distance separating these four genes is shown below;
P-Q = 24% R-S = 8%
R-P = 14% S-P = 6%
Draw the chromosome map to show the position of these chromosomes.
Answer. Draw the chromosome map for these genes
a. Insert the positions of the genes with the smallest cross over value in the middle of the chromosome map.
b. Examine the next largest cross over value and insert both possible positions of its genes on the
chromosomes relative to either S or P.
c. Repeat the procedure for all the remaining cross over values until you reach the largest cross over values.
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
…………………………………………………….………………………………………………………………
………………………………………………………………………………………………….…………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
FACTORS THAT AFFECT CROSSING OVER

1) The relative distance between the genes on the chromosome. When the genes are far apart from each
other on the chromosome, they have high chances of forming chiasmata in between thereby leading to
genetic exchange on the other hand when genes are very close to each other on the chromosome, their
chances of forming chiasmata is limited.
2) The position of the centromere on the chromosome. If the genes are very close the centromere there
chances of undergoing genetic exchange are limited. However, if the genes are far away from the
centromere, there are high chances that they can be exchanged by crossing over.
3) Temperature. Crossing over decreases with increase in temperature because the process of meiosis
requires suitable temperature that can promote efficient crossing over.
4) Age of the organism. Increase in age lowers the chances of crossing over. Meiosis is more efficient in
grown up adults before menopause stage in females and before senescence in male.
5) Mutagens. These can decrease or increase the rate of crossing over. The chances of crossing over are
greatly reduced by presence of chemical substances that inhibit chiasmata formation thereby preventing
cross over e.g. in drosophila flies.
Page 141 of 447

INHERITANCE OF SEX
The sex of an organism is determined by two factors namely; environmental conditions and genetic factors.
Environmental determination of sex, In lower animals, sex can be determined by environmental factors
such as temperature, salinity, type of food e.t.c. for example in tadpoles the eggs laid in cool places develop
into males while those laid in warm places develop into females.
Genetic sex determination; the sex organs development can be determined by the sex genes or the
chromosomes. Under chromosomal sex determination, sex can be determined by;
a. The number of chromosomes. E.g. in bees the females are diploid and have 32 chromosomes while the
males are haploid and have 16chromosome. In grasshoppers the females have 24 chromosomes while the
males have only 23 chromosomes.
b. The sex chromosomes. In heterogametic organisms,, such as human beings, there are two sex
chromosomes that determine the sex of an individual namely the X and Y chromosomes e.g. the females
are XX and are described as homogametic while the males are XY and are described as heterogametic.
Therefore in these organisms it is the presence of the Y chromosome that makes one a male and its
absence makes one a female. This implies that it is the type of sperm (whether X or Y) that fertilizes the
egg which determines the sex of the offspring.
The X chromosome is large with many genes
on it that are essential in both male and
female development. The Y chromosome is
smaller, with far fewer genes. Part of the Y
chromosome has the same sequence of genes
as the X chromosome (homologous part), but
the genes on the remainder of the Y
chromosome are not found on the X
chromosome (non-homologous part) and are
not needed for female development.
One gene, the tdf gene, is only found on the Y

chromosome. It initiates the development of
male features, including testes and
testosterone production. The homogametic
females lack the Y chromosome with its tdf
gene hence ovaries develop instead of testes
and female sex hormones are produced
instead of testosterone.
In birds sex is determined by the X and Y chromosomes except that the females are XY while males are
XX.
In grasshoppers sex is only determined by X chromosomes where by the males are XO i.e. they have only
one X chromosome (XO) while the females are XX.
SEX LINKED CHARACTERISTICS

This is the transmission of characteristics (from parents to offsprings) whose genes are located on the sex
chromosomes i.e. the genes controlling such a character are transmitted along with those that determine sex on
the same chromosome.
Most sex linked characters are controlled by genes located on the X chromosome and very few are controlled
by genes located on Y chromosome. Examples of sex-linked characteristics include; haemophilia, colour
blindness e.t.c. Sex linked characteristics can therefore be defined as those whose genes controlling them
occur on the sex chromosome and yet they do not determine sex.
Sex linked characters are often expressed more in males than females.
Page 142 of 447

This is because the males being heterogametic In the case of females, the X chromosomes are
have a non-homologous portion of the X completely homologous to each other and this gives
chromosome while the sex linked allele is chances of development of carrier females (heterozygous
located and therefore such an allele cannot be females) for sex linked characters who may never express
suppressed in the phenotype by any other the characteristic in the phenotype as the recessive sex
dominant allele. linked allele would be suppressed by the dominant allele
on the counterpart X chromosome.
Sex linked characters are determined by recessive alleles.
However sex linked characteristics undergo a
characteristic cross pattern of inheritance i.e. the
fathers transmit there sex linked characters to their
grandsons through their daughters who are carriers this
implies that the father will not transmit the sex linked
character to his sons but instead to his daughters. This is
This implies that the genes of the sex linked because the son only inherits the father’s Y chromosome
characters of the males are located in the non- and not the X chromosome that controls the sex-linked
homologous portion and therefore whenever a characteristics.
recessive allele of these characters appears, it Although sex linked characters are mostly carried on the
has to be expressed in the phenotype since it X chromosome there are a few of them which are carried
does not have a counterpart allele that can on the Y sex chromosome and these are called holandric
suppress its phenotypic expression. characters i.e. development of many hairs in the nostrils
and ears.
In Drosophila, females are XX and males are XY. The gene for eye colour is located on the X chromosome.
The wild type flies have red eyes and are either homozygous or heterozygous for the alleles. Male flies are
hemizygous, carrying only one allele for eye colour in the single X chromosome. When mutant white-eyed
female Drosophila flies are crossed with wild-type (red-eyes) male, all the F1 male offspring have white eyes
while the female offsprings have red eyes.
Let R represent the allele for red eyes
Let r represent the allele for white eyes
Let XR represent the X chromosome with the allele for red eyes
Page 143 of 447

Let Xr represent the X chromosome with the allele for white eyes
Let Y represent the Y chromosome
Parental phenotype red-eyed male X white-eyed female
Parental genotype X RY XrXr
Meiosis
Gametes all Xr
XR Y
Fertilisation
F1 offspring genotype XRXR YXr

F1 offspring phenotype Red-eyed females White eyed male
Ratio 1 : 1
1 1
Probability 2
: 2
Complete the cross to show the probability of the obtaining a red-eyed female when heterozygous red-eyed
female flies are crossed with white-eyed male flies.
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………..,…..
Worked examples
Consider a normal man who marries a female whose father was having haemophilia and the mother was
homozygous normal.
- Using suitable genetic symbols, workout the phenotypes and genotypes of their offsprings?
- What is the probability that this couple will produce a haemophilic boy?
Page 144 of 447

……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
Note:
1. Haemophilia is a condition whereby blood takes too long to clot after an injury leading to excessive
bleeding of the victim. This makes hemophiliac individuals rear in population as most of them die before
reproductive age. Although haemophiliac females are known, the condition is almost entirely confined to
males.
Page 145 of 447

2. The females rarely survive beyond their first menstrual periods

3. Today people with haemophilia are treated with intravenous injections of the missing protein
Color blindness is a condition whereby an individual fails
to see the colors or fails to distinguish between particular
colors e.g. red, green color blindness where the red and
green cone cells of the eye retina are defective due to some
sex linked gene in an individual which does not allow such
an individual to distinguish between red and green. Colour
blind individuals are more common in the population than
haemophiliacs because haemophiliacs have higher chances
of dying before reaching reproductive age to pass on their
genes to the next generations whereas colorblind
individuals survive and reach reproductive age in most
cases which enables them to reproduce and pass on their
gene of colour blindness to the next generation which
increases their number in the population. Besides,
haemophiliac people may both choose not to marry due the
lethal gene they have, thereby becoming unable to pass on
their genes to the next generation.
Example 2
Green color blindness is sex linked in man. A normal man married a color blind woman. Using suitable
genetic symbols workout the genotypes and phenotypes of their children?
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………..
Page 146 of 447

Pedigree charts
Pedigree analysis is a systematic listing (using symbols or words); to trace the ancestors of a give individual,
of a ‘family tree’ for a large number of individuals, of the genetic pattern of inheritance of a particular
characteristic. Various symbols are used in pedigree charts
Page 147 of 447

Inheritance of sex-linked characters are traced by use of pedigree charts. In these a male is represented by a
square and a female by a circle. Shading within either shape indicates the phenotypic presence of a character
such as haemophilia. A dot within a circle signifies a normal phenotype who carries the allele for non-
production of factor VII.
A famous pedigree chart showing the inheritance of haemophilia from Queen Victoria in members of various
European royal families is shown
Page 148 of 447

SEX LIMITED AND SEX INFLUENCED CHARACTERISTICS
Sex limited characteristics are the ones which occur particularly in one sex. These characteristics occur at a
later stage in the life of an organism e.g. in human beings they normally occur at puberty.
In human beings the males have the following sex limited characteristics beard, deep voice, hairs in the ears
and nostrils, porcupine characteristics e.t.c. In human females these characteristics include development of
breasts, widening of the hip girdles e.t.c.
Sex influenced characteristics are those whose dominancy is determined by the sex of the bearer e.g. baldness
of the head occurs in males and not in females because the genes determining it are dominant in males and
recessive in females.
VARIATION
This is the description of the differences in phenotypic and sometime genotypic characteristics shown by
organisms belonging to the same species or natural population due to interaction between the genes and the
environment.
Variations can be clearly seen among sexually reproducing organisms due to some differences in genetic
constitution that occur during meiosis.
Variations are important because they make organisms better adapted to their environment. This is because
some variations within the population are favorable (beneficial) to the organisms possessing them making
such organisms better adapted or fit to survive in their environment and this gives a selective advantage to
those organisms possessing them. Other variations are unfavorable because they are disadvantaged in the
environment and organisms possessing such. The first organisms therefore survive, grow and reproduce and
pass on their favuorable characteristics to the next generation. If this continues for a long time it leads to the
emergency of new species in the population having good characteristics and therefore better adapted to the
environmental change. Variation is therefore a raw material for evolution during which new species are
formed.
TYPES OF VARIATION
There are two types of variation namely;
 Continuous variation
 Discontinuous variation
Continuous variation
This is the type of variation whereby characteristics in a given population show a smooth gradation between
two offsprings with the intermediate phenotype being the majority in the population and few individuals being
at the extremes of the characteristics. This implies that organisms do not show any clear cut differences
among themselves.
It is brought about by the influence of many genes but can also be influenced by environmental factors.
Continuous variation characteristics are therefore influenced by both environmental conditions and genetic
factors.
Examples of continuous variation characteristics include skin color, height, weight, intelligence e.t.c. These
characteristics are quantitative i.e. they can be measured and are controlled by many genes. These
characteristics are therefore described as polygenic characteristics i.e. characteristics which are controlled by
Page 149 of 447

a number of genes during their transmission i.e. many genes control a single characteristic. These genes are
sometimes referred to as multiple genes. Each dominant allele has a small quantitave effect individually on
the phenotype and these allelic effects are additive. Although these genes may determine a single
characteristic each of them has its own alleles which occur on different loci. These genes have an additive
effect.
The transmission of characteristics that are controlled by many genes from one generation to another is called
polygenic inheritance and such characteristics are known as polygenic characteristics. The statistical analysis
of these characteristics gives a normal distribution curve shown below;
The above graph shows that continuous variation characteristics appear in the graded pattern and therefore
show a smooth graduation. It also shows that most of the individuals in the population lie along the normal.
Discontinuous variation
This is the type of variation where individuals show clear cut differences among themselves in the population
with no intermediate phenotypes between them but instead they are grouped into distinct categories. These
characteristics are therefore qualitative and cannot be measured. Such characteristics include sex, blood
groups in man, tongue rolling, e.t.c.
This variation is controlled by a single gene and cannot be influenced by environmental conditions i.e. they
are purely genetically controlled.
CAUSES OF GENETIC VARIATION
This is caused by the gene reshuffling and mutations.
Reshuffling of genes refers to the random orientation of chromosomes at the equator of the spindle during
meiosis which changes the positions of the genes on the chromosomes.
Page 150 of 447

Reshuffling of genes include the following;
I. Crossing over: this is the exchange of genetic material between the non-sister chromatids of homologous
chromosomes during pachytene stage of prophase I of meiosis. This produces new linkage groups and so
provides a major source of genetic recombination of alleles on chromosomes which results into formation
of recombinant gametes and leads to variation in the offsprings formed. When the gametes undergo
random fertilisation, offsprings with different genetic constitution are produced.
II. Independent assortment. During independent assortment in during metaphase 1, chromosomes are
distributed randomly at the equator and segregate (separate). It is by pure chance as to which chromosome
from each homologous pair ends up in a daughter cell at the end of meiosis and therefore all sorts of allele
combinations are possible in the gametes. This reshuffles the existing alleles thereby producing new
genetic recombination’s in of the gametes and the offsprings formed from these gametes when they fuse
randomly during fertilisation.
Independent assortment can therefore be defined as the random orientation of the chromatids of
homologous chromosomes (bivalents) on the equator of the spindle during metaphase 1 of meiosis which
determines the direction in which the pairs of chromatids move during anaphase 1. This is so because after
random arrangement on the equator of the spindle the chromosomes subsequently segregate (separate)
independently thereby leading to the mixing of genes in addition during metaphase II the orientation of
the pairs of chromatids is again random at the equator of the spindle and determines which chromosomes
migrate to the opposite poles of the cell during anaphase II.
III. Fertilization. Fertilization occurs randomly between the male and female leading to mixing of genes in
different combinations.
1. Mutation. Mutations change the genotype of an organism with respect to a specific
characteristic as it produces new alleles in the population hence making it to vary due to
the combination of mutant and non-mutant gametes during random fertilisation.
2. Genetic drift. This refers to a loss of genes from a small population or the change of gene
frequency of a small population by chance alone and not natural selection which results
into the change of the gene frequency of the small population. This changes the
phenotypic appearance of the organisms thereby making them to vary.
3. Cross breeding. This mixes genes from different individuals resulting into the formation
of hybrids (heterozygotes) with improved qualities compared to the parents. Cross
breeding can be defined as mating of organisms that are pure breeding in which one has
better x-tics than another which results into the formation of the hybrid offspring.
MUTATIONS
This refers to the sudden or spontaneous genetic changes which occur in the genetic constitution of an
organism. These changes are brought about by mutagens. Mutations change the genotype of an organism with
respect to a given characteristic as it produces new alleles in the population. Mutations cause permanent
genetic variations unlike reshuffling of genes whose genetic variations are temporary as they can be undone
(removed) in subsequent generations due to chromosomes rearranging themselves alongside with their genes.
During mutation, some genetic material may be lost, doubled, inverted, translocated (moved), and mixed,
resulting into mutants having different genetic constitution from the non-mutants. The mutants formed
transmit these mutated genes to their offsprings through random fertilisation which makes the offsprings
become different from the non-mutants.
Individuals or cells resulting from mutations are known as mutants. The sudden changes in the genetic
constitution of an organism are brought about by substances called mutagens. The common mutagenic
Page 151 of 447

agents include the following;
o Gamma rays o Chemicals such as o Opium

o Alpha and beta particles caffeine and heroin o Cocaine
o X-rays o DDT and other o Formaldehyde
o Cosmic rays insecticides o Some food preservatives,
o Ultra violet rays o Colchicine color and sweeteners
o Excessive heat o Marijuana o Accidents
Note: mutations usually occur in germ cells during gamete production and so lead to the formation of mutant
gametes. When these gametes fuse randomly with mutant or non-mutant gametes of another parent, a mutant
offsprings is formed which must have unique characteristics compared to the parents. Such mutations are
known as gametic mutations. Some mutations may occur in somatic cells and are therefore known somatic
mutations e.g. cancer.
TYPES OF MUTATIONS
There are two types of mutations (germ mutations) namely;
1. Chromosomal mutations
2. Gene mutations (or point mutations)
Somatic mutations cannot be inherited while gametic mutations can be transmitted from parents to the
offsprings indeed most of the gene and chromosomal mutations are gametic and can therefore be inherited or
they are usually recessive.
Chromosomal mutations
This refers to the changes that occur in the chromosome number or chromosome structure but can be
transmitted from the parents to the offspring.
Chromosome mutations usually occur during prophase I of meiosis where a number of mistakes are made on
the chromosome structure i.e. chromosomes break and join wrongly. It can also arise during anaphase I and II
where by some chromosomes may fail to separate and move to opposite poles which brings about an increase
in the number of chromosomes or polyploidy i.e. an increase in the number of chromosomes beyond the
normal diploid number. The process by which chromosomes fail to separate during anaphase I of meiosis is
known as non-disjunction.
Chromosomal mutations are divided into the following categories;
a. Mutations that change the chromosome structure.
b. Mutations that change the chromosome number.
a. Mutations that change the chromosome structure

Such mutations include the following;
i. Deletion
This is a form of mutation where part of a chromosome breaks and gets lost leading to the formation of a
number of chromosomes that is shorter than the original chromosome. This is the most dangerous form of
mutation because it leads to loss of genes from the chromosome.
In human beings deletion leads to cat cry syndrome where the voice box fails to develop properly
Page 152 of 447

ii. Inversion
This is the form of mutation where part of a chromosome breaks, rotates through 180 degrees and rejoins
in the reverse way, this in turn changes the sequence of genes on the chromosomes as well as the sequence
of bases on the DNA strand which makes the offspring vary.
iii. Duplication
This is a form of mutation whereby a portion of chromosomes baring certain gene is doubled. This form
of mutation causes over amplification of certain phenotypes whose genes have been duplicated. This form
of mutation is of importance in crop and animal husbandry since it increases yields and improves other
characteristics.
iv. Translocation
This is a form of mutation whereby a portion of a chromosome breaks and is moved to join another
chromosome which maybe homologous or non-homologous.
b. Mutations that change the chromosome number

These are mutations that affect the whole chromosome and change the chromosome number in the cell.
This normally occurs during meiosis at anaphase I and II where two of the same type of chromosomes or
chromatids fail to separate and are transmitted together into a single gamete leaving the other gamete
empty a concept known as non-disjunction.The other chromosomes not affected by non-disjunction are
usually distributed normally by meiosis into gametes formed.
Non-disjunction results into formation the formation of gametes either with an extra number of
chromosomes (n+1) (n+2) (n+3) e.t.c. or a less number of chromosomes (n-1). Therefore, this condition
where by half of gametes contain extra number of chromosomes while the other half of gametes formed
during meiosis contain a chromosome missing is known as aneuploidy.
If a chromosome is present in triplicate in the fertilised egg (so that the cell has a total of 2n+1
chromosomes), the aneuploidy cell formed is said to be trisomic and if a chromosome is missing, so that
the zygote cell formed has 2n-1 chromosomes, the aneuploid cell is said to be monosomic. Mitosis will
subsequently transmit this variation to all embryonic cells (somatic cells) leading to the formation of an
organism with variation in the form of a set of symptoms caused by the abnormal dose of genes associated
with extra or chromosomes missing.
Some organisms have more than two complete sets of chromosome missing in each of their cells and such
organisms are called polyploids.
Illustration
Note: The zygote produced with odd number of chromosomes in the above cross containing less than the
diploid number of chromosomes usually fails to develop. But those with extra sets of chromosomes though
odd numbered or even numbered usually develop and in most cases this produces severe abnormalities. In
humans, non-disjunction causes the following abnormalities.
Page 153 of 447

1. Down’s syndrome (mongolism)

This disease is also referred to as mongolism and it is caused by an extra autosomal chromosome in
position number 21 of the homologous chromosome pairs.
Mongolism occurs as result of non-disjunction in chromosome number or during anaphase I of meiosis in
gametes formation leading to formation of abnormal gametes with 24 chromosomes. In such a case if a
normal gamete fertilizes an abnormal gamete, the zygote formed will have 47 chromosomes instead of the
normal 46. This form of syndrome occurs in both males and females. It can take place during sperm
production but it’s more common during cogenesis.
Mongolism is due to a type of mutation known as translocation in which chromosome 21 is translated or
moved to chromosome 14 in most cases or chromosome 22 in some cases.
Most non disjunction occurs in meiosis I where it causes failure of the whole chromosome 21 to separate
if it occurs in meiosis II the chromatids fail to separate leading to Down’s syndrome.
Mongolism causes miscarriage in mothers and the chances of it to occur increases with age of the females.
This form of syndrome results in individuals having the following characteristics;
 They are mentally retarded
 They have a low resistance to infections and therefore have a short life span
 They have frequent saliva flow from the mouth
 They have slit eyed appearance.
2. HETEROSOME NON-DISFUNCTION
This is non-disjunction of sex chromosomes which produces a variety of aneuploid conditions which include
the following;
a. Klinefelter’s syndrome (XXY)

This results from non-disjunction of the sex chromosomes pair in either males or females resulting into a
male with 47 chromosomes and genotype XXY. It may occur if a normal Y sperm fertilizes an abnormal
egg. It may also occur if an abnormal sperm (XY) fertilizes a normal egg (X). This is illustrated below;
Parental phenotype: male X female

Parental genotype (2n) XY X XX
Meiosis
Gametes n+1 XY n-1 all n X
Fertilisation
Offspring genotype
XXY(2n+1) XO (2n-1)
Offspring phenotype Klinefelter’s Turner’s
syndrome (47 syndrome (45
chromosomes) chromosomes)
Individuals suffering from Klinefelter’s syndrome have the following characteristics;

 They are infertile (sterile) and therefore they don’t produce sperms although erection and
ejaculation occurs.
 Their testes are usually very small compared to the normal males i.e. the testes fail to grow to the
expected size.
 They have very subnormal intelligence
 They tend to be taller than the average height
 They have little facial hair
Page 154 of 447

 They possess some female secondary sexual characteristics e.g. developed breasts and wide hips.
 Their trunk may show signs of obesity
b. Turner’s syndrome(XO)
This monosomy condition occurs in females resulting into a female with 45 chromosomes and genotype
XO. It occurs as a result of non-dysfunction which results into an abnormal egg or an abnormal sperm. If
an abnormal sperm fertilizes a normal egg this condition occurs and if an abnormal egg that is empty is
fertilized by a normal sperm containing X chromosomes this condition again occurs. Individuals with
Turner’s syndrome vary from others by having the following characteristics;
 They do not show female secondary characteristics such as developed breasts, menstruation,
widening of hips e.t.c.
 They are infertile with no ovaries but with a small uterus.
 They have a short height than the women average height.
 They have a webbed neck.
 They are usually of normal intelligence
c. Triple X syndrome(XXX)
This also occurs in females due to non-disjunction which results into formation of an abnormal egg
containing XX chromosomes and if such an egg is fertilized with a normal X sperm the triple X female
occurs with 47 chromosomes and these individuals have the following characteristics;
 They are fertile females
 They are mentally normal
 They are physically normal
 They have a very high sex libido
d. XYY syndrome
This condition occurs in males with genotype XYY. It occurs in case the Y chromosome undergoes
duplication and fails to separate at anaphase I. This may result into production of abnormal sperms
containing YY which if they fertilize a normal Xegg and XYY syndrome occurs. Individuals with this
syndrome vary from other males by having the following characteristics;
 They are usually very aggressive and therefore common in prisons and security forces.
 They are fertile males.
 They are giants.
 They are mentally and physically normal
POLYPOIDY (EUPLOIDY)
This is a condition whereby cells of organism possess extra sets of chromosomes beyond the normal diploid
number. Polyploidy therefore makes the genetic constitution of an organism multiplied to become 3n, 4n, 5n,
6n e.t.c.
Polyploidy is a useful phenomenon in plant bleeding where the chromosome number is increased so as to
improve on the vigor (characteristic) of the plant i.e. plants acquire better characteristics such as high yields,
high resistance to dieses, quick maturity, high resistance to pests e.t.c. It is more common in plants than
animals because the increased number of chromosomes in animal polyploidy causes errors in gamete
formation unlike in plants which usually reproduce vegetatively without such errors.
Polyploidy brings about genetic variation with in a population as it results into the formation of new and
different genetic combinations within some individuals of the population. This makes some polyploids with in
the population better adapted than their original parents and so are favoured by a selection pressure of nature
to survive, reproduce and transmit their adaptive variations to their offsprings. However, better adapted
polyploids may fail to interbreed with diploid organisms, thereby forming a new specie due to possession of
extra sets of chromosomes beyond the diploid number.
Page 155 of 447

Polyploidy also results into formation of new species via interspecific hybridisation, a process in which the
F1 hybrids formed are sterile because their chromosomes cannot form homologous pairs being that they arise
from organisms of different species, but when a diploid number of chromosomes of F1 hybrids is doubled due
to non-disjunction, tetraploids (4n) can be formed as F2 hybrids which can inter breed among themselves to
produce fertile offsprings. The F2tetraploids formed by interspecific hybridisation can interbreed among
themselves to form fertile offsprings but cannot interbreed with any of the original parents because of having
extra sets of chromosomes thereby becoming a new species.
The F2 offsprings formed are described as allopolyploids because they are formed by a type of polyploidy
known as halopolypolidy. Halopolypolidy is the one which occurs when two different species interbreed and
produce a sterile F1 hybrid whose chromosome number gets doubled by non-disjunction thereby changing the
sterile F1 hybrids into fertile F2 hybrids. The halopolypolids formed are fertile with each other but cannot
interbreed with a diploid parental species. The F1 hybrids are sterile because the set of chromosomes from one
species cannot pair during meiosis with another set of chromosomes from another species.
Illustration
Parental phenotype: cabbage X radish
Parental genotype (2n) 2n=18 X 2n=18
Meiosis
Gametes n=9 n=9
Fertilisation
Offspring genotype
2n=18
Offspring phenotype
When selfed, (after non-disjunction)
Parental phenotype: F1 hybrid X F1 hybrid
Parental genotype (2n) 2n=18 X 2n=18
Meiosis
Gametes 2n=18 2n=18
Fertilisation
Offspring genotype
(Allopolyploids) 4n=36
The tetraploids formed is fertile because homologous pairing of chromosomes can occur in meiosis, as the two
sets of parental chromosome present in diploid gametes are produced which contain nine chromosomes from
the parental cabbage and 9 chromosome of the parental radish.
There are two forms of polyploidy namely; autopolyploidy and allopolyploidy
1. Autopolyploidy
This is where the chromosome number of some individuals in a given species is increased either naturally or
artificially by preventing cytokinesis or preventing the formation of spindle fibres during cell division. This
can be done artificially using colchicines which prevents formation of spindle fibers thereby increasing the
Page 156 of 447

chromosome number. This mutation prevents the tetraploids from successfully interbreeding with the diploid
plants of the original population leading to the reproductive isolation. However, the tetraploids can still
produce fertile offsprings by self-pollination or by mating with other tetraploids
Autopolyploid can be as fertile as diploids if they have an even number of chromosomes and they can be
infertile if they have odd number of chromosomes because they cannot form homologous pairs.
Colchicines and other related drugs have been used in breeding in certain varieties of tobacco and tomatoes
whose cells have a large nucleus.
2. Allopolyploidy
This is a condition which arises when the chromosome number in the sterile hybrids gets doubled and
produces fertile hybrids. Sometimes the F1 offsprings formed may be sterile but if these individuals are
crossed with another related
During meiosis in F1 hybrids chromosomes from each parent cannot pair together to form homologous
chromosomes hence the F1 hybrids produces gametes with a diploid set of chromosomes. This brings about
allopolyploid as illustrated below;
The allopolyploid is fertile because homologous pairing of chromosomes can occur in meiosis as the two sets
of parental chromosomes are present. Allopolyploid is an example of interspecific hybridization i.e. form of
sympatric speciation which occurs when a new species is produced by the crossing of individuals from two
unrelated species.
Gene mutations (point mutation)
This is a sudden change in the sequence of nuclear nucleotides or bases of DNA. These mutations are of the
following types;
1. Substitution. This type of mutation whereby or more bases of nucleotides may be replaced with wrong
nucleotides or basses. If we consider the base sequence of GTC, a change to a single base could result in
one of the following;
 A silent mutation occurs if the substitution results in a different base occurring in a DNA triplet
but one that still codes for the same amino acid. The final polypeptide produced is identical to the
original and no effects on the final i.e. Cytosine is replaced by Thymine, GTC becomes GTT.
Page 157 of 447

However, as both these replicates code for glutamine, there’s no change to the polypeptide
produced.
 A nonsense mutation occurs if the base change results in the formation of one of the three stop
codons that mark the end of a polypeptide chain e.g if Guanine is substituted with Adenine, GTC
becomes ATC. The final protein would certainly be shortened and the protein could not perform
its usual function.
 A mis-sense mutation arises when the base change results in a different amino acid being coded
for. In the example above, if the final base Cytosine is substituted by guanine, then GTC becomes
GTG. The amino acid histidine is coded for by GTG and this then replaces the original amino
acid, glutamine. If the original amino acid is vital in the formation of bonds that determine the
three-dimensional shape of the final protein then the new protein may not function as the original
protein. Sickle cell anaemia is an example of a mis-sense mutation.
2. Deletion. This is a form of gene mutation where a section of DNA is lost. This results into wrong
transcription and wrong translation processes in protein synthesis. This mutation is dangerous because it
leads to absence of certain structures or wrong physiological processes taking place in an organism.
Additions or deletions lead to a frame shift in the DNA code, whereby every triplet of bases that follows
the change is altered. An example of deletion mutation is cystic fibrosis. This causes very sticky mucus
that causes lung congestion, reduced gaseous exchange and blocked pancreatic ducts.
3. Insertion. This is where one or more nucleotides may be fixed in a particular DNA strand. This also
causes a frame shift. Every single triplet genetic code after the mutation point is altered. This results in
either a different DNA strand synthesised during semi-conservative replication or an altered mRNA with
many different codons is produced during transcription. This leads to formation of an incorrect series of
amino acids in the polypeptide chain. Frame shift mutations cause severe effects on the phenotype and are
sometimes lethal to the organisms.
4. Inversion. This is where a group of nucleotides in DNA becomes reversed after rotating through 180
degrees. Non-frame shift mutations does not cause the alteration of the whole nucleotide base sequence
(reading frame). Only the mutated single code in DNA and a single codon in mRNA are affected.
Therefore only one amino acid is different in the resultant polypeptide that is synthesised. If the different
amino acid is located within the active site of an enzyme or involved in the folding of a particular protein,
this would affect the functioning of the enzyme or enzyme. Substitution and inversion are two types of
non-frame shift mutations.
OUT OF CLASS EXERCISES

1. (a) State Mendel’s law of dihybrid inheritance. (01 mark)
(b) In garden peas, a cross between plants with yellow-round seeds and those with green-wrinkled seeds
produces all offspring being with yellow-round seeds.
(i) Suggest an explanation about the inheritance of seed colour and seed shape in peas. (02 marks)
(ii) State the genotypes of the parental and F1 offspring all with yellow-round seeds? (Use suitable
symbols) (02 marks)
(c) Work out the F2 phenotypic ratio if the F1 plants are self-pollinated (05 marks)
2. A man and a woman who are both normal for blood clotting have a haemophiliac son. Haemophilia is a
sex-linked recessive disorder.
(a) What is meant by sex-linked gene?
(b) Draw a single pedigree diagram top show the genotypes of the three individuals. Explain how the
pedigree is obtained.
(c) What is the probability that their second child would be
(i) a boy suffering from haemophilia?
Page 158 of 447

(ii) a daughter who is a carrier?

(d) (i) Could a woman be a haemophiliac? Give reasons for your answer.
(ii) Explain why a human population will contain more colour-blind individuals than
haemophiliacs
3. (a)Describe how DNA replicates semi conservatively (10 marks)

(b) Describe the experiments carried out by Meselson and Stahl to demonstrate that replication of DNA is
semi conservative. (10 marks)
4. (a) In snapdragon flower colour is determined by two alleles of R for red and W for white which are
Incompletely dominants
A population has the following individuals distributed as shown in table 3 below
Flower colour Number of individuals
Red 450
Pink 500
White 50
(b) Using the information provided determine the
(i) total number of the R and W alleles in the population. (03 marks)
(ii) Genotype frequency for each genotype. (03 marks)
(iii) Allele frequencies of each allele. (02 marks)
(c) State two causes of change in the allele frequencies and genotype frequencies in population. (02 mar)
5. (a) What do you understand by the following terms?

i) Allele (01 mark)
ii) Co-dominance (01 mark)
b) In cats the allelic gene for black colour and ginger colour shows co-dominance. This gene is sex-linked.
A male cat with ginger coat colour was crossed with a female with black coat colour. Using genetic
symbols work out the genotypes of across of the F1 off springs. (08 marks)
6. In cats, short hair is dominant over long hair, the gene involved is autosomal. Another gene which is sex-
linked produces yellow coat colour, its allele produces black coat colour and the heterozygous
combination produces tortoise shell coat colour.
a) If along haired black male is mated with a tortoise shelled female homozygyous for short hair, what kind
of offspring will be produced in F1. (08 marks)
b) i) If the F1 cats are allowed to interbreed freely among themselves what are the chances of obtaining
long haired female.
ii) Apart from being sex linked what else can you say about the inheritance of the gene for coat colour.
7. Chickens with shortened wings and legs are called creepers

(i) When creepers are mated to normal birds, the offspring ratio is creepers: normal birds are in equal
frequency.
(ii) When creepers are mated to creepers the offspring ratio is two creepers: one normal
(iii) Crosses between normal birds produce only normal progeny.
(a) Explain these results using your knowledge of genetics (4 marks)
(b) Using suitable symbols work out the genotype and phenotypes of the offspring of the second cross. (5
(c) State the genotypes of the parents in the third cross (1 mark)
Page 159 of 447

8. In humans, the inheritance of skin pigmentation is controlled by two genes a and B, such that the
presence of both genes in the genotype results in black pigmented skin. Presence of gene A in absence of
B results in dark brown pigmented skin and absence of gene A when B is present results in light brown
pigmented skin. Absence of both genes results in white skin (albino).
(i) What does the above information indicate about the inheritance of skin pigmentation in humans?
(02 marks)
(ii) Determine the phenotypic ratio of a cross between a black man and a dark brown woman that
results in offspring of all skin colours with light brown and white skins being fewer but equal
proportions and black and dark brown being more but also in equal proportions. (07 marks)
9. The genetic code contains punctuation codons to mark the start and end of synthesis of polypeptide chains
on ribosomes
a) State the codes for;
i) Start codon (01 mark)
ii) Stop codon (01 mark)
b) Outline the process of the formation of mRNA from DNA (03 marks)
c) State two structural differences between mRNA and DNA (02 marks)
d) Explain the role of mRNA in protein synthesis (02 marks)
e) What is the fate of the proteins made in a cell? (01 mark)
10. (a) state where each of the following is found in a cell (01 marks)
i. DNA
ii. RNA
(b) Give three structural differences between DNA and RNA (03 marks)
DNA RNA
(c) What is the genetic significance of DNA replication? (03 marks)

(d) Give two evidence that suggest DNA as a hereditary material (02 marks)
11. (a) State two situations where Mendel’s laws would not apply (02 marks)
b) In an animal species, individuals that are homologous for gene A or its alleles die. Another
independent gene B in the homozygous state, blocks this lethal effect, otherwise gene B has no
other effect on the organism.
i) Workout the expected phenotypic ratio of the viable offspring in a cross of individuals AaBb
and AaBB genotypes (05 marks)
ii) State the type of gene interaction in b (i) (01 mark)
c) Explain why a rhesus negative mother of blood group O is carrying a rhesus positive child of any
blood other than O, haemolytic disease of the newborn does not arise (02 marks)
12. A cross between two fruit flies with long wings and red eyes yields in the progeny mutant phenotypes
called curved wings and lozenge eyes as follows
Females Males
600 long wing red eyes 300 long wings red eyes
200 curved wings red eyes 100 curved wings red eyes
300 long wings red lozenge eyes
100 red wings lozenge eyes
Page 160 of 447

(a) Suggest what type of allele causes (02 marks)

(i) Curved wings
(ii) Lozenge eyes
(b) Using suitable symbols state the genotypes of the parents (03 marks)
(c) Work out the genotypes of the offspring of this cross (05 marks)
13. (a) The figure below shows the number of bases found in the sense strand and the antisense strand of a
short piece of DNA, and the mRNA transcribed from it
i) Identify the bases represented by each of the following letters P,Q,R and S (02 marks)
ii) Explain why the total number of bases in the DNA sense strand and the total number of bases in
the DNA antisense strand are the same (02 marks)
iii) Explain why the total number of bases in the DNA sense strand and the total number of bases in
the mRNA strand are different (02 marks)
(b) The mRNA has a sequence of 1824 bases. How many amino acids will join to form the polypeptide
chain? (03 marks)
(c) Although DNA is double stranded only the sense strand determines the specific amino acid sequence
of a polypeptide chain. Suggest one role of the antisense strand (02 marks)
14. (a). What is meant by the term Co-dominance. (02 marks)

(b). In mice, coat colour is determined by alleles A for agouti (black) and a for brown. Another allele C, at
another locus, however, determines the expression of colour and its recessive allele c lack of it i.e. albino.
Show the phenotypic ratio of the F2offsprings of a cross between two homozygous brown and agouti mice
(08 marks)
(d) The following is an account of tongue rolling (due to a dominant autosomal gene T) in a family.
A man has both of his parents unable to roll their tongues and a wife who can roll her tongue and so is
her grandparents and mother but whose father and sister cannot.
They produce a son who can roll his tongue, marries a wife who can roll her tongue too and they
produce five children two of these are sons and neither can roll their tongues and three daughters two
of whom can roll and the other cannot. Express the transmission of tongue-rolling in this family
through the generations in the form of a pedigree. (04 marks)
15. a) What is meant by the term linkage? (02 marks)

In one species of tomato, the stems can be of different colours (green or purple) and may be hairy or hairless.
In an experiment, a homozygous tomato plant with green and hairless stem was crossed with another plant
homozygous for purple and hairy stem. The resulting F1 plants had purple and hairy stems. By using symbols
A for dominant allele for colour and a for recessive allele for colour, and B for dominant allele for hairlines
and b for recessive allele for hairiness
b) State the genotypes of each parent and F1 offspring (03 marks)
c) When F1 plants were selfed, the phenotypes and number of offspring is given in the table below.
Page 161 of 447

Phenotypes Number of offspring

Purple, hairy 293
Purple, hairless 15
Green, hairy 12
Green, hairless 98
i) In this dihybrid cross what would be the expected ratio of phenotypes in the offspring? (01 marks)
ii) Explain the difference between the expected ratio and the numbers shown in the table (02 marks)
iii) Calculate the crossover value and explain how it may affect the numbers of plants having
phenotypes purple-hairless and green-hairy (03 marks)
d) If a tomato breeder wanted to find out which of the purple, hairy plants and homozygous for both
characters
i) State the genotype of the plant which should be crossed with the purple, hairy plants in the
test cross (01 marks)
ii) Explain why this genotype should be used (01 marks)
16. In guinea pigs, the gene that controls the production of enzyme tyrosinase to synthesise melanin is
epistatic to the gene at another locus that regulates deposition of melanin. Deposition of melanin in the
hair produces a coat colour and is regulated by the gene with allelic pair B and b. B represents the
dominant allele for black coat and b the recessive allele for brown coat. Another gene (alleles A and a)
located on a different locus, control the production of melanin. The alleles A codes for enzyme tyrosinase
which converts a colourless precursor to melanin. The recessive allele a codes for an in active form of the
enzyme.
a) What is meant by the term epistasis? (02 marks)
b) Describe what happens if the animal is homozygous recessive aa? (02 marks)
c) If a cross is carried out between a male guinea pig with genotype AaBb and a female guinea pig with
Aabb, what is the possible phenotypic ratio of the F1 generation?
Use a genetic diagram to show the results of the above cross. (06 marks)
17. (a) A biochemical analysis of a sample of DNA showed that 33% of the nitrogenous bases were guanine.
Calculate the percentage of the bases in the sample which would be adenine. Explain how you arrived at
your answer? (06 marks)
(b) (i) What name is given to the triplet of bases which designate an individual amino acid.
(ii) If the triplet of mRNA which designates amino acid lysine is AAG (Where A= adenine and G =
guanine), what is the complementary triplet of three bases on the tRNA molecule? Give a key for the
letters that you use. (03 marks)
18. (a) Phenylketonuria (PKU) is an inherited disease caused by a recessive allele. If a woman and her
husband who are both carriers have three children, what is the probability of having the following?
i. All three children being normal (01 mark)
ii. One or more of the three children having the disease (01 mark)
iii. All three children having the disease (01 mark)
iv. At least one child being normal (01 mark)
(b) In some pea plants a true breeding red flowered strain with terminal flowers gives all pink flowers.
Flowers can be positioned terminally or in the axis irrespective of flower colour. Work out the F2
phenotypic ratio resulting from a cross involving true breeding axial red and terminal white parents
(06 marks)
19. (a) What is meant by the term mitosis? (02 marks)
Page 162 of 447

Figure 5 shows four animal cells in different stages of the mitotic cell cycle.
cell 1 cell 2
centriole
cell 3 cell 4 Fig 5
(b) Using the number given to each cell in Fig. 5, arrange the stages as they occur in the mitotic cell cycle. (01
(c) (i) State what is occurring at A in cell 2. (01 marks)
(ii) Label B is pointing to a region of the chromatid that contains repetitive nucleotide sequences. State the name
given to this region. (01 marks)
(d) The centriole labelled in Fig. 5 is composed of microtubules.
(i) Suggest why a student would not be able to see a microtubule using a good quality light
microscope. (02 marks)
(ii) Outline the role of microtubules in mitosis. (02 marks)
(e) What is the significance of mitosis to eukaryotic cells (01 marks)
20. (a) What is meant by the term linkage? (02 marks)

(b) In man the gene for red blood corpuscle shape is represented by alleles E (elliptical) and e (normal),
while another gene for rhesus blood is represented by alleles R for rhesus positive and r for rhesus
negative. The two genes are linked. A person may have alleles E and R on one chromosome and e and r
on its homologous partner.
(i) State possible genotypes of the gamete if
Page 163 of 447

there’s crossing over (02 marks)

there’s no crossing over (01 marks)
(ii) If a man with genotype EeRr marries a woman with genotype eerr, what is the chance that
this couple will produce a child with the genotype Eerr if linkage is complete? (Show your working)
(04 marks)
(c) Give one disadvantage of linkage. (01 mark)
21. A farmer crossed sweat pea plants with purple flowers and long pollen grains with those red flowers and
round pollen grains. The F1 plants had purple flowers and long pollen grains. However in F2 the off
springs were:
4891 purple, long plants
390 purple , round plants
393 red, long plants
1338 red, round plants.
(a) (i) Determine the phenotypic ratio of the F2 off springs. (½ marks)
(ii) What theoretical phenotypic ratio would you have expected among the F2 off springs? (½ marks)
(iii) What explanation can you give to the farmer for the experimental results obtained. (2 marks)
(b) (i) By means of a genetic diagram explain how the results in F2 were obtained. (6 marks)
(ii) Determine the cross over value for flower colour and pollen shape characteristics. (2 marks)
22. In tomatoes the allele for red fruit R is dominant to that for yellow fruit r. The allele for tall plant T is
dominant to that for short plant t. The genes assort independent of each other during their transmission.
(a) A tomato plant is homozygous for allele R. Giving a reason for your answer in each case, how
many copies of this allele would be found in:
(i) a male gamete produced by this plant? (02 marks)
(ii) a leaf cell from this plant? (03 marks)
(b) A cross was made between two tomato plants.
(i) The possible gametes of the plant chosen as the male parent were Rt, Rt, rT and rt.
What was the genotype of this plant? (½ mark)
(ii) The possible gametes of the plant chosen as the female were rt and rT. What was the
phenotype of this plant? (½ mark)
(iii) What proportion of the off spring of this cross would you expect to have red fruit?
Use a genetic cross to explain your answer. (05 marks)
23. In Drosophila, the gene for wing length and shape of the abdomen are sex linked. The genes for long
wing and broad abdomen are dominant over those for vestigial wings and narrow abdomen
(a) Work out the phenotypes resulting from a cross between a vestigial winged and a broad abdomen
male and a homozygous long winged and narrow abdomen female fly in the
(i) F1 generation (06 marks)
(ii) F2 generation (04 marks)
(b) A cross between a female from the F1 generation in (a) (i) with a vestigial winged and narrow
abdomen male fly gave the following results;
Long winged, narrow abdomen flies = 35
Long winged, broad abdomen flies = 17
Vestigial winged, narrow abdomen flies = 36
Vestigial winged, broad abdomen flies = 18
Account for the phenotypes and their relative numbers in the cross (05 marks)
(c) Explain why Drosophila are commonly used in genetic experiments (05 marks)
Page 164 of 447

24. (a) How is sex determined in humans? (04 marks)

(b) A woman has four sons, one of whom is a haemophiliac and the other three are normal.
i. What are the possible genotypes of the woman and her husband? (12 marks)
ii. Is it possible for the couple to have a haemophiliac daughter? Explain your answer (04 marks)
25. The following experiments were performed using the fruit fly Drosophila melanogaster.
Experiment 1
Male flies showing two recessive sex linked characteristics: white eyes and vestigial wings were mated
with females which were true breeding for wild type eyes and wild type wings.
The female offsprings of this first cross were then mated with their parents. The results of this second
cross had 72 flies with white eyes, vestigial wings, 80 flies with white eyes, wild type wings, 76 flies with
wild type eyes, vestigial wings and 84 flies with wild type eyes, wild type wings
Experiment 2
Male flies showing two recessive sex linked characteristics: white eyes and vestigial wings were mated
with females which were true breeding for wild type eyes and wild type wings.
The female offsprings of this first cross were then mated with their parents. The results of this second
cross had 128 flies with white eyes, vestigial wings, 21 flies with white eyes, wild type wings, 17 flies
with wild type eyes, vestigial wings and 136 flies with wild type eyes, wild type wings.
a) i) Using suitable symbols, explain the crosses responsible results obtained in Experiment 1 .
ii) What is the cross over value?
iii) What are the positions of the alleles of white eyes and vestigial wings on the Drosophila
chromosome in Experiment 1?
b) i) Why are the results observed in the second cross of Experiment 2 different from those observed in a
similar cross in Experiment 1?
ii) What are the positions of the alleles of white eyes and vestigial wings on the Drosophila
chromosome in Experiment 2 ?
c) Briefly comment on the significance of these results in relation to Mendel’s law of Independent
Assortment.
26. (a) Give any

i) Two similarities between DNA and RNA molecules.
Similarities:-
ii) Three structural differences between DNA and RNA MOLECULES
DNA RNA
1.
2.
b) The diagram shows the sequence of bases on one strand of a short length of DNA
Page 165 of 447

ACC CGACCCCAG
This sequence should read from left to right
i) Give the base sequence that will be produced as a result of transcription of the complete length of DNA
shown in the diagram.
ii) Give the bases of the transfer RNA which will correspond to the sequence of bases shown in the box on the
diagram.
c) Of what importance are the following during protein synthesis?
i) Nonsense triplets.
ii) Ribosomes.
iii)ATPase enzyme.
27. (a) Distinguish between sex-linked and sex-limited characteristics, giving one example in each case.
(02 marks)
(b) In drosophila melanogaster the inheritance of eye colour is sex-linked. The gene for red eye is
dominant to that for white eye. A cross was made between a white eyed female and red eyed male, what
are the phenotypes and genotypes of the F1? (Show your working) (04 marks)
(c) What are the phenotypes and genotypes when a reciprocal cross in carried out? (show your working)
(02 marks)
(d) Give two characteristics of sex-linked characters. (02 marks)
28. What is a sex linked trait?

b) Name any three sex linked traits.
c) Coat colour in cattle is inherited by Co-dominance. A red bull was mated with a white cow. Two of the
produced were then crossed to get better varieties. Using well define genetic symbols, carry out
diagrammatic crosses and then give the genotypic phenotypic ratios of Calves in the second filial
generation.
29. (a) Mention any two reasons why Mendel chose to use Pisum sativum, in his experiments.
(b) Manx cats do not have tails. When a manx cat is mated with a normal long tailed cat, approximately
half of the offsprings are long tailed and approximately half are manx. When two Manx are mated, the
ratio of offsprings is 2 Manx to 1 log tailed cat.
(i) What does this suggest about the inhentance of the Manx condition in cats
(ii) Show by means of a cross, the inheritance of the Manx condition when two Manx cats are
mated.
30. In mice fur colour is controlled by a gene with multiple alleles as shown below;
Black & tan =Cbt yellow =Cy
Agouti =Ca black =Cb
(a) Explain the following crosses

i. Mice with agouti fur crossed with mice with black fur product all agouti offspring or some
agouti and some black. (03 marks)
Page 166 of 447

ii. Heterozygous parents with genotype Cy Cb produce a ratio of two yellow mice to one black
mouse. (03 marks)
(b)(i) What is a test cross? (01 marks)
(ii) Describe how you would carry out a test cross to determine the genotype of a black and tan
mouse. (03 marks)
31. A single dominant gene I blocks the action of gene C for colour formation at another locus in some
species of chicken such that presence of gene C results in coloured feathers in the absence of gene I. The
recessive alleles have no effect on colour. When white Plymouth rock and white leghorn chickens are
crossed the F1 offspring are all white as expected but the F2 have both white and coloured birds in the
ratio 13 white: 3 coloured.
a) Explain why
i) Leghorns are white (03 marks)
ii) Plymouth rock are white (02 marks)
iii) F1 offspring are all white (01 mark)
iv) Some F2 offspring are coloured (03 marks)
b) of the F2 offspring how many are plymouth rock (01 mark)
32. Two genes A and B located on different chromosomes interact to determine three coat colours in mice. i.e.
Grey, Black and Chocolate. Each gene has a recessive allele.
The table below shows the phenotypes and genotypes of some of the mice
Genotype Phenotype
AABb Grey
Aabb Grey
aaBb Black
Aabb Chocolate
(a) State the expected coat colour of the genotypes given below (02 marks)
Genotype Expected coat colour
AA BB
AAbb
AaBb
aaBB
(b) Work out the genotypic and phenotypic ratios of the F2 offspring of a pure breeding Grey and Chocolate
mice. (08 marks)
33. (a) Biochemical analysis of a sample of DNA showed that 33% of the nitrogenous bases were guanine.
Calculate the percentage of bases in the sample which could be adenine. Explain how you arrived at the
answer. (03 marks)
1
(b) (i) What name is given to the triplet of bases which designate an individual amino acid?(2 𝑚𝑎𝑟𝑘𝑠)
1
(ii) State three properties of the feature given in (b)(i) above. (1 𝑚𝑎𝑟𝑘𝑠)
2
Page 167 of 447

(c) The following table gives the amount of DNA in a cell at various stages of cell division. The least
amount of DNA present at any stage is taken as 1.0 and this is used as a basis for comparison of other
stages.
DNA content of the cell Stage of cell division
1.0 (Meiosis) late telophase II
2.0 (Mitosis) early interphase, late telophase.
(Meiosis) metaphase II
3.0 (Mitosis) prophase
(Meiosis) anaphase I
Explain the differences in DNA content between:

i. Mitosis early interphase and mitosis prophase (02 marks)
ii. Meiosis anaphase I and meiosis metaphase II (02 marks)
iii. Meiosis metaphase II and meiosis late telophase (01 mark)
34. A pure breeding tall tomato plant with green leaves was crossed with a pure breeding dwarf plant with
molted yellow and green leaves. -All the F1 were tall and had green leaves.
a) Using genetic symbols ^show the results of the test cross of the F1offsrping
b) The actual results of the test cross gave the following off springs
A. Tall with green leaves - 43
B. Tall with mottled leaves - 07
C. Dwarf with green leaves - 05
D. Dwarf with mottled leaves- 45
Explain the difference in the results of the two crosses
(c) State three harmful genetic effects of inbreeding.
35. In domestic poultry the character of the comb is controlled by two genes R for rose comb and P for pea
comb. If the dominant allele R is present in the genotype with a dominant P then a walnut comb is
produced. If an individual is homozygous recessive for booth alleles a single comb is produced. If an R is
present without a P in the genotype the comb is rose whereas a P without an R produces a pea comb.
(a) Determine the phenotypic ratio among the offspring of a cross between two birds whose
genotypes are RrPp X Rrpp (06 marks)
(b) A walnut crossed with a single produced among the progeny only one single combed offspring.
What were the possible genotypes of the parents? Show your reasoning (03 marks)
(c) Suggest a cross between two birds of different comb shapes that produce offspring among which
all four combs are represented in equal proportions (01 mark)
36. (a) In an oil seed plant species, the allele for tallness is dominant over that for dwarfness. Meanwhile the
allele for chlorophyll production and non-chlorophyll show incomplete dominance. The heterozygous
plants are variegated.
(i) Using suitable symbols, construct a diagram of a cross between a tall plant with green leaves
and a dwarf plant with variegated leaves, to show the genotypes and phenotypes of the
offspring. (05 marks)
(ii) Explain why 25% of the offspring of the cross in (a) would fail to survive (02 marks)
Page 168 of 447

37. A cross was carried out between two maize plants, one true breeding for brown pericarp and shrunken
endosperm, the other true breeding for white pericarp and full endosperm. The resulting F1 plants, all
white pericarp and full endosperm, were backcrossed against double recessive plants.
The F2 results were;
81 white pericarp, full endosperm
89 brown pericarp, shrunken endosperm
14 white pericarp, shrunken endosperm
16 brown pericarp, full endosperm
a) Present the information given in a diagrammatic form (05 marks)
(Symbols, W-white pericarp, F-full endosperm)
b) State which members of the F2 generation are recombinants (02 marks)
c) Calculate the percentage number of recombinants present in F2 generation (02 marks)
d) How many units apart on the chromosomes are the genes that determine pericarp colour and
endosperm type? (01 mark)
38. a)Explain the following terms

(i) Recessive allele (01mark)
(ii)Co dominant allele (01 mark)
b) The figure below shows inheritance of the Rhesus blood group in one family
Explain one piece of evidence from the diagram which shows that:
(i)the allele for Rhesus positive is dominant (02 marks)
(ii) The gene is not on the X chromosome (02 marks)
c) Sixteen percent of the population of Europe is Rhesus negative .What percentage of individual would you
expect to be heterozygotes for Rhesus gene? (04 marks)
39. Wild rats are grey coloured while albinos rats are white in colour. The results below are for breeding
experiments involving the two species of rats
I. Mating albino rats with wild rats produced equal proportions of wild and albino rats
Page 169 of 447

II. Mating wild offsprings from I produced litter of wild and albino rats in the ratio of 2:1
respectively
III. Mating albino rats produced only litter of albino rats
a) Using suitable symbols, work out the mating in I showing the phenotypic and genotypic
proportions of the off springs (04 marks)
b) From your answer in (a) above explain;
i. How colour in rats is controlled (01 mark)
ii. The results for the matings I-III above (03 marks)
c) Give two ecological significance of colour in organisms (02 marks)
40. (a) What are sex limited characters? (02 marks)

b) Explain why sex linked characters are more common in males than females (03 mark)
c) Although the frequency of the gene for abnormal haemoglobins in human population is low, that gene
cannot completely disappear from the population. Explain that observation (03 marks)
d) What is meant by a pure line? (02 marks)
41. The diagram below shows the nucleus of an animal cell
(a) Name the structures seen inside the nucleus. (01 mark)
(b) Draw the correct number and types of these structures as they would appear;
(i) After mitosis
(ii) After meiosis ,of the nucleus
(c) What’s meant by;
(i) F1 hybrids
(ii) Breeding true
(d) Explain why F1 hybrids will not breed true if they are self-fertilized. (04 marks)
42. (a) Describe the following genetic abnormalities.

i) Mongolism (04 marks)
ii) Klinefelter’s syndrome. (04 marks)
(b) Explain the formation of new species, from organisms of two different species. (12 marks)
43. (a) State and explain the law of segregation of Mendel.

(b) Explain how the following are determined in man.
(i) Sickle cell anemia (ii) sex
44. (a) Distinguish between continuous and discontinuous variation. (05 marks)
(b) In oats, the grain is enclosed by the dried remains of the outer parts of the flower called the hull. In a
cross between two pure breeding varieties of oats, one with black hulled grains and the other with white
hulled grains, the F1 off spring all had black hulled grains. Allowing the F1 plants to self fertilize gave an
F2 with the phenotypes shown below.
Page 170 of 447

Phenotype
Black hulled grains 418
Grey hulled grains 106
White hulled grains 36
The data shows evidence of epistasis

(i) What is meant by the term epistasis? (01 mark)
(ii) How does epistasis differ from mendalian dominance? (04 marks)
(iii) What genetic ratio is suggested from the figures given? (01 mark)
(iv) Set out the crosses to show the gametes, genotypes and phenotypes in each generation. State the
phenotypic ratio obtained. (09 marks)
45. (a) How does meiosis explain Mendel’s first law of inheritance? (04 marks)
(b) (i) What is meant by the term genetic drift? (02 marks)
(ii) Describe how genetic drift affects the amount of genetic variation within very small populations.
(c) Explain the cause of genetic variation. (10 marks)
46. (a) Describe the structure of the tRNA molecule

(b) How does the body make insulin hormone?
47. (a) Describe the contribution of meiosis towards variation (10 marks)
(b) Compare mitotic prophase and prophase I of meiosis (10 marks)
48. (a) Describe the process of semi conservative DNA replication (06 marks)
(b) State the difference between continuous variation and discontinuous variation (06 marks)
(c) Describe the role of DNA in controlling polypeptide chain synthesis (10 marks)
(d) Explain the relationship between the Watson–Crick DNA structure and its function (10 marks)
49. a) Describe how you would carry out and record the results of a dihybrid cross to obtain F1 and F2
generations in a named organism emphasizing reasons for the procedures (07 marks)
b) Consider an F1 generation of a dihybrid cross. Explain how the results of F2 are dependent on the
behaviour of chromosomes during meiosis in F1. To what extent can the genotype interact with the
environment to give phenotypes? (08 marks)
c) A typical 9:3:3:1 phenotype ratio is obtained in F2 of a dihybrid cross. What effect do the following
have on the ratio;
i. Linkage
ii. Incomplete dominance
Explain your answer (05 mark)
50. (a). How does meiosis explain Mendel’s law of inheritance? [06 marks]
(b). Describe how abnormal haemoglobin arises in the human population [09 marks]
(c).Explain the effects of the gene for abnormal haemoglobin in the human population. [05 marks]
51. (a). Distinguish between continuous and discontinuous variation in a species. [05 marks]
(b). Hoe does epistasis differ from Mendelian dominance? [04 marks]
(c). Account for the existence of genetic variation in a population [11 marks]
52. (a).Explain how sex is determined in mammals. [05 marks]

(b).Describe how mongolism a rises in the human population. [08 marks]
Page 171 of 447

(c).Explain interspecific hybridization. [07 marks]
53. (a) Explain the meaning of each of the following terms with examples:
i. Test cross iv. Pure breeding line
ii. Reciprocal cross v. Sex-linked traits
iii. Pedigree vi. Sex limited traits.
(b. A daughter of a couple where the woman was a carrier for red-green colour blindness and the husband
had red-green colour-blindness got married to a man with normal colour vision.
(i) What are the possible genotypes and phenotypes of the grand children of the original couple?
(ii) What is the probability that any of the grand children will be colour-blind?
54. a). Describe the mechanism of semi conservative replication. [09 marks]
b) Describe the formation of polypeptide chains in the cytoplasm of the cell. [11 marks]
55. (a) What is the genetic basis of

(i) hybrid vigour?
(ii) Determination of the ABO blood groups.
(b) In fruit flies the body colour and eye colour genes are sex linked. A Cross between yellow bodied
white- eyed females and grey bodied red-eyed males produced females which were grey bodied red-eyed.
When the F1 females were back crossed with their male parents, the resulting males comprised the
following:
1330 yellow bodied white-eyed flies
26 yellow bodied red eyed flies
35 grey bodied white eyed flies

1297 grey bodied red eyed flies.
(i) Explain with the aid of diagrams the results obtained

(ii) What would be the genotypes and phenotypes of the female offspring from mating F1 females
with yellow-bodied white-eyed males?
56. (a) Describe the general structure and chemical composition of a transfer ribonucleic acid (t-RNA)
molecule. (09 Marks)
(b) Describe how the genetic information stored in the DNA is translated into a protein. (11 Marks)
57. (a) In cats the allele for short hair is dominant to the allele for long hair; the gene involved is autosomal.
Another gene which is sex-linked produces hair colour; its alleles produce black or white coat colour, and
the heterozygote combination produces tortoise-shell colour. If a long-haired black male is mated with a
tortoise-shelled female homozygous for short hair, what kind of offspring will be produced in Fl? (08 m
(b) Explain the inheritance of the ABO blood groups in man. (08 marks)
(c) What is the role of mutation in evolution? (04 marks)
58. (a) Explain how semi conservative DNA replication occurs in an eukaryotic cell. [12 marks]
(b). Explain the role of DNA in controlling the process of polypeptide synthesis in a cell.
59. (a) Describe the structure of the tRNA molecule. [06 marks]
(b) What are the characteristics of a genetic code? [05 marks]
Page 172 of 447

(c) Compare the structure of tRNA and DNA. [09 marks]
60. (a) Describe the structure of nucleic acids. (06 Marks)

(b) Compare the processes of eukaryotic DNA replication and transcription. (06 marks)
(c) Explain how meiosis can result in an almost infinite genetic variety. (08 marks)
61. In the garden pea, Pisum sativum, the dominant alleles of two unlinked genes, A/a and B/b, are needed to
make the pods tough and inedible. All other genotypes result in soft, edible ‘sugar-snap’ pea pods.
• Pods with genotypes including the dominant allele A have a thin layer of cells lining the pod.
• Pods with genotypes in which the recessive allele a is homozygous have no thin lining layer.
• Pods with genotypes including the dominant allele B have lignin added to the thin lining layer, when it is
present.
• Pods with genotypes in which the recessive allele b is homozygous do not have added lignin.
(a) Explain the phenotypes of pea pods with the following genotypes: (04 marks)
(i) AAbb
(ii) aaBB
(b) Two pea plants of genotypes AAbb and aaBB were interbred to give an F1 generation and these in turn
were interbred to give an F2 generation. Using an appropriate genetic cross, including gametes, show the
genotypes and phenotypes of the F1 and F2 generations. Give the ratio of phenotypes expected in the F2
generation. (10 marks)
(c) Gene R for red flower colour can only express itself phenotypically in the presence of gene C which
complements its action to form colour. When two white-flowered plants with genotypes CCrr and ccRR
were crossed, the F1 generation all had red flowers. What would be the phenotypic ratio of the F 2 progeny
when the F1 progeny are selfed? (Show your working). (06 marks)
62. (a) Explain how interaction at one loci and between loci can affect phenotypic variation. (09 marks)
(b) How does natural selection increase adaptation of a species of the environment? (11 marks)
63. (a) Distinguish between translation and transcription (03 marks)

(b) Give an account of the process of translation in a cell (12 marks)
(c) What is the significance of translation in a living cell (03 marks)
64. (a) What is meant by the term linkage as used in inheritance (02 marks)
(b) Briefly explain why identical twins are very important in genetics (02 marks)
(c) In tomatoes, the allele for red fruit, R is dominant to that for yellow fruit, r. The allele for tall
plant, T is dominant to that for short plant, t.
i. A cross was made between two tomato plants and the possible genotypes of the
gamete of the male were: RT, Rt, rT and rt while that of the female were rt. What are
the genotypes and phenotypes of the male and female parents? (02 marks)
ii. What proportions of the resulting offspring from the genetic cross in (c) (i) above
would you expect to have red fruits?
Use a genetic diagram to explain your answer (05 marks)
(d) In cats the gene controlling the coat colour are carried on the X-chromosomes and are co-
dominant. Female cats are usually homogametic while males are heterogametic.
A black coat male produced a litter consisting of only black males and tortoise shell female
kittens. What is the expected F2 phenotypic ration? Explain your answer (09 marks)
65. (a). What is meant by the term linkage? [03 marks]
Page 173 of 447

(b) A homozygous purple-flowered short stemmed plant was crossed with a homozygous red flowered,
long stemmed plant. The F1 phenotypes had purple flowers and short stems. When F1 generation was
test crossed, the following genotypes were produced;
Phenotype Number of plants

Purple flowered - short stemmed 52
Purple flowered – long stemmed 47
Red flowered – short stemmed 49
Red flowered – long stemmed 45
(i) Using relevant illustrations, account fully for the results [14 marks]
(ii) Do the results indicate linkage, explain your answer. [03 marks]
66. Both haemophilia and colour blindness are transmitted in the same way
a) What are the effects of each disease? (04 marks)
b) Describe the transmission of the diseases. (08 marks)
c) Explain why there are more colourblind individuals than hemophiliacs among the human
population inspite of the similar way of transmission. (08 marks)
67. (a) Distinguish between codominance and pleiotropy. (04 marks)

(b) In cats, the genes controlling fur colour are carried on the X chromosomes and are codominant. The
homozygous conditions are black fur and ginger fur. The intermediate condition is tortoise shell. A black
female mated with a ginger male. What is the expected phenotypic ratio in the F2 generation? (10 marks)
(c) Explain why sex-linked traits are most common in males among humans. (06 marks)
68. In the fruit fly, Drosophila melanogaster, the genes for broad abdomen and long wing are dominant
over the genes for narrow abdomen and vestigial wing. Pure-breeding strains of the double dominant
variety were crossed with a double recessive variety and a test cross was carried out on the F 1 generation.
(a) Using suitable symbols, work out the expected phenotypic ratio of the test cross of the F 1 generation,
if the genes for abdomen width and length of wings are linked. (07 marks)
(b) It was however observed that when the test cross of the F 1 generation was carried out, the
following results were obtained:
Broad abdomen, long wings 380
Narrow abdomen, vestigial wings 396
Broad abdomen, vestigial wings 14
Narrow abdomen, long wings 10
(i) Explain the above results (03 marks)
(ii) Using appropriate genetic crosses show how the above results are obtained. (07 marks)
(iii) Calculate the distance in units between the genes for abdomen width and length of wing
(03 marks)
69. (a). How does meiosis explain Mendel’s law of inheritance? [06 marks]
(b). Describe how abnormal haemoglobin arises in the human population [09 marks]
(c).Explain the effects of the gene for abnormal haemoglobin in the human population. [05 marks]
70. A sex-linked gene controls fur colour in cats. Ginger-coloured fur is controlled by the allele G, and
black-coloured fur is controlled by the allele g. Some cats, exclusively females are described as
tortoiseshell because of having ginger and black patches of fur.
(a) Using suitable genetic symbols, workout the genotypes and the ratio of phenotypes expected in the
offspring of the cross between a male cat with genotype XgY and a tortoiseshell female cat. (06 Mark)
Page 174 of 447

(b) The effect of the G and g alleles is modified by another gene which is not sex-linked but has two
alleles. The allele d changes the ginger colour to cream and the black colour to grey. The dominant
allele D does not modify the effect of G or g.
Using suitable genetic symbols, workout the genotypes and the ratio of phenotypes expected in the
offspring of the cross between a cream-coloured male cat and black female whose genotype was XgXgDd
to produce male kittens of two different colours. (07 Marks)
(c) (i) With examples from humans, distinguish between sex-linked and sex-limited genes. (04 Marks)
(ii) Explain why sex-linked features are more common in men than in women. (03 Marks)
71. In Drosophilia the genes for wing length and for eye colour are sex-linked. Normal wing and red eye are
dominant to miniature wing and white eye.
(a) In a cross between a miniature wing, red-eyed male and a homozygous normal wing, white-eyed
female, explain fully the appearance of;
i. the F1 and (05 marks)
ii. the F2 generations (05 marks)
(b) crossing a female from the F1 generation above with a miniature wing, white eyed male gave the
following results:
normal wing, white-eyed males and females 35

normal wing, red-eyed males and females 17
miniature wing, white-eyed males and females 18
miniature wing, red-eyed males and females 36
i. Account for appearance and numbers of the phenotypes shown above (06 marks)
ii. Determine the cross-over value for the results above (03 marks)
72. (a) State eight situations where Mendel’s laws would not apply. (08 marks)
(b) How does meiosis explain Mendel’s first law of inheritance? (04 marks)
(c) (i) In corn plants a dominant allele A inhibits Kernel colour while the recessive allele a permits
colour formation when homozygous. At a different locus, the dominant allele R causes purple Kernels
colour while the homozygous recessive genotype rr causes red Kernels.
i. If plants heterozygous at both loci are crossed, what will be the phonotypic ratio of the
offspring? (07 marks)
ii. State the type of gene interaction in C(i) (01 mark)
73. (a)(i) Using examples, differentiate between sex-linked and sex-limited characters. (03 Marks)
(i) Haemophilia is a condition caused by a recessive gene carried on the X-chromosomes.
Determine the chances of producing a normal boy from a carrier mother and a normal father.
(04 Marks)
(b) Suggest three reasons why female haemophiliacs are very rare. (03 Marks)
74. (a) What is the significance of the genetic code to the life of an organism? (08 marks)
(b) Describe how polypeptides are made at the ribosomes in a cell (08 marks)
(c) Describe the evidence to show that DNA is a genetic material (04 marks)
75. (a)Compare mitotic prophase and prophase I of meiosis (12 marks)

(b) State evidences for DNA as the hereditary material (04 marks)
Page 175 of 447

(c) State the differences between continuous and discontinuous variation (04 marks)
76. (a) Explain why a single base deletion from one DNA molecule usually cause greater effect than
replacement of one base by another different base (08 marks)
(b) Describe how sickle-cell anaemia arises in a population (12 marks)
77. (a) Distinguish between polygenic and pleiotropic traits (02 marks)
b) Give an example of a human trait due to (02 marks)
(I) polygenic inheritance (ii) Pleiotropic inheritance
b) What is the meaning of the following terms as used in the study of genetics? (02 marks)
(i) Segregation
(ii) Independent assortment
c) Two unlinked gene loci interact to give variation in coat colour in certain breeds of cats. The
dominant allele for colour B produces black pigment, the recessive allele b gives a cinnamon coat.
Allele D prevents dilution of coat colour pigment which occurs in animals homozygous for the
recessive allele d. dilution of the black gives a blue coat and dilution of cinnamon produces fawn.
Work out the expected probability, phenotypes and genotypes of the offspring from a cross between a
heterozygous blue cat and a cinnamon cat. (04 marks)
END
Page 176 of 447

P530 (2020) By Nakapanka J Mayanja 0704716641
TOPIC 4: CYTOLOGY, MICROSCOPY AND HISTOLOGY

Syllabus extract
SPECIFIC OBJECTIVES
Content
The learner should be able to:
 Identify plant and animal cell structures visible

Cell structure under the light microscope
 Structure of the animal cell and plant cell as  Draw and label detailed animal and plant cells.
seen under a light microscope.  State functions of cell structures as seen under an
 Detailed animal and plant cells. electron microscope.
 Functions of detailed cell structures indicating  Distinguish between the plant cell and animal cell
ultra-structures as visible under the electron
the inter
relationship. rnicroscope
 Differences between plant cell and animal cell  Explain the theory behind the structure of the
ultra-structures. plasma membrane.
 The theory behind the structure of the plasma
membrane.
 The fluid mosaic model-plasma membrane.
Diversity of Specialized cells and tissues.

 Distinguish between prokaryotes and eukaryotes.
 Differences between Eukaryotic cells and  Explain cell and tissue specialization.
Prokaryotic cells.  Explain how epithelial tissues are adapted to
 Specialised cells and tissues: xylem, phloem, diversity of functions in the body.
tracheids, sclerenchyma, collenchyma,  Distinguish between the different levels of
parenchyma, connective and skeletal tissue. organization.
 Function and adaptation of epithelial tissues  State the advantages and disadvantages of being
 Levels of organization: cell, tissue, organ and unicellular.
organ system.  State the advantages of being multicellular.
 Advantages and disadvantages of being
unicellular
 Advantages of multicellular state.
cell structure and microscopy Practical  Explain the functioning principles of a light and
 The functioning principles of a light and electron microscope
electron microscope: resolving power, e.t.c.  Prepare temporary mounts of cells and tissue
 Preparation of temporary mounts of cell anti slides.
tissue slides.  Use simple stains in studying cells and tissues
 Simple staining methods  Identify different plant tissues using different
 Staining plant tissues. laboratory stains.
 Estimation of cell size.  Determine cell size
 Epithelial tissues classification.  Draw and label epithelial tissues.
Page 177 of 447

CELL BIOLOGY
This deals with cell structure, function and cell physiology all at the unit level of a living organism called a
cell. The study of the structure of cells, cytology, is part of a major branch of biology known as cell biology.
The main functions of the cell include
1. Basic unit of life. The cell is the smallest part to which an organism can be reduced that still retains the
characteristics of life.
2. Protection and support. Cells produce and secrete various molecules that provide protection and support
of the body. For example, bone cells are surrounded by a mineralized material , making bone a hard tissue
that protects the brain and other organs and that supports the weight of the body.
3. Movement. All the movements of the body occur because of molecules located within specific cells such
as muscle cells
4. Communication. Cells produce and receive chemical and electrical signals that allow them to
communicate with one another. For example, nerve cells communicate with one another and with muscle
cells, causing them to contract
5. Cell metabolism and energy release. The chemical reactions that occur within cells are referred to
collectively as cell metabolism. Energy released during metabolism is used for cell activities, such as the
synthesis of new molecules,muscle contraction, and heat production, which helps maintain body
temperature.
6. Inheritance. Each cell contains a copy of the genetic information of the individual. Specialized cells are
responsible for transmitting that genetic information to the next generation
The cell of a living organism

The cell can be defined as the basic unit of structure and function in a living organism. This generalisation is
known as the cell theory and it embraces four ideas;
A. The cell is the building block of structures in living cells
B. The cell is derived from other cells by cell division
C. The cell contains hereditary information that is passed from parent cell to daughter cell
D. The cell is the functioning unit of life i.e. the chemical reactions of life takes place within cells
The cell theory states that “a cell is the fundamental and functional unit of life” i.e. the cell is the basic unit
of the structure and function in living organisms.
Factors that limit cell size

1. Surface area to volume ratio
Small cells have large surface area: Volume ratio (SA: V ratio) while large cells have a small SA: V ratio.
A large SA: V ratio enables fast rate of diffusion while a small SA: V ratio slows the rate of diffusion.
Small cells have low metabolic demands and form low amount of wastes while large cells have higher
metabolic demands and form much amount of wastes.
Therefore, the large SA: V ratio in small cells enables adequate supply of oxygen and nutrients and
expulsion of wastes e.g. carbon dioxide via the surface of the cell by simple diffusion while the small SA:
V ratio in large cells limits diffusion hence the supply of nutrients by simple diffusion is inadequate to
meet the metabolic demands of the cell.
2. Nucleocytoplasmic ratio
DNA in the nucleus provides instructions for protein synthesis hence controls activities of the whole cell.
Each nucleus can only control a certain volume of cytoplasm.
Specialization forms some long / large cells, therefore to overcome this limitation such cells are modified
to become multinucleate / coenocyte e.g. skeletal muscle cells and fungal hyphae.
3. Fragility of cell membrane
Page 178 of 447

As cell size increases, the risk of damage to the cell membrane also increases. This limits the maximum
size of cells, especially animal cells. Hence;
(i) In animals, some large sized cells take in substances in bulk by endocytosis and expel bulk
substances by exocytosis to supplement on simple diffusion.
(ii) Some animal cells increase their surface area by forming many tiny projections called microvilli.
(iii) Some cells divide when they reach a certain size to maintain suitable SA: V ratio.
Note: SA: V ratio particularly limits the size of bacterial cells, i.e. prokaryotic cells which are incapable
of endocytosis and exocytosis.
4. Mechanical structures that hold the cell together
Cells with tough cell walls e.g. plant cells are larger than cells with only the fragile cell membrane e.g.
animal cells because the tough walls provide support and maintain cell shape.
Cells with complex internal cytoskeleton are larger than cells with little cytoskeleton because the
cytoskeleton protects and supports the cell structure and maintains cell shape.
TYPES OF CELLS
There are two fundamentally different types of cells, the prokaryote cell and eukaryote cell.
A. Prokaryote cell (Pro, before; karyon, nucleus)
Characteristics of prokaryotic cells
- These are cells that do not have a true nucleus.
- They have no membrane bound organelles. An organelle can be defined as a membrane-enclosed
structure with specialised functions, suspended in the cytosol of eukaryotic cells.
- Their nuclear material lies in a free region known as a nucleoid e.g. in bacteria. They were probably the
first organisms on earth
- The cell has no distinct nucleus. The nucleoplasm appears scattered in the cytoplasm or the nuclear
materials e.g. DNA.
- The cell lacks a nuclear membrane
- Each cell has got very few cell organelles (cell parts) e.g. they do lack the chloroplasts and mitochondria.
- The cell has a single circular chromosome in the form of a ring, of Deoxyribonucleic Acid (DNA) in the
cytoplasm, not contained in a nuclear membrane
- They are extremely small, ranging in size between 1-10milimetres in diameter
- Duplication of the chromosomes occurs but not on the spindle i.e. their cells are capable of multiplication
- The cell has got a unique cell wall containing a polysaccharide
Examples include bacteria and cyanobacteria i.e. first organisms on earth.
Diagram of a generalised structure of a bacterium Fig 2.5 pg 9 Soper
Page 179 of 447

Function of the parts

Structures which are always present
1. Cell wall
This lies external to the cell membrane, it’s rigid and strengthened by presence of murein (a molecule
consisting of parallel polysaccharide chains cross-linked at regular intervals by short amino acid chains)
The cell wall is a physical barrier which:
(i) protects the internal parts from mechanical damage
(ii) prevents the cell bursting when it takes in water by osmosis
(iii) allows entry of some substances, such as water, ions and small molecules
The cell wall cannot grow and for growth to occur the cell wall is forts dissolved at intervals for materials
to be added.
2. Cell membrane
It lies immediately below the cell wall and has a fluid mosaic structure. It’s hydrophobic and
impermeable to most water soluble molecules.
It has enzymes involved in the synthesis of the capsid and cell wall components. Enzymes for respiration
and those which facilitate flagella mobility.
Note: a damaged cell membrane leads to the death of the cell.
3. Ribosomes
Prokaryotes have 70S ribosomes which are slightly smaller than the 80S eukaryotic ribosomes.
Ribosomes are site of protein synthesis.
4. DNA
The DNA comprises of a single circular molecule possessing the genetic information needed to replicate
new cells
5. Food reserves
Food reserves include lipids and glycogen
6. Cytoplasm
This is enclosed by the cell membrane and is divided into three divisions (bacteria only) i.e.
(i) Cytoplasmic area which contains ribosomes and it is also a site for protein synthesis.
(ii) Chromatin area, a dense area which is rich in nucleic acid material. The nuclear region is called
nucleoid. Bacteria have single circular strand of chromatin material.
(iii) The fluid area, an area with dissolved substances. In the bacteria the rough endoplasmic
reticulum is lacking. The ribosomes are generally smaller than in an eukaryotic cell and are free
or attached to the cell membrane.
Structures sometimes present
7. Flagellum (Plural. flagella)
This occurs in many species of bacteria. They are hair like helical appendages protruding through the cell
wall. They are used for propulsion. Bacterial flagella are smaller, thinner and simpler than eukaryotic
flagella. Their location and number may be used in identification of bacteria.
8. Pili
Pili are numerous fine protein rods projecting from the walls of some bacteria. The pili are for attachment
to specific cells or surfaces. The F.pilus is used in sexual reproduction.
9. Capsule
This is an enveloping layer of viscous substances around the cell wall. This layer can be detected under
the light microscope after staining the bacteria with Indian ink. Its uses include;
(i) Protecting against infecting phages
(ii) Resist engulfment by white blood cells
(iii) Prevents agglutination of bacteria
(iv) Used by bacteria to stick firmly onto substances e.g. bacteria on teeth
Page 180 of 447

(v) The capsular secretions are in some cases used to unite bacteria into colonies
10. Plasmids
Plasmids are small self-replicating strands of extra DNA. Plasmids possess only a few genes, and are
generally concerned with survival in adverse conditions.
Plasmids are known which;
a) Confer resistance to antibiotics
b) Confer resistance to disinfectants
c) Cause disease
d) Are responsible for fermentation of milk to cheese by lactic acid bacteria
e) Confer the ability to use complex as chemicals such as hydrocarbons as fuel
11. Mesosomes
Bacteria lack membrane bound organelles such as mitochondria and chloroplasts. Instead they have
invaginations of cell membranes forming a system referred to as mesosomes. There are 2 types; the
central and the peripheral mesosomes.
(i) Central mesosomes
These are invaginations which penetrate deep into the cytoplasm. They appear to be linked to the
nuclear material and play a role in cell division.
(ii) Peripheral mesosomes
These are shallow invaginations formed by infoldings of the cell membrane. They are associated
with export of secretions such as cellular secretions or enzymes. They are site of respiration.
12. Photosynthetic membranes
Photosynthetic bacteria possess sac-like, tubular or sheet-like infoldings of the cell surface membrane
containing photosynthetic pigments, always including bacteriochlorophyll
13. Spores
Some bacteria form endospores (spores produced inside cells). The spores are thick-walled, long-lived,
and extremely resistant (particularly to heat, drought, and shortwave radiations)
14. Membranes for nitrogen fixation
B. Eukaryotic cell (Eu, true; Karyon Nucleus)
These are cells with a true nucleus. Their nuclear materials are found inside the nucleus surrounded by two
membranes. They probably evolved about 1000 million years ago, 2 million years after the prokaryotes. There
are 2 main types of eukaryotic cells; the plant cell and the animal cell.
Cells as seen with the light microscope
A light microscope is a microscope that uses light as a source of radiation. Under the microscope, cells are
described as a small unit of living protoplasm and always surrounded by cell surface membrane and
sometimes as in plants, surrounded by a non-living cell wall made of cellulose. The most conspicuous
structure is the nucleus which contains a deeply staining material know as chromatin. When loose it is referred
to as chromosome. Chromosomes appear as thread like structures just before nuclear division. The living
material between the nucleus and the cell surface is known as the cytoplasm which contains a variety of
organelles.
A generalised cell is a cell which shows all the typical features found in a cell.
a) Animal cell
An animal cell as seen in a light microscope contains protoplasm (nucleus and cytoplasm) surrounded by a
thin plasma membrane.
Each cell has a relatively large central nucleus surrounded by the cytoplasm. The nucleus contains coiled
threads called chromatin. Chromatin contains DNA and proteins called histones which together condense to
Page 181 of 447

form chromosome during cell division. DNA carries genetic material which controls cell activities and
determines the organism’s characteristics. The cytoplasm contains organelles suspended within.
The structure of a generalised animal cell Fig 5.1 a pg 129 Soper
b) Plant cell
Many of the structures found in an animal cell also occur in the plant cell. A typical plant cell has
additional specialised structures.
The structure of a generalised plant cell (Fig 5.2 pg 130 Soper)
There’s a protective, rigid, cellulose cell wall surrounding the cell. Plant cells have a nucleus and cytoplasm
which are usually peripheral. The cytoplasm contains chlorophyll pigments which carry out photosynthesis.
A large central vacuole filled with cell sap is present in mature plant cells. The vacuole is surrounded by the
tonoplast
Page 182 of 447

Description of a generalised structure of a eukaryotic cell.

A cell is a small unit of living protoplasm, always surrounded by a cell surface membrane and sometimes by a
non-living cell wall (as in plants and fungi).
The most conspicuous structure of the cell is the nucleus which contains chromatin. Chromatin is the loosely-
coiled form of chromosomes. Chromosomes contain genetic material in the form of DNA. The nucleus is
separated from the cytoplasm by its nuclear membrane
The cytoplasm contains organelles.
Comparison between prokaryotic and eukaryotic cells

a. The protoplasm is Prokaryotes Eukaryotes
surrounded by a The nuclear material is not enclosed The nuclear material is enclosed by
membrane that is by nuclear membrane nuclear membrane
selectively permeable Genetic material is circular double Most DNA is linear and associated
(protoplasm = strand of DNA with histones proteins to form
nucleoplasm + chromosomes
cytoplasm) No membrane bound organelles Has membrane bound organelles
b. The binding protein is No mitosis or meiosis Mitosis, meiosis or both can occur
made up of lipid- No spindle formation There’s spindle formation
protein complex Ribosomes are smaller (70S) Ribosomes are bigger (80S)
c. The cells have got Rigid cell wall containing murein Cell walls of plants and algae contain
ectoplasmic and (peptidoglycan) cellulose, fungi contain chitin and
nuclear regions animal cells have no cell walls
No mitochondria (mesosomes in Mitochondria present and function as
bacteria and plasma membrane of sites for cellular respiration to produce
cyano bacteria contain respiratory ATP.
enzymes)
Use mesosomes for respiration No mesosomes
Flagella if present, contain flagellin Flagella, if present, have a ‘9+2’
and lack microtubules arrangement of microtubules
Average diameter of cell is 0.5-5µm Average diameter of cell is 10-100µm
Some bacteria have small circular Plasmids are absent
DNA plasmids
Few organelles Many organelles
No chloroplasts (some prokaryotes Chloroplasts containing grana
are photoaoutotrophs with the
photosynthetic membranes not
stacked into grana)
NOTE: the only organelle found in animal cells which is absent from plant cells is the centriole
Advantages of having membrane bound organelles
1. Potentially harmful reactions (enzymes) can be isolated inside an organelle so that they do not harm the
rest of the cell
2. The rate of any metabolic reactions inside an organelle can be controlled by regulating the rate at which
the membrane allows the first reaction to occur or to enter
3. The containment of enzymes for a particular metabolic pathway within the organelle means that the
products of the reaction will always be in close proximity to the next enzymes within the sequence. This
increases the rate of metabolic reactions
4. Many metabolic processes which involve enzymes occur in the membrane.
Page 183 of 447

MICROSCOPY
Microscopy is the science that studies structure, magnification, lenses and techniques related to the use of
microscopes. A microscope is an instrument that magnifies images of very tiny objects to show great details.
Units of measurements and magnification
Magnification is the number of times that an image is larger than the specimen i.e. the ration of an object’s
𝑠𝑖𝑧𝑒 𝑜𝑓 𝑖𝑚𝑎𝑔𝑒
image size to its real size and is usually given by the formula: Magnification = 𝑠𝑖𝑧𝑒 𝑜𝑓 𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛
The units of measurement used in cell biology are shown in the table below
Units for measurement of a cell
Unit Relation to a milimetre Relation to a metre
Angstrom (Å) 10-7 10-10
Nanometer (nm) 10-6 10-9
-3
Micrometer (µm) 10 10-6
Milietre 10-3
Centimeter 101 10-2
Or 1 meter = 102 cm = 103 mm = 106 µm = 109 nm = 1010 Å
Worked examples
1. An animal cell of 60µm length is enlarged 2. A plant cell is magnified X2000 and the length of the
photographically. An enlargement print is chloroplast in the diagram is 16mm.
made showing the cell at 12cm. Calculate the actual length of the chloroplast in µm
What is its magnification? 1mm = 1 x 103µm = 1,000 µm
1cm = 1x104 µm = 10,000µm 16 cm = 16 X 1000 = 16,000 µm
12 cm = 12 X 10,000 = 120,000 µm 𝑠𝑖𝑧𝑒 𝑜𝑓 𝑖𝑚𝑎𝑔𝑒
Magnification = 𝑠𝑖𝑧𝑒 𝑜𝑓 𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛
120,000
Magnification = = X2,000 16000
60 X2000 =
𝑠𝑖𝑧𝑒 𝑜𝑓 𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛
16,000
Actual size of specimen = 2000
= 8 µm
Resolution (resolving power)

Resolution is a measure of the clarity of the image i.e. the minimum distance two points can be separated and
still be distinguished as two points. For example, what appears to the unaided eye as one star in the sky may
be resolved as twin stars with a telescope.
The limit of resolution of a microscope is the minimum distance between two points at which they are still
distinguished as two separate points. If the two points cannot be resolved, they will be seen as one point. A
microscope with a high resolving power will enable two small objects close together to be seen as two
separate objects. A microscope with a low resolving power will cause the two small objects close together to
be seen as one object.
Light microscope (LM)/optical microscope
All the microscopes you are likely to use in the labaratory
are all LMs. In the LM, light rays passing through a
specimen are brought to focus by a set of glass lenses. The
resulting image can be seen by the human eye.
Visible light used in the LM has a wave length of about
400-700nm. The maximum resolution of an optical
microscope is about 200nm, which gives a maximum
magnification of about 1500 times.
Page 184 of 447

Organelles such as chloroplasts (about 3000nm in diameter) are large enough to interfere with the light waves
and can been seen. Ribosomes (about 20nm) are too small to interfere with the light waves and cannot be seen
under a light microscope.
Phase contrast microscope
Many cell details cannot be seen using an ordinary optical microscope. This is because there is very little
contrast between structures. They have similar transparency and are not coloured. A third important
parameter in microscopy is contrast, which emphasizes or makes more noticeable, differences in parts of the
sample. New methods of improving contrast include staining or labelling cell components to stand out
visually.
Special phase contrast condensers and objective lenses are added to the optical microscope. Light rays
travelling through material of different densities are bent and altered giving a better contrast (cell components
having different densities show variation in the refractive index, those of a higher refractive index can bend
light to greater angle than other cell components of lower refractive index).
Phase contrast microscopes enable living, non-pigmented specimen to be studied without fixing and staining.
Phase contrast microscopes give better contrast but do not improve resolution. This is similar to the optical
microscope.
Just as the resolving power of the human eye is limited, the light microscope cannot resolve detail finer than
1bout 0.2µm 0r 200nm, the size of a small bacterium, regardless of the magnification factor. The poor
resolution of the LM could only be overcome by using a form of radiation with a wave length less than of
light. This led to the development of the electron microscope (EM). Since electrons have a shorter
wavelength than light (about 0.005nm), they couple their higher magnifying power with much greater
resolution and contrast. They can resolve two objects which are only about 1nm apart.
While the light microscope uses glass lenses to focus the light rays, the electron beam of the electron
microscope is focused by powerful electromagnets. The image produced by electron microscopes cannot be
seen by the unaided eye. Instead the electron beam is directed unto a screen giving black and white images
(photographs). A photograph taken with an electron microscope is called an electron micrograph or
photomicrograph.
There are two types of electron microscopes; the Transmission Electron Microscope (TEM) and the
Scanning Electron Microscope (SEM).
A TEM is an electron microscope in which the electron beam is transmitted through the specimen before
viewing. The principle is the same as in the light microscope in that a beam of radiation is focused by
condenser lenses through the specimen, and the image is magnified by further lenses. The TEM has a
resolving power of about 1nm. It used to study the ultrastructure of a cell.
The electron beam is heated using a cathode and passed through ultra-thin dehydrated sections of dead
specimen. Electrons are absorbed by heavily stained (due to treatment with heavy metals) parts but pass
through the lightly stained parts. This provides contrast between different parts of the specimen.
Drawing of the pathway of the electron beam in the TEM (Soper pg 133)
Page 185 of 447

A SEM is an electron microscope in which the electron beam is scanned to and from across the specimen. The
electrons reflected from the surface are collected and used to form a TV-like image on a cathode ray tube.
This enables the studying of the surfaces of structures and gives three-dimensional images. The SEM has a
resolving power of about 5nm higher than that of a light microscope, but lower than that of a TEM. Larger
and thicker specimens can be examined.
Note: the fine structure of the cell as seen with the electron microscope is called the ultra-structure.
Differences between the light and electron microscopes
TEM Compound light microscope
1. Source of radiation are electrons 1. Source of radiation is light
2. Electrons have a shorter wavelength of about 2. Light has a longer wave length of about 400-
0.005nm 700nm
3. Maximum resolution is greater (about 0.5nm) 3. Maximum resolution is lower (about 20nm)
4. Maximum useful magnification on screen is 4. Magnification is low (about X1500)
higher (about X250,000)
5. Uses powerful electromagnets as lenses 5. Uses glass lenses
6. The specimen is dead, dehydrated and 6. The specimen maybe living or non-living
relatively small or thin
7. The specimen is supported on a small copper 7. The specimen is supported on a glass slide
and in a vacuum
8. The stains used contain heavy metals to reflect 8. The stains used are coloured dyes
electrons
9. The image is black and white 9. The image is usually coloured
A comparison of radiation pathways in light and transmission electron microscopes
Page 186 of 447

A comparison of the relative advantages and disadvantages of the light and electron microscopes
Light microscope Electron microscope
Advantages Disadvantages
1. Cheaper to produce and operate 1. Much more expensive to purchase and operate
2. Smaller and more portable; thus can be used 2. Much larger and fixed, and must be operated in
almost anywhere special rooms
3. Not affected by magnetic fields 3. Affected by magnetic fields
4. Preparation of material is relatively quicker and 4. Preparation of material is lengthier and requires
simpler, and requires only a little expertise and more expertise and more complex equipment
simpler equipment
5. Material rarely distorted by preparation 5. Material is usually distorted by the preparation
(preservation and staining may change or
6. The natural colour of the material can be damage the structure)
observed 6. All images are in black and white
7. The specimen may be living 7. The specimen must be dead because it is viewed
in a vacuum
8. The specimen does not deteriorate easily, 8. The specimen gradually deteriorates in the
allowing more study time electron beam, and thus photomicrographs must
be taken and observed on the screen
Disadvantages Advantages
1. Lower magnification of up to X1,500 1. Higher magnification of up to X250,000
2. Has a restricted depth of field 2. Enables investigation of a greater depth of field
3. Lower resolution of about 200nm 3. Higher resolution of about 0.5nm
CELL STRUCTURE
Differences between plant and animal cells
Plant cells Animals cells
1. Have tough slightly elastic cellulose cell wall 1. Have no cell wall, only a cell surface membrane
outside the cell surface membrane surrounds the cell
2. Have pits and plasmodesmata in the cell wall 2. Have no cell wall, and therefore have no pits
and plasmodesmata
3. Have middle lamellae joining the cell walls of 3. Middle lamellae are absent, the cells are joined
adjacent cells by intercellular cement
4. Possess plastids, such as chloroplasts 4. Lack plastids
5. Mature cells possess a large single, central 5. Possess only small vacuoles scattered
vacuole filled with cell sap throughout the cells
6. The cell vacuole is enclosed by a tonoplast 6. Vacuoles lack tonoplasts
7. Have a thin layer of cytoplasm confined to the 7. Have much cytoplasm spread throughout the
edge of the cell cell
8. The nucleus is located at the edge of the cell 8. The nucleus is usually placed centrally in the
cell
9. Higher plant cells lack centrioles 9. Possess centrioles
10. Higher plant cells lack cilia and flagella 10. Often possess cilia and flagella
11. Store food as starch grains 11. Store food as glycogen granules
Page 187 of 447

12. Only some cells are capable of division 12. Almost all cell are capable of division
13. Plant cells produce secretions 13. Animal cells produce a wide variety of secretion
14. Have a regular shape 14. Have an irregular shape
a) The ultra-structure of a generalised animal cell (Soper fig 5.10 page 135)
b) The ultra-structure of a generalised plant cell (Soper fig 5.11 page 135)
Page 188 of 447

Detailed study of animal and plant cells.

Cell membrane
The cell membrane is invisible with a light microscope.
Danielli and Davison proposed a membrane Diagram of the cell membrane based on the
structure in which a lipid bilayer was coated on Danielli-Davison hypothesis
either side with a protein so as to provide the Fig 2.21 pg 27 Roberts
mechanical strength (elasticity and surface tension
properties) to the cell membrane. This hypothesis
proposes that the plasma membrane is made up of
three layers: a bimolecular layer of lipid sandwiched
between two layers of protein, the lipid molecules
being set at right angles to the surface i.e. the
sandwich model of the cell membrane
From the speeds at which various molecules penetrate the membrane, they predicted the lipid layer to be about
6.0nm in thickness, and each of the protein layers about 1.0nm giving a total thickness to the membrane of
about 8.0nm.
Robertson (1960) used an electron microscope to observe a cell membrane and proposed that a cell
membrane is actually a unit membrane. According to his proposal, all membranes have the same structure. A
unit membrane has protein molecules with lipid molecules inside. The head of the lipid molecules are in
mutual electrostatic attraction with the protein molecules, this increases on the mechanical strength of the unit
membrane.
Fig 2.22
Fig 2.22 pg
pg 27
27 Roberts
Roberts The unit membrane has pores that are lined with protein
molecules which enable water soluble substances to
enter or leave the membrane. Such substances include,
water molecules, mineral salts, simple sugars, vitamins,
gasses and excretory products. The membrane has got
lipid layers which enable the lipid molecules to enter and
leave the membrane.
Most of the proteins on the cell membranes are called
carrier proteins i.e. they enable the transport of
substances across the membrane. Other proteins are
enzymes in nature i.e. they catalyse biochemical
reactions at the cell surface
In 1972 Singer and Nicolson suggested that the unit membrane has a fluid mosaic model
- The fluid mosaic model proposes that the basic structure for the unit membrane is a phospholipid bilayer
with various protein molecules embedded and attached to it.
- The hydrophilic phosphate heads of the phospholipids face outwards into the aqueous environments inside
and outside the cell and form hydrogen bonds with water molecules.
- The hydrocarbon tails face inwards and create a hydrophobic interior through Van der Waal forces and
hydrophobic interactions. The phospholipids are fluid and move about rapidly by diffusion in their own
layers. Some of the fatty acid tails are saturated and some are unsaturated. Unsaturated tails are bent and
fit together more loosely. Therefore the more unsaturated the tails are, the more fluid the membrane is.
- Most protein molecules float about in the phospholipid bilayer forming a fluid mosaic pattern and these
proteins stay in the membrane because they have regions of hydrophobic amino acids which interact with
- the fatty acid tails to exclude water.
Page 189 of 447

- The membrane proteins most of which Fig 5.16 b pg 142 Soper OR Fig 7.15 pg 159 Cross-sectional
float individually in the fluid bilayer, view Clegg
forming the mosaic part of the fluid
mosaic model. The rest of the protein is
hydrophilic and faces into the cell or out
into the external environment, both of
which are aqueous. Some proteins
penetrate only part of the way into the
membrane (extrinsic proteins) while
others penetrate all the way through
(intrinsic proteins).
- Some proteins and lipids
(phospholipids) have short branching
carbohydrate (oligosaccharides) chains
forming glycoproteins and glycolipids
respectively, more glycoproteins are
formed than glycolipids. These are
important for cell recognition.
- Membranes also contain cholesterol
which disturbs the close parking of
phospholipids and regulates membrane
fluidity. This is important for organisms
living at low temperatures where
membranes can solidify. Cholesterol
also increases flexibility and stability of
membranes, without it, membranes
break up.
A comparison of the sandwich model (Daniel-Danielli) and the fluid-mosaic model (Singer-Nicholson) of
the cell membrane
a) Similarities
(i) Both comprise of a bimolecular layer of phospholipids
(ii) Both contain protein molecules
(iii) In both, the phospholipids possess hydrophilic heads and hydrophobic tails
(iv) In both, the phospholipid tails extend inwards, while the heads lie at the periphery
(v) In both, the main structural skeleton of the membrane comprises lipids and proteins
b) Differences
Sandwich model Fluid-mosaic model
1. Proteins regularly arranged to form a 1. Proteins arranged irregularly in a mosaic pattern
continuous layer covering both sides of 2. Some globular proteins lie on the surface, some
the membrane extend into the lipid layer to varying degrees,
2. Proteins lie on the surface, and do not get and others extend through it
in the membrane 3. Lipids and proteins capable of much movement
3. Lipids and proteins are rigid and cannot like a fluid
move 4. Proteins molecules are of different sizes
4. Protein molecules are of the same size 5. Some proteins have pores
5. Proteins lack pores 6. Proteins may be structural, carrier proteins or
6. All proteins offer structural support only enzymes
Page 190 of 447

How phospholipid properties maintain cell membrane structure

1. Hydrophilic / hydrophobic layers restrict entry/ exit of substances.
2. Phospholipids are held together by hydrophobic interactions
3. Phospholipid layers are stabilized by interaction of hydrophilic heads and surrounding water
4. Phospholipids with short fatty acids and those with unsaturated fatty acids are more fluid. Fluidity is
important in breaking and remaking membranes (e.g. endocytosis / exocytosis)
5. Phospholipids can move about / move laterally (horizontally) / "flip flop" (move transversely) to increase
fluidity
6. Phospholipids allow for membrane fluidity/ flexibility. Fluidity/ flexibility enables membranes to be
functionally stable
Membrane fluidity
Membranes are fluid, dynamic structures whose fluidity/viscosity is affected by their composition.
a) An increase in temperature increases the fluidity of the membrane. Low temperature decreases membrane
fluidity because lipids are laterally ordered, the lipid chains pack well together, mobility reduces to allow
many stabilising interactions. Increase in temperature increases membrane fluidity because lipids acquire
thermal energy to become mobile and reduce stabilising interactions.
b) At moderate warm temperatures, the cholesterol molecules reduce the free movement of phospholipid
molecules and make the membranes less fluid. At low temperatures, cholesterol molecules prevent the
close packing of phospholipid molecules and slow down solidification of the membrane.
c) Lipid chains with double bonds (unsaturated fatty acids) are more fluid because the kinks caused by
double bonds make it harder for the lipids to pack together. Lipids that have single bonds only (saturated
fatty acids) have straightened hydrocarbon chain which pack together to reduce membrane fluidity.
d) Lipids with shorter chains are more fluid because they quickly gain kinetic energy due to their smaller
molecular size and have less surface area for Van der Waals interactions to stabilise with neighboring
hydrophobic chains. Lipids with longer chains are less fluid because their large surface area enables more
Van der Waals interactions hence increasing the melting temperature.
Functions of the unit membrane

1) Surface membrane forms a protective barrier between cell contents and external environments, and
determines the shape of the cell.
2) They form membrane organelles e.g. mitochondria, chloroplasts e.t.c.
3) Membranes are selectively permeable and regulate movement of substances in and out the cell
4) Some membrane proteins act as enzymes e.g. ATP synthase
5) Cell surface receptor proteins are involved in signal-transduction
6) Some membrane proteins act as electron carriers in the electron transport chain
7) Glycoproteins with branching oligosaccharides act as antigens
8) Glycolipids are involved in cell-cell recognition
9) Folding of cell membranes enables the cell to carry out phagocytosis and pinocytosis which enables the
cell to obtain nutrients or to engulf and destroy foreign particles.
10) Folding of membranes also increases the surface area for reactions e.g. the epithelium villus of the ileum
11) Cell adhesion proteins join cells together forming tissues which carry out specific functions
12) Cholesterol molecules stabilise the membrane structure and reduce entry or exit of polar molecules
through the membrane
Note; the various membranes of an eukaryotic cell are different because only certain proteins are unique to
each membrane.
Page 191 of 447

Molecules and ions can pass through the plasma membrane in four ways:
a) Directly through the phospholipid membrane. Molecules that are soluble in lipids, such as oxygen,
carbon dioxide, and steroids, pass through the plasma membrane readily by dissolving in the lipid bilayer.
The phospholipid bilayer acts as a barrier to most substances that are not lipid-soluble; but certain small,
nonlipid-soluble molecules, such as water, carbon dioxide, and urea, can diffuse between the phospholipid
molecules of the plasma membrane.
b) Membrane channels. There are several types of protein channels through the plasma membrane. Each
channel type allows only certain molecules to pass through it. The size, shape, and charge of molecules
determines whether they can pass through a given channel. For example, sodium ions pass through
sodium channels, and potassium and chloride ions pass through potassium and chloride channels,
respectively. Rapid movement of water across the cell membrane apparently occurs through membrane
channels.
c) Carrier molecules. Large polar molecules that are not lipidsoluble, such as glucose and amino acids,
cannot pass through the cell membrane in significant amounts unless they are transported by carrier
molecules. Substances that are transported across the cell membrane by carrier molecules are said to be
transported by carrier-mediated processes. Carrier proteins bind to specific molecules and transport them
across the cell membrane. Carrier molecules that transport glucose across the cell membrane do not
transport amino acids, and carrier molecules that transport amino acids do not transport glucose.
d) Vesicles. Large nonlipid-soluble molecules, small pieces of matter, and even whole cells can be
transported across the cell membrane in a vesicle, which is a small sac surrounded by a membrane.
Because of the fluid nature of membranes, the vesicle and the cell membrane can fuse, allowing the
contents of the vesicle to cross the cell membrane.
Role of proteins within the plasma membrane

1. Anchoring cells. Membrane proteins anchor cells to the cuticle membrane, and also to
microfilaments within the cell.
2. Transport. Membrane proteins form channels that allow selective passage of ions or molecules,
for example carrier proteins during facilitated diffusion.
o Some carrier proteins pump solutes across membranes by active transport
3. Enzyme activity. Some membrane proteins are enzymes that catalyse reactions that are placed
within or along the surface of the membrane.
4. Signal transduction. Receptor proteins bind with signal molecules such as hormones and
neurotransmitters, and transmit information into the cell
5. Cell recognition. Proteins function as identification tags for cells
6. Junction between cells. Cell adhesion proteins of different cells together
7. Energy transducers and electron carriers. In photosynthesis and respiration, membrane proteins
take part in energy transfer
8. Structural support. The various proteins dotted throughout the biphospholipid layer provide
structural support to the cell membrane.
Evidence for the fluid-mosaic model of the cell membrane
a. Pieces of the cell membrane treated from one side with chemicals which react with the proteins
but cannot pass through the membrane behave differently.
In some cases, the reactions are confined to the side of the membrane to which the chemicals are
applied, while in other cases they occur on both sides, suggesting that this particular proteins
span the entire membrane.
Page 192 of 447

b. Using freeze-fracture technique, a piece of the cell membrane is frozen, then split down the
middle longitudinally. If there’s inner surface is then viewed in the electron microscope, globular
structures of the same size as the membrane proteins can be seen scattered about as shown below
c. Experiments on membrane viscosity suggest that it is of a fluid consistency rather like oil, and
shows considerable side movements of the lipid and protein molecules within it.
The cytoplasm
The cell organelles are contained within the cytoplasmic matrix (cytoplasm). The cytoplasm is an aqueous
material forming a solution or colloidal suspension of many fundamental biochemicals of life, including ions
such sodium, phosphates and chlorides; organic molecules such as amino acids, ATP, fatty acids, nucleotides,
vitamins; dissolved gases and storage material such as oil droplets.
The cytoplasm is capable of mass flow in a process cytoplasmic streaming. The cytoplasm is important for
important biochemical processes.
The nucleus
This is the central region in both plant and animal cells with a diameter of 4-10µm. In this region, all the cell
activities are directed e.g. cell division and protein synthesis. A nucleus can be seen with the ordinary
microscope. The nuclei have got various shapes depending on the cells e.g. oval, spherical or lobed
Mammalian red blood cells (erythrocytes) and phloem sieve tube elements don’t have a nucleus.
A distinct nucleus is present at some stage in the cells of all forms except in bacterial cells, blue green- algae
and viruses.
- The nucleus has a double layered nuclear membrane (unit membrane). The outer membrane is continuous
with the endoplasmic reticulum. The perinuclear space occurs between the two membranes.
- The nuclear membrane has got nuclear pores, which regulate exchange of substances between the
nucleoplasm and the cytoplasm. The nuclear membrane pores are routes for the passage of large
molecules such as mRNA, from the nucleus to the cytoplasm and this happens during protein synthesis.
The nuclear pores can only be seen using an electron microscope.
- Inside the nuclear membrane, we find nucleic acids (DNA and RNA) and proteins. The nuclear DNA is
bonded to a number of proteins which are called histones which appear as chromatin in a non-dividing
cell. During nuclear division, the chromatins become visible as chromosomes and the nuclear membranes
disappear. During Interphase, some of the chromatin strands are tightly coiled and are called
heterochromatin. The remaining loosely coiled chromatin is called euchromatin. Inside the nucleus, there
is a nucleolus which makes ribosomes and ribosomal RNA.
- The nucleus contains one or more small spherical bodies called nucleoli which manufacture ribosomal
RNA (rRNA) and assemble ribosomes. A nucleolus contains RNA and DNA.
Page 193 of 447

Functions of a nucleus
1. Chromosomes in a nucleus contain the genetic

material of the cell
2. The nucleus acts as the centre to control cell
activities and cell division
3. Production of ribosomes and RNAs needed for
protein and enzymes synthesis
4. Formation of the ribosomal RNA by nucleolus.
5. Nuclear division gives rise to cell division hence
reproduction.
Adaptations of the nucleus to its function

1) DNA is long to store many genes
2) Nuclear membrane has pores for exchange of DNA and RNA between the nucleus and cytoplasm
3) Presence of nucleolus that produces ribosomes which are protein factories
4) Nuclear envelope that isolates nucleus from interference by processes in cytoplasm
5) Nuclear pores are narrow to regulate entry and exit of substances
Mitochondria
Mitochondria appear as rod-shaped or cylindrical organelle, although occasionally they are more variable in
shape with a length of about 2.5-5µm and a diameter of 1µm.
Each mitochondrion is bound by a double membrane, the outer layer being a continuous smooth boundary.
Between the two membranes is the intermembrane space. The inner membrane is extensively in folded to
form partitions called Cristae (consisting of a head piece, stalk and base piece), which partially divide the
interior. The Cristae in plants are commonly tubular and villus-like; in animal cells they are sheet-like plates.
The inner membrane holds the oxysome and encloses are fluid filled space called the matrix. The matrix
contains enzymes and DNA, the DNA directs or codes the synthesis of proteins within the mitochondria i.e.
mitochondria multiply during cell division. Functions
Fig 7.21 pg 163 Clegg OR Fig 162 B & C 1. They are sites of ATP formation
pg 162 Monger 2. They are sites of aerobic respiration
NOTE; Mitochondria are prominent in organs where
there’s a lot of metabolic activity e.g. kidney nephron,
muscle fibres, neurone axons, tail of the sperm and root
hairs.
Adaptations of the mitochondria to its function
(energy production)
1. The double membranes separate the mitochondrion
from interference by processes in the cytoplasm
2. small size gives a large surface are to volume ratio for
the rapid uptake / release of materials
3. matrix contains enzymes of the Krebs cycle
4. inner membrane invaginates (in-folds) forms cristae
to increase the surface area for electron transport
chain (oxidative phosphorylation)
5. inner membrane has cristae with oxysomes that
contain ATP synthetase (ATPase) on stalked
Page 194particles
of 447 that make ATP
6. narrow intermembrane space (gap between inner and outer membranes) enables pH / H+ / proton
concentration gradient to be rapidly established / steeper chemiosmosis therefore more efficient /
chemiosmosis can occur
7. inner membrane contains molecules for electron transport pathway
8. DNA is present to act as genetic material for synthesis of some protein / control of metabolism
9. Presence of many ribosomes for protein synthesis to reduce on importation of some proteins
10. Phosphate used in glycolysis thru protein carriers (not clear)
Comparison between the structure of the nucleus and the mitochondria

a. Similarities
- Both contain DNA
- Both contain RNA
- Both contain ribosomes
- Both contain enzymes
- Both are bound by a double membrane
b. Differences
Nucleus Mitochondrion
1. Liner DNA 1. Circular DNA
2. DNA contained in chromosomes 2. DNA not contained in chromosome
3. Larger 80S ribosomes 3. Smaller 70S
4. Membrane has pores 4. Membrane has no pores
5. Inner membrane not folded 5. Inner membrane folded to form cristae
6. Oval or spherical 6. Sausage shaped, spiral or cup-shaped
7. Outer membrane continuous with endoplasmic 7. Outer membrane not continuous with any
reticulum organelle
8. Ribosomes may be attached to outer membrane 8. Ribosomes not attached on outer membrane
Chloroplast
Chloroplasts are members of a group of organelles known as plastids. Plastids normally contain pigments
such as chlorophylls and carotenoids and bound by 2 membranes. They develop from small bodies called
protoplastids found in the meristematic regions. There are mainly two types of plastids and they are both
found in plant cells.
The leucoplasts are colourless and are found in plant parts which are not exposed to sunlight; these parts
include roots and underground stems. They are the food storage organelles. There are three types of
leucoplasts;
1. In the amyloplasts, sugar is converted into starch
2. In the elaioplasts, there’s synthesis and storage of lipids
3. In the aleuroplast, there’s synthesis and storage of proteins
Chromoplasts are coloured pigments containing non-photosynthetic pigments, common in fruits, carrot root
tissue and in flower petals
Structure
- Chloroplasts are biconvex in shape, 4-10µm in diameter, 2-3µm thick.
- They are bound by a double unit membrane, like chloroplasts, but in addition chloroplasts have a third
membrane called the thylakoid membrane. This is folded into thin vesicles (the thylakoids), enclosing
small spaces called the thylakoid lumen (lamellae). The thylakoid vesicles are often layered in stacks
called grana, which contain photosynthetic pigments. The thylakoid membrane contains the same ATP
synthase particles found in mitochondria.
Page 195 of 447

- The interior of the chloroplast is divided into the grana which are surrounded by an aqueous matrix called
stroma, into which the lamella is suspended.
- Chloroplasts contain DNA, tRNA and ribosomes, and they often store products of photosynthesis as
starch grains and lipid droplets.
Fig 4.5 a & b pg 56 Toole. Adaptations of chloroplasts to their function
1. Chloroplasts of flowering plants have a biconvex
shape which increases the surface area for the
exposure of the photosynthesis pigments.
2. It has a double membrane with an outer
membrane (surface) membrane which prevents
the photosynthetic reactions from mixing with
those in the cell cytoplasm.
3. The surface membrane is permeable to gases like
carbon dioxide which is a raw material for
photosynthesis.
4. The internal membrane also contains electron transport systems which synthesize ATP.
5. It contains chlorophyll for trapping sunlight energy.
6. It has thylakoids that increase the surface area for holding chlorophyll molecules.
7. The thylakoid granum is connected by intergrana membranes thus maintaining the thylakoids and
chlorophyll stationary in position.
8. The stroma of the chloroplast has DNA and ribosomes for protein synthesis.
9. The stroma contains the necessary enzymes for protein synthesis.
Comparison of chloroplast and mitochondrion
Similarities: Differences
Both: Chloroplast Mitochondrion
a. are enclosed by double Site of photosynthesis Site of respiration
membrane Contains thylakoid membranes Lacks thylakoid membranes
b. contain DNA Contains photosynthetic pigments Lacks photosynthetic pigments
c. contain 70S ribosomes that absorb light
d. have electron transport chain There is light generated ATP ATP production by oxidation of
e. produce ATP by production organic molecules
chemiosmosis H+ gradient across thylakoid H+ gradient across inner
f. contain ATP synthase membrane membrane
/ATPase Cristae absent Cristae present
Larger size Smaller size
Microvilli
Microvilli are tiny finger-like extensions of the cell surface membrane or certain animal cells, such as those of
the intestinal epithelium. Microvilli are massed together forming a brush bonder at the edge of cell bearing
them.
Each microvillus contains bundles of actin and myosin filaments, causing the microvilli to contract.
Microvilli provide a large surface area for absorption and digestion.
Page 196 of 447

Ribosomes
Ribosomes are tiny organelles with a diameter of about 20nm, made

of ribosomal RNA and protein. They carry out protein synthesis.
Each ribosome consists of two sub-units, one large and one small.
There are two types of ribosomes: 70s are found in prokaryotes,
mitochondria, chloroplasts and are the slightly larger 80s ribosomes
occur in the cytoplasm of eukaryotes.

When several ribosomes occur along a
common strand of mRNA, the whole
structure is known as a polysome or a
polyribosome.
Bound and free ribosomes are
structurally identical, and ribosomes can
alternate between the two roles.
Ribosomes lying free in the cytoplasm are the site of synthesis of proteins that are retained within the cells,
e.g. enzymes that catalyse the first steps of sugar breakdown and haemoglobin in young red blood cells.
Ribosomes bound to endoplasmic reticulum produce proteins that are subsequently secreted outside the cell
e.g. proteins inserted into membranes for packaging within certain organelles (lysosomes) or for export from
the cell.
Cells that specialise in protein synthesis have a high proportion of bound proteins and a prominent nuclei e.g.
cells of the pancreas that secrete digestive enzymes.
Endoplasmic reticulum
Endoplasmic reticulum (ER) consists of a network of folded membranes forming sheets, tubes or flattened
sacs in the cytoplasm. It forms a cytoplasmic skeleton called a cytoskeleton. The tubules and sacs are called
cisternae.
ER is flexible and mobile since it occupies much of
the cytoplasm of many cells, including those in which
streaming movements of the cytoplasm occur. It
therefore forms an intracellular transport system and
a cytoplasmic skeleton of the cell.
Functions of ER
1. Offer increased surface area for cellular reactions.
2. Form part of the cell’s skeletal framework
3. Transporting proteins and carbohydrates to other
organelles like lysosomes, Golgi apparatus, and
plasma membrane.
4. Form the nuclear membrane during cell division.
Rough ER (RER) consists of an interconnected system of membrane-bound flattened sacs. It is continuous

with the outer membrane of the nucleus and has many minute globular bodies called ribosomes. The RER
isolates and transports proteins manufactured by ribosomes, mainly secreted proteins for export i.e. those that
the cell does not need but are needed elsewhere e.g. enzymes and hormones. RER is abundant in cells which
are rapidly growing or secretory cells e.g. pancreatic cells.
Page 197 of 447

Functions of RER
1. Rough ER is concerned with the transport of proteins which are made by ribosomes on its surface
2. The protein is extensively modified as it passes through the cisternae e.g. converting it into a glycoprotein.
3. Checks the quality of proteins formed, especially correct ordering and structure.
Smooth ER (SER) is a system of interconnected tubules and it lacks ribosomes. SER is abundant in cells
involved in lipid and steroid hormone synthesis e.g. cells in the testes and ovaries or cells involved in
detoxification e.g. liver cells.
Functions of SER
1. Enzymes of SER are important in the
synthesis of lipids including oils,
phospholipids and steroids e.g. lipids from
fatty acids and glycerol in the epithelium of
the intestine. Testes and ovaries are rich in
SER because they secrete steroid hormones
2. Other enzymes of SER detoxify drugs,
alcohol and poisons, especially in the liver
3. SER becomes modified to form the
sarcoplasmic reticulum surrounding the
muscle myofibrils
4. SER attaches receptors to cell membrane
proteins in plant cells
5. Synthesis and repair of membranes by producing cholesterol and phospholipids
6. For metabolism of glycogen in the liver e.g. glucose-6-phosphatase enzyme in SER converts glucose-6-
phosphate to glucose.
7. Contains enzymes that detoxicate lipid soluble drugs, alcohol and metabolic wastes from the liver
8. The SER also stores calcium ions
9. Pathway for the transport of materials through the cell
Adaptations of ER to its function
1. The interconnected network provides the cell with skeletal framework.
2. Forming an extensive network increases the surface area for metabolic reactions e.g. protein synthesis at
RER.
3. The endoplasmic reticulum membrane compartmentalizes the cytoplasm (isolates lumen from cytosol),
which;
 Enables transporting soluble and well packaged substances to their specific destinations.
 Prevents interference of different metabolic processes taking place in the cell at the same time.
4. Contains a variety of enzymes for performing diver roles in cell metabolism.
5. The SER is modified into sarcoplasmic reticulum storage and release of calcium ions.
6. The membrane has a variety of proteins that offer unique properties including signal reception.
7. The RER membrane has sites for attachment of many ribosomes for protein synthesis
Lysosomes (suicide bag)

These are tiny membrane bound organelles that contain hydrolytic enzymes. Lysosomes occur only in animal
cells and there are primary and secondary lysosomes. The primary lysosomes are tiny vesicles from the Golgi
body while the larger secondary lysosomes are formed when the primary lysosomes fuse with small vacuoles
of the animal cells.
They are usually absent in plants except insectivorous plants e.g. Nepenthes, Dionaea.
Page 198 of 447

Lysosomal enzymes work best in the acidic environment found in lysosomes. If a lysosome breaks open or
leaks its contents, the released enzymes are not very active because the cytosol has a neutral pH. However,
excessive leakage from a large number of lysosomes can destroy a cell by autodigestion.
Particles taken in by cells or made in the cell are digested on the lysosome. Lysosomes contain enzymes e.g.
lipase which hydrolyses lipids to fatty acids and glycerol, carbohydrases which hydrolyse carbohydrates to
simple sugars, peptidases which hydrolyse peptides to amino acids, RNA-ase, DNA-ase, and others.
Fig 5.32 pg 155 Soper OR Fig

Functions 7.20 pg 162 Clegg
1. Digestion of materials taken in by endocytosis. The digestion
may be for nutrition or defensive purposes. After its action, the
products of digestion are absorbed, assimilated and the vacuole
migrates to the cell surface membrane and releases its contents.
2. Release of enzymes outside the cell. This occurs during
replacement of cartilage by bone during development or bone
remodeling after injury.
3. On the sperm head is an organelle called acrosome which is
actually a lysosome, it contains enzymes that enable the sperm to
penetrate the ova.
4. Autolysis. This is the process by which the lysosome releases its
contents into the cell i.e. a suicide bag. Autolysis occurs during
reabsorption of the tail of a tadpole and returning the nucleus to
its normal size after delivery
5. Autophagy. This is the process by which unwanted structures within the cell are engulfed and digested
within the lysosomes.
Page 199 of 447

Golgi apparatus/body Fig 4.10 pg 60 Toole OR Fig 2.10 pg 19 Roberts OR

It is called a dictyosome in plants. Fig 2.19 pg 162 Clegg
Golgi consist of a stack of flattened, membrane
bound sacs called cisternae, together with a
system of associated vesicles called Golgi
vesicles. They are abundant in secretory cells
and in rapidly dividing cells e.g. pancreatic
cells, goblet cells, cells in testes and ovaries.
NOTE: at one end of the stack, new cisternae
are constantly being formed by vesicles from
the SER.
Mode of action of the Golgi
i. Proteins made at RER have, as part of
their amino acid sequence, a signal that
directs them where to go:
ii. Proteins arriving at cis-Golgi but
having RER retention signal (were
wrongly sent), are repackaged into
vesicles then returned to RER.
iii. Soluble or properly folded
macromolecules (proteins, lipids and
polysaccharides) from RER enter cis-
Golgi network via transport vesicles
iv. Within cis-cisternae, macromolecules
are partly modified i.e. carbohydrates
are added to proteins (glycosylation),
phosphate is added to protein
(phosphorylation) e.t.c.
v. After partial modification, coated
vesicles bud (pinch) off the swollen
ends of cis-cisternae and fuse with ends
of medial cisternae.
From trans-cisternae, the transformed macromolecules exit the Golgi and are sorted into different transport
vesicles destined for lysosomes, plasma membrane or storage vesicles for secretion.
Within medial-cisternae, different enzymes further transform macromolecules differently, depending on their
structures and destination i.e. some are modified for secretion, others for the membrane, and some for
lysosomes.
After further modification within the medial-cisternae, coated vesicles bud (pinch) off the swollen ends of the
medial-cisternae and fuse with the ends of trans-cisternae for further transformation
a) Vesicles containing hydrolase enzymes fuse with membranes of growing lysosomes so that the contents of
both structures fuse.
b) Vesicles containing hormones e.g. insulin remain until when signaled by the cell, the vesicles then fuse
with plasma membrane to release (secrete) the hormone outside the cell by exocytosis.
c) Vesicles containing membrane proteins fuse with the cell membrane and some of the modified proteins
become part of the cell membrane e.g. protein receptors.
Page 200 of 447

Functions neurotransmitters, hormones, mucin, zymogen

1. The function of the Golgi apparatus is to e.g. pepsinogen, etc.
transport and chemically modify the materials 10. Fusion of Golgi vesicles with cell membrane
contained within it. maintains the membrane which is used to form
2. Golgi apparatus forms lysosomes containing phagocytic vacuoles and Pinocytic vesicles
hydrolytic enzymes 11. They form lysosomes if or when they contain
3. Golgi is involved in the formation of digestive enzymes
peroxisomes Adaptations of the Golgi
4. Golgi apparatus in the leaf glands of some a) Cisternae are enclosed by permeable
insectivorous plants e.g. sundews secrete a membranes, which isolate the inside cavity
sticky slime and enzymes which trap and from cytosol for efficient functioning.
digest insects b) Tubular structure enables transportation of
5. The membranes of the vesicles from the Golgi soluble protein and lipids from the
apparatus for the first layer of the new cell endoplasmic reticulum for modification.
wall that develops between the two daughter c) Variety of enzyme systems for modifying
cells as they divide. proteins by adding carbohydrates and
6. Golgi is sometimes involved in the secretion phosphate by the process of glycosylation and
of carbohydrates e.g. polysaccharides are phosphorylation respectively.
attached to a protein to form proteoglycans d) Many cisternae increase the surface area for
present in the extracellular matrix of the modifying synthesised macromolecules.
animal cell e) There are many compartments at the cis,
7. To form carbohydrates. located at the beginning of the Golgi apparatus
8. Transport of lipid molecules around the cell. to facilitate passage of proteins through the
9. Secretory vesicles produced by Golgi contain Golgi apparatus
a variety of important substances e.g.
Page 201 of 447

Plant cell wall

It has fibres of cellulose that contain several units of glucose cellulose fibres. Each fibre has several
microfibrils. These are strands of cellulose in a crystalline state and these cellulose molecules are held in a
matrix by hydrogen bonds. The matrix consists of pectic acid, calcium and magnesium pectate and
hemicelluloses. Hemicelluloses are polymers of various pectose and hexose sugars. Pectic substances also
make up most of the middle lamella. The middle lamella binds adjust plant cells to one another. The cell wall
is interrupted by pores which carry strands of cytoplasm called plasmodesmata. This cytoplasm facilitates the
movement of substances between adjacent cells as well as the deposition of cellulose during the thickening of
secondary cell wall.
The young plant cells are made up of the primary wall. These cells are usually found in the growing regions of
plants i.e. the meristems e.g. the shoot and root apex. The primary wall is thin, plastic and it allows the cell to
grow. Inside the primary wall develops the secondary wall which is thicker due to more cellulose fibres being
laid down as the cell grows. The cellulose fibres are closely packed and are laid down in an orderly way.
The secondary wall is impregnated with lignin which is an alcohol polymer and this lignin gives strength to
cells of the xylem and the sclerechyma. The secondary cell wall tends to be rigid and tangile. This
characteristic brings about the death of the cell because the essential nutrients from the cytoplasm can no
longer move across the pores through the cell wall.
In the cork tissue, the tissue between primary and secondary wall, is a fatty substance called suberin. The cork
(phloem) cells are formed by the cork cambium (phellogen) prevents the passage of water and gases into and
out of the woody plants.
The outer walls of the leaves and young stems are made up of cells called epidermal cells. These cells are
covered by a waxy polymer called cutin. Which is secreted by the cytoplasm and it passes through the primary
Page 202 of 447

wall and the middle lamella to appear on the epidermis. Cutin provides a water proof covering to the aerial
surface of the plant.
Functions
1. Mechanical strength and skeletal support is provided for individual cells and for the plant as a whole
2. Cell walls are fairly rigid and resistant to expansion and therefore allow development of turgidity when
water enters by osmosis
3. Orientation of cellulose microfibrils limits and controls cell growth and shape because the cell’s ability to
stretch is determined by their arrangement
4. The system of interconnected cell wall (apoplast) is a major pathway of movement for water and
dissolved salts
5. Cell walls develop a coating of waxy cutin, the cuticle, on exposed epidermal surfaces reducing water loss
and risk of infection
6. The walls of xylem vessels and sieve tubes are adapted for long distance translocation of materials
through the cells
7. The cell wall of root endodermal cells are impregnated with suberin that forms a barrier to water
movement
8. Some cells walls are modified as food reservoirs as in storage of hemicelluloses in some seeds.
9. The cell walls of transfer cells develop an increased surface area and the consequent increase in surface
area of the cell surface membrane increases the efficiency of transfer by active transport
10.
Adaptation of the cell wall to its function
a) The cell wall has cellulose polymers associate through very many H-bonds whose cumulative bonding
energy provides high tensile strength of the cell wall for providing support and preventing rupturing
b) The cell wall has relatively thick multiple wall layers provide mechanical support
c) The cell wall has secondary walls which may be cutinized / suberinised for preventing water loss
d) The variety of functional proteins like oxidative enzymes (peroxidases), hydrolytic enzymes (pectinases,
cellulases) enable performing several functions like protection against pathogens, cell expansion, cell
wall maturation
e) The cell wall has extremely rigid secondary walls that provide compression strength
f) Deposition of cellulose fibrils in alternating layers enables some degree of flexibility
g) The cell wall is semi-permeable in nature to allows exchange of water, dissolved salts and small protein
molecules
Comparison between pant cell wall and plasma membrane
(Group assignment ) Cell wall Plasma membrane
Number of main layers / regions Number of main layers / regions
varies (2 or 3) constant
Skeleton mainly made of Skeleton mainly made of
carbohydrates / polysaccharides phospholipids
More permeable to molecules Less permeable to molecules
Lacks transmembrane proteins Transmembrane proteins present
Plasmodesmata present Plasmodesmata absent
May be lignified and suberinised Lacks lignification and suberinisation
Has middle lamella Lacks middle lamella
Secondary thickening occurs Lacks secondary thickening
Page 203 of 447

Flagella and cilia Centrioles

These are organelles that project from the surface of Centrioles are found in animal cells. The centrioles are
cells but are connected to a basal body just below located outside the nucleus in a material of poorly
the membrane. defined structure called the centrosome. Centrioles are
Flagella occur singly or in small numbers, whereas paired cylinders of about 0.3-0.5µm long and 0.24µm
cilia occur in large numbers on larger cells, and are in diameter which are held at right angle to each other.
typically shorter than flagella. Each cylinder consists of nine triplets of microtubules
Flagella and cilia are enclosed by a plasma in a ‘9+0’ arrangement.
membrane. Internally, they consist of microtubules Functions
arranged in an outer ring of nine pairs surrounding a. Centrioles act as organizers of spindle fibres and
two central pairs (‘9 + 2’ arrangement). are involved in the separation of chromosomes or
Flagella and cilia move by means of sliding chromatids during cell division
movements of one member of a microtubule pair b. In some cells, centrioles divide to produce basal
relative to the other. bodies from which flagella and cilia develop.
Fig 4.15 b pg 63 Toole OR Fig 2.16 pg 23 Roberts Microfilaments
These are long fibres of about 6-7nm in diameter. They
are made up of two actin protein strands intertwined
together.
Functions
1. Component of cytoskeleton; give support and
maintain cell shape
2. Actin and myosin filaments are needed for muscle
contraction
3. Constriction of filaments causes cleavage and
furrow formation in cytokinesis of animal cells
Functions 4. They play a role in cellular movements e.g.
1. In moving cells e.g. sperm, chlamydominas spp cytoplasmic streaming, cell motility, involved in
e.t.c. phagocytosis and pinocytosis.
2. To propel fluids across cells of ciliated cells that
move mucus along the brachial lining
3. To acquire food e.g. the feeding current
generated by paramecium in its oral groove
4. To sense the environment e.g. sensory hair cells
Microbodies
These are small spherical membrane-bound bodies of 0.5-1.5µm in diameter. The two common types of
microbodies are peroxisomes and glyoxysomes.
Peroxisomes contain oxidative enzymes e.g. catalase. Glyoxysomes are found in the fat tissues of germinating
seedling such as those of peanut plants. Glyoxysomes contain enzymes that catalyse the conversion of fats and
oils into sugars until the germinating seedlings can produce their own sugars through photosynthesis.
Middle lamella
This is a membrane that holds adjacent plant cells together. It is called the basement membrane in animal cells
to form a tissue. The middle lamella is made up of calcium pectate.
Vacuoles
Vacuoles are fluid cavities bound by a single membrane. They are formed either by infolding or pinching off
of part of the cell membrane, or by enlargement of a vesicle cut off by the Golgi apparatus.
Young plant cells usually contain several small vacuoles which, in the mature cell, have united to form a large
permanent, central vacuole. The plant vacuole is filled with a liquid known as a cell sap, an aqueous solution
Page 204 of 447

of dissolved food materials, ions, waste products and pigments. The membrane around this type of vacuole is
known as the tonoplast.
The vacuoles of animal cells are usually very small and less permanent, called vesicles. They may contain
engulfed solids or liquids.
Functions of vacuoles
1. In plants, the vacuole functions to store food substances e.g. sugars
2. The concentrated cell sap causes water to enter by osmosis and the cell becomes turgid. Turgidity brings
about support in herbaceous plants and plays a role in enlargement and growth of young plant cells
3. Vacuoles of come plant cells e.g. petals of flowers; contain coloured pigments to attract insects for
pollination.
4. Vacuoles in leaves accumulate waste products e.g. tannins and are removed when the leaves fall
5. Food vacuoles formed by endocytosis enable bulk intake of large food particles
6. Contractile vacuoles in unicellular organisms e.g. amoeba and paramecium, regulate water content in the
cell.
Protoplasm
This is the living material that comprises of the cytoplasm and the nucleoplasm. The cytoplasm is the
protoplasm outside the nucleus and it has all other organelles e.g. mitochondria, RER and other cell contents
e.g. glycogen in animal cells, liquid droplets, starch granules in plant cells, salts e.g. NaCl.
The nucleoplasm is the cytoplasm bound by the nuclear membrane. Chromatins are found within the
nucleoplasm and later form the chromosomes.
The protoplasm is a colloidal system i.e. a solution with suspended particles in it e.g. cell organelles and food
nutrients
Microtubules
These are straight unbranched hollow cylinders, 25nm wide and usually short in strength. They are made of
protein and constantly being built up and broken down.
Functions
1. They are involved in the movement of cytoplasmic components within the cell.
2. Microtubules appear to direct the passage of Golgi vesicles to deposition sites.
3. Along with the microfilaments, the microtubules constitute the cytoskeleton, which controls the shape and
movement of the cell
4. They are used in cell wall formation
5. They also occur in basal bodies, centrioles, in the spindle, in cilia and flagella
Distribution and function of membranes of cells
a) Membranes of cells is not limited only to the cell membrane (plasma membrane), which forms the cell
boundary plus its various modifications, it also includes all other membranes enclosing some organelles
and some cytoplasmic inclusions within cells.
b) Plasma membrane: Forms a protective barrier between the cell inside and outside. Determines cell shape
and provides cell stability. Selectively regulates entry and exit of substances.
c) Nuclear envelope: Separate nuclear contents from cytoplasm hence limits DNA within the nucleoplasm
but allows exit of RNA. Controls flow of information to nucleus and DNA that are carried by the
macromolecules.
d) Outer mitochondrial membrane: Allows entry of ATP, NADH and from glycolysis
e) Inner mitochondrial membrane: Contains electron carriers in electron transport chain
f) Rough Endoplasmic Reticulum: Intracellular transport and sites for ribosome attachment
g) Smooth Endoplasmic Reticulum: intracellular transport
h) Outer chloroplast membrane: Allows photosynthetic products out and substrates in
Page 205 of 447

i) Thylakoid membranes of chloroplasts: Store photosynthetic pigments e.g. chlorophyll. Contains electron
carriers
j) Golgi complex membrane. Storage of glycoprotein. Synthesis of polysaccharides e.g. cellulose in plants
k) Lysosomes. Isolates autolytic enzymes from unnecessary digestion of cell components
l) Tonoplast. Limits cell sap within the vacuole
m) Membranes surrounding vesicles: Limit the contents of the vesicles within until when ready for exit e.g.
calcium ions and neurotransmitters in neurones, undigested materials in phagocytic vesicles, etc.
n) Neurilemma of neurones. Contains protein pumps for Na+ and K+ which bring about impulse propagation
o) Myelin sheath membrane. Insulates nerve fibre to increase transmission speed.
Advantages of having membrane-bound organelles (importance of possession of numerous internal
membranes)
1. Internal membranes maintain pH and temperature of internal membranes for reactions to proceed
optimally
2. Increases proportion of membrane area to cell volume, increasing surface area over which metabolic
reactions occur, for metabolic pathways with membrane-embedded enzymes.
3. Internal membranes partition the cell into compartments, providing different local environments for
specific metabolic pathways so that incompatible processes can proceed simultaneously inside the same
cell
4. Inner membranes provide attachment sites for specific enzymes, metabolites and molecules, regulating
the occurrence of specific metabolic processes.
5. Enzymes and metabolites for particular metabolic pathways are enclosed within organelles, causing close
proximity of products of one reaction to the next enzyme in the sequence, thereby increasing the rate of
metabolic reactions
6. Internal membranes regulate the entry of metabolites into the organelle, controlling the rate of metabolic
activity
7. Potentially harmful metabolites and enzymes are isolated inside organelles, preventing damage to the rest
of the cell, such as lytic enzymes in lysosomes.
8. Internal membranes provide a supporting cytoskeleton to the cell, and serve as an intracellular transport
system
9. Internal membranes protect the genetic material (DNA) from digestion and chemical alternation,
preventing harmful mutations
10. Internal membranes maintain optimal conditions in specific organelles for specific metabolic pathways to
proceed optimally
Page 206 of 447

HISTOLOGY
Histology is the study of tissue structure, largely by various methods of staining and microscopy.
A tisssue is a group of cells of similar apearance and a common function. Broadly tissues consist of cells
physically linked and associated intracellular substances that is specialised for particular function.
There are both plant and animal tissues to be looked at in this study;
HISTOLOGY OF PLANTS
Plant tissues can be divided into;
a) Meristems, these include apical meristems, lateral meristems and intercalary meristems.
b) Permanent tissues, which are divided into two major groups;
i. Ground tissues, which include; ii.Vascular tissues, they include;
 Parenchyma tissues  Xylem
 Collenchyma tissue  Phloem
 Sclerenchyma tissue
MERISTEMS
A plant meristem is a group of cells which retain the ability to divide by mitosis. These are three types of
meristems namely; apical, lateral & intercalary meristems.
a) Apical meristems
Are located at the growing shoot and root apex and are responsible for primary growth
b) Lateral meristems (cambium)
Occur as cylinders into the older parts of the plants are responsible for secondary growth of
dicotyledonous plants
c) Intercalary meristems
These meristems occur at the nodes of the plants
Functions of meristematic cells

Meristematic cells retain the ability to divide by mitosis to produce new cells. The cells elongate and
differentiate to form specialised cells to carry out specific functions. Some examples are growth, reproduction
and replacement of old and damaged cells.
i.Apical shoot and root meristems produce new cells for growth of shoot and root
ii.Vascular cambium produces new cells to increase the diameter of stems and roots during secondary
growth
iii.Cork cambium (phellogen) produces the outer cork layer (phellem) which consists of suberized cells.
The cork layer reduces evaporation of water from the plant and protects against entry of pathogens.
iv.The intercalary meristems allow growth and increase in length in regions other than the tip
PERMANENT TISSUES
Parenchyma
Parenchyma tissue consist of living cells. They are usually isodiametric or elongated cells. However, their
shape may be distorted by pressure from adjacent cells.
Parenchyma cells have thin cell walls containing cellulose, hemicellulose and pectin. There are no secondary
walls. The walls are permeable to water and permit the passage of solutes.
The cells have a large central vacuole with a nucleus and a thin layer of cytoplasm pushed to the membrane.
Page 207 of 447

Transverse section of parenchyma cells
(Roberts fig 3.8A page 40 OR Soper fig 6.2a page 169)
Functions of parenchyma tissue

i. They are unspecialized tissues which form major component of tissue of stems and roots especially in
herbaceous plants.
ii. When the cells are turgid and tightly packed, they provide support for herbaceous plants
iii. Some parenchyma cells, like mesophyll cells, contain chloroplasts and carry out photosynthesis
iv. They store food substances such as starch and malic acid that are stored temporarily in the vacuoles of
CAM parenchyma cells
v. The parenchyma cells in flowers and pericarps contain chromoplasts to attract pollinating agents and
dispersal agents of seeds and fruits.
vi. The parenchyma tissues can be modified or differentiated to form specialised cells to carry out specific
functions. These include epidermis, mesophyll, endodermis, pericycle, aerechyma and secretory cells.
Modifications of the parenchyma tissue
Epidermis
This is a layer of flattened cells, one cells thick. The cells secrete cutin which forms a layer of waxy cuticle on
the outer surface of the epidermis.
Functions of the epidermis
i. It’s a protective layer to the inner tissue
ii. The waxy cuticle reduces water loss through evaporation from the plant and entrance of pathogens
iii. Stomatal pores in the epidermis allow gaseous exchange
iv. Epidermis is transparent and allows light to reach the mesophyll layers of leaves for photosynthesis.
 Endodermis
It consists of a single-celled ring which is a selective barrier between the outer cortex and the inner
pericycle tissues.
In roots the endodermis is impregnated with suberin to form a distinct casparian strip and prevent the
movement of water via the apoplast pathway. Non-suberized passage cells in the endodermis permit
lateral movement of water and mineral salts.
Page 208 of 447

 Pericycle  Aerechyma
This is made up of one to several layers Parenchyma tissues that surround large air spaces form
of parenchyma cells. reservoirs of oxygen and permit gaseous exchange in
Pericycle is found between the submerged parts. The large air spaces also provide
endodermis and central vascular tissues. buoyancy.
It can divide to produce the lateral roots  Secretory cells
and it is involved in the secondary Some parenchyma tissues are modified to form secretory
growth of roots. tissue for example nectary glands, hydathodes and resin
ducts.
 Palisade mesophyll  Spongy mesophyll
This is made up of column shaped cells It has isodiametric or irregular shaped cells.
and its found below the upper They are loosely packed with many intracellular space
epidermis. for gaseous exchange.
Function They have fewer chloroplasts than the palisade cells to
It contains many chloroplasts that carry out photosynthesis
enable a leaf to carry out
photosynthesis.
Collenchyma
These consist of living cells. Cells are Diagram showing a longitudinal section of the
polygonal shaped and they are elongated. collenchyma cells
They are closely packed together with very (Roberts fig 3.8B page 40 OR Soper fig 6.5b page 173)
small intracellular air spaces.
The cell walls are unevenly thickened at the
corners of cell walls (angular collenchyma).
Pits are present in the cell walls.
Collenchyma tissues are usually found in
herbaceous plants below the epidermis, midrib
of the leaves and leaf petioles.
Functions of collenchyma tissues
It acts as a supporting tissue to provide support
to herbaceous plants. With mechanical strength
and flexibility.
It allows the cell to expand and be stretched as
the young stem grows.
Some of the collenchyma cells contain
chloroplasts which carry out photosynthesis
Diagram showing a transverse section of collenchyma cells
Soper fig 6.5a page 173
Sclerenchyma
They are two types of sclerenchyma
Page 209 of 447

i. Sclerenchyma fibres
ii. Sclereids (stone cells)
Diagram showing transverse section through sclerenchyma cells (Soper fig 6.5a page 173)
Sclerenchyma fibres
These are polygonal shaped cells with tapering ends. Functions
Mature sclerenchyma cells have thick lignified
i. It acts as a supporting tissue. Collectively the tightly
secondary cell wall impermeable to water, solutes and
gases. (Acidified phylorogucinol can be used for packed sclerenchyma fibres with thick lignified
staining lignin red). The cells have protoplast with walls provide the plant with mechanical strength
narrow empty lumen. Pits are present in cell walls. and rigidity.
Sclerenchyma fibres are found below the epidermis of ii. The tapering ends of sclerenchyma fibres overlap
the stems or roots or around the vascular bundles and and interlock with one another, further increasing
in the midrib of the leaves. their combined strength
Sclereids (stone cells). Drawing of the structure of the sclerenchyma sclereids

(Roberts fig 3.8Cpage 40 OR Soper fig 6.6b page 175
They have different shapes
but are usually shorter than
sclerenchyma fibres. They
consist of dead cells with
thicker lignified walls. Simple
branching pit are present in
the walls.
A simple pit is formed in an It is a protective tissue which gives strength and support to the plant
area where lignin is not structures or organs.
deposited on the primary Drawing of the structure of a boardered pit (Soper fig 6.12 page 180)
wall. A boardered pit is
formed when lignin arches
over the area.
Sclereids are found singly or
in groups in stems, leaves,
fruits e.g. pears, guavas and
in the hard endocarp of
coconuts and seeds e.g. testa
of beans.
THE VASCULAR TISSUE
Vascular tissue consists of xylem and phloem which are specialised for the internal transport of substances in
the plant.
Page 210 of 447

THE XYLEM
The xylem tissue is made up of two conducting tissues or cells known as xylem vessels and xylem tracheids.
(Roberts fig 12.7A page 187 OR Soper fig 6.9B page 77)
A xylem vessel is formed from a chain of elongated cylindrical cells formed end-to-end. The horizontal end-
walls break down partially or completely during the course of development to allow open communication
between cells. During development of the xylem tissue, the cellulose side walls of this tissue become
impregnated or coated with lignin, a very hard layer which makes them impermeable to water, solutes and
gases hence leading to the death of the protoplasm of the tissue. This leads to the formation of a hollow tube
called xylem vessel hence more water can flow through the hollow continuous tube with less friction.
The lignified walls are perforated by numerous pits which allow horizontal movement of the lumen of water
in and out of the lumen of vessels. Most pits are bordered by a lignified rim. In conifers, the bordered pits
contain a valve-like plug called torus which controls the passage of water through the pits.
Lignification of the side walls (replacing cellulose with lignin) gives the xylem vessel extra mechanical
strength, which prevents its walls from curving in or collapsing during the passage of water under a high
tension. This lignification of the side walls occurs in four different patterns which include, annular
lignification, simple spiral lignification, multi spiral lignification and reticular lignification
The protoxylem is the first xylem vessel to develop, just behind the apical meristem in the shoot and root.
There is incomplete lignification in the walls of protoxylem vessels. Lignin is deposited in rings to form
annular vessels in spirals to form spiral vessels. These annular and spiral vessels can be stretched to provide
support for vessels during elongated and growth of the young stems and roots.
Different types of thickenings

As growth proceeds, new vessels are formed with
more extensive lignification, these are called
metaxylem. Metaxylem have bigger lumen and are
able to transport more water and mineral ions to the
older plant. The presence of pits in the metaxylem
vessels allow lateral movement of water and
mineral ions to the surrounding living cells (from
one tracheid to another). Secondary xylem is
formed from the activity of the vascular cambium
of dicotyledonous plants during secondary growth.
Although the vessels and tracheids of the xylem tissues are meant for transportation of mineral salts from
roots to leaves. They also provide mechanical strength and greatly offer support to the plant.
Page 211 of 447

The table below shows a comparison between the xylem and the tracheids.
Xylem vessels Tracheids

Cylindrical shape Are 5 to 6 sided in cross section
Have open ends at their sides Perforated end walls
Have non tapering (pointed) ends Have tapering ends
Offers less resistance towards Offer more resistance towards water

water passage passage
Adaptations of the xylem tissue for water transport

1. Xylem vessels have a narrow lumen which enables the upward movement of water from roots to leaves
due to the high capillarity
2. Xylem vessels and tracheids have lignified cell walls which enable upward movement of water through
them at a high tension as lignin prevents curving in of the walls due its tensile strength and it makes the
xylem water proof.
3. Xylem vessels and tracheids lack living protoplasmic contents which enable them to remain hollow so
that they allow water to move through with minimum resistance.
4. Xylem vessels and tracheids have partially or completely broken down to allow open communication of
one cell to another such that there’s free passage of water through them.
5. Xylem vessels and tracheids side walls are perforated by lateral pits to allow horizontal movement of
water in and out of the xylem tubes.
6. In some plants like conifers, the bordered pits of the tracheids have a plug-like torus which controls the
lateral passage of water through the pits
7. In the protoxylem, annular and spiral thickenings allow the stretching of the walls and further elongation
of the stem is possible. In the metaxylem, scalariform, reticulate and pitted thickenings provide
additional mechanical strength to older stems. Metaxylem vessels have bigger lumens than protoxylem to
transport more water.
Roberts fig 12.7b page 187
Page 212 of 447

THE PHLOEM
The phloem tissue consists of sieve tube elements, companion cells, parenchyma cells, fibres and schlereids.
Translocation of organic food molecules from the leaves where they are manufactured takes place through
sieve tubes of the phloem. The sieve tubes are long tubes formed from the fusion of end to end sieve tube
elements, where end walls break down to a greater or lesser extent to allow the passage of materials.
Roberts fig 12.5A page 293
Sieve tube elements are mature sieve tubes without a nucleus. When still young, the sieve tube cells contain a
nucleus and other organelles such as ribosomes, mitochondria, endoplasmic reticulum e.t.c.
At maturity, the cell organelles of the sieve tube elements including the nucleus degenerate, though some few
such as the mitochondria, plastids, endoplasmic reticulum e.t.c persist immediately adjacent to the cell walls
only.
During differentiation i.e. specialisation for a
particular function, the end walls of the adjacent
sieve tube elements get coated with cellulose, and
form a sieve plate that is perforated by sieve pores.
Through the sieve pores are cytoplasmic filaments or
trans-cellular strands which run across and are
continuous with all the sieve tube elements in the
tissue. The cytoplasm of these trans-cellular strands
is structurally very simple without organelles
because all the organelles degenerate during its
development. The side walls of the sieve tube
elements are impregnated or coated with a lot of
cellulose and pectic acid.
Alongside each sieve tube element is one or more companion cells made of thin cellulose walls, enclosing a
protoplast with a dense cytoplasm. The companion cells are the metabolically active cells of this tissue with a
prominent nucleus, numerous mitochondria, ribosomes and many other organelles. Therefore most important
processes which involve active metabolism are conducted within the companion cells and all the required
materials for these processes and all the required materials for these processes are passed via plasmodesmata
from the companion cells or to the sieve tube elements and vice versa. The sieve tube elements are therefore
living cells.
Adaptations of the phloem tissues for its function
1. It has sieve plates which are perforated to enable a continuous of food
2. The sieve tubes have large pits (plasmodesmata) for lateral movement of organic substances.
3. Companion cells have numerous mitochondria so as to produce large quantities of energy in form of
ATP needed for transport of food.
Page 213 of 447

4. Sieve tube elements have fewer cell organelles so as to provide more space for the flow of food
materials
5. Some sieve tube elements have cytoplasmic strands or filaments (trans-cellular strands) which allow
peristaltic movement of food through the sieve tubes
Phloem collenchyma cells have thick cellulose fibres for providing mechanical support to avoid collapse.
HISTOLOGY OF ANIMALS
Animals are multicellular and they need to have a combination of the individual cells so as to form tissues.
This enables the animal to function properly. Animals have a small surface area to volume ratio as compared
to unicellular organisms. Therefore, simple processes of diffusion, osmotic uptake of molecules, phagocytosis
e.t.c. are not adequate in their function and therefore, there’s need for the cells to combine together to form
tissues and even in the complex animals, tissues form organs so as to carry out the various functions over a
surface with a small surface area to volume ratio.
Animal tissues fall into four main categories;
1. Epithelial tissues 3. Muscle tissues
2. Connective tissues 4. Nervous tissues
EPITHELIAL TISSUE
This is the tissue found lining the free surface of animal internally and externally. If the tissue is internal, then
it’s called an endothelium. The endothelium is found lining all the internal body cavities and lumen. Such
activities include the mouth lining, trachea, blood vessels, tubules, oviducts e.t.c.
Epithelial cells are attached to the underlying tissue by a basement membrane, made of a network of white
wavy, non-elastic collage fibres.
Epithelial tissues are mainly protective and secretory. However, they are of a variety of forms and differ in
shape and number of layers and may perform different functions e.g. in the
Page 214 of 447

a. skin they are protective
b. lungs they are respiratory
c. gut they are secretory
d. kidney they are excretory
e. tongue they are sensory
f. gonads they are reproductive where they form the germinal epithelium
Characteristics of epithelial tissue cells
a. they are attached to the basement membrane which is produced by the cells themselves
b. the adjacent cells are joined by intracellular cement
c. there may be interconnecting bridges of cytoplasm within cells
d. they undergo rapid cell division to resist wearing away (abrasion)
e. they may build up large numbers of layers to resist wearing away e.g. in the epidermis of the skin
Epithelial tissues are classified according to the number of cell layers and the shape of the individual cells in
longitudinal section. They are usually classified into two categories;
a. Simple epithelium, this is usually one cell thick. It includes;
i. Squamous epithelium
ii. Columnar epithelium
iii. Ciliated epithelium
iv. Cuboidal epithelium
v. Pseudostratified epithelium
Simple squamous epithelium
This is the simplest type of epithelial tissue, sometimes called the pavement epithelium. It has the following
characteristics;
Surface view (Roberts fig 3.1
 The cells are very thin and contain little cytoplasm page 33)
 It consists of delicate cells usually less than 20mm thick.
 The cells are loosely packed
 The nucleus is centrally placed
 The cells are flat in nature and fixed on a basement
membrane.
 They have little intercellular substance (matrix) in which
the cells are embedded.
The above characteristics enable the epithelium to perform the following functions;
i. In the lumen or blood vessels, the tissue offers a smooth surface for the efficient passage of fluids.
ii. In the Bowman’s capsule and glomerulus, the tissue is permeable to fluids and there’s quick diffusion
of the fluids
iii. Between the two surfaces that slide over each e.g. between the ribs, the epithelium reduces friction
Simple columnar epithelium
The characteristics of the columnar epithelium include;
 Cells which are found at right angles to the basement membrane.

 The cells are tall and narrow in shape (elongated with a length much more than the width)
 The nuclei of the cells are at the base of the cells
Page 215 of 447

 Sometimes the cells have microvilli at their end and this makes the cells have a striated or divided
border
 The cells may at times have the mucus secreting cells (goblet cells) interspersed among them and the
tissue is known as glandular columnar epithelium.
 The micro villi increase the surface area of the cells which are in contact with the fluids
 The goblet cells produce mucus for the reducing friction and for protection against digestive enzymes
The columnar epithelium offers an extensive surface area and it is often found where there’s absorption of
materials in the form of gases and solutions e.g. lining the small intestines and stomach.
It is a component of the gall bladder and thyroid gland and it protects many kidney ducts.
Longitudinal view Diagram of a goblet cell

Soper fig 6.16a page 185 OR Roberts fig 3.1 page 33 Soper fig 6.17 page 185
Simple ciliated Columnar epithelium

The cells of the tissue have the same characteristic as those of the columnar epithelium except that they have
cilia instead of the micro villi and are associated with mucus secreting goblet cells.
The ciliated epithelium is found; Longitudinal section
i. Lining cavities in charge of movement or Roberts fig 3.1 page 33 OR Soper fig 6.17a page 185
those of in charge of tiny particles.
ii. In the respiratory tract (trachea) where
dust particles are trapped by mucus and
then waffled or moved by cilia into the
pharynx.
iii. In the oviduct where they transport the ova
iv. In the brain cavities and the spinal cord
where they keep the fluids in motion
Longitudinal view
Simple cuboidal epithelium
Roberts fig 3.1 page 33 OR Soper fig 6.15 page 184
This is the least specialised of all epithelia.
 The cells are cuboid in shape

 The nucleus is usually centrally positioned
 Some cells may have microvilli in the surface
to increase surface area
Such cells perform the following functions;
i. Excretion of waste products in the human kidney i.e. lining the Loop of Henle and collecting duct
Page 216 of 447

ii. It is also found in the salivary glands, sweat glands, collecting duct of the pancreas, mucus glands,
germinal epithelium of the ovary and the thyroid gland
NOTE: such epithelia is located in areas where there’s secretion of fluids
Pseudo-stratified epithelium Longitudinal view
Stratified means layers. This epithelium has the following Soper fig 6.18a page 186
characteristics;
 Its cells don’t reach the free surface uniformly
 The cells are of unequal size
 The nuclei of the epithelium appear at different levels
 The cells are usually columnar in shape and ciliated
This type of epithelia is found lining the cavities e.g. the
urinary bladder and nasal passages.
b. Compound epithelium, this is more than one cell thick. They include;
i. Transitional epithelium
ii. Stratified epithelium
iii. Glandular epithelium
Transitional epithelium
It consists of three to four layers of cells thick. Its cells can alter or change shape when put under pressure i.e.
they are intermediate between stratified and cuboidal (when relaxed) and stratified squamous (when
contracted or stretched)
This type of epithelium is found lining/covering organs Drawing of longitudinal view
that constantly experience pressure and distensions e.g.
in the urinary bladder, ureter and in the pelvic region
of the kidney.
Note: epithelial cells are frequently interspaced with
secretory cells and in this case the form a glandular
epithelium which secrete materials like mucus,
hormones, enzymes into cavities, spaces which it is
.
lining
Stratified epithelium
It has got layers/strata of cells with only one layer resting on the basement membrane. The cells continue to
divide by mitosis and push other layers of cells outwards which look thin and flattened. The epithelium is very
thick, consisting of more than four layers of cells.
Roberts fig 3.1 page 33 OR Soper fig 6.19a
In some stratified epithelium, the outer most cells page 186
(squamous) may be transformed into a dead horny
layer of keratin and in this case the epithelium is said
to be cornified which makes it tough and impervious to
water and gases.
Its superficial/surface cells wear off from the surface
while new cells are regenerated from the basement
membrane.
Location
Page 217 of 447

This type of epithelia is found covering regions/surfaces that are exposed to constant wear and tear. It’s the
thickest and toughest regions that experience friction most often, such regions include the sides of the feet and
palms, outer skin, linings of the oesophagus, anus and vagina.
Note: stratified cuboidal epithelium consists of more than one layer of cuboidal cells and it is found lining the
excretory ducts of exocrine glands such as salivary glands, sweat glands and the pancreas.
Glandular epithelium
Glandular cells are secretory in function. They produce Roberts fig 3.1 page 33
secretions such as sweat by the sweat glands, sebum by
the sebaceous glands, tears by the tear glands e.t.c.
Glandular epithelia can exist in two ways i.e. the epithelia
can bear a single layer of cells or it can bear an aggregate
or group of glandular cells in one place forming a
multicellular gland
An example of a single glandular epithelia is the goblet gland. If the gland discharges its secretions into a duct,
then it is described as an exocrine gland e.g. the pancreas. If there’s no duct in the gland, so that the secretions
are discharged directly into the blood stream, then it is called an endocrine gland (ductless gland). Most
hormone producing glands are endocrine glands while those producing enzymes and secretions are exocrine
glands.
Development of exocrine and endocrine glands
NOTE; the pancreas and the stomach are both exocrine and endocrine.
During formation of these glands, a patch of the epithelium is folded inwards forming an invagination which
becomes secretory hence developing into a gland which may be exocrine or endocrine.
Diagrams showing formation of endocrine and exocrine glands
Secretions produced by glandular cells are released in three different ways;
Page 218 of 447

a) Merocrine b) Apocrine c) Holocrine
In merocrine glands, the In apocrine glands, the In holocrine, the cell(s) breaks down to
secretions produced in cells are portion of cell’s distal release its secretions (secretory products)
passed through the cell membrane cytoplasm is lost as the and the cell is excluded from the epithelial
at the cell’s surface and there is no secretion is lost e.g. in the layer e.g. in the sebaceous glands which
loss of cytoplasm. This occurs in secretion of the mammary produce sebum for the softness of hair.
the simple goblet cells, sweat glands.
glands and vertebrate pancreas.
Sometimes a cell may secrete different materials each by a different method e.g. in a mammary glands, the
lipid is secreted by apocrine mechanism and the protein secretion is by merocrine or mucocyte. If the
secretion produced is clear/watery and contains enzymes, the gland is called a serocyte. If both secretions are
produced from within the same glands, then it is called a mixed gland.
Certain glandular epithelia contain so many densely packed secretory cells that are folded in various ways to
increase the surface area from which secretions takes place. Folding of glandular epithelia results in the
formation of glands whose sole function secretion. The different types of exocrine glands include;
Glandular epithelia can exist in two ways i.e. the epithelia can bear a single layer of cells or it can bear an
aggregate or group of glandular cells in one place forming a multicellular gland. An example of a single
glandular epithelia is the goblet gland. If the gland discharges its secretions into a duct, then it is described as
an exocrine gland e.g. the pancreas. If there’s no duct in the gland, so that the secretions are discharged
directly into the blood stream, then it is called an endocrine gland (ductless gland). Most hormone producing
glands are endocrine glands while those producing enzymes and secretions are exocrine glands.
NOTE; the pancreas and the stomach are both exocrine and endocrine.
Certain glandular epithelia contain so many densely packed secretory cells that are folded in various ways to
increase the surface area from which secretions takes place. Folding of glandular epithelia results in the
formation of glands whose sole function secretion using either tube-shaped or sac-shaped portions of the
epithelia for secretion. The different types of exocrine glands include;
Simple tubular gland e.g. Simple saccular gland Simple branched tubular Coiled tubular gland e.g.
crypts of Lieberkühn in the e.g. mucus glands in gland e.g. Brunner’s gland the sweat gland in the
ileum and the fundic the skin of the frog and and gastric glands skin of man
regions of the stomach other amphibians
Page 219 of 447

Compound tubular gland Simple branched Compound Compound saccular
e.g. salivary glands saccular gland e.g. the tubulosaccular e.g. gland e.g. mammary
oil-secreting sebaceous salivary glands glands and the part of
glands in mammalian the pancreas which
skin secrete digestive
enzymes
The secretory portions are black
CONNECTIVE TISSUE
These are tissues which bind other tissues together e.g. in the muscles. They include the adipose (fat) tissue,
collagen tissue, and skeletal tissue which is composed of bones and cartilage. They bind or support other
tissues of the body.
Main characteristics of connective tissue
a. They possess a considerable number of fibres in the intercellular substances
b. They have a large amount of intercellular substances
c. They are all developed from the mesoderm
Page 220 of 447

AREOLAR TISSUE Structure of areolar connective tissue
This tissue contains cells which are widely dispersed in (Roberts fig 3.5 page 36
the matrix and has fibres that are loosely woven in a
random manner. The matrix is a transparent semi-fluid
which consists of gelatinous glycoproteins containing
many types of cells and protein fibres. The fluid also
contains abundant mucin, hyalorinic acid and
chondroitic sulphate.
The areolar tissue is strong and tough due to two types

of fibres i.e.
a) The unbranched collagen (white) fibres which run
parallel to each other in a bundle
b) The branched elastic (yellow) fibres which form a
dense network in the matrix
The cells of the areolar tissue include;
i. Cells responsible for synthesis and maintenance of extracellular material e.g. the fibroblasts which
secrete both collagen and elastic fibres and are usually associated with the fibres they secrete
ii. Cells responsible for storage and metabolism of fats e.g. the fat cells
iii. Cells which defend and have immune functions e.g. phagocytes/phagocytic macrophages
iv. Amoeboid mast cells which are oval shaped and they secrete a matrix and anticoagulants such as heparin
and histamine
v. Plasma cells which produce antibodies that are important components in the body’s immune system
vi. Chromatophores which are present in some specialised areas of the skin and eye. The cells are densely
packed with melanin which gives the skin its characteristic colour and in the retina of the eye they prevent
back reflection of light
vii. Mesenchyme cells which act as reservoirs of undifferentiated cells in the tissue. They can be stimulated to
transform into any one of the above cells when need arises.
THE COLLAGEN (white fibrous) TISSUE

This consists of glycoprotein matrix which
contains mainly densely packed collage fibres.
Each collagen strand has three chains of
tropocollagen plaited together as in a rope. The
tissue is comparatively inelastic and has great
tensile strength.
They are located in areas with great tension such
as tendons, sclerotic and cornea of the eye, the
kidney capsule, in some ligaments and in the
perichondrium.
ELASTIC TISSUES (yellow elastic tissue)

This consists of glycoprotein matrix containing only elastic fibres and they combine strength with
elasticity.
They are found in ligaments, walls of arteries, lungs and in the neck chords.
ADIPOSE TISSUE (fatty tissue)
The matric contains densely packed fat cells. The tissue is important in storage and in the skin it insulates
the body against heat loss.
They are found around the heart and kidney and act as energy reservoirs and shock absorbers.
Page 221 of 447

P530 CELL PHYSIOLOGY By Nakapanka Jude Mayanja 0704716641
SKELETAL TISSUES
The vertebrate skeletal tissue is composed of cartilage only like in elasmobranch fishes e.g. dogfish and
sharks or both cartilage and bone covered by a muscular system. It also includes ligaments and tendons.
Bones form the larger component of the skeleton and cartilage is only found at joints.
Ligaments connect bones together and fit them in position while tendons connect muscles to bones.
All skeletal tissues consist of living cells surrounded by a non-living matrix secreted by the cells themselves.
FEATURES OF CARTILAGE
It consists of a firm translucent matrix of muco-polysaccharide called chondrin. The matrix is produced by
cells called chondroblasts which are distributed in the matrix in groups of single pairs or fours.
The cartilage is surrounded by a connective tissue made up of a dense network of fibres and cells called
perichodrion from where new chondroblasts are produced.
Cartilage is non vascularised i.e. not supplied with blood and therefore materials and nutrients diffuse in and
out via the matrix.
Each chondroblast occupy its own space called lacuna and this chondroblast enclosed in a lacuna is referred
to as a chondrocyte.
The matrix may be impregnated with collagen as in the vertebral disc and elastic fibres as in the ear and the
nose
Cartilage tissue is of three types; Hyaline, Yellow / elastic and White/ fibro-cartilage.
Hyaline cartilage
It’s the most common type of cartilage, its matrix is translucent and contains very fine collagenous fibres.
Location: nose, ends of long bones, ribs, trachea rings, foetal skeleton.
● It’s a solid flexible connective tissue composed NB: Chondroblasts that become embedded in the matrix are
of a translucent mucopolysaccharide matrix called chondrocytes.
(chondrin) in which are distributed cartilage cells Roberts, et.al Adv. Biol. Pg. 67 fig. 4.9
(chondroblasts) and many intercellular substances
like fibres.
●Each chondroblast lies in a small chamber called
lacuna surrounding by a capsule.
●Chondrin lacks direct blood supply except in the
Perichondrium; a tough fibrous membrane
surrounding cartilage.
●In some cases chondroblasts occur in cell
nests i.e. a pair or 2 pairs of cells encased by
one capsule.
Yellow / elastic cartilage White / fibro-cartilage
It’s more flexible than hyaline cartilage because Contains dense collagenous fibres embedded in matrix,
the matrix contains many elastic fibres in it absorbs shock and reduces friction between joints
addition to collagen fibres. and can withstand tension and pressure.
Location: frame work of Pinna (outer ear), Location: intervertebral discs, wedges in the knee joint,
epiglottis. insertion of tendon on patella.
THE MAIN FUNCTIONS OF CARTILAGE TISSUE

Reducing friction at the joints; supporting tracheal and bronchial tubes; acting as shock absorbers
between vertebrae; maintaining the shape and flexibility of ear and nose.
Main features of bone tissue
●It’s a rigid, tough, connective tissue composed mainly of calcified substance.
●Bones occur in a variety of shapes, have complex internal and external structures, are lightweight yet strong.
Page 222 of 447

●Several tissue types make up bone; including the mineralized bone tissue that gives it rigidity and brittleness,
collagen fibres that provide slight elasticity, marrow, endosteum, periosteum, nerves, blood vessels and
cartilage.
●All bones consist of living and dead cells embedded in the mineralized organic matrix called osteon that
makes up the bone tissue.
Description of bone structure
External structure Draw from: Soper (VS of femur head)
●A tough, fibrous, vascularised connective tissue
called periosteum encloses a compact outer layer.
●The epiphyses (ends of bone) are usually expanded
while the diaphysis / shaft (portion between two
epiphyses) is slightly narrow.
●Each epiphysis is covered by articular cartilage.
Internal structure
● Filling the interior of the bone is the cancellous or
spongy bone or trabecular bone tissue (an open cell
porous network), which is composed of a network of
rod- and plate-like elements that make room for blood
vessels and marrow.
●Fatty yellow marrow fills the medullary cavity in
the diaphysis while red marrow occurs in the spongy
bone at the epiphyses.
Molecular and Cellular structure

● Bone matrix (osteon) of compact bone is made up of organic substances mainly collagen fibres, and inorganic
materials like calcium, phosphorus and magnesium salts arranged in concentric layers called lamellae around
Haversian canals that contain blood vessels and nerves.
●Osteoblasts (immature bone forming cells) and Osteoclasts (bone breakdown cells) are located at the bone
surface.
●Osteocytes (mature bone forming cells) occupy lacunae (spaces in the lamellae) and bear many canaliculi
(fine protoplasmic extensions) that span across lacunae.
Osteocyte functions include, to varying degrees: formation of bone; matrix maintenance; and calcium
homeostasis, act as mechano-sensory receptors — regulating the bone's response to stress and mechanical load.
NB: Osteoclasts are closely related to macrophages
Draw from: Roberts, Pg. 68 fig. 4.10 D (Haversian systems in detail) OR Soper fig 6.26 (b) page 191
Page 223 of 447

HOW STRUTURE IS RELATED TO FUNCTION IN COMPACT BONE

● Bone matrix (osteon) is made up of organic substances mainly collagen fibres, and inorganic materials
which provide great tensile strength.
● There is continual remodeling which enables compact bone to respond to mechanical stress of varying loads
placed on it.
● Haversian canals contain blood vessels for efficient supply of nutrients to and draining of wastes from
bone cells.
●Osteocytes (mature bone forming cells) bear many canaliculi (fine protoplasmic extensions) that span across
lacunae to improve on material exchange between the bone cells.
●Bundles of collagen fibres originate from the bone surface and act as a firm base for tendon insertions.
MAIN FUNCTIONS OF BONES

A. Mechanical
●They protect internal organs, e.g. the skull protects the brain, the rib cage protects the heart and lungs
●They provide a frame work to keep the body supported.
●Bones, skeletal muscles, tendons, ligaments and joints function together to generate and transfer forces to
cause movement.
●Bones in the ear (ossicles) transmit vibrations that result in hearing.
B. Synthetic
●Bone marrow, located within the medullary cavity of long bones and interstices of cancellous bone, produces
blood cells in a process called haematopoiesis.
C. Metabolic
●Bones act as reserves of minerals important for the body, most notably calcium and phosphorus.
●Mineralized bone matrix stores important growth factors such as insulin-like growth factors, transforming
growth factor, etc.
●The yellow bone marrow acts as a storage reserve of fatty acids.
●Bone buffers the blood against excessive pH changes by absorbing or releasing alkaline salts.
●Bone tissues can also store heavy metals and other foreign elements, removing them from the blood
and reducing their effects on other tissues. These can later be gradually released for excretion
●Bone controls phosphate metabolism by releasing fibroblast growth factor – 23, which acts on
kidneys to reduce phosphate reabsorption. Bone cells also release a hormone called osteocalcin,
which contributes to the regulation of blood glucose and fat deposition. Osteocalcin increases both
the insulin secretion and sensitivity, in addition to boosting the number of insulin-producing cells
and reducing stores of fat
COMPARISON OF BONE AND CARTILAGE
Similarities
●Both bone and cartilage consist of living cells and extracellular matrix
●Cells reside in lacunae in both.
●Both are capable of growth.
●Both have collagen fibres
Differences
CHARACTERISTIC CARTILAGE BONE

Mechanical properties
●Stiff but flexible and incompressible ●Rigid and brittle.
Page 224 of 447

Innervation ●Lacks nerve stimulation ●Has nerve fibers
External covering ●Covered by perichondrium ●Covered by periosteum

●There is both appositional growth ●There is only appositional growth
Nature of growth (addition of new cells and matrix onto (addition of new cells and matrix onto the
the outside of the growing structure) outside of the growing structure)
and interstitial growth (cell division
and secretion of new matrix within an
established structure).
●Mature cartilage is relatively ● Internal remodelling (continual
permanent. destruction and renewal) occurs throughout
life.
Internal anatomy ●Cartilage is compact, no marrow ● Most mature bones have a marrow-filled
cavity.
● Occurs in 3 forms; hyaline, fibro- ●Occurs in 2 forms; compact and spongy
cartilage and elastic cartilage. bone.
●No lamellae, no Haversian canals. ●Organic and inorganic substances are
arranged in concentric layers called
lamellae around Haversian canals
●No Haversian systems. ●Compact bone has lamellae organized
into sets of Haversian systems.
●Matrix is gel-like and non-calcified ● Matrix is highly calcified.
● Chondrocytes are spherically- ●Osteocytes bear canaliculi (fine
shaped. protoplasmic extensions).
● Vascular (has blood vessels).
● Avascular (no blood vessels). ● Matrix impermeable to tissue fluid
● Matrix allows tissue fluid diffusion.
diffusion.
SAMPLE QUESTIONS
1. Figure one below shows the structure of the plasma membrane
(a) Name molecules A, B and E (03 marks)
Page 225 of 447

(b) Explain how the features of molecules of A cause them to form a layer in the membrane as seen in the
figure above (03 marks)
(c) State the functions of C and D (02 marks)
2. a) State the physiological importance of the following structural components of the plasma
membrane.
i. Proteins (03 marks)
ii. Carbohydrates (02 marks)
iii. Cholesterol (03 marks)
b) Explain why non polar (lipid soluble) molecules diffuse more rapidly through membranes than polar
(lipid insoluble) molecules. (02 marks)
3. (a) Describe the formation of Golgi bodies in the cell (06 marks)
(b) What are the functions of this organelle to the cell? (04 marks)
4. a) State the components of the cell theory? (04 marks)
(b) The figure below shows part of a membrane
(i) Name the structure labelled X (01 mark)

(ii) Explain briefly the role of X in the membrane when the surrounding temperature is low or at
moderately warm conditions (05 marks)
5. Mitochondria and chloroplasts are cell organelles that change energy from one form to another
a) What is meant by the term cell organelle? (02 marks)
b) Describe how the membranes of the two enable them to carry out their respective functions
i. Mitochondria (03 marks)
ii. Chloroplasts (03 marks)
6. The diagram below is drawn from an electron micrograph and shows the structure of parts of a cell
and the barrier between it and two of its neighbours.
Page 226 of 447

(a) (i) Identify the structures A and B (02 marks)

(ii) What role do structures A and B play in the life of the cell?
(b) Give two features of the cell which show that it is NOT a prokaryotic cell (02 marks)
(c) (i) Is it a plant cell or animal cell? (1 mark)
(ii) Give two reasons for your identification in (c) (i) above (2 marks)
Measure the length XY of the organelle C and calculate the actual length in µm. Show your working.
(2mks)
7. (a) Give three properties of the cell membrane (03 marks)
(b) Name two other membranes in the cell with similar properties as the cell membrane
(c) (i) What is the name given to membrane bound cell inclusions (01 marks)
ii) What purpose is served by membranes in such cell inclusions? (02 marks)
(d) (i) What constituents the cell’s protoplast? (01 marks)
ii) List three processes carried out by the cell’s protoplast? (03 marks)
8. a) State the physiological importance of the following structural components of the plasma
membrane.
i. Proteins (03 marks)
ii. Carbohydrates (02 marks)
iii. Cholesterol (03 marks)
b) Explain why non polar (lipid soluble) molecules diffuse more rapidly through membranes than polar
(lipid insoluble) molecules. (02 marks)
9. (a) Briefly describe the structure of the mitochondrion, without drawing (04 marks)
(b) How are mitochondria suited for their functions? (02 marks)
(c) State two structural differences between mitochondria and chloroplasts (02 marks)
(d) Mitochondria and chloroplasts are said to be semi-autonomous. Explain this statement. (02 marks)
10. (a) Distinguish between prokaryotic and eukaryotic cells (01 mark)
(b) State eight (8) structural and two (2) functional major differences between prokaryotic and eukaryotic
cells.
(i) Structural differences (08 marks)
(ii) Functional differences (02 marks)
11. The diagram below shows the structure of the cell surface membrane of an animal cell
(a) (i) State the name given to this model of the plasma membrane (1 Mark)
Page 227 of 447

(ii) Give two reasons why it is so called (2 Marks)

(b) (i) State where the cytoplasm can be found, on side M or N?
(ii) Give a reason for your answer in (i) above
(c) (i) Name the structures labelled A to E
(ii) Give the average value for the width of D
(d) (i) State one function each o structures C, D and E
12. Explain how the following are suited for their functions
a) nucleus (3 marks)
b) Chloroplast (3 marks)
c) Mitochondrion (3 marks)
d) Endoplasmic reticular (1 mark)
13. (a) How do the components of the plasma membrane ensure its fluidity? (06 marks)
(b) Relate the function (s) of each of the following to the fluidity and porosity of their membranes?
(i) Plasma membrane (06 marks)
(ii) Rough Endoplasmic Reticulum (05 marks)
(iii) Golgi apparatus (03 marks)
14. In an investigation pea plants were dug up from the field and washed thoroughly. The nodules were
removed surface sterilized and transferred aseptically to a sterile liquid culture medium. After two
weeks incubation, small samples of culture media were removed and added to trays each containing a
batch of pea plants growing in an inert medium. Each batch was watered regularly with a nutrient
solution containing a particular concentration of sodium nitrate for four weeks, at the end of four
weeks the mean number of root nodules and biomass were obtained from the investigation are shown
in the table below.
Nitrate concentration of nutrient Mean number of nodules Biomass of pea plants
solution (arbitrary units ) per plant /gm-2
0 82 140
1 70 200
2 68 230
3 40 350
3.5 20 400
4 10 460
5 0 440
5.5 0 400
6 0 350
(a) Represent the results of the table above graphically (08 marks)
(b) Explain the changes in mean number of nodules per plant and changes in the biomass of pea plants
with increasing nitrate concentration of nutrient solution (20 marks)
(c) How was accuracy of results to be obtained ensured throughout the experiment (05 marks)
(d) (i) on the graph draw a graph to represent the plot for biomass you would expect if the experiment
was repeated and in this case the sample culture medium was not added to the trays containing pea
plants (02 marks)
(ii) Suggest reason(s) for the appearance of the graph drawn in d (i) above (03 marks)
(e) How can the information from the investigation be beneficial in crop production? (02 marks
15. (a) Describe the structure of a chloroplast (08 marks)
(b) Compare the structure of a chloroplast with that of a mitochondrion. (12 marks)
16. a) With the help of well labelled diagrams, describe the structures of the following:
i. bone
ii. cartilage
iii. areolar tissue
b) (i) How is cartilage replaced by bone? (05 marks)
Page 228 of 447

(ii) Describe how locomotion instabilities are overcome in a bony fish such as Tilapia marks)
17. a) Describe the structure of meristematic tissue in plants (06 marks)
b) Explain the role of each of the following in the formation of meristematic tissue in higher plants
i. Vascular cambium (07 marks)
ii. Cork cambium (07 marks)
18. Relate the structure and function of these tissues
a) voluntary muscle (08 marks)
b) parenchyma (06 marks)
c) xylem (06 marks)
d) phloem
19. (a) Describe how each of the following tissues are related to their functions.
i. Parenchyma (03 marks)
ii. Collenchyma (03 marks)
iii. Sclerechyma (06 marks)
(b) Explain the distribution pattern of mechanical tissue in a stem and root of a dicotyledonous plant
REFERENCES
8. D. T. Taylor, N.P.O. Green, G.W. Stout and R. Soper. Biological Science, 3rd edition, Cambridge
University Press
9. M. B. V. Roberts, Biology a Functional approach, 4th edition, Nelson
10. C. J. Clegg with D. G. McKean, ADVANCED BIOLOGY PRICIPLES AND APPLICATIONS, 2 nd
thornes
END
Page 229 of 447

TOPIC 5: MOVEMENT IN AND OUT OF CELLS

Syllabus extract
SPECIFIC OBJECTIVES Content
The learner should be able to: Movement in and out of cells
 Describe diffusion, osmosis, active transport,  Diffusion and osmosis, active transport,
phagocytosis and pinocytosis. phagocytosis and pinocytosis: exocytosis and
 State the factors that affect the processes of diffusion. endocytosis.
 Describe the processes of osmosis.  Diffusion: factors affecting rate of diffusion,
 Explain the significance of diffusion and osmosis in  process of osmosis: including; turgidity,
organisms. plasmolysis, water potential, osmotic
 Explain how solvents and solutes are exchanged in potential, wall pressure.
animal and plant tissues or cells across the cell  Significance of diffusion process and
membrane in relation to its structure. significance of osmosis in organisms.
 Describe how unicellular organisms obtain water and  Exchange of solvents and solutes in plant
food. and animal tissues or cells across the cell
 Explain the relationship between structure and membrane in relation to its structure.
function of a cell membrane.  How unicellular organisms obtain water and
food.
 Relationship between structure and function
of a cell membrane.
Movement in and out of cells practical
 Identify habitats with suitable media for organisms’  Habitats with suitable media for organism’s
survival. survival
 Demonstrate use of salt in food preservation, use of  Use of salt in food preservation, use of
visking tubing, glass columns, microscope in diffusion visking tubing, glass columns and
and osmosis experiments. microscope in diffusion experiments.
 Demonstrate conditions affecting the rate of diffusion.  Conditions affecting the rate of diffusion.
 Demonstrate effects of osmosis on the cell/ tissues.  Effect of osmosis in living tissues.
Introduction
The plasma membrane isolates the inside of the cell protoplasm from its extracellular environment. Materials
are exchanged between the protoplasm and the extracellular environment across the plasma membrane. The
plasma membrane is selectively permeable and allows transport of materials across it.
The transport of substances is important to;
a. Supply cells with oxygen for respiration and raw materials for anabolism (synthesis of biological
molecules)
b. Regulate the pH and solute concentration for maintaining a stable internal environment for enzymes
to function optimally
c. Excrete toxic waste substances
d. Secrete useful substances for cell activities
Note: the transport of substances across the cell membrane takes place by two major fundamental processes.
Substances move in and out of cells by the following processes:

1. Simple diffusion i. Phagocytosis
2. Facilitated diffusion ii. Pinocytosis
3. Osmosis iii. Receptor mediated endocytosis
4. Active transport 6. Exocytosis
5. Endocytosis
Page 230 of 447

SIMPLE DIFFUSION
Diffusion is the random movement of ions or molecules from a
region where they are at higher concentration to a region of their
lower concentration. That is, to move down a concentration
gradient until equilibrium is reached. The phospholipid bilayer
is permeable to very small and uncharged molecules like oxygen
and carbon dioxide. These molecules diffuse freely in an out of
the cell through the phospholipid bilayer.
Hydrophobic substances (lipid-soluble) e.g. steroids, can also diffuse through. These non-polar molecules do
not require the aid of membrane proteins (channel or carrier) to move across the cell membrane.
The rate of diffusion depends upon;
a) The concentration gradient
This refers to the difference in the relative concentration on either side of the membrane or between two
points. The greater the difference between the points, the faster the rate of diffusion and if the difference is
less, the slower the diffusion rate. Therefore a reduced concentration gradient causes a reduced rate of
diffusion and vice versa.
b) Distance over which diffusion takes place
This is the distance over which the molecules are to travel i.e. the surface thickness across which the
molecules move. The greater the distance the lower the rate of diffusion. This is another factor which limits
cell size.
Note: the inverse square law states that the rate of diffusion is proportional to the reciprocal of the distance.
Diffusion is therefore only effective over very short distances.
c) Surface area over which diffusion occurs
The larger the surface area over which the molecules are exposed, the faster the rate of diffusion.
Fick’s law summarises the three factors. It states that ‘the rate at which one substance diffuses through
𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 𝑋 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑖𝑛 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛
another is directly proportional to
𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓 𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒
d) Temperature
When increased, temperature causes an increased rate of diffusion because the particles acquire increased
kinetic energy which causes increased speed of movement hence increased rate of diffusion.
At low temperatures, the kinetic energy is very low and the speed of movement by particles is equally very
low.
e) Size and nature of diffusing molecules
The smaller the size of the diffusing particles, the faster they diffuse i.e. smaller particles move very fast while
the large ones will move slowly.
Fat soluble molecules (non-polar substances) diffuse more rapidly through the cell membrane than water
soluble (polar) molecules.
f) Permeability
The more porous a surface is, the greater the number of particles that diffuse through it hence the greater the
rate of diffusion. Diffusion rate increases with increase in size of the pores.
Significance of diffusion
1. It’s a means by which gaseous exchange occurs in plants and animals e.g. in plants diffusion of gases
occur through the stomata and in animals, in gills of fish, , the skin and buccal cavity of amphibians
alveoli of reptiles, mammals and birds.
2. Absorption of certain digested food materials e.g. glucose in the ileum.
3. A means of exchange of materials between blood in capillaries and the tissues
Page 231 of 447

4. Movement of chlorides and hydrogen carbonate ions into and out of red blood cells during the chloride
shift occurs by facilitated diffusion
5. During formation of the nerve impulse, sodium ions diffuse into the nerve cells facilitating generation of
nerve impulses and ensures transmission of nerve impulses from one neurone to another i.e. diffusion
facilitates synaptic transmission
6. It ensures excretion of waste products e.g. ammonia in fresh water fishes
7. It’s the main means of transportation of materials within the cell’s cytoplasm e.g. in unicellular organisms
8. Absorption of mineral salts by plants from the soil is effected by diffusion as one of the mechanisms
Fick’s law suggests that structures are adapted to maximise the rate of diffusion by;
(i) Having a steep concentration gradient
(ii) Having a high surface area to volume ratio
(iii) Being thin to minimise the distance over which diffusion occurs
In order maximize the rate of diffusion, tissues where diffusion occurs attained special adaptations. These
include;
a) The lungs are ventilated by the respiratory tract (trachea, bronchus, bronchioles) which maintain a steep
concentration gradient between the lung alveoli and blood in the capillaries.
b) Respiratory surfaces like the lung alveoli and intestine epithelial lining possess a rich supply of blood
vessels which transport away the diffusing materials hence maintaining a steep gradient which sustains the
fast diffusion
c) Diffusion surfaces e.g. lung alveoli and intestines (ileum) are covered by a thin epithelium lining which
reduces the distance over which diffusion takes place.
d) The epithelial lining covering the alveoli and rumen of the ileum is very permeable to allow molecules to
travel across them
e) In lungs there are numerous alveoli and in the ileum infoldings known as villi and microvilli which is
coupled with a very long ileum also increases the surface area along which particles move into cells hence
increase the rate of diffusion.
f) Flattened body e.g. platyhelminthes (flatworms) which increases the surface area for movement of
materials by diffusion
g) Some organisms are of small size e.g. unicellular organisms which increases the surface area to volume
ratio of the surface that permits increased rate of diffusion
FACILITATED DIFFUSION
This refers to the passive transport of molecules and ions across a membrane by specific transport proteins,
carrier and channel proteins, found within the membrane in the direction of lower concentration of the ions or
molecules i.e. in favour of the concentration gradient (difference) of ions.
Facilitated diffusion is a faster form of movement than simple
diffusion and it involves transport of large polar molecules and
ions that cannot be transported by simple diffusion. Even though
water is an extremely small, its polar therefore it does not move
across the cell membrane by simple diffusion.
A charged molecule or atom and its surrounding shell of water,
find the hydrophobic layer (non-polar) of the membrane more
difficult to penetrate thus the lipid bilayer partly accounts for the
membrane’s selective permeability by preventing very large
molecules and small polar molecules of ions to move across it.
Page 232 of 447

Trans-membrane proteins form channels or act as transport proteins to facilitate and increase the rate of
diffusion across the semi permeable membrane. The transport protein molecules involved in facilitated
diffusion include channel and carrier proteins.
Facilitated diffusion by carrier proteins
Some small hydrophobic organic molecules e.g. amino acids and glucose pass through the cell membrane by
facilitated diffusion using carrier proteins. These proteins are specific for one molecule, so substances can
only cross a membrane if it contains the appropriate proteins i.e. they are specific.
The transport of glucose across the plasma membrane of fat
cells, skeletal muscle fibres, the microvilli of the ileum
mucosa and across proximal convoluted tubule cells of
vertebrate kidneys is brought about by a change in the shape
of the carrier protein once the glucose molecule bonds to it.
The binding state is called the ping state and the releasing
state is the pong state.
Carrier proteins alter their conformation/shape when
moving the solute across the membrane.
The solute molecule is released on the other side of the membrane, down its concentration gradient. The
carrier proteins bind molecules to them at the binding site and then change shape so as to release the
molecules on the other side.
Facilitated diffusion by protein channels:
These trans-membrane proteins form water-filled hydrophilic functional pores in the membrane whose shape
is specific for the passage of particular ions or polar molecules.
This allows charged substances, usually ions, and polar molecules Fig 2 & 3 pg 69 Kent OR Fig 5.17 pg
to diffuse across the cell membrane. Most channels have fixed 144 Soper OR Fig 4.18 pg 67 Toole Fig
shapes and can be gated (opened or closed), allowing the cell to 2
control the entry and exit of the ions, these include the ligand-
gated and voltage gated channels. Transport proteins allowing the
passage of ions are called ion channels. The proteins form specific
water filled hydrophilic channels that permit the diffusion of
various ions such as K+, Na+, Ca2+,Cl-,HCO-3.
There are also specialised channels for water known as aquaporins
found in both plant and animal cells. The aquaporins speed up the
rate of diffusion of water molecules down its water potential
gradient.
Comparison between simple and facilitated diffusion

Both move molecules from a Simple Facilitated
region of high concentration Diffusion can occur in either Diffusion occurs in only one
to a region of low direction direction
concentration through a Similar molecules diffuse at the Specific molecules diffuse faster than
partially permeable same rate others
membrane Does not require special transport Occurs via special channels or carrier
proteins proteins
Page 233 of 447

ACTIVE TRANSPORT
It is the movement of molecules or ions across a cell membrane against their concentration gradient aided by
the protein pump with specific binding sites, involving the expenditure of energy.
Cells which carry out active transport have a high respiratory rate and a large number of mitochondria to
generate a high concentration of Adenosine Tri Phosphate (ATP). The energy from ATP can be directly or
indirectly used in active transport.
Active transport can be slowed or inhibited by respiratory poisons (inhibitors) e.g. cyanide or lack of oxygen.
Mechanism of active transport
This can be direct active transport if the energy from ATP is used directly to transport the substances, ions or
molecules, or it can be indirect active transport if the energy is not directly used to transport a substance
across a membrane.
Types of membrane proteins involved in active transport. Three main types of membrane proteins exist;
a. Uniport carriers. They carry (transport) a single ion or molecule in a single direction.
b. Simport carriers. They carry two substances in the same direction.
c. Antiport carriers. They carry two substances in opposite directions.
One common example of active transport is the sodium-potassium which actively removes sodium ions from
cells, while actively accumulating potassium ions into the cell from their surroundings.
Direct active transport (e.g. Na+ - K+ pump)

The sodium potassium pump is a carrier protein which spans the cell membrane from one side to the other.
The Na+ - K+ pump accepts sodium ions and ATP on the inside, while it accepts potassium ions on the outside.
ATP is hydrolysed to ADP and an Illustration Fig 5.21 pg 146 Soper
inorganic phosphate by enzyme ATPase.
The binding of the phosphate and sodium
to the inside of the protein pump changes
the protein conformation. The protein
pump actively transports three sodium
ions (3 Na+) out of the cell for every two
potassium ions (2K+) pumped against
their concentration gradient into the cell.
This generates a difference in ionic charge on the two sides of the membrane i.e. the inside of the cell
becomes negative with respect to the outside. This potential difference across the membrane is important for
the transmission of nerve impulses. The Na+ gradient is also used in the coupled uptake of solutes such as
glucose into the cells against its concentration gradient.
Importance of the sodium-potassium pump
(i) Maintains electrical activity in nerve and muscle cells
(ii) Drives active transport of other substances such as sugars and amino acids
(iii) Provides the high concentration of potassium ions needs inside cells for protein synthesis, glycolysis and
other vital processes
Page 234 of 447

(iv) Controls osmotic balance of animal cells during osmoregulation

Note: if the pump is inhibited the cell swells and bursts, because buildup of sodium ions inside the cells
results in excess water entering the cells by osmosis. However, bacteria, fungi and plants which have cell
walls do not need the sodium potassium pumps.
Indirect active transport mechanism (secondary active transport)
This is also known as co-transport i.e. a form of active Illustration Fig 2 pg 70 Kent
transport in which the pumping of one substance indirectly
drives the transport of one or more other substances against a
concentration gradient e.g. the coupled uptake of glucose into
cells lining the ileum in mammals where glucose and Na+ ions
are absorbed into the cells. Sodium ions move down a
concentration gradient while the glucose molecules against the
concentration gradient. In co-transport of Na+ and glucose, ATP
is used by the protein pump to pump Na+ out of the cell creating
a Na+ concentration gradient. The Na+ and glucose molecules
then bind to trans-membrane protein (carrier protein), also called
co-transport proteins/coupled transport proteins.
They are then moved by the proteins inside the cells i.e. the Na+ moves down its concentration gradient while
the glucose molecules moves down against its concentration gradient.
A similar process transports glucose and amino acids into the cells having the digestive tract in mammals. But
here, absorption of nutrients is dependent on the sodium-potassium pump.
The factors required for active transport to take place;
1. Temperature
Increase in temperature increases the rate of transport of substances by active transport, so long as the increase
in not above the optimum. The increase in temperature makes respiratory enzymes more active, having their
speeds of movement increased (kinetic energy) with that of substrate molecules which results into collisions
of molecules at a faster rate thus forming enzyme substrate complexes that form products. In this case, ATP is
required to power active transport.
At very high temperatures, above the optimum, respiratory enzymes are denatured in the carrier proteins in the
membrane. This reduces the rate of active transport.
At very low temperatures, below the optimum, the respiratory enzymes together with the carrier proteins are
inactive and this reduces the rate of active transport.
2. Availability of oxygen
Oxygen is required for aerobic respiration to generate ATP. Increase in oxygen concentration results into
increased rates of active transport as more ATP molecules are available for the process. In circumstances of
very little or no oxygen, the rate of active transport is reduced since in the case of anaerobic respiration,
there’s very little or no ATP molecules available for active transport
3. Concentration of respiratory substrates e.g. glucose
If the concentration of respiratory substrate is increased, the rate of active transport also increases and if it is
lowered, the rate of active transport lowers. This is because increase in the amount of the substrate increases
the rate of ATP generation during respiration. If the amount of substrate is reduced, the rate of ATP
generation is also lowered.
Importance of active transport
1. It is a means of absorption of food materials in the mammalian gut
2. It is the means of absorption of mineral salts by plant root hairs and the root epidermal cells of the
peliferous layer
3. Selective reabsorption of glucose and sodium ions from the proximal convoluted tubule, and sodium ions
from kidney cortex occurs by active transport
Page 235 of 447

4. It facilitates the excretion of waste materials from the cells to the extracellular fluids against a
concentration gradient e.g. excretion of urea
5. It is important in muscle contractions and relaxations where there’s active pumping in and out of calcium
ions inside the cytoplasm (sarcoplasm) of the muscle.
6. It is used in the loading and unloading of materials in the plants phloem tissue which creates pressure
differences in the phloem tissue that maintain mass flow of materials.
7. Active transport is vital in transmission of nerve impulses along nerve cells where it creates a membrane
action potential using the potassium-sodium pumps.
8. It plays a part in the opening and closure of stomata where differential pumping of potassium ions
between the guard cells and neighboring subsidiary cells lead to turgidity changes hence causing stomatal
movements (opening/closure).
9. Removal of excess water from amoeba by contractile vacuoles occurs by active transport
10. Fresh water fish carry out the active uptake of mineral ions from the external environment by special cells
in the gills
Note: metabolic poisons (inhibitors), inhibit the enzymes and carrier proteins required to bring about active
transport by either changing the active sites/binding sites for the enzymes/carrier proteins for the molecules to
be transported. The poisons also inhibit ATP synthesis hence cutting off the source of energy needed to effect
the active transport.
Differences between the functioning of carrier proteins in facilitated and those in active transport
Carrier proteins in facilitated diffusion Carrier proteins in active transport
1. Do not use ATP 1. Use energy in form of ATP
2. Carry substances from a region of their lower 2. Carry substances usually from a region of their
concentration lower concentration to a region of their higher
concentration
3. Not affected by metabolic rate/ respiratory 3. Affected by metabolic rate/ respiratory
inhibitors/ oxygen concentration/ concentration inhibitors/ oxygen concentration/ concentration
of respiratory substance e.g. sugar, glucose of respiratory substances e.g. sugars
4. Carry substances slower 4. Carry substances faster
5. Carry substances in both directors across a 5. Carry a particular substance in one direction
membrane
Differences between diffusion and active transport

Diffusion Active transport
1. Materials move down their concentration 1. Materials move usually against their
gradient concentration gradient
2. Does not require energy in form of ATP 2. Energy in form of ATP is used
3. Slower 3. Faster
4. Allows all transmiscible molecules and ions to 4. Causes selective uptake of materials
pass through cell membranes
5. Not affected by metabolic rate 5. Affected by metabolic rate
6. Not affected by lack oxygen 6. Affected by lack of oxygen
7. Not affected by metabolic reactions 7. Affected by metabolic reaction
OSMOSIS
This the passive movement of water molecules, across a partially permeable membrane, from a region of
lower solute concentration to a region of higher solute concentration. It may also be defined as the passive
Page 236 of 447

movement of water molecules from a region of higher water potential to a region of lower water potential
through a partially permeable membrane.
A selectively permeable membrane is one that allows Fig 1c pg 72 Kent
unrestricted passage of water molecules but no
passage of solute molecules. Different concentrations
of solute molecules lead to different concentrations of
free water molecules on either side of the membrane.
On the side of the membrane with a high
concentration of free water molecules (low solute
concentration), more water molecules will strike the
pores in the membrane in a given interval of time,
water molecules pass through the pores resulting in net diffusion of water molecules from the region of high
concentration of free water molecules to the region of low concentration of free water molecules.
A net flow of free water molecules is maintained because in the side with more solute molecules, water forms
hydrogen bonds with solutes which are charged or polar forming a hydration shell around them in solution,
making water molecules unfree and therefore cannot flow back across the membrane.
Osmosis and aquaporins
In living cells, transport of water across the cell membrane is facilitated by channel proteins called aquaporins
which have specialised channels for water.
Water molecules are small but they are polar and
therefore cannot interact with hydrophobic phospholipid
layers easily and therefore diffusion through the lipid
bilayer is extremely rare (such as areas of the fluid
mosaic membrane rich in phospholipids with unsaturated
carbon tails) or not there at all, and water molecules can
quickly enter with ease through aquaporins in the cell
membrane.
Water potential
This is the net tendency of any system to donate water to its surroundings OR the term given to tendency of
water molecules to enter and leave a solution by osmosis. The symbol for the water potential is , the Greek
letter psi, and is usually measured in kilopascals (Kpa).
The higher the concentration of water molecules in a system, the higher the total kinetic energy of water
molecules in that system, and the higher is its water potential. The water potential of pure water is zero
pressure units and any addition of solute to pure water reduces its water potential and makes its value negative
i.e. pure water has the highest water potential.
In pure water or dilute solution with very few solute molecules, the water molecules have a high free kinetic
energy and can move very freely. A dilute solution therefore has a higher water potential than a concentrated
solution. This is because the movement of the water molecules is restricted by the attraction between solute
and water molecules i.e. there are fewer water molecules with a high kinetic energy to move across the
membrane. This is because water is a polar molecule which attracts the positive part of the solute (cation) to
its partially negatively charged oxygen atom, negative part of the solute (the anion) is attracted to the slightly
positively charged hydrogen part of the water, forming hydrogen bonds. This reduces the mobility of the
water molecules, lowering their kinetic energy, and decreasing the tendency of the system to lose water
molecules.
Page 237 of 447

The greater the concentration of solutes, the more Fig 44 pg 51 Roberts

negative is the water potential. Water potential of
a plant cell is the algebraic sum of its wall
pressure (pressure potential) and its osmotic

(solute) potential . A concentrated solution has
a low water potential and water therefore moves
down a water potential gradient i.e. water diffuses
from a region of high water potential (less
negative or zero value) to a region of lower water
potential (more negative value). The water
potential of pure water W at atmospheric
pressure is arbitraily given the value of 0 Kpa W
= 0 Kpa. The water potential of solutions is
therefore less than 0 i.e solution < 0 Kpa.
Water potential if affected by amount of solutes and external pressure.

When an external pressure is applied to pure water or a solution, its water potential increases. This is because
the pressure forces water molecules out of the system.
Solute potential ( s)
This is the potential or force of attraction towards water molecules caused by dissolved substances (solutes)
inside the solution. That is to say, a change in water potential of a system in the presence of solute molecules.
The attraction between solute molecules and water molecules reduces the random movement of water
molecules. The addition of more solute molecules lowers the water potential of a solution.
 Solute potential/osmotic potential is denoted by ( S) and its equal to 0 for pure water
 Solute potential is always negative for solutions because the forces of attraction between the solute
molecules and water molecules reduces the movement of water molecules.
 For a solution, water potential is equal to solute potential and its always negative
Pressure potential ( p)
This is the pressure exerted on a fluid by its surrounding. At any one time, the water potential of a plant is the
sum of the solute potential and pressure potential. Pressure potential is usually, though not always, positive.
W
= s
+ p
Water potential Solute / osmotic Pressure

of plant cell potential potential
When water enters the cell by osmosis, the pressure of the cytosol builds up, pushing out against the cell
membrane. This pressure is called hydrostatic pressure. In plant cells, this pressure builds up pushing the cell
membrane against the cell wall. Because the cell wall is capable of only very limited extension, a pressure
builds up that resists further entry of water. The cell wall begins to resist the swelling caused by the influx of
water. The pressure that the cell wall develops is the pressure potential. For plants therefore, pressure potential
is the pressure exerted on the cell contents by the cell wall and cell membrane.
Pressure potential is usually positive, but in the xylem of a transpiring plant the water column is under tension
and the pressure potential is negative.
Osmotic pressure and cell relationship

Osmotic pressure is the hydrostatic pressure needed to stop osmotic flow. If the membrane is strong enough,
the cell reaches an equilibrium, a point at which the osmotic pressure drives water into the cell exactly
Page 238 of 447

counterbalanced by the hydrostatic pressure which tends to drive water back out of the cell. However, the
plasma membrane itself cannot withstand the large internal pressures and an isolated cell under such
conditions would just burst. In contrast, cells of prokaryotes, fungi, plants and many protists are surrounded
by a strong cell wall which can withstand high internal pressure without bursting.
If a cell is surrounded by pure water or a solution whose concentration is lower than that of the cell contents,
water will osmotically flow into the cell; such a solution with a lower osmotic pressure than that of the cell’s
cytoplasm is said to be hypotonic. If the cell is surrounded by a solution whose solute concentration exceeds
that of the cell cytoplasm, water flows out of the cell. In this case the outer solution is said to be hypertonic to
the cell cytoplasm. If the cell concentration of the cell cytoplasm and the surrounding medium are the same
and there would be no net flow of water in other directions and the external solution is said to be isotonic.
The osmotic flow of water into the cell is endosmosis and the osmotic flow of water out of the cell is
exosmosis.
Hypertonic solution Hypotonic solution
1. Higher concentration of solute molecules 1. Lower concentration of solute molecules
2. Lower solute potential 2. Higher solute potential
3. Lower concentration of water molecules 3. Higher concentration of water molecules
4. Lower water potential 4. Higher water potential
5. Higher osmotic pressure 5. Lower osmotic pressure
6. More negative water potential 6. Less negative water potential
7. More negative solute potential 7. Less negative solute potential
Osmosis and plant cells

A plant cell will be divided into three main parts
(i) The cell wall, which is freely permeable, except when impregnated with lignin
(ii) The cytoplasm, which is surrounded internally by the tonoplast and externally by the plasma membrane.
Both the tonoplast and plasma membrane are partially permeable.
(iii) The cell vacuole, which contains an aqueous solution of salts, sugars and organic acids.
(iv)
a. Turgidity
When the external solution is hypotonic e.g. distilled water, the cell’s cytosol has a lower water potential,
causing an influx of water into the cells. The water enters into the cells vacuole, by osmosis, through the
partially permeable plasma membrane and tonoplast. The volume of the cell protoplasm increases, the
protoplast swells causing an internal hydrostatic pressure developed by the cell hence the cell wall stretches.
The pressure potential reaches its maximum when the cell wall is stretched to its maximum. At this point, the
cell is described as a fully turgid or it has full turgor reached and the water potential at this point equals to 0
i.e. =0 and no more water can enter the cell.
Turgor pressure plays part in supporting plants and maintains their shape and form of herbaceous plants by
being filled with fully turgid cells tightly packed together. It is also responsible for holding leaves in flat and
horizontal position as well as the opening of the stomata. This is because the cell wall is tough and rigid hence
resisting expansion of the protoplast so the cell wall exerts an equal and opposite pressure against the
protoplast, and the rapidly increasing hydrostatic pressure inside the cell causing a buildup of the pressure
potential. The pressure of the cell wall against the expanding protoplast is called wall pressure.
Importance of turgidity in plants

1. Turgor pressure maintains the shape and form of a plant
2. Stems of herbaceous plants and non-woody plants are maintained in an erect position by fully turgid cells
tightly packed together to provide support
3. Turgor pressure holds leaves on a flat and horizontal position to receive sunlight.
Page 239 of 447

4. Turgor pressure maintains the floral whorls of flowers open for pollination
5. Turgor pressure causes cell enlargement and stretches stems causing increase in girth
6. The rapid nastic responses of some plants, e.g. thigmonastic collapses of leaves and stems of Mimosa
pudica are due to changes in turgidity
7. Closing and opening of stomata by guard cells
Fig 4.5 pg 52 Roberts
b. Plasmolysis
When a plant cell is immersed in a hypertonic solution, than its cytosol, the cell decreases in volume as water
moves out osmotically from its vacuole through the partially permeable plasma membrane and tonoplast. The
protoplast shrinks, pulling away from the cell wall and leaving gaps between the cell wall and plasma
membrane. A cell in this condition is said to be plasmolysed and the cell becomes flaccid.
Plasmolysis is the shrinking of a plant cell’s protoplast away from the cell wall leaving gaps between the cell
wall and the plasma membrane.
When a plant cell is placed in hypertonic solution, it loses water by exosmosis. The protoplast shrinks and
pulls away from the cell wall. Also on a dry and hot day, the plant cells lose their way evaporation and the
turgor pressure of the plant cells is reduced with the result that the plant droops. The phenomenon is called
wilting. This is the drooping of leaves and stems as a result of plant cells losing water exosmotically and
becoming flaccid. A plant suffers from water stress when it loses more water by transpiration than it
absorbs by the roots.
Page 240 of 447

Effects of water stress in plants

(i) Excessive water loss causes drooping of shoots and leaves, which produce abscisic acid so as to close the
stomata and reduce on the rate of transpiration. This leads to reduction in the rate of photosynthesis due to
lack of carbon dioxide
(ii) There’s reduced growth and stunting due to reduced photosynthesis
(iii) Excessive water loss causes dessication and drying up of the plant
Plant-water relations
This takes into account of three forces which include;
a. Solute potential s
b. Pressure potential ( p).
c. Water potential of the cell sap w
Graphical illustration of a relationship between s (osmotic potential), w (water potential of the cell) and
pressure potential ( p) of a plant cell at different stages of turgor and plasmolysis is shown below
Fig 4.6 p.g. 54 Roberts
From full plasmolysis to full turgidity (full turgor) the solute potential increases gradually because the
osmotic entry of water into the cell gradullay reduces the concentration of solutes in the cell. The attraction
between solute molecules and water molecules reduces which increases the random movement of water
molecules hence increasing the solute potential. At full turgidity, the water potential is equal to 0Kpa and
solute potential is equal to pressure potential.
Considering a fully plasmolysed cell, its pressure potential is 0Kpa since the protoplast is completely pulled
away from the cell wall, so the cell wall does not exert pressure on the protoplast.
From full plasmolysis to incipient plasmolysis, the pressure potential remains constant at 0Kpa. This is
because the protoplast remains pulled away from the cell wall, so the cell wall does not exert any pressure
against the protoplast. When immersed in pure water, water enters the sap osmotically and the protoplasm
begins to expand. As the osmotic influx of water continues, the protoplast goes on expanding until the cell
membrane comes slightly into contact with the cell wall, incipient plasmolysis, but it is not pressed against it,
so the protoplast exerts no pressure against the cell wall so the pressure potential remains 0.
Page 241 of 447

From incipient plasmolysis to full turgidity (full turgor) the pressure potential increases rapidly (becomes
positive). As the cell continues to expand, due to the osmotic influx of water, the volume of the protoplasm
increases. The osmotic influx of water into the cell is opposed by the inward pressure of the cell wall i.e.
pressure potential. The protoplast exerts pressure against the cell wall and the rigid cell wall exerts pressure
back against the protoplast causing a rapid increase in the pressure potential of the cell.
At full plasmolysis, the water potential of the cell is low (more negative) since the protoplast has a high
concentration of solutes and a low concentration of water molecules. From full plasmolysis to incipient
plasmolysis, the water potential increases gradually (becomes less negative) because the osmotic entry of
water into the cell gradually increases the concentration of water into the cell.
From incipient plasmolysis to full

turgidity (full turgor) the water
potential increases rapidly
(becomes less negative) due to
further osmotic entry of water. The
volume of the protoplasm increases,
the protoplast swells further and
presses against the rigid cell wall.
The rigid cell wall presses back
against the protoplast, causing a
rapid increase in the pressure
potential of the cell.
The water potential of increases rapidly due to the rapidly increasing pressure potential, until the water
potential becomes 0Kpa at full turgidity at which point the cell cannot take in more water.When full turgor is
reached, the cell cannot expand anymore and at this point s (osmotic potential) is exactly outbalanced by the
pressure potential ( p). If the solution produces no change within the volume of the cell, it has a solute
concentration similar to that of the cell sap or tissue and therefore water potential of the solution equals to the
water potential of the cell or tissue.
In general:
 cell = s (always negative) + p (always positive)

 At total plasmolysis; the vacuole almost disappears, minimum hydrostatic pressure, cell membrane
completely not attached to the cell wall. Cell generally small and described as flaccid.
 At incipient plasmolysis; cell membrane begins to leave cell wall and water is lost from the cell.
 At full turgidity; the cell vacuole with maximum volume and no more water can enter.
Differences between wilting and plasmolysis

Wilting Plasmolysis
1. Occurs due high temperature 1. Occurs due to an osmotic gradient
2. The entire cell, including the cell wall, shrink 2. Only the protoplast shrinks away from the cell
wall
3. Water loss is serious leading to dessication 3. Water loss is not serious and can hardly result
which causes death into death
4. Results in drooping of shoots and leaves of the 4. Does not result into drooping of leaves and
plant shoots of the plant
Page 242 of 447

Osmosis and animal cells

If the red blood cells are placed in a hypotonic solution i.e. 0.5% sodium chloride, water enters the cells by
osmosis. The cell expands (swells) and the thin plasma membrane bursts, releasing the cell contents, this is
called haemolysis. Haemolysis is due to red blood cells lacking cellulose cell walls which would prevent red
blood cells expansion and therefore stops bursting.
If a human red blood cell is placed in an isotonic solution i.e. 0.9% sodium chloride solution, the cell neither
shrinks nor swells/no change in shape or volume because there’s no net movement of water molecules .
If the red blood cells are placed in a hypertonic solution i.e. 1.2% sodium chloride, there is a net outflow of
water by osmosis. The cell shrinks and the cell membranes appears crinkled and this is called crenation.
Note: unicellular protists e.g. Amoeba and Paramecium have contractile vacuoles to regulate the water
content in the cell.
Role of osmosis in living organisms

1. It is the main form by which root hairs and piliferous layer cells on roots absorb water from the soil
2. Movement of water from the root via the root cortex to the xylem
3. In herbaceous plants, osmosis brings about turgidity in plant cells due to presence of cell wall leading to
provision of support and shape in a whole plant body.
4. Osmosis causes plant structures (organs) like leaves and flowers to determine their form for example
holding the leaf in flat and horizontal position enabling it to trap maximum sunlight.
5. Osmosis bring about opening and closure of petals of flowers and osmosis bring about the opening and
closure of stomata in plant leaves when the guard cells become turgid facilitating gaseous exchange in
plants
6. Movement of water from the gut into the blood stream
7. Kidney nephrons (tubules) re-absorb water back into the blood stream via the blood capillaries
osmotically leading to water conservation in the body hence bringing about osmoregulation
Factors affecting osmosis in physical systems

1. Temperature
Provided pressure is constant, when a partially permeable membrane separates pure water on two
sides, the molecules of water will move from a region with a higher temperature to a region with a
lower temperature.
2. Pressure
Provided temperature is constant, when pure water in two sides of a partially permeable membrane is
subjected to diffusion pressures, water molecules move from the side with a higher pressure to the
side with lower pressure.
3. Solute molecule
Water molecules from a dilute solution to a concentrated solution
Differences between diffusion and osmosis

Diffusion Osmosis
1. Involves movement of solute or gas molecules 1. Involves movement of solvent
2. Involves movement of solute molecules or ions 2. Involves movement of solvent molecules from a
from a region of their higher concentration to a region of their higher concentration to a region
region of their lower concentration of their lower concentration
3. Occurs where there are no barriers to movement 3. Occurs through a partially permeable membrane
Differences between active transport and osmosis
Page 243 of 447

Active transport Osmosis

1. Involves movement of solute molecules 1. Involves movement of solvent molecules
2. Solute molecules move against their 2. Solvent molecules move down their
concentration gradient concentration gradient
3. Energy is required in form of ATP 3. No energy is required
4. Occur only in living tissues 4. Can occur in non-living tissues
5. Occurs across both partially permeable 5. Occurs across a partially permeable membrane
membrane and freely permeable membranes
6. Affected by lack of oxygen/ affected by 6. Not affected by lack of oxygen/ not affected by
metabolic poisons/ affected by the metabolic metabolic poisons/ not affected by the metabolic
rate rate
7. Concentration equilibrium is not obtained at the 7. Concentration equilibrium may be obtained at
end the end
8. Carrier proteins required 8. No carrier proteins required
An experiment to determine the water potential of plant materials such as a potato tuber
Apparatus and materials
 Sucrose  Beakers
 Distilled water  Stop clock
 Potato tubers  Razor blade
 Cork borer  Ruler
Procedure
1.
Prepare a series of sucrose solutions on known concentrations e.g. 0.1M, 0.2M, 0.3M, 0.4M, 0.5M
and 0.6M.
2. Place the same volume of each solution in six labelled beakers
3. Set up another beaker containing the same volume of distilled water, which is 0.0M
4. Using a cork borer, make seven cylindrical pieces of the potato tuber
5. Make all the cylindrical pieces 3cm long using a razor blade and a ruler
6. Add a potato cylinder to each of the labelled beakers containing the sucrose solutions, including the
0.0M solution
7. Leave the potato cylinders completely immersed in the sucrose solutions for 1 hour
8. Remove the potato cylinders from the sucrose solutions and measure their lengths accurately to the
nearest mm
Treatment of results
1. Calculate the percentage increase or decrease in length of the potato cylinders in each of the solutions
2. Plot a graph of percentage change in length against molarity of sucrose solutions
3. From the graph determine the molarity of sucrose solution at which there is no change in length
4. Record the value, and from a set of tables determine the osmotic potential of this solution
Conclusion
The water potential of the potato tuber is equal to the osmotic potential of the sucrose solution at which there
is no change in length of the potato cylinder
Page 244 of 447

P530 TRANSPORT By Nakapanka Jude Mayanja 0704716641
Determination of the mean solute potential of the cell sap in a sample of plant cells using the method of
incipient plasmolysis
The incipient plasmolysis method involves counting the number of plasmolysed cells in a given field of view
under the microscope for different concentrations of sucrose solutions then determining the percentage
plasmolysis and plotting a graph of percentage of plasmolysed cells against molarity of sucrose solutions.
By interpretation from the graph, the sucrose concentration which occurs when 50% of the cells to be
plasmolysed is read off.
Using the relationship:
(a) cell = s cell + p cell, and

(b) solution = s solution
When the two are in equilibrium, cell = solution, when p cell = 0
At incipient solution the protoplasts have shrunk to the point where they begin to pull away from the cell
wall and the pressure potential is zero, since no pressure is exerted by the protoplasts against the cell wall,
therefore; cell = s cell = solution, from (a) and (b) above.
Hence the solution causing incipient plasmolysis has the same solute potential as the cell sap.
So, at 50% plasmolysis the average cell is said to be at incipient plasmolysis, and solute potential of the
solution causing this plasmolysis can be obtained to give the mean solute potential of the cell sap, from
the tables of the relationships between molarity of sucrose solutions and solute potential of sucrose
solutions.
BULK TRANSPORT ACROSS THE CELL MEMBRANE

Cytosis
This is a form of active transport involving infoldings and out-folding of sections of the cell surface
membrane resulting into the bulk transport of materials into a cell (endocytosis) or out of the cell (exocytosis).
The flexibility of the cell membrane is an important factor in the bulk transport of materials into the cell.
Cytosis involves the contractile proteins in cellular microfilaments and microtubules pulling a small region of
a membrane from the rest of the membrane using energy in the form of ATP. Cytosis results in bulk transport
of materials into the cell or outside the cell, thus cytosis is divided into two main types i.e.
a. Endocytosis
b. Exocytosis
Endocytosis
This is bulk transport of materials inside
the cell. It involves a small area of plasma
membrane folding inwards (invaginating)
to surround a material to be taken in and
moves deeper inside the cell. There are
three types of endocytosis;
a. Phagocytosis
b. Pinocytosis
c. Receptor-mediated endocytosis
Page 245 of 447

Phagocytosis (cellular eating)

This is called cellular eating and it involves the cell taking in large solid substances. Phagocytosis involves
invagination of cell membrane, forming a cup-shaped depression, surrounding the organism or particle
forming a phagocytotic vesicle or vacuole which pinches off the cell membrane and moves into the cytoplasm.
Lysosomes fuse with vacuoles and release hydrolytic enzymes into the vacuole which break down the
substances in the vacuole. The protein substances are absorbed into the surrounding cytoplasm across the
lining of the vacuole. Any undigested material may be got rid of by the vesicles of vacuoles moving into the
cell surface membrane and fusing with it.
Cells specialised for phagocytosis are called phagocytes and are said to be phagocytic, as in white blood cells
and amoeba.
Mechanism of phagocytotic killing by white blood cells

The engulfing cells detect chemo-active molecules (usually small peptide molecules) released by the target
matter, and respond by moving towards it. White blood cells form cytoplasmic extensions to form
pseudopodia which surround and engulf micro-organisms. The microorganisms attach onto the white blood
cell by some ‘lock and key’ mechanism involving receptor proteins on the cell surface membrane.
Micro-organisms are completely surrounded by [Clegg and Mackean Pg 240 fg 11.23]
pseudopodia due to the activation of the contractile
processes of the cell’s cytoskeleton. These proteins
react with ATP to form phagocytotic vesicles or
phagosomes which pinch off the cell membrane into
the cytoplasm. The phagosome fuses with the
lysosome to form a phagolysosome. Inside the
phagolysosome are microbes which are broken down
by hydrolytic enzymes
Pinocytosis (cellular drinking)

Pinocytosis is the process by which the cells takes in bulk liquid material. It is also called cellular drinking, it
is similar to phagocytosis only that the infoldings forming the vesicles are much smaller.
Liquid and large macro molecules BS page 147
such as proteins are taken in via
small pinocytotic vesicles. Smaller
pinocytotic vesicles may be
formed, in which case the process
is called micro-pinocytosis. The
process is highly specific
involving the binding of the
molecules with corresponding
receptor molecules in the plasma
membrane.
A summary of the role of the plasma membrane in endocytosis and exocytosis

UNEB 2012
Page 246 of 447

Receptor mediated endocytosis

This involves receptor molecules on a cell membrane which binds with specific substance from extracellular
fluid i.e. it is selective.
The receptor proteins are usually already clustered in Illustration (M. Kent pg 71)
regions of the membrane called coated pits.
Extracellular substances (ligands) bind to these
receptors (a ligand is a molecule that binds
specifically to a receptor site of another molecule). As
the receptor sites are filled, the surface folds inwards
until the coated vesicles finally separates from the cell
surface membrane forming a coated vesicle containing
ligand molecules. After ingested material is liberated
from the vesicle, the receptors are recycled to the
plasma membrane by the same vesicle.
One common example is the binding of cholesterol molecules to specific receptor proteins on the plasma
membrane triggers the inward folding of the cell membrane. A vesicle is formed that carries the cholesterol
molecule into the cell.
Exocytosis
This involves the vesicles or vacuoles moving to the cell membrane fusing with the releasing their contents to
the outside of the cell.
Exocytosis provides a means by which enzymes, hydrochloric acid in the gastric glands, hormones in the
various ductless glands, antibodies, sweat-secreting cells of sweat glands of human skin, and cell wall
precursors are released from the cell.
The vesicles are often derived from the Golgi apparatus or
endoplasmic reticulum, which move along microtubules of the
cytoskeleton of the plasma membrane. When the vesicles get into
contact with the plasma membrane, the lipid molecules of the two
bilayers rearrange and diffuse. The content of the vesicles spill to the
outside of the cell and the vesicle membrane becomes part of the
plasma membrane
Note: Vesicle and food vacuole formation are active processes, which require energy from respiration.
Importance of cytosis
1. Many secretory cells use exocytosis to release their excretory products outside themselves e.g. pancreatic
cells manufacture insulin and secrete it into blood by exocytosis and many other hormones are secreted in
this form by the gland cells
2. Exocytosis facilitates synaptic transmission during which neuro-transmitter substances like acetylcholine
in synaptic vesicles of synaptic knobs fuse with the pre-synaptic membrane to release neuro transmitter
substances into the synaptic cleft of the synapse.
3. Exocytosis delivers cell wall materials to the outside of the cell from the Golgi apparatus/body through
vesicles which contain proteins and certain carbohydrates
4. Exocytosis leads to replenishment of the plasma membrane as the vesicle membrane become part of the
plasma membrane become part of the plasma membrane after spilling/discharging their contents to the
outside.
Page 247 of 447

Summary
Features Simple diffusion Facilitated diffusion Active transport
Concentration Down the concentration Down the concentration Against a concentration
gradient gradient from high to low gradient from high to low gradient from low to high
Energy expenditure None None Energy expenditure is in the
form of ATP
Carrier protein/ Not required Required Required
transporter
Speed Slowest mode Fast Fastest
SAMPLE QUESTIONS
1. The table below shows results of an experiment to determine the solute potential of onion epidermal
cells using incipient plasmolysis method. In each case, the total number of cells observed in one field
of view was eighty (80).
Concentration of sucrose Number of cells plasmolysed Percentage plasmolysis
solution (mol /dm3)
0.1 0
0.2 0
0.3 2
0.4 3
0.45 10
0.50 60
0.55 80
0.60 80
(a) Copy and complete the table by working out the percentage of cells which are plasmolysed.
(04 marks)
(b) What is meant by the terms?
(i) Solute potential. (03 marks)
(ii) Incipient plasmolysis. (03 marks)
(c) (i) Plot a graph to show the relationship between percentage of plasmolysed cells and sucrose
concentration. (08 marks)
(ii) From the graph, determine the concentration of the onion epidermal cells to be used to determine
their solute potential. (02 marks)
(iii) Briefly explain how you arrived at your answer in (c) (ii) above. (08 marks)
(d) Explain the ecological significance of osmosis to plants. (06 marks)
2. (a) (i) Define the term active transport. (02 marks)

(ii) Describe the sodium-potassium pump as an example of active transport. (07 marks)
(b) Define the terms uniport carrier, symport carrier and antiport carrier. (06 marks)
(c) With an example, explain the process of cotransport. (05 marks)
3. An experiment was carried out with cells of the carrot tissue which was first thoroughly washed in pure
water. The slices of carrot tissue were immersed in aerated potassium chloride solution of known
concentration at varying temperatures. At the fourth hour, the carrot tissue at 25 0C was treated with
potassium cyanide. The results are shown in the table below.
Time in minutes Potassium ion uptake in mg g-1
Page 248 of 447

At 20C At 250C
0 0 0
60 90 170
120 105 360
240 130 480
300 130 500
360 130 50
(a). Represent the above data graphically. [6marks]
(b). Describe the changes in the rate of potassium ions absorption within the first four hours at 250C.
[3marks]
(c). During the first hour, some potassium ions enter the carrot cells passively. Suggest any two
possible means of their movement and any two conditions needed for one of them to occur.
[4marks]
(d). (i). Calculate using minutes, the mean rate of absorption of potassium ions at 250C
between the 2nd and 6th hour [3marks]
(ii). Compare the rates of absorption of potassium ions at 20C and 250C during the
experiment. [4marks]
(iii). Suggest an explanation for the differences of potassium at the two temperatures.
[6marks]
(e). Explain the effects of treating the carrot with potassium cyanide on the rate of their absorption
of potassium ions. [4marks]
(f). Suggest
(i). the aim of the experiment. [1mark]
(ii). why the carrot tissue was first washed in pure water [2marks]
(iii). why the potassium chloride solution was aerated. [2marks]
(g). Briefly explain the significance of the existence of the casparian strip within endodermal cells
of the root. [5marks]
4. Figure 1 shows changes in the different potentials of a fully plasmolysed plant cell placed in a
hypotonic solution.
Figure 2 shows the rate of movement of two different substances across a phospholipid membrane;
glucose by facilitated diffusion and water by simple diffusion, at varying extracellular concentration.
Figure 1 Figure 2
Page 249 of 447

(a) From figure 1, compare the changes in pressure potential and water potential from full
plasmolysis to full turgor. (05 marks)
(b) As indicated in figure 1, explain the change in water potential from full plasmolysis to full turgor.
(15 marks)
(c) From figure 2, describe the effect of increasing extracellular concentration:
(i) on glucose uptake. (07 marks)
(ii) on water uptake (05 marks)
(d) Explain the observed rates of uptake of glucose and water, from figure 2 above. (08 marks)
5. The graph below shows the effect of concentration difference on three transport processes of molecules
or ions across a cell surface membrane. Study the information and answer the questions that follow.
Active transport
Rate of trasnport
Simple diffusion
Facilitated diffusion
Concentration difference
a) From the graph;

(i) state one similarity between the three transport processes (02 marks)
(ii) compare the rate of transport by facilitated diffusion and active transport (05 )
(iii) explain the rates of transport observed when the concentration difference is zero
(04 marks)
(iv) explain the changes in the rate of transport by facilitated diffusion (10 marks)
(v) what is the basis of the difference in the graphs for simple diffusion and facilitated
diffusion (02 marks)
b) (i) Which one of the processes would stop if a respiratory inhibitor was added? (01)
(ii) Explain your answer in b (i) above (03 marks)
c) Outline the differences between the functioning of carrier proteins in facilitated diffusion and
those in active transport (04 marks)
d) Describe the sodium potassium pump as an example of active transport (09 marks)
e) state the composition and major function of the animal’s cell surface.(03 marks)
6. Two investigations concerning movement of substances in and out of cells were carried out in 2
different organisms and results were summarized in tables 1 and 2 as indicated below.
Page 250 of 447

The first investigation had 2 experiments. In the first experiment the marine ciliate corthurnia was
placed in a series of dilutions of sea water and the output of its contractile vacuole was measured. In
another experiment, the change in volume of the organism in different dilution of sea water was
recorded.
Added fresh water/% 0 10 20 30 40 50 60 70 80 90
Contractile vacuole out put/dm3s-1 0.7 0.6 1.1 1.0 1.5 2.4 6.3 18.2 35.1 9.5
Relative body volume 1.0 1.1 1.2 1.3 1.4 1.6 1.8 2.0 2.1 2.0
In the second investigation, the relative rate of uptake of glucose and xylose ( a pentose) from living intestine
and from intestine which had been poisoned with cyanide, was determined and results recorded in table 2
Sugar Without cyanide With cyanide
Glucose 100 28
xylose 18 18
a) Represent graphically the results in table 1 using a single set of axes (06 marks)
bi) Explain the effects of dilutions on the activity of the contractile vacuole(04 arks)
ii) what do changes in relative body volume indicate about the effect of the contractile vacuole activity?
c) Some species of marine protozoa form contractile vacuoles only the protozoan begins to feed . Suggest an
explanation for this observation. (03 marks)
d) How is active transport:
i) similar to facilitated diffusion (02 marks)
ii) different from facilitated diffusion ( 03 marks)
e) Explain the relative uptake of the sugars by the intestines (05 marks)
f) How do the following factors affect the rate of diffusion across a membrane
i) concentration difference, (02 marks)
ii) the size of the molecules(02 marks)
iii) temperature (02 marks)
iv) polarity of the molecules(02 marks)
7. In an experiment a set of young cereal roots were washed thoroughly in pure water and transferred
into culture solutions containing potassium chloride solution under varying oxygen concentrations (at
point M on the graph below). After 160 minutes solution of unknown substance was introduced (at
point N on the graph below). The rate of oxygen uptake and potassium chloride uptake were
measured and recorded graphically as shown in the figure below.
Page 251 of 447

a) Compare the rate of oxygen uptake with the rate of chloride uptake between 60 and 240 minutes.
(04 marks)
b) Explain the rate of oxygen and potassium chloride uptake as shown in the graph above?
(06 marks)
8. The graph below shows the percentage change in length of cylinders of potato which had been placed
in sucrose solutions of different concentrations for 12 hours.
10
6
PERCENTAGE CHANGE IN LENTGH
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
-2
CONCETRATION ON SUCROSE/moldm-1
-4
-6
-8
-10
a) What is meant by the term water potential? (02 marks)

b) In terms of water potential, explain the change in length which occurred when the cylinder of potato
was placed in a sucrose solution of concentration of 0.3mol dm-1
c) With a reason, state the concentration of sucrose in the potato tubers used in the experiment above
(03 marks)
Potato tubers store starch. As they start to grow or sprout, some of this starch is converted to sugars. Sketch a
graph on the one plotted above to represent the changes in length you would expect if the investigation had
been carried out with sprouting potatoes
9. Define the term facilitated diffusion
c) State three ways how facilitated diffusion differs from simple diffusion
d) Describe one way how facilitated diffusion occurs across membranes
e) State two ways how the action of carrier proteins is similar to that of enzymes
10. State the parameters listed in Fick’s law of diffusion (03 marks)
b)Explain how each parameter in Fick’s law of diffusion is reflected in the structure of the mammalian lung
c) Explain the changes in oxygen delivery to the tissues that occur as a person proceeds from a
resting state to intense exercise (04 marks)
11. The table below shows the results of an experiment on the rate of absorption of sugars by a
mammalian intestine. Study it carefully and answer the questions that follow.
Sugar Relative rates of absorption taking normal glucose uptake as 100
By living intestine By intestine poisoned with cyanide
Page 252 of 447

Hexose Glucose 100 30

sugars Galactose 106 35
Pentose Xylose 32 32
sugars Arabinose 30 31
(a) Suggest a reason for the difference between the rates of absorption of hexose and pentose sugars in
the living intestines (03 marks)
1
(b) Mention the mechanism by which hexose sugars are absorbed by living intestines (02 mark)
(c) What is the advantage to the individual of having hexose sugars absorbed in the way mentioned
above?
(d) What could be the effect of cyanide on the mechanism of hexose absorption? (02 marks)
(e) In an intact mammal, absorption of fatty acids is drastically curtailed by any clinical condition which
leads to a reduction in bile salt excretion or release. Explain why this is so.
12. Beet root cells contain a pigment that cannot normally escape from the cells through the cell surface
membrane. The graph below shows the results of an investigation into the effect of temperature on the
permeability of the cell surface membrane of beet root cells. The permeability was measured by using
a calorimeter to measure the absorbance of green light by the solution in which samples of beet root
had been immersed. The greater the absorbance, the more red pigment had leaved out of the beet root
cells.
120
Absorbance of green light in arbitrry units
100
80
60
40
20
0
0 10 20 30 40 50 60 70 80 90
Temperature in oC
(a) Describe the changes in the absorbance of green light with temperature. (4 marks)
(b) What is the general effect of temperature on the absorbance of light? (1 mark)
(c) With reference to the structure of cell membranes, explain the effect of temperature on absorbance. (4 m
(d) State one other way in which membrane permeability could be altered. (1 mark)
13. In a physiological investigation, screened red blood cells were placed in different concentrations of
aqueous sodium chloride solution. In each case an average total of five thousand (5000) cells
were viewed and the total number of haemolysed cells recorded. The results of this
investigation are shown in the table below.
Page 253 of 447

Sodium chloride concentration ( g /100ml) 0.33 0.36 0.38 0.39 0.42 0.44 0.48
Number of cells haemolysed 4900 4500 4000 3400 1500 800 100
Percentage cells haemolysed/ %
(a) (¡) calculate the percentage cells haemolysed at each sodium chloride Concentration using the formula
below and fill in the table. (31/2 marks
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑒𝑙𝑙𝑠 ℎ𝑎𝑦𝑚𝑜𝑙𝑦𝑠𝑒𝑑
Percentage cells haemolysed = 𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑡𝑜𝑡𝑎𝑙 𝑐𝑒𝑙𝑙𝑠 𝑣𝑖𝑒𝑤𝑒𝑑 (5000) X 1000
(b) Plot a graph to show variation of percentage cells haemolysed with sodium chloride Concentration.
(c) Describe the changes in the percentage cells haemolysed. (04 marks)
(d) Explain the shape of the graph. (06 marks)
(e) From the graph, determine the sodium chloride concentration:
( ¡) At which 100% haemolysis occurs.
( ¡¡) Isotonic to the red blood cells and explain your answer. (04 marks)
(f) Suggest what would happen if the red blood cells were placed in sodium chloride Concentration of
(¡) 0.6g/100ml
(¡¡) 0.1g/100ml (04 marks)
(g) Give reasons why the red blood cells haemolyse over a wide range of salt concentration.
(03 marks)
(h) Briefly describe five ways by which green plants obtain Nitrogen. (08 marks)
14. In an experiment, the rate of uptake of glucose by the blood using simple and facilitated diffusion
at varying extracellular concentration of glucose, was measured. The results are shown in the
table below. Study the information and answer the questions that follow.
600
Maximum rate
Rate of glucose uptake in µmol/ml/hr
500
facilitated diffusion
400
300
200
100
Simple diffusion
0
0 2 4 6 8 10 12 14 16
Extracellular concetration of glucose in mM
a) Describe the rate of glucose uptake with increasing extracellular concentration when diffusion is
facilitated. (09 marks)
b) Compare the rate of glucose uptake when diffusion is facilitated and when it is not.
(08 marks)
Page 254 of 447

c) Explain the effect of increasing extracellular concentration of glucose on the uptake of glucose, when
diffusion is facilitated. (09 marks)
d) Suggest what would happen to the rate of glucose if a respiratory poison was introduced into the cell
membrane. Give an explanation for your answer. (03 marks)
e) Explain why:
i. Facilitated diffusion occurs (06 marks)
ii. The cell membrane is able to carry out facilitated diffusion (12 marks)
REFERENCES
1. D.T.Taylor, N.P.O. Green, G.W. Stout and R. Soper. Biological Science, 3rd edition, Cambridge
University Press
3. C.J.Clegg with D.G.Mackean, ADVANCED BIOLOGY PRICIPLES AND APPLICATIONS, 2 nd
thornes
Page 255 of 447

TOPIC 6: TRANSPORT IN LIVING ORGANISMS

Syllabus extract
Specific objectives: The learner should be able to: Content
Necessary for transport systems  Limitations of simple diffusion process:

 Explain the limitation of simple diffusion in the concept of surface area: volume ratio and
transport process. its effect on diffusion rate.
Water as a medium in plants and animals  Significance of water in transport: solvent
 Explain the significance of water in transport. medium of transport
Circulatory systems in animals  Circulatory systems in insects, annelids and
 Describe the circulatory systems in insects, mammals.
annelids and mammals.  Types of circulatory systems: open and
 Compare the structure and function of veins closed, single and double.
arteries and capillaries.  Advantages and disadvantages of open and
 Describe types of circulatory system closed systems in animals.
 Explain the advantages and disadvantages of  Structure of transport systems in fish and
open and closed systems in animals. mammals.
 Compare the circulatory systems of fish and  Functioning of the mammalian heart: cardiac
mammals. cycle blood pressure changes, myogenic,
 Describe the functioning of the mammalian heart. myogenic property , control of the
 Explain the response of the heart to body activities. heartbeat.
 Interpret information on the effects of drugs and  Response of heart to body activities
variation of temperature on the cardiac frequency.  Effects of drugs and temperature variations on
 Explain how the heart beat rate is controlled. the cardiac frequency.
 Relate the action of adrenalin and acetylcholine to  Action of adrenalin and acetylcholine on the
the innervations of the hear innervation of the heart.
 Describe the role of blood components in the  Blood constituents and functions.
transport process.  Common diseases of the blood and heart,
 Explain the diseases related to the circulatory including sickle cell anaemia and coronary
system. artery diseases.
 Control of heart beat rate.
Circulatory systems in animals practical  Structure of blood vessels (veins, arteries
 Identify structural features of blood vessels. capillaries).
 Display and draw major structures of the  Circulatory systems in insects, toads and
circulatory systems in insects, toads and mammals. mammals gross structure and fine structure.
 Describe the insects, toad and mammals  Insect, toads and mammals circulatory
circulatory system in relation to their functions. systems in relation to functions.
 Structural adaptation of cardiac muscle and
smooth muscle of the circulatory system of
mammals .
Defence against diseases  Mechanism of blood clotting
 Describe the mechanism of blood clotting.  Immune response in humans: definition,
 Describe immune responses in humans. primary secondary).
 State the role of the thymus gland in immunity.  The role of the thymus gland in immunity.
 Blood transfusion and blood groups.
Page 256 of 447

 Explain the immune responses during blood  Effect of the Rhesus factor during pregnancy.
transfusion.
 Describe the effects of the Rhesus factor
Vascular system of flowering plants  Structure and functional adaptation of vascular
 Describe the structural and functional adoption tissue in monocotyledonous and
of the vascular tissues to transport process of dicotyledonous plants.
materials in monocotyledonous and  Mechanism of transporting materials in plants.
dicotyledonous plants.  Evidence for the path of materials in plants.
 Explain the mechanism of transporting materials  Uptake of water and mineral salts in plants.
in plants.  Role of transpiration in transport of water and
 Describe the evidence for the path of materials in dissolved mineral salts in plants.
plants.
 Describe translocation and uptake of water and
mineral salts in plants
 Explain the role of transpiration in transport of
water and dissolve mineral salts in plants.
Vascular system of flowering plants. Practical  Structure and distribution pattern of the
 Identify types and the pattern of distribution of vascular tissue in monocotyledon and
vascular bundles in the plant organs. dicotyledonous plants.
 Stain and make temporary mounts of transverse  Transverse T.S and longitudinal sections
section (T.S) and longitudinal section ( L.S) of : L.S. of stems, roots and T.S of leaves of
stems, roots and T.S of leaves. monocotyledonous and herbaceous
 Draw and label low power plans to show dicotyledonous plants.
distribution of issues in T.S and L.S of stems,  Labeled diagrams of T.S of stems, root and
roots and T.S of leaves. T.S of leaves.
 Make high power labeled drawings of vascular
tissues in T.S of leaves.
Page 257 of 447

Need for a transport system

Many materials including oxygen, carbon dioxide, soluble food substances, hormones, urea e.t.c.
need to be transported from one point to another using a transport network and medium.
The transport system in animals is mainly made up of blood vessels consisting of blood as the
medium circulating through them to the various body tissues. The transport system is also made up
of the pump i.e. the heart which brings about circulation of blood throughout the body, by pumping
it. The transport system is also composed of the lymph vessels containing the lymph fluid.
The larger, compact and more active an organism is, the more the need for a transport system due to
a small surface area to volume ratio which reduces the rate of diffusion of materials from the body
surface to the cells in the middle of the organism. There are however some organisms which lack the
transport system e.g. protozoa and platyhelminthes e.t.c. This is because, being small in size and
being flattened in shape gives these animals a large surface area to volume ratio, this enables free and
rapid diffusion of materials from one part of the body to another. Consequently large multi-cellular
organisms have an elaborate transport system that carries useful substances such as oxygen and
glucose to the cells and carries away the waste products of metabolism. An elaborate transport
system has two major features;
a. An increased surface area of the sites of exchange of materials. Such sites include the lungs
and the gills where oxygen is absorbed and the villi of the ileum where food nutrients are
absorbed along the alimentary canal.
b. A system whereby the circulating medium carries the absorbed substances at a faster rate than
diffusion. In some organisms with a blood circulating system, blood flow is not confined to
blood vessels but instead it flows within a blood filled cavity called Haemocoel e.g. in
arthropods and molluscs. In other organisms with the blood circulatory system, blood flow is
confined to blood vessels only e.g. in vertebrates and some invertebrates such as the earth
worm.
Importances of a blood circulatory system (functions of blood)

1. Tissue respiration. It enhances the formation of energy in the tissues by transporting oxygen and
soluble food substances to the tissues to be used as raw materials for respiration. Carbon dioxide
is also transported away from the tissues mainly in the form of bicarbonate ions (HCO3-) as a by-
product of respiration and then taken to the lungs for its removal from the body. Oxygen is
transported in the form of oxyhaemoglobin from the respiratory surfaces to the tissues.
2. Hydration. Blood transports water from the gut to all tissues.
3. Nutrition. Blood transports the soluble well digested food materials from the gut to the body
tissues.
4. Excretion. Blood transports metabolic waste products from the tissues to the excretory organs for
their removal from the body e.g. blood transports urea from the liver to the kidney in order for it
to be removed from the body.
5. Temperature regulation. Blood distributes heat from the organs where it is mainly generated e.g.
the liver and the muscles, uniformly throughout the body.
6. Maintenance of constant pH. Blood maintains a constant pH through the maintenance of
circulation of the plasma proteins manufactured by the liver which act as buffers to maintain the
pH of the body fluids constant. This enables enzymes to function efficiently as charges will
denature the enzyme.
Page 258 of 447

7. Growth, development and co-ordination. Blood transport different metabolites such as glucose,
amino acids and hormones needed for the growth and development of the body.
8. Defence. Blood defends the body against diseases through the following ways;
 By using some white blood cells (leucocytes) which phagocytotically ingest and destroy
pathogens that cause diseases.
 By formation of a blood clot around the wound so as to prevent entry of microbes or
pathogens into the body.
 By use of the immune response mechanism towards infection e.g. by use of the different
types of antibodies to destroy the microbes

BLOOD
This is a highly specialized fluid tissue which consists of different types of cells suspended in a pale
yellow fluid known as the blood plasma
Blood plasma
This is a pale yellow fluid component of blood composed of the plasma proteins and blood serum
where the blood cells are suspended. Blood plasma carries the biggest percentage of blood and
consists of a colourless fluid known as serum and also plasma proteins. It is in the blood serum that
all the different soluble materials are dissolved e.g. urea, hormones, soluble food substances,
bicarbonate ions e.t.c.
The plasma proteins are manufactured by the liver and include the following;
a. Fibrinogen. This protein is important for normal blood clotting by changing into fibrin in the
presence of thrombin enzyme.
Fibrinogen (soluble) Fibrin (insoluble)
b. Prothrombin. This is the inactive form of the proteoltyic enzyme, thrombin, used in
converting fibrinogen to fibrin during the clotting of blood.
c. Globulin. Both Prothrombin and globulin play important roles in the homeostasis. All the
plasma proteins maintain pH of the body fluids constant by acting as buffers.
d. Blood cells. There are three main types of blood cells which include;
i. Erythrocytes (Red blood cells)
ii. Leucocytes (White blood cells)
iii. Platelets
ERYTHROCYTES (Red blood cells)

These are small numerous bi-concave disc shaped Diagram showing the shapes of erythrocytes
cells mainly important in transportation of oxygen
as oxyhaemoglobin from the respiratory surfaces
e.g. lungs and gives it to the tissues as well as
transporting carbondioxide from tissues back to
lungs. Erythrocytes are manufactured by the bone
marrow in adult and by the liver in the foetus.
Note; Erythrocytes have a life span of 120 days.
Page 259 of 447

Adaptations of erythrocytes
1. They have a red pigment called haemoglobin in their cytoplasm which has a high affinity for
oxygen and therefore rapidly transports oxygen.
2. They have a thin and permeable membrane which enables faster diffusion of oxygen and carbon
dioxide into them
3. They have a pliable membrane (flexible membrane) which can enable them change their original
shape and squeeze themselves into the blood capillaries in order to allow the exchange of
respiratory gases
4. They have an enzyme known as carbonic anhydrase within their cytoplasm which enables most
of the carbon dioxide to be transported in form of bicarbonate ions (HCO3-), by catalyzing the
reactions between carbon dioxide and water to from carbonic acid.
CO2 + H2O H2CO3
Carbonic anhydrase
5. They lack a nucleus so as to provide enough space for haemoglobin in order to carry a lot of
oxygen in form of oxyhaemoglobin.
6. They have a bi-concave disc shape which provides a large surface area that enhances maximum
diffusion of enough oxygen into them.
LEUCOCYTES (white blood cells)

They are amoeboid cells having a nucleus and a colourless cytoplasm important for defense of the
body against infections. They are fewer than erythrocytes i.e. they are about 7000/m3 of blood. They
are mainly manufactured by the bone marrow. They are classified into two main types which
include;
Granulocytes (polymorphonuclear leucocytes)
These are leucocytes with granules in there cytoplasm and a lobed nucleus. They originate in bone
marrow. There are three types of granular leucocytes which include;
i. Basophils (0.5%)
ii. Eosinophils (1.5%)
iii. Neutrophils (70%)
Basophils (0.5%) produce heparin and histamine. Heparin is an anti-coagulant which prevents blood
clotting in blood vessels. Histamine is a substance that is released during allergic reactions e.g. hay
fever. Histamine brings about allergic reactions by causing dilation (widening) and increased
permeability of small blood vessels which results in such symptoms as itching,, localized swellings,
sneezing, running nose, red eyes e.t.c.
Eosinophils (1.5%) possess anti-histamine properties and their number increases in people with
allergic reactions such as high fever, asthma e.t.c. so as to combat the effects of histamine.
Neutrophils (phagocytes) (70%) engulf pathogens phagocytotically and digest them actively inside
to defend the body against diseases.
Page 260 of 447

Agranulocytes (mononuclear leucocytes)
These are leucocytes with no granules in there cytoplasm usually with a spherical or bean shaped
nucleus. They originate in bone marrow and lymph nodes. They are divided into two types;
i. Monocytes (4%)
ii. Lymphocytes (24%)
Monocytes (4%) are leucocytes which enter the tissues from which they develop into macrophages
which carry out Phagocytosis to defend the body against pathogens.
They have a bean shaped nucleus.
Lymphocytes (24%) they are produced in the

thymus gland and lymph nodes. The
precursor cells of lymphocytes in the bone
marrow form a tissue which is called the
lymphoid tissue. Lymphocytes are usually
round and they possess a small quantity of the
cytoplasm. Lymphocytes produce antibodies,
agglutins, lysins, opsonins and antitoxins.
In adults they are produced and develop in the
bone marrow and lymph glands while in
embryos they are produced in the thymus
gland, liver and spleen.
They have a life span of 21 days
Adaptations of white blood cells to their function

a. They are larger than the pathogens
b. They are numerous
c. Some lymphocytes produce antibodies which attack pathogens
d. They have a sensitive cell surface membrane that detects micro organisms
i. They have enzymes in their cytoplasm to digest the engulfed micro organisms
ii. They do not have a fixed shape and hence the amoebic movements used to engulf pathogens.
iii. They have an irregular shaped nucleus which allows them to squeeze through the narrow
capillaries
iv. They have a large nucleus which contains many genes for the control of antibody production.
BLOOD PLATELETS (thrombocytes)

These are irregularly shaped, membrane bound cell fragments lacking the nuclei and are formed
from the bone marrow cells. They are responsible for starting up the process of blood clotting. There
are abound 250,000 blood platelets per mm3 of blood.
Page 261 of 447

TRANSPORT OF OXYGEN
The equation below shows how haemoglobin combines with oxygen.
As shown by the equation above, each haem group combines with one oxygen molecule and
therefore 1 haemoglobin molecule carries four oxygen molecules.
Hb + 4O2 ↔ HbO8
Haemoglobin
Haemoglobin is a large and complex molecule that is composed
of four polypeptide chains (therefore it has a quaternary
structure) arranged around four haem groups. Two of the
polypeptide chains are coiled to form α-helix, and this in turn is
folded on itself into a roughly spherical shape, the other two
chains are called β-chains due to unique primary structures in
both types of chains. Various kinds of chemical bonds, together
with electrostatic attraction, keep the folds of the chain together
and maintain the shape of the molecule. Haemoglobin is an
example of a conjugated protein: attached to the hydrophobic
crevice of the polypeptide chain is a flat group of atoms, the
prosthetic group, consisting of a central iron atom held by rings
of nitrogen atoms, which are part of a large structure known as
porphyrin rings
The prosthetic group is haem and it is to the iron atom in the middle of it that the oxygen molecule
becomes attached. The presence of four haem groups means that a single molecule of haemoglobin
can carry four molecules of oxygen. Haem belongs to a class of organic compounds known as the
porphyrins.
Other oxygen carrying pigments

There are several other groups of blood pigments and they differ mainly in the nature of prosthetic
group. Chlorocruorin and haemoerythrin both contain iron, and haemocyanin contain copper. These
three pigments are confined to invertebrate groups, particularly annelids and molluscs.
Pigments differ in their oxygen0-carrying capacities and are located in different areas
Haemoglobin Chlorocruorin Haemocyanin (snails Haemoerythrin
(some annelids) and crustaceans) (some annelids)
Colour of Red Green Blue Red
pigment
Metal in Iron Iron Copper Iron
prosthetic group
Molecule of 1:1 1:1 1:2 1:3
oxygen carried
per atom metal
Location in Cells or Plasma Plasma Cells or plasma
blood plasma
Page 262 of 447

Oxygen tension and oxyhaemoglobin formation

The ability of erythrocytes to carry oxygen to the tissues is due to haemoglobin having a high affinity
for oxygen i.e. it can readily combine with oxygen and becomes fully saturated with it at relatively
low partial pressures of the gas. Partial pressure of a gas is the measure of the concentration of a gas
expressed in Kilo Pascals (Kpa) or milimetres of mercury (mmHg)
The high affinity of haemoglobin for oxygen is measured experimentally by determining the
percentage saturation of haemoglobin with oxygen. When the percentage saturation of blood with
oxygen is plotted against the partial pressure of oxygen an S-shaped curve or sigmoid curve is
obtained and this curve is called the oxygen dissociation curve which is shown on the right
The curve indicates that a slight increase in the partial pressure of oxygen leads to a rapid increase in
the percentage saturation of haemoglobin with oxygen. This indicates that haemoglobin has a high
affinity for oxygen in that it readily combines with it and become saturated with it at low partial
pressures of oxygen.
(Toole fig 21.3 pg 414 OR Kent fig 3 pg 129 The S-shaped curve is due to the way in which
haemoglobin binds to oxygen. The first molecule
of oxygen combines with a haem group with
difficulty and distorts the shape of the
haemoglobin molecule during the process. The
remaining three haem groups bind with three
oxygen molecules more quickly than the first one
which increases rapidly the percentage saturation
of haemoglobin with oxygen.
When oxyhaemoglobin is exposed to regions
where the partial pressure of oxygen is low, e.g. in
the respiring tissues, the first oxygen molecule is
(Toole fig 21.3 pg 414 OR Kent fig 3 pg 129 released easily and faster but the last one is
released less readily with a lot of difficulty and
least readily.
The steep part of the curve corresponds to the
range of oxygen partial pressures found in the
tissues. Beyond this part of the curve, any small
drop in oxygen partial pressure results into a
relatively large decrease in the percentage
saturation of blood due to the dissociation of
oxyhaemoglobin to release oxygen to the tissues.
Beyond this part of the curve any small drop in the oxygen partial pressure results into a relatively
large decrease in the percentage saturation of blood with oxygen, due to the dissociation of
oxyhaemoglobin to release oxygen to the tissues.
Page 263 of 447

In conclusion, the curve indicates that haemoglobin has a high affinity for oxygen where the oxygen
tension is high e.g. in the alveolar capillary of the lungs. However, the affinity of haemoglobin for
oxygen is lower where the oxygen tension is low and instead it dissociates to release oxygen e.g. in
the blood capillaries serving blood to respiring tissues.
The oxygen supply can be distributed according to the requirements of different times, with skeletal
muscles getting more during exercise or the intestinal tract getting more during digestion. Of
particular importance is the constant flow of blood to the brain. For example, falling during fainting
actually prevents serious damage to the brain cells as a result of inadequate blood supply. (These
responses are often thwarted by well-meaning bystanders anxious to get the affected individual ‘back
on his feet’. In fact, holding a fainting person upright can lead to severe shock and even death).
Note: loading tension is the partial pressure of oxygen at which 95% of the pigment is saturated with
oxygen, and the unloading tension is the partial pressure at which 50% of the pigment is saturated
with oxygen.
Affinity of haemoglobin for oxygen under different conditions
Region of the Oxygen tension Carbon dioxide Affinity of Result
body (concentration) tension haemoglobin for
(concentration) oxygen
Gaseous High Low High Oxygen is
exchange surface absorbed
Respiring tissue Low High Low Oxygen is
released
There are many different oxygen dissociation curves because:

 there are a number of different respiratory pigments
 haemoglobin exist in a number of different forms
 the characteristics of each pigment change under different conditions
The many different oxygen dissociation curves are better understood if two facts are always kept in
mind:
 the more to the left the curve is, the more readily the pigment associates with oxygen but
the less easily its dissociates with it
 the more to the right the curve is, the less readily the pigment associates with oxygen but
the more easily it dissociates from it
Effect of carbon dioxide on the oxygen dissociation curve (Bohr’s effect)
Within tissues there is a high concentration of carbon dioxide produced during aerobic respiration
C6H12O6 + 6O2 6CO2 + 6H2O
Increase in carbon dioxide concentration decreases the affinity of haemoglobin for oxygen, by
making the pH of the surrounding medium more acidic (low), thereby shifting the oxygen
dissociation curve to the right. This shifting of the curve to the right is known as Bohr’s effect i.e. the
shifting of the oxygen dissociation curve to the right due to the increase in partial pressures of carbon
dioxide which results into haemoglobin having a low affinity for oxygen and a high affinity for
carbon dioxide.
Page 264 of 447

Bohr’s effect may be defined as ‘the lowering of the affinity of blood’s haemoglobin for oxygen due
to increased acidity caused by increase in carbon dioxide concentration’.
From the dissociation curves above,

shifting the oxygen dissociation curve to
the left means that haemoglobin has a
higher affinity for oxygen and therefore
becomes fully saturated with oxygen it at
very low partial pressures of oxygen.
It also means that haemoglobin has a low rate of
dissociation to release oxygen to the tissues but a high
rate of combining with oxygen.
Shifting of the oxygen dissociation curve to the right
means that haemoglobin has a lower affinity for oxygen
and a higher rate of dissociation to release oxygen to
the tissues rapidly to support tissue respiration
Effect of carbon monoxide on the affinity of haemoglobin for oxygen

There’s a loose and reversible reaction between oxygen molecules and iron (II) atoms of haem
groups of haemoglobin to from oxyhaemoglobin. This means that iron (II) is not oxidized to iron
(III) as haemoglobin combines with oxygen.
In the presence of carbon monoxide and oxygen, haemoglobin combines readily with carbon
monoxide to form a permanent compound known as carboxyhaemoglobinrather than combining with
oxygen.
A permanent carboxyhaemoglobin compound is formed because carbon monoxide oxidizes iron (II)
to iron (III). This reduces the free haemoglobin molecules available to transport oxygen molecules to
the tissues, which makes the tissues develop symptoms of anoxia (total lack of oxygen in the tissues).
Page 265 of 447

Therefore, carbon monoxide is referred to as a respiratory poison because it can readily combine
with haemoglobin much more than oxygen and the product formed i.e. carboxyhaemoglobin does not
dissociate.
Note; smokers have 10% of their total haemoglobin in form of carboxyhaemoglobin.
Myoglobin and other pigments

The oxygen dissociation curves for myoglobin lies to the left of that of haemoglobin as shown in the graph
Myoglobin is a respiratory pigment which also contains (Kent fig 3 pg 131 OR Clegg fig
iron containing haem groups mostly found in the muscles 17.31 pg 360)
where it remains fully saturated at partial pressures below
that required for haemoglobin to give up its oxygen.
Myoglobin has a higher affinity for oxygen than
haemoglobin in a way that it combines readily with
haemoglobin and it becomes fully saturated with oxygen at
a lower partial pressure of oxygen.
Myoglobin acts as a store of oxygen in resting muscles in
form of oxymyoglobin and only releases the oxygen it
stores only when oxyhaemoglobin has been exhausted i.e.
many vigorous activities because myoglobin has a higher
affinity for oxygen than haemoglobin.
Note;
1. High affinity refers to low rate of dissociation of oxyhaemoglobin to release oxygen and a higher
rate of association of haemoglobin with oxygen.
2. Low affinity refers to higher rate of dissociation of oxyhaemoglobin to release oxygen and a
lower rate of association of haemoglobin with oxygen.
Comparison between the oxygen dissociation curve for Lugworms’ (Arenicola) haemoglobin
and that of Man
The oxygen dissociation curve of the lugworm’s (Clegg fig 17.32 pg 360 OR Toole fig
haemoglobin lies on the left of that of man’s 21.5 pg 416)
haemoglobin as shown in the graph below This
indicates that the haemoglobin of the lugworm has a
higher affinity for oxygen than that of man. This is
because the lugworm lives in oxygen deficient mud and
so in order to extract enough oxygen from that
environment of low oxygen tension, the haemoglobin
of the lugworm must have a higher affinity for oxygen
than that of man thriving in a well-supplied
environment with oxygen.
This implies that the lugworm’s haemoglobin
dissociates to release oxygen to its tissues compared to
that of man which makes the lugworm less active than
man, who releases much oxygen rapidly to the tissues.
Page 266 of 447

Comparison between the oxygen dissociation curves of different sized mammals

The smaller animal size, the higher the surface
area to volume ration. Small animals therefore
lose a lot of heat from their surfaces and in
order to maintain a constant internal body
temperature, they have to produce a lot of heat
to compensate for the lost heat. Such animals
therefore higher metabolic rates and so need
more oxygen per gram of tissue than larger
animals. Therefore they have blood that gives
up oxygen more readily i.e. their dissociation
curves are on the right of the larger animals
Comparison between the oxygen dissociation curves at rest and during exercise
During exercise, the oxyhaemoglobin releases
oxygen more readily hence the oxygen
dissociation curve during exercise is to the
right of the curve when at rest.
Comparison between the oxygen

dissociation curve of maternal haemoglobin
and that of the foetal haemoglobin
The oxygen dissociation curve of foetal
haemoglobin lies to the left of maternal (Clegg fig 17.36 pg 363 OR Toole fig 21.7 pg
haemoglobin as shown in the diagram besides; 416 OR Soper fig 14.32 pg 481)
This indicates that the foetal hemoglobin has a
higher affinity for oxygen than that of the
mother. This enables the foetal haemoglobin
to pick sufficient oxygen from the mother via
the placenta and also increases on the oxygen
carrying capacity to the tissues, especially
when the foetus needs a lot of energy.
It also increases on the oxygen carrying
capacity to the tissues of the foetus in the
situation whereby deoxygenated and
oxygenated blood are mixed due to the
bypasses of ductus arteriosus and foramen
ovale in the foetus.
Effect of temperature on haemoglobin oxygen dissociation curve
A rise in temperature lowers the affinity of haemoglobin for oxygen thus causing unloading from the
pigment i.e. a rise in temperature increases the rate of dissociation of oxyhaemoglobin to release
oxygen to the tissues.
Page 267 of 447

Increased tissue respiration which occurs in the skeletal muscles during exercise generates heat. The
subsequent rise in temperature causes the release of extra oxygen from the blood to the tissues. This
is so because increase in temperature makes the bonds which combine haemoglobin with oxygen to
break, resulting into the dissociation of oxyhaemoglobin.
Oxygen dissociation curve for haemoglobin at different temperatures
Effect of changing altitude on oxygen carriage

There is a decrease in the partial pressure of oxygen in the atmosphere with increase in altitude from
sea level. Therefore the volume of oxygen is less at high altitudes than at sea level. When an
organism moves from the sea level to high altitudes, very fast, such an organism tends to develop
symptoms of anoxia (lack of oxygen) which include headache, fatigue, nausea, and becoming
unconscious. However, when an organism moves slowly from sea level to high altitudes like the
mountain climbers, such an organism can at first develop symptoms of anoxia but later on such
symptoms disappear due to adjustments in the respiratory and circulatory systems in response to
insufficient oxygen reaching the tissues from the surrounding.
The amount of haemoglobin and the red blood cell count increases together with the rate of breathing
and the heartbeat. More red blood cell formation occurs in the bone marrow under the control of the
hormone called erythropoietin secreted by the kidney. Secretion of erythropoietin is stimulated by
lower oxygen tension in the tissues. Increase in the amount of haemoglobin and red blood cells
together with increase in the breathing rate and heart beat increases the oxygen carrying capacity of
the blood to the tissues which leads to the disappearance of the symptoms of anoxia and which also
makes the individual organism to be acclimatized.
Acclimatization is therefore a condition whereby an organism carries out a series of physiological
adjustments in moving from a low altitude area to a high one to avoid symptoms of anoxia so that
such an organism can survive in an environment of low oxygen content.
Page 268 of 447

The graphs below show the oxygen dissociation curves of people living at sea level and at high altitude
(Clegg fig 17.37 pg 363 OR Toole fig 21.4 pg 415 OR Soper fig 14.31 pg 481 OR Simpkins fig 8.19 pg 145)
The mammals that live in regions of the world beyond
the sea level e.g. mountains solve the problem of lack
of enough oxygen in the atmosphere by possessing
haemoglobin with a higher affinity for oxygen than
that of mammals at sea level. This enables the high
altitude mammals to obtain enough oxygen through
the oxygen deficient environment e.g. the llama. This
explain why the oxygen dissociation curve of the
haemoglobin of the llama lies to the left of that of
other mammals at sea level. The vicuna long necked
member of the camel family that stays in the high
alpine areas of the Andes
Mammals living at high altitudes

1. These possess an improved capillary network in the lungs which coupled with their deeper
breathing (hyperventilation) insures increased oxygen uptake.
2. They have an increased red blood cell which increases the amount of oxygen transported by
blood.
3. Increased haemoglobin concentration in the red blood cells which improves the amount of
oxygen transported by the blood.
4. Changes in haemoglobin affinity for oxygen. Here the oxygen dissociation curve is shifted to the
right to facilitate release of oxygen to the tissues. This particularly occurs at relatively lower
altitudes.
5. Mammals living at altitudes about 3500m have their oxygen dissociation curves shifted to the left
this favours their survival by promoting an increased affinity for oxygen by haemoglobin.
6. Increased myoglobin levels in muscles myoglobin has a higher affinity for oxygen than
haemoglobin. This facilitates the exchange of oxygen from the blood to the tissues making
oxygen available to the tissues.
Diving mammals e.g. seals, dolphins and whales.
1. They have a large spleen which can store large volumes of blood e.g. the seals spleen stores 24l
of blood after the dive has begun, the spleen contracts and supplies the blood in circulation with
additional erythrocytes that are highly leached with oxygen.
2. Have high concentration of myoglobin in their muscles. Myoglobin is an oxygen storing protein.
3. Mammals during the diving reflex slow down the pulse as the heart beat is also slowed down in
order to effect an overall reduction on oxygen consumption since there is reduced cardiac output
to the tissues.
4. Store oxygen in their blood as oxyhaemoglobin and this they achieve by having concentration of
haemoglobin.
5. Blood supply to muscles is restricted and completely cut off during the longest dives hence
encouraging anaerobic instead of aerobic respiration.
6. In this way, the muscles use sparingly oxygen stored in their myoglobin.
Page 269 of 447

TRANSPORT OF CARBON DIOXIDE

Carbon dioxide is transported from the body tissues mainly inform of bi-carbonate ions in blood
plasma to the lungs for removal.
Although carbon dioxide is mainly transported inform of bi-carbonate ions i.e. 85%, carbon dioxide
can also be transported in the following ways;
a. About 5% of carbon dioxide is transported in solution form. Most of the carbon dioxide
carried in this way is transported in physical solution. A very small amount is carried as
carbonic acid. In the absence of haemoglobin, the plasma proteins buffer the hydrogen ions to
form weak proteionic acids.
b. About 10% of carbon dioxide combines with the amino group of haemoglobin to form a
neutral compound known as carbamino haemoglobin (HbCO2).If less oxygen is being
carried by haemoglobin molecule, then more carbon dioxide is carried in this way as HbCO2.
Transportation of carbon dioxide inform of hydrogen carbonate ion

When carbon
dioxide is formed
during respiration, it
diffuses from the
tissues into the
erythrocytes, via
their thin and
permeable
membrane. Inside
the erythrocytes,
carbon dioxide
reacts with water in
the presence of
carbonic anhydrase
enzyme to form
carbonic acid as
shown below;
H2O (l)+ CO2 (g) H2CO3 (aq)

Carbonic anhydrase
The formed carbonic acid then dissociates into hydrogen ions and bicarbonate ions as shown below
H2CO3 (aq) H+ + HCO-3 (aq)
The formed hydrogen ions decrease the pH in erythrocytes which results into the dissociation of
oxyhaemoglobin being carried from the lungs to the tissues into the free haemoglobin molecules as
free oxygen molecules.
The free oxygen molecules diffuse into the tissues to be used in respiration. The free haemoglobin
molecules buffer the hydrogen ions (H+) inside the red blood cells into a weak acid known as
haemoglobinic acid
H++ Hb HHb
Page 270 of 447

In case of excess H+ plasma proteins are used to buffer them into another weak acid called proteinic
acid.
The formed hydrogen carbonate ions within the erythrocytes diffuse out into the plasma along the
concentration gradient and combine with sodium to form sodium hydrogen carbonate which is then
taken to the lungs.
HbO8 Hb + 4O2 (g)
The outward movement of bicarbonate ions from the erythrocytes into the plasma results into an
imbalance of positively charged and negatively charged ions within the cytoplasm. In order to
maintain electrochemical neutrality, to remove this imbalance in the red blood cells, chloride ions
diffuse from the plasma into the red blood cells, a phenomenon known as the chloride shift
When the bicarbonate ions reach the lungs, they react with H+ to form carbonic acid which
eventually dissociates into carbon dioxide and water.
H++ HCO3- H2CO3
H2CO3- H2O + CO2
The carbon dioxide and water formed from the dissociation of carbonic acid in the lung capillaries
are then expelled out by the lungs during exhalation so as to maintain the blood pH constant
VASCULAR SYSTEMS IN ANIMALS

In animals, every vascular system has at least three distinct characteristics.
a. It has a circulating fluid e.g. blood
b. It has a pumping device inform of a modified blood vessel or a heart.
c. It has tubes through which the fluid can circulate e.g. blood vessels
Note; animals require a transport system because of;
1. Surface area of the organism
2. Surface area: volume ratio of the organism
3. Activity of the organism
4. The diffusion distance for the transported substances between the tissues to and from their
sources.
5. There are two types of vascular systems, the open vascular system and the closed vascular
system.
Open vascular system not on syllabus

Open circulation is the flow of blood through the body cavities called Haemocoel instead of flowing
in blood vessels. This exists in most arthropods, molluscs and tunicates.
In this system, blood is pumped by an aorta which branches into a number of arteries which open
into the haemocoel. From the haemocoel, blood under low pressure moves slowly to the tissues
where there’s exchange of materials e.g. gases, nutrients e.t.c. from the haemocoel blood percolates
back into the heart via the open ended veins.
In insects the haemocoel is divided into two parts by a transverse pericardial membrane forming a
pericardial cavity dorsally and the ventral perivisceral cavity.
Page 271 of 447

Aorta Arteries Haemocoel Veins
Heart
In the body of the insects there are no blood vessels except the tubular heart which is suspended in
the pericardial cavity by slender ligaments and extends through the thorax and abdomen. The heart is
expanded in each segment to form a total of 13 small chambers which are pierced by a pair of tiny
tubes called ostia. The ostia allow blood to flow from one segment of the chamber to another. Alary
muscles are located at each chamber of the heart.
Closed vascular system

In a closed vascular system, blood flows in blood vessels or sinuses. It oc curs in all vertebrates,
annelids such as earthworms, cephalopods and echinoderms. The distribution of blood in this system
is therefore adjustable e.g. blood from the heart is at high pressure and that to the heart is at low
pressure. Closed vascular systems are further divided into single and double circulation.
Single and double circulation

Single circulation is the flow of blood through the heart once for every complete circulation around
the body. Single circulation occurs in fish and the deoxygenated blood from the body tissues is
pumped by the heart to the gills from where it flows back to the body tissues and eventually returns
to the heart.
Diagram showing single circulation in fish
The problem of single circulation is
that blood tends to move very slowly at
the venous side due to the significant
drop in pressure before completing the
circulation. The drop in pressure is as a
result of capillaries having a
considerable resistance to blood flow
i.e. capillaries in the gills and body
tissues. The sluggishness of blood flow
at the venous side is solved by
replacing the veins with large sinuses
Vascular system of the earthworm (annelid)

The earthworm belongs to phylum annelida. Annelids are coelomate animals i.e. they have a body
cavity that separates the muscular wall of the animal from the internal organs
Page 272 of 447

The largest vessel is the longitudinal Transverse section of the annelid vascular system
muscular-walled dorso vessel and it is Clegg fig 17.6 pg 344
above the alimentary canal (gut). The
peristaltic contraction from the posterior
end of the vessel drives blood forward to
the anterior end of the animal. The
backflow of blood is prevented by valves.
Each valve originates from a fold of an
internal membrane or tissue of any blood
vessel that is called an endothelium.
The dorso vessel collects and receives
blood from the body wall, the gut, the
nerve cord and the nephridia via
capillaries.
The dorso vessel connects with the smaller more contractile ventral vessel via five pairs of
contractile pseudo hearts.
Each pseudo heart has four valves which permit the blood to flow towards only the ventral vessel
and back to the posterior end of the animal.
Between the ventral vessel and the organs in the coelom e.g. nephridia and gut, there are a series of
segmented blood vessels which run between them and they end up forming capillaries where there is
exchange of materials between the organs and the blood in the capillaries. From the capillaries, blood
fills its way back to the dorso vessel for its flow to the anterior side due to the peristaltic movement
of the dorso vessel. The blood is red in colour with haemoglobin.
Double circulation
Double circulation is the flow of blood through the heart twice for every complete circulation around
the body.
In double circulation deoxygenated blood from body Diagram showing double circulation in
tissues is pumped from the heart to the lungs from a frog and a mammal
where it returns to the heart after being oxygenated and
it is then re-pumped to the body tissues so as to supply
oxygen to the body tissues. A double circulation serves
as one of the solutions towards the sluggish flow of
blood at the venous side in single circulation
In double circulation, the heart must be divided into the
left and right chambers to prevent oxygenated blood
from mixing with deoxygenated blood e.g. in reptiles,
birds and mammals have a four chambered heart made
up of the right atrium and ventricle and the left and
atrium and ventricle.
Page 273 of 447

The frog experiences double circulation although its heart has three chambers namely; one ventricle
and the two atria i.e. the left and right atria.
Both deoxygenated and oxygenated blood in the frog flow through the same ventricle and conus
arteriosus at the same time without mixing. This is achieved due to the folding in the walls of the
ventricle which enhances the separation of deoxygenated blood from oxygenated blood and this
separation is also facilitated by the spinal valves in the conus arteriosus.
Some organisms e.g. the octopus and squids solve the problem of sluggish flow of blood of the
venous side by possessing brachial hearts which pump deoxygenated blood from the body tissues of
the gills and eventually back to the main heart. The main heart pumps, oxygenated blood to body
tissues from the gills.
(Roberts fig 11.16 pg 175)
Page 274 of 447

MAMMALIAN BLOOD CIRCULATION

The mammalian blood circulation is a double blood circulation which is mainly based on the heart
and blood vessels,
THE MAMMALIAN HEART
Structure of the mammalian heart
The heart is the muscular organ pumping blood to
all body organs using its chambers. It is made up
of four chambers which include the right and left
atria (auricles) and the right and left ventricles.
The four chambers enhance the blood flow
through the heart at the same time without mixing
it i.e. the deoxygenated blood is separated from
oxygenated blood oxygenated blood flows through
the left atrium and ventricle while the
deoxygenated blood flows through the right atrium
and ventricle.
The heart is composed of the cardiac muscles within its walls which are myogenic in nature, in a
way that, the initiation of their contraction is not under the control of the central nervous system but
is within the muscles themselves.
This enables them to contract continuously and rhythmically without fatigue and therefore enables
the heart to beat and pump without stopping. The heart consists of atrioventricular valves/ pocket
valves and semi lunar valves. The atrioventricular valves include the following;
The three (3) flapped tricuspid valves found between the right atrium and the right ventricle
Page 275 of 447

The two (2) flapped bicuspid valves which prevent back flow of blood from the left ventricle to the
left auricle
The semi lunar valves are prevented from turning inside out by connective tissues called tendinous
cords
The heart is linked with four blood vessels (1) The venacava which transports deoxygenated blood
from body tissues through the right atrium of the heart. (2) The pulmonary artery which transports
deoxygenated blood from the right ventricle of the heart to the lungs. (3) The pulmonary vein
which transports oxygenated blood from the lungs into the left atrium of the heart. (4) The aorta
which is the biggest vessel and it transports oxygenated blood from the left ventricle of the heart to
the body tissues.
The left ventricle is more muscular (thicker) than the right ventricle because the left ventricle has to
contract more powerfully than the right ventricle in order to enable oxygenated blood with high
pressure to move for a long distance to the body tissues unlike the right ventricle which pumps
deoxygenated blood with low pressure for a short distance to the lungs.
Initiation of the heart beat

The cardiac muscle within the walls of the heart is myogenic in nature in a way that the initiation of
its contraction is within the muscle itself, but not under the control of the central nervous system
(brain and spinal cord). This enables the muscles to contract continuously and rhythmically without
fatigue to enable the heart to beat continuously and rhythmically without stopping. The intrinsic
initiation of the heart beat enables the heart to remain beating even it is surgically removed from the
body, provided it is under ideal conditions.
The rhythmic contraction of the cardiac muscles Clegg fig 17.8 pg 347
is initiated by specialized network of fine
cardiac muscles network found inside the wall
of the right atrium close to the entrance of blood
from venacava into the right atrium.
This network of fine cardiac muscle fibre is
known as Sino Atrial Node (SAN) and it serves
as a pace maker by giving off a wave of
electrical excitations similar to impulses, which
spread out very rapidly over both atria causing
them to contract and force blood into the
ventricles via the open atrial ventricular valves.
When the electrical excitations reach the
junction at the boundary of the atria, they excite
another specialised plexus of other cardiac
muscle chambers known as Atrio Ventricular
Node (AVN)
When excited, the AVN sends waves of electrical excitations down to another bundle of cardiac
muscle of fibres formed along the inter-ventricular septum called the Purkinje tissue or Bundle of
His to the apex of the heart. This conducts and spreads the excitement to both ventricles which
eventually pump blood into the arteries.
NOTE;
Page 276 of 447

1. The closing of the atrioventricular valves during ventricular systole produces the first heart
sound, described as lub.
2. The closing of the semi lunar valves causes the second heart sound, described as dub.
3. The pulse in the arteries is due to ventricular systole and elastic recoil of the arteries due to high
pressure of blood.
4. The pulse is more pronounced in the arteries
5. The PCG (phonocardiogram) is a recording of the sound the heart makes. The cardiac muscle
itself is silent and the sounds are made by the valves when closing. The first sound (lub) is the
atrioventricular valves closing and the second sound (dub) it is the semi lunar valves closing.
6. The ECG (electrocardiogram is a recording of the electrical activity of the heart. There are
characteristic waves of electrical activity marking each phase of the cardiac cycle. It begins with
a P wave, atrial depolarisation and the spread of the through the atria. The QRS complex
indicates ventricular depolarisation. The T wave represents ventricular repolarisation. Changes in
these ECG waves can be used to help diagnose problems with heart.
Page 277 of 447

The cardiac cycle (Sequence of the heart beat)

This is the sequence of events of heart beat by which blood is pumped around the body. The
pumping action of the heart consists of alternate contractions of heart muscles (cardiac muscles)
called systoles and relaxations called diastoles. The term cardiac output refers to the volume of
blood pumped from each ventricle.
The cardiac cycle begins with the contractions of the atria i.e. atrial systole, which is initiated by
SAnode and it which causes the atria volume to decrease and the atria pressure increases. As the atria
contracts, the ventricles relax i.e. undergo ventricular diastole, causing the bicuspid and tricuspid
valves to close. The contraction of the atria due to blood entering the atria forces the bicuspid and
tricuspid valves to open so that blood moves from atria into the ventricles.
Contraction of atria walls has an effect of sealing off the venacava and pulmonary veins, thereby
preventing the back flow of blood into the vessels as the blood pressure rises within the atria. It takes
0.1 seconds.
When the ventricles are filled with blood from atria, their walls contract simultaneously i.e.
ventricular systole, and the atria relax i.e. atrial diastole. Ventricular systole is initiated by
impulses from AVnode to the bundle of His, Purkije fibres and rapidly through the ventricle muscles.
The ventricles’ volume reduces while the pressure increases, forcing the bicuspid and tricuspid
valves to close and prevent the back flow of blood into the atria. The increased pressure in the
ventricles also forces blood to be pumped into the pulmonary artery via the open semi lunar valves
from the ventricles. This enables the blood to be pumped into the lungs via the pulmonary artery and
into the body tissue via the aorta.
The ventricular systole is more powerful than the atrial systole because the ventricles are more
muscular than the atria and therefore generate more pressure. The powerful ventricular systole forces
blood into the atria and pulmonary artery.
After ventricular systole, there’s a short period of simultaneous atrial and ventricular relaxations. In
the ventricular diastole, the high pressure developed in the ventricles causes a slight back flow of
blood which closes the semi lunar valves, thereby reducing blood back flow.
Relaxation of the atrial wall and contraction of the ventricle, initiates the refilling of the atria by
blood under relatively low pressure i.e. deoxygenated blood in the venacava flows into the right
atrium and oxygenated blood from the lungs flows into the left atrium via the pulmonary vein.
Intrinsic control of the heart beat
The cardiac muscle in the heart is myogenic. It contracts and relaxes automatically and does not
depend on stimulation by nerves. The initial stimulus originates from the sino-atrial node (SAN),
often called the pacemaker. The pacemaker is found in the right atrium wall at the entrance of the
superior venacava. The membranes of the cells of the SAnode are permeable to sodium ions. Sodium
ions enter into these cells and the cell membranes are depolarized.
An excitatory wave of depolarization is generated which spreads rapidly from the SA node across the
two atria causing them to contract simultaneously. A slowing down occurs as depolarization of the
atrio-ventricluar node (AVN) is delayed for about 0.1s to allow the atria to complete their contraction
and empty the blood into the ventricles. Impulses from the AV node are conducted by specialized
muscle fibres called bundle of His in the inter-ventricular septum towards the heart apex. Impulses
are conducted by Purkinje fibres (Purkyne tissue) throughout the ventricular walls. This causes the
contraction of both ventricles forcing blood into the pulmonary arteries and the aorta.
Page 278 of 447

Characteristics of the cardiac muscle in relation to excitation and contraction

1. The absolute relative refractory period is longer than that of other muscles i.e. the heart cannot be
fatigued easily
2. The generation of the wave from the SAN has a refractory period between contraction of the
heart and relaxation of the heart i.e. the waves are not generated continuously.
Intrinsic control of the heart beat

The cardiac muscle in the heart is myogenic. It contracts and relaxes automatically and does not
depend on stimulation by nerves. The initial stimulus originates from the sino-atrial node (SAN),
often called the pacemaker. The pacemaker is found in the right atrium wall at the entrance of the
superior venacava. The membranes of the cells of the SAnode are permeable to sodium ions. Sodium
ions enter into these cells and the cell membranes are depolarized.
An excitatory wave of depolarization is generated which spreads rapidly from the SA node across the
two atria causing them to contract simultaneously. A slowing down occurs as depolarization of the
atrio-ventricluar node (AVN) is delayed for about 0.1s to allow the atria to complete their contraction
and empty the blood into the ventricles. Impulses from the AV node are conducted by specialized
muscle fibres called bundle of His in the inter-ventricular septum towards the heart apex. Impulses
are conducted by Purkinje fibres (Purkyne tissue) throughout the ventricular walls. This causes the
contraction of both ventricles forcing blood into the pulmonary arteries and the aorta.
Characteristics of the cardiac muscle in relation to excitation and contraction

1. The absolute relative refractory period is longer than that of other muscles i.e. the heart cannot be
fatigued easily
2. The generation of the wave from the SAN has a refractory period between contraction of the
heart and relaxation of the heart i.e. the waves are not generated continuously.
Hormonal control of the heat rate

A number of hormones affect the heart rate by stimulating the SAnode, either directly or indirectly.
Those with a direct effect are considered
a) Adrenaline hormone. Adrenaline is secreted by the medulla (middle) of the adrenal glands.
The adrenal medulla also secretes smaller amounts of hormone noradrenaline which has
similar effects to adrenaline. Both stimulate the heart, although adrenaline is more effective.
Cardiac output and blood pressure are increased heart rate. The two hormones also have other
effects on the body which prepare the body for action (the ‘flight or fight’ response).
b) Thyroxine. This is produced by the thyroid gland which raises the basal metabolic rate. This
in turn leads to greater demand for oxygen production of more heat. As a result, vasodilation
(dilation of the blood vessels) followed by increased blood flow occurs, and this leads in turn
to increased cardiac output. Heart rate is also directly stimulated by thyroxine.
Control of the rate of the heart beat

Through the initiation of the contraction of cardiac muscle and hence initiation of heart beat are not
under the control of the central nervous system, the rate at which the heart beats to pump blood is
under the control of the autonomic (Involuntary) nervous system.
Page 279 of 447

The heart is innervated by the sympathetic nerve from (Clegg fig 17.13 pg 350 OR Soper fig
the sympathetic autonomic nervous system and by the 14.24 pg 475)
vagus nerve, a branch of a parasympathetic autonomic
nervous system. The nerves modify the rate at which
the pace maker gives waves of electrical excitations
hence controlling the speeding up or slowing down of
the rate of the rate of heart beat.
When the rate of heart beat increases beyond the
normal rate, the vagus nerve (parasympathetic nerve)
is stimulated to release acetylcholine such that it
lowers rate of the heart beat back to normal
If however, the rate of the heart beat lowers below the
normal rate or if there’s need for higher rate of heart
beat the sympathetic nerve releases noradrenaline to
bring back or increase to the cardiac frequency
usually to the normal rate. Therefore the sympathetic
and vagus nerves are antagonistic, functionally.
Cardiac output
It refers to the volume of blood pumped out from the heart, per minute by one ventricle.
Cardiac output (volume of blood Rate of heart beat X Cardiac frequency
going out of the heart) =
a) Stroke volume is the strength of the heart beat measured in volume of blood per heart beat
b) Heart rate is the number of heart beats per minute
Cardiac output is regulated by the autonomic Soper fig 14.25 pg 476
nervous system. The output increases when there
is an increase in body activity. This serves to
supply more oxygen and glucose to respiring
cells and remove waste products.
Prolonged athletic training strengthens the heart,
increasing the heart muscles and enlarging the
heart chamber. This leads to an increase in
strength of cardiac muscle contraction and an
increase in the stroke volume. Thus, at rest, the
trained athlete has a higher cardiac output than
an untrained person Explain the relationship between;
- Stroke volume and heart beats
- Cardiac output and heart beats
Short term effects of exercise on the cardiovascular system
a) Cardiac output increases
b) Vasodilation or vasoconstriction of different blood arterioles redistributes the blood towards
muscles and away from organs such as kidneys and intestines, whose need is less immediate.
The heart needs more blood to maintain the higher cardiac output and the brain must continue
Page 280 of 447

to function normally in order to coordinate activities – its supply is therefore largely

unchanged during exercise. As exercise generates heat, the blood supply to the skin is
increased during exercise to help dissipate this heat to the environmet.
c) Increase in systolic blood pressure although diastolic pressure is largely unchanged.
Long term effects of exercise
a) Hypertrophy of the heart i.e. increase in size of the heart (cardiac muscle)
b) Increase in the stroke volume
c) Decrease in the resting heart rate
d) Increased maximum cardiac output
e) Increased volume of blood
f) Decrease in blood pressure when at rest
g) Increased in the number of blood capillaries
Internal factors affecting the heart beat

1. Body temperature 6. Salt balance
2. Blood pH 7. Blood pressure
3. Carbon dioxide concentration 8. Emotional situations
4. Partial pressure of oxygen 9. Impulses from the venacava and aorta
5. Hormonal balance
QN. Explain how change in each of the above factors may affect the heart beat
BLOOD VESSELS
There are three main types of blood vessels; arteries, veins and capillaries. The walls of these blood
vessels occur in three layers, namely; (1) Tunica externa (outer most layer), (2) Tunica media
(middle layer) & (3) Tunica interna (inner most layer)
Tunica externa, this is the outermost Diagrams showing the transverse sections of the
layer which is tough and made up of thick vein, artery and capillary
collagen fibres which provide strength and
prevents extensive stretching.
Tunica media is the middle layer which
consists of smooth muscles, collagen and
elastic fibres. The structural proteins allow
for the stretching of the walls of blood
vessels during vasodilation. The smooth
muscles allow for the distension and
constriction of the walls of the blood
vessels.
Tunica interna is the innermost layer composed of a single layer of squamous endothelium. It is
found in all walls of blood vessels. Capillaries have only the tunica interna
Arteries transport oxygenated blood from the heart to the tissues except the pulmonary artery which
transports deoxygenated blood from the heart to the lungs while veins transport deoxygenated blood
from tissues to the heart except the pulmonary vein which transports oxygenated blood from the lungs
to the heart. Therefore arteries can be defined as blood vessels which transport blood away from the
heart and veins are defined as blood vessels which transport blood from the tissues to the heart.
Page 281 of 447

Comparison between arteries and veins

1. Both tunica media and tunica externa are more developed in arteries than veins and therefore
arteries have thicker walls than those of veins. Arteries have thicker walls than veins because
blood flows through them at a higher pressure than in the veins, due to the pumping action of
blood by the heart. Arteries therefore have thicker walls to counteract the pressure by which
blood moves through them. The capillaries lack both the tunica externa and the tunica media.
2. In addition the walls of the arteries are more elastic than those of veins, in order to overcome the
pressure by which blood flows through them by rapidly stretching without bursting.
3. Also arteries have a narrower lumen than veins, which increases the pressure of the blood
flowing through them.
4. Arteries also lack valves while veins haves valves which prevent the backflow of blood in veins.
However, arteries do not need valves since they transport blood under high pressure, which
pressure ensures that blood flows forward
Adaptations of blood Diagram showing the capillary network
capillaries Clegg fig 17.18 pg 353
1. They possess the capillary
sphincter muscles which
contract and relax so as to
regulate the amount of
blood entering into the
capillary network.
2. Some capillaries have a
bypass arterio-venous
shunt vessel which links
the arterioles and venules
directly so as to regulate
the amount of blood
which flows through the
capillary network e.g. in
the capillaries of the feet,
hands, stomach e.t.c.
3. They are numerous in
number to provide a large
surface area which
increases the rate of
diffusion and allows rapid
exchange of materials
between blood and the
tissue fluid.
4. The capillary network offers maximum resistance to blood flowing through them hence
decreasing the speed of blood flow which allows the maximum diffusion and exchange of
materials between blood and the tissues. Blood capillaries are the smallest blood vessels found in
close contact with tissues in form of a dense network which allows a high rate of diffusion of
materials during their exchange between the blood circulatory system and the tissues.
Page 282 of 447
5. They have a thin and permeable membrane which is made up of thin flattened pavement cells
which allow rapid diffusion and exchange of materials between blood and tissues with minimum
resistance.
Adaptations of veins to their function
1. The elastic layer is relatively thin because blood is under low pressure, cant cause them to
burst and the pressure is too low to create a recoil action
2. The muscular wall is relatively thin because veins carry blood away from tissues and
therefore their dilation and constriction cannot control the flow of blood to the tissues
3. The collagen fibres provide a tough outer layer in order to prevent the veins bursting from the
external forces
4. There are semilunar valves throughout to ensure that blood does not flow backwards, which it
might otherwise do because the pressure is so low.
5. The overall thickness of the wall is small because there’s no need for a thick wall as the
pressure within the veins is too low to create any risk of bursting.
6.
Blood flow velocity
The speed of blood flow reduces as it moves from arteries to arterioles to capillaries. Each artery
conveys blood to so many capillaries that the total cross-sectional area is much greater in capillary
beds than in the arteries or any part of the circulatory system. The result is an decrease in velocity
from the arteries to capillaries than in the aorta.
The reduced velocity of blood flow in capillaries is critical to the function of the circulatory system.
Capillaries are the only vessels with walls thin enough to permit the transfer of substances between
the blood and interstitial fluid. The slower flow of blood through these tiny vessels allows time for
exchange to occur. After passing through the capillaries, the blood speeds up as it enters the venules
and veins, which have smaller total-sectional areas
Page 283 of 447

Blood pressure
Contraction of the heart ventricle
generates blood pressure, which
exerts a force in all directions. The
force directed lengthwise in artery
causes the blood to flow away from
the heart, the site of highest pressure.
The force exerted against the elastic
wall of an artery stretches the wall,
and the recoil of the arterial wall
plays a critical role in maintaining
blood pressure, and hence blood
flow, throughout the cardiac cycle.
The numerous arterioles and
capillaries offer resistance to blood
flow hence reducing the blood
pressure.
Changes in blood pressure during the cardiac cycle
Blood in arteries moves inform of pulses while
in veins is flows smoothly without any pulse.
A pulse is a series of waves of dilation that pass
along the arteries caused by the pressure of the
blood pumped from the heart through
contractions of the left ventricle. Arterial blood
pressure is highest when the heart contracts
during ventricular systole, this is systolic
pressure, which causes the expansion of the
arterial wall. This is also due to the narrow
openings of arterioles impeding the exit of blood
from arteries. Hence, when the heart contracts,
blood enters the arteries faster than it can leave,
and the vessels stretch from the rise in pressure.
During diastole, the elastic walls of the arteries snap back. As consequence, there’s a lower but still
substantial blood pressure when ventricles are relaxed (diastolic pressure). Before enough blood has
flowed into the arteries to completely relieve pressure in the arteries, the heart contracts again.
Because the arteries remain pressurized throughout the cardiac cycle blood continuously flows into
arterioles and capillaries.
NOTE:
Blood is expelled from the heart only when it contracts. Blood flow through the arteries is therefore
intermittent, the blood flowing rapidly during systole and slowly during diastole. However, by the
time the blood reaches the capillaries it is flowing evenly. The gradual change from intermittent to
even flow is made possible by the elasticity of the of the arterial walls which contain elastic tissue
and smooth muscles
Page 284 of 447

Control of blood pressure

Small receptors which are sensitive to stretching, called baro receptors are found in the walls of
aortic arc, carotid sinuses, vena cava and the right atrium become stimulated when blood pressure
increases above the norm. They fire impulses to the vasomotor centre and cardio vascular centre
found in the medulla oblongata of the brain via the afferent nerves (sympathetic nerves). The cardio
vascular centre sends impulses to the heart via the efferent nerves (vagus nerves), which results into
reduction of the cardiac output. The vasomotor centre on receiving impulses, its sympathetic output
is suppressed and this lowers the blood pressure by causing vasodilation of the arterioles
When the blood pressure lowers below the norm, the baro receptors stop being stimulated and this
leads to impulses being fired from the cardio vascular centre to heart. The cardiac output is then
increased. Decrease in blood pressure also increases the vasomotor centre sympathetic output which
results into vasoconstriction of the arterioles hence increasing the blood pressure back to normal.
NOTE: When the arterioles constrict (vasoconstriction) blood pressure is raised and when they
dilate (expand) the blood pressure decreases.
Stimulus Receptor Regulator Effectors Response
Pressure Baro- Brain Muscles, glands,

receptors arterioles & Response
change
cardiac muscle
s
The brain includes the vasomotor, cardiovascular

Note:-Blood pressure depends on the
centre and following
the medulla factors;
oblongata
1. Blood volume
2. Force of the heart
3. Blood vessel radius/ diameter of the lumen
4. Blood volume is adjusted to some extent through contraction of the spleen and liver which bring
stored blood into circulation. The stored blood is due to the regulation of the fluid intake and
fluid loss by organs such as the kidney and the skin during homeostasis.
Blood vessels offer resistance Clegg fig 17.17 pg 352 OR Soper fig 14.26 pg 477]
(R) to blood flow. The
resistance is inversely
proportional to the fourth
power of the radius (r) of the
1
vessel (R α 4 ). Therefore,
r
the resistance increases as the
vessel becomes narrower and
since we are dealing with the
fourth power of the radius,
small changes in the arterioles
radius will make a large
difference to the resistance.
Page 285 of 447

Note:-Blood pressure depends on the following factors;
 Blood volume
 Force of the heart (cardiac output) i.e. blood from the ventricles
 Blood vessel radius/ diameter of the lumen i.e. resistance to blood flow
Blood volume is adjusted to some extent through contraction of the spleen and liver which bring stored
blood into circulation. The stored blood is due to the regulation of the fluid intake and fluid loss by
organs such as the kidney and the skin during homeostasis.
Blood vessels offer resistance (R) to blood flow. The resistance is inversely proportional to the fourth
1
4
power of the radius (r) of the vessel (R α r ). Therefore, the resistance increases as the vessel
becomes narrower and since we are dealing with the fourth power of the radius, small changes in the
arterioles radius will make a large difference to the resistance.
Blood pressure is increased by; Blood pressure is decreased by;
1. Increased cardiac output e.g. during 1. decreased cardiac output e.g. during
exercise sleep or rest
2. Increased resistance to blood flow e.g. 2. decreased resistance to blood flow e.g.
vasoconstriction and atherosclerosis vasodilation
3. Increased blood volume e.g. due to 3. decreased blood volume e.g. during loss
retention of water by the kidney under the of blood due to injury
influence of ADH
TOPIC 7: DEFENCE AGAINST DISEASES

Every mammal is equipped with a complex system of defensive mechanisms which are designed to
enable it prevent the entry of microbes into it, to withstand attacks by pathogens (disease causing
micro-organisms) and to remove foreign materials from the system.
The defensive mechanisms of blood include the following;
1. Clotting of blood
2. Phagocytosis
3. Immune response to infection
Clotting of blood
When a tissue is wounded, blood flows from it and eventually coagulates to form a blood clot which
covers the entire wound. This prevents further blood loss and entry of pathogens. The process of
blood clotting is described below.
When blood platelets and damaged tissues are exposed to air, the platelets disintegrate and release an
enzyme called thromboplastin or thrombokinase, which in the presence of plasma proteins and
calcium ions catalyses
Thrombin is a proteolytic enzyme that hydrolyses a plasma protein called fibrinogen into an
insoluble protein called Fibrin forms fibres at the wounded area. Within the fibrous network of fibrin
Page 286 of 447

blood cells become trapped, thereby forming a fibrin clot or a blood clot. The clot not only prevents
further blood loss, but also prevents the entry of bacteria and other microbes which might otherwise
cause infection
Note: (Clegg fig 17.41 pg 365)
Heparin is an anticoagulant which inhibits the
conversion of prothrombin to thrombin thereby
preventing blood clotting.
Apart from blood clotting, the entry of microbes
into the body can be prevented by the following;
1. Using impermeable skin and its protective
fluid called sebum (oily secretion in the skin)
2. Using mucus and cilia to trap the microbes and
then remove them
3. By using hydrochloric acid in the stomach
4. By using lysozyme enzyme in the tears and
nasal fluids
5. By vomiting and sneezing
Why blood does not clot in the vessels
Connective tissue plus the liver produce chemical, heparin, which prevents the conversion of
prothrombin to thrombin, and fibrinogen to fibrin.
Blood vessels are smooth to the flow of blood. Damage to the vessel’s endothelium can lead to
platelets breakdown which leads to clotting of blood.
BODY DEFENCE SYSTEM AND MECHANISM IN MAMMALS (HUMANS)

An animal must defend itself against unwelcome intruders e.g. dangerous viruses and other
pathogens it encounters in the air, water and food. The body also deals with abnormal cells (cancer
cells) that develop periodically in the animal’s body.
A summary of defence mechanisms in an animal’s body
Immunity
Non-specific defence mechanism Specific defence mechanism
Internal
External (first line of defence) Lymphocytes Antibodies
Composes of physical barriers
Skin and its secretions e.g. sweat Mucus membranes and their secretions
e.g. respiratory tract and digestive tract
e.g. stomach (HCl), saliva, tears e.t.c.
Phagocytes (neutrophils, macrophages, Natural killer cells Inflammation Antimicrobial

eosinophils) fever proteins
Page 287 of 447

Two comparative defensive systems are used to fight pathogenic and abnormal cells in the body. One
of the system is non-specific in nature i.e. it does not distinguish one infectious agent from another.
The other defence system is specific in nature and constitutes the immune system. The non-specific
system includes two lines of defence which an invader encounters in sequence. The first line of
defence is external comprising of epithelial tissues that cover and line our bodies (skin and mucus
membranes) and other secretions these tissues produce. The second line of non-specific defence is
internal. It is triggered by chemical signals and uses antimicrobial proteins and phagocytic cells that
indiscriminately attack any invader that penetrates the body’s outer barrier (inflammation is a sign
that the second line of defence has been deployed).
The immune system constitutes a third line of defence which comes into place simultaneously with
the second line of specific defence. However, the immune system responds specifically to a
particular type of invader. This immune response includes the production of specific defence proteins
called antibodies. It also involves participation of several different types of cells that are derived
from the white blood cells called lymphocytes.
NOTE: the non-specific defence system which involves use of phagocytes, natural killer cells and
antimicrobial proteins is said to offer innate immunity (defence) which is abroad defence mechanism
against infection. The immune response offers a specific defence against infection. It is also
described as acquired immunity. Immunity is the ability of an organism to resist infection or to
counter the harmful effects of toxins produced by infecting organisms.
NON SPECIFIC DEFENCE MECHANISM

The non-specific defence mechanism act in 6 ways i.e
1. Through physical barriers e.g. skin. 4. Anti-microbial proteins.
2. Phagocytosis. 5. Inflammation.
3. Natural killer cell. 6. Fever
THE SKIN AND MEMBRANES

This is the first line of defence and it takes a different number of forms.
The intact skin is a (1) barrier that cannot be penetrated by bacteria or viruses, although minute
abrasions allow their passage [some pathogens such as the malarial parasite Plasmodium, use a
vector, the mosquito, to penetrate this covering and so gain entry to the body]. In the same way, the
(2) (a) mucus membranes which line the digestive, respiratory and urinal genital tracts prevent the
entry of potentially harmful microbes [such areas cannot be covered by a thick layer of skin due to
the body’s need to obtain and lose substances by diffusion]. Mucus, which is a viscous secreted by
cells of the mucus membranes also traps particles that contact it. Microbes entering the upper
respiratory system are caught in the mucus and are then swallowed or expelled. The lining of the
trachea has specialized epithelial cells equipped with cilia which sweep out microbes and other
particles trapped by mucus, preventing them from entering the lungs.
Apart from their role as physical barriers, the skin and mucus membranes produce secretion that
counter pathogens e.g. in humans, secretions from the (b) oil and sweat gland give the skin a pH
ranging from 3-5 which is acidic enough to discourage micro-organism from colonizing there,
bacteria that make the normal flora of the skin are adapted to its acidic relatively dry environment.
(c) Saliva, tears and mucus secretions that bathe the surface of the exposed epithelia wash away
Page 288 of 447
many potential invaders and in addition to these secretions contain various antimicrobial proteins.
For example the enzyme cysozyme which digests the cell walls of many bacteria, destroys many
microbes entering the upper respiratory system and openings around the eyes.
Microbes present in food or trapped in swallowed mucus, from the upper respiratory system pass,
through the highly (3) acidic gastric juice produced by the stomach lining which denatures the
enzymes of most of the macrobes before entering the intestinal tract.
Despite these precautions, pathogens still frequently gain entry and therefore the body has a second
line of defence, a series of specific cellular and chemical defecnces designed to;
 neutralise any toxins produced by the pathogens
 prevent the pathogen multiplying
 kill the pathogen
 remove any remains of the pathogen
Phagocytic defence mechanism

Phagocytosis is the process by which large particles are taken up by cells, in the form of vesicles
formed from the cells surface membrane. Two types of white blood cells (1) neutrophils and (2)
monocytes are attracted by chemicals released by body cells which have been damaged by invading
pathogens. These white blood cells show amoeboid movements which engulf, ingest and destroy
pathogens. They are known as phagocytes and are produced in the marrow of the long bones.
Neutrophils can squeeze through blood capillary walls a process called diapedesis and move about
in tissue spaces. The monocytes migrate out of blood stream then become larger white blood cells
(leucocytes) called macrophages. Some macrophages are permanently located in tissues and organs
such as the liver, spleen, kidney and lymph nodes while other circulate throughout the body. The
term macrophage means “big eater” and these cells are long lived phagocytes which even engulf
much larger particles like old red blood cells and protozoan parasites.
 Antibodies attach themselves to A drawing to summarize the phagocytic process
antigens on the surface of the affected by neutrophil, macrophage or monocytes.
bacterium
 Proteins, found in the plasma,
attach themselves to the
antibodies
 As a result of a series of reactions,
the surface of the bacterium
becomes coated with proteins
called opsonins. This process is
called opsonisation.
 Complement proteins and any chemical products of the bacterium act as attractants, causing
neutrophils to move towards the bacterium.
 Neutrophils attach themselves to the opsonins on the surface of the bacterium
 Neutrophils engulf the bacterium to form a vesicle, known as a phagosome.
 Lysosomes move towards the vesicle and fuse with it
Page 289 of 447

 The enzymes within the lysosomes breakdown the bacterium into smaller, soluble materials
 The soluble products from the breakdown of the bacterium are absorbed into the cytoplasm of the
neutrophils.
Note: The eosinophils have low phagocytic activity but are critical to defence against multicellular
parasitic invaders such as the blood fluke (Schistosoma mansoni) they rarely engulf such a large
parasite but position themselves against the parasites body and then discharge destructive enzymes
which damage the invader
Inflammation
An inflammation is a localized non-specific response initiated by the defence system of the body due
to physical damage to the skin or mucus membranes by bacteria. This physical damage causes (1)
release of chemical signals such as histamine and prostaglandins. (2)The chemical signals induce
increased permeability of the blood capillaries to blood components and (3) the flow of blood to the
affected area respectively [having increased blood flow causes the area to swell]. (4) They also
attract phagocytic cells and lymphocytes which on arrival at the site of injury, the phagocytes
consume pathogen (the area becomes warm and pale red in colour) and the cells debris and
consequently the tissue heals
Note. it is the damaged cells and certain leucocytes that produce histamine and prostaglandins. The
histamine cause vasodilatation i.e. the capillaries dilate and the walls become leaky. As more fluid
collects around the wound, the site becomes red, swollen and warm. The localized swelling is called
oedema. The prostaglandins are the ones that promote blood flow to the site of injury and increase
the sensation of pain.
Natural killer (N.K) cells
This is a class of white blood cells which attack virus injected body cells and abnormal cells that
could form tumours.
The virus infected cells have viral proteins displayed on their surfaces and these are recognized by
the natural killer cells contains perforin – filled vesicle.
When an N.K encounters a virus infected cell, perforin molecules are released by exocytosis.
Perforin molecules make large holes of pores in the turgid cells plasma membrane, causing leakage
of the cytoplasmic contents. This results into cell death. The membrane of NK cell is not affected by
these membranes dissolving molecules
FEVER
Fever refers to increase in body temperature. It is triggered if microbes infect larger areas of the body
in response to infection, certain leucocytes releases pyrogens which are also anti-microbial protein of
the complement system. The pyrogen stimulate the hypothalamus to rise the body temperature set
point from its normal value about 390C hence casing a fever. The fever has several beneficial effects;
It increases the activity of phagocytes which then attack the invading microbes more efficiently.
It increases the production of interferon in virus infected cells. Interferons are proteins which inhibit
viral replication, activate natural killer and stimulate macrophages to destroy tumour cells and virus
infected cell
Page 290 of 447

ANTIMICROBIAL PROTEINS
These are proteins that function in the mechanisms by attacking microbes directly of by impeding the
production e.g. lysozyme.
Other antimicrobial proteins include about 30 serum proteins that make up the complement system
proteins through a sequence of stops, leading to lysis (bursting) of invading cells.
Some complement proteins initiate inflammation and also play a role in acquired defence
(specific defence system) interferon is one of the proteins of the complement system which provides
innate defence against viral infection the interferon protein is secreted by virus infected body cells
and induce neighbouring uninfected to produce other substances that inhibit viral reproduction. In
this way, inteferons limit the cells spread of viruses in the body helping control of viral infections
such as colds and influenza.
SPECIFIC DEFENCE SYSTEM /IMMUNE SYSTEM

The specific immune response confers immunity against specific microbes and it depends on a type
of whte blood cells called lymphocytes. Immunity is the capacity of an organism’s body to
recognize the intrusion of foreign materials in the body and mobilize cells and cell products
(antibodies) to remove a particular sort of foreign material to a greater speed and effectiveness. The
specific defence system involves immune system whose response result from the interaction among
several types of lymphocytes, the molecules they produce (antibodies) and the foreign material
introduced by microbes (antigens)
Molecules of the immune system

a) Antigens
An antigen is any organism or substance that is recognised as non-self (foreign) by the
immune system and provokes an immune response. Antigens are usually proteins that make
up the cell surface membranes of invading cells, such as microorganisms, or diseased ones,
such as cancer cells. The presence of an antigen triggers the production of an antibody as part
of the body’s defence system.
b) Antibody
This is a specific protein
(immunoglobin) which recognizes and
binds to specific antigens. Antibodies
either neutralise antigens or tag cells that
are antigens for easy attack by
macrophages. They are synthesised by
cells in the blood called B-lymphocytes.
Note: Macrophages are also taken to be
part of the immune response i.e.
involved in specific defence mechanism
through indirectly since they are
phagocytes which destroy microbes and
alert other immune cells the infection.
How antibodies work

An antibody does not directly destroy an antigenic invader. However specific antibodies bind
to specific antigens, in the same way a key fits a lock, to form an antigen antibody complex
Page 291 of 447

which is the basis for several effector mechanisms which make macrophages recognize the
antigens and destroy them. The binding of antibodies to antigens is very specific and takes
various forms, some of which include the following:
 Neutralisation
Here the antibody blocks certain sites on an antigen or toxins (chemicals that cause many of
the symptoms of a disease) making it ineffective. Antibodies neutrallise a virus by attaching
to the sites the virus uses to bind to its host cell. Also bacterial toxins become coated with
antibodies hence getting neutralised, eventually, phagocytic cells (macrophages) destroy
these antigen-antibody complexes.
 Agglutination (clumping)
This is when antibodies cross link adjacent antigens. This is made possible because certain
antibodies possess at least two antigen binding sites. The clumping of antigens e.g. bacteria
makes it possible to be recognized by macrophages and other phagocytes which destroy the
antibody-antigen complex
 Precipitation
This is a similar mechanism to agglutinations, except that here the antibody-antigen
complexes are formed with soluble antigen molecules rather than cells are linked to form
immobile precipitates which are captured by phagocytes and macrophages that destroy them
i.e. soluble antigens are precipitated out so that they are easily destroyed by phagocytes.
 Opsonisation
Here, the antibody molecule oats the surface of a microbe making it easier for phagocyte and
leucocytes to engulf it.
 Lysis
Having attached themselves to antigens on foreign cells, antibodies then attract other
compound which bind to them. These include enzymes which help to break down the
foreign cells.
 Complement fixation
Here, the antibodies activate the complement proteins which then leads to lysis of foreign
cells.
c) Epitopes
These are antigens determinants with specific sequences of amino acids that confer as
specific shape to the antigen molecules which is then recognized by an antibody or T-cell
receptor. An antigen can have several different epitopes on its surface and different
antibodies can therefore bind a single antigen.
d) Cytokines (lymphokines)
These are peptides and proteins that regulate many cell activities (growth and repair) and act
as signal in both the specific and non-specific immune responses
Examples of cytokines include
 Interferons  Interleukin
e) Complement system.
This is a group of about 20 proteins found in plasma and other body fluid. These are inactive
until the body is exposed to antigens e.g. histamines.
Page 292 of 447

Mechanism of immune responses

The immune response depends on a type of white blood cell called a lymphocyte formed in stem
cells found in the bone marrow. There are two types of lymphocytes each with its own immune
response to antigens, namely:
a) B lymphocytes (B cells) - humoral response which involves antibodies which are present in
body fluids or ‘humour’. B-cells mature in the Bone marrow.
b) T lymphocytes (T cells) - cell-mediated response. Which involves cells. T cells mature in the
Thymus gland
Humoral response
The humoral immunity results in the production of antibodies which are secreted by B-cells, the
antibodies circulate as soluble proteins in blood plasma and lymph, the fluids that were once called
humors.
There are many different types of B- lymphocytes (close to 10 million) each producing a
specific/different antibody which respond to one antigen.
When an antigen, e.g. a protein on the surface of a pathogen cells, enters the blood, or tissue fluids, it
encounters numerous B cells. A few of these will carry the appropriate immunoglobin on their
surface membrane and will attach to antigens on the surface of the pathogen or toxin. This
attachment has a dramatic effect on the B-cell. If given the right signal from the T-helper cells, the
B-cell divides rapidly by mitosis to produce a large number of daughter cells i.e. a clone. This is
referred to as polyclonal activation. Some of these daughter cells develop into plasma cells which
produce and secrete up to 2000 molecules of their specific antibody per second. These antibodies
destroy the pathogen, and any toxins it produces. The plasma cells are therefore responsible for the
immediate defence of the body against infection. This is known as the primary immune response.
Finally some daughter cells develop into memory cells which remain in the circulation without
secreting antibodies. They can live for decades circulating in the blood and tissue fluid until they
encounter the same antigen at some future date.
Memory and secondary immune response

Memory cells function in secondary immune response. In primary immune response there is
selective proliferation (multiplication) of lymphocytes to form clones of effector cell upon the first
exposure to an antigen. Here there is a lag period between initial exposure to an antigen and
maximum production of effector cells. During the lag period, the lymphocytes secreted by the
antigen differentiates into effector T-cells
(TH and TC) and antibody producing plasma cells. If the body is exposed to the same antigen at a
later time, the response is faster one/more prolonged than the primary immune response. This is the
secondary immune response.
Secondary immune response is the rapid response that results in faster production of effector T cells
and antibody-producing plasma cells, when the body is exposed to subsequent infection of the same
antigen that has ever invaded the body. Antibodies produced during the secondary immune response
are more effective in binding to the antigen than those produced during the primary immune
response. The immune systems’ ability to recognize an antigen as previously encountered is called
immunological memory. The ability is based on long lived effector cells of the immune response,
memory cells. These cells are not active, survive for long periods and proliferate rapidly when
expose to the same antigen that caused their formation. Secondary immune gives rise to a new clone
of memory cells as well as effector cells.
Page 293 of 447

Graph to illustrate changes in antibody concentration during primary and secondary immune
responses to antigens
Page 294 of 447

Cell mediated immune response.

In the cell mediated response, the immunity depends on the direct action of the T-lymphocytes rather
than antibodies. T-lymphocytes only respond to antigens that are attached to a body cell (rather than
those within body fluids). T lymphocytes respond to an organism’s own cells that been invaded by
non-self material e.g. virus or cancer cell. They also respond to transplanted materials, which is
genetically different. Invader cells are distinguished from the normal cells because:
a) macrophage cells that have engulfed a pathogen and broken it down, present some of the
proteins produced on their own surface
b) body cells invaded by a virus also manage to present some of the viral proteins on their own
cell surface membrane, as a sign of distress
c) cancer cells likewise display non-self proteins on their cell surface membrane.
The non-self materials on the surface of these cells act as antigens therefore the term antigen-
presenting cells is used to describe them. There are many different versions of the two main types of
T lymphocytes each with a different receptor protein on its surface. Although these receptors
function in a similar way, they are not antibodies, because they remain attached to cell rather than
being released into the blood plasma.
Note: The circulating antibodies of the humoral branch of the immune response defends the body
against toxins, free bacteria and viruses present in the body fluids. In contrast, lymphocytes of the
cell mediated branch are active against bacteria and viruses inside the body’s cells and against fungi,
protozoa and worms. The cell mediated immunity is also involved in attacks on transplanted tissue
and cancer cells both of which are perceived as non self.
Major cells in the immune system.

1. B-cells (B-lymphocytes)
These are lymphocytes that produce antibodies when stimulated. They are produced and mature in
the bone marrows from the stem cells. They have glycoprotein receptors on their cell surface
membranes which bind specific antigens. Mature B-cells become plasma cells and memory cells
produce much more antibodies in terms of quantity and effectiveness than plasma cells.
2. T-cells (T-lymphocytes)
The T-lymphocytes regulate the immune response (in case of TH-cells) or kill certain types of cells
(Tc-cells) the T cells are produced in the bone marrow but mature in the thymus gland where they
develop specific receptors which recognise specific antigens. These are two main categories of T cell
namely:
a) T cytotoxic cells (T killer cells), recognize and destroy cells with foreign antigens on their
surface. They mainly attack virus infected cells, cancerous body cells and foreign grafted
tissues. They kill not by phagocytosis but by making holes in the cell surface membrane
using proteins called perforins. These holes allow water to rush into the cell, causing it to
burst. Since viruses need living cells within which to reproduce, this sacrifice of body cells
prevents viruses multiplying.
b) T-helper cells, play a key in the immune system. When they attach to an antigen-presenting
cell, T helper cells secrete chemicals called cytokines. These cytokines:
 stimulate macrophage cells to engulf pathogens by phagocytosis
Page 295 of 447

 stimulate B lymphocytes to divided and develop into antibody producing plasma cells
 activate T cytotoxic cells (T killer cells)
Both T helper and T cytotoxic cells produce their own type of memory cells, which circulate
in the blood in readiness to respond to future invasions by the same pathogen.
Another type of T lymphocyte is the T suppressor cells, suppress the activity of the killer T-cells
and B-cells after the microbes have been cleared out of the body to prevent these cells from attacking
and destroying the body cells. Suppressor T-cells therefore regulate the immune response and
prevents antibodies from being produced by the B-cells.
3. Memory cells
These are derived from B cells and T-cells. They are long lived and confer future immunity against
subsequent infections by the same antigen i.e. they are the ones responsible for causing the
secondary immune response.
Characteristics of the immune system

The immune system develops specific response against each type of foreign microbes, toxin or
transplanted tissues.
The immune system has 4 features i.e.
a. Specificity. c. Memory
b. Diversity d. Self/non self-recognition.
a. Specificity
The immune system has the ability to recognize and eliminate particular microorganism, and foreign
molecules. The immune system responds to an antigen by activating specialized lymphocytes and
producing specific proteins called antibodies.
Antigens that trigger an immune response include molecules belonging to viruses, bacteria, fungi,
protozoa and parasitic worms.
Anti-bodes recognize antigens using epitopes which are antigenic determinants on the surfaces of the
antigens. If an antigen has several epitopes, it stimulates several different B cells which secrete
specific distinct antibodies against it. Therefore each antigen has a unique molecular shape and
stimulate the production of the very type of antibody that defends against that specific defences, each
response the immune system targets a specific invader distinguishing it from other foreign molecules
that may be very similar.
b. Diversity
The immune system has the ability to respond to very many kids of invaders each recognized by its
antigenic markers. This diversity of response is possible because the immune system is equipped
with an enormous variety of lymphocyte population among the antibody producing lymphocytes (B-
lymphocytes) each population is stimulated by a specific antigen and response synthesizing and
secreting the appropriate type of antibody.
c. Memory
The immune system has the ability to “remember” antigen encountered and react more promptly and
effectively on the subsequent exposures. This characteristics also known as acquired immunity.
d. Self/non self-recognition
Page 296 of 447

The immune system distinguishes the body’s own molecules from foreign molecules (antigens).
Failure of self/non self-recognition leads to anti immune disorders in which the immune system
destroys the body’s own tissues
Types of immunity
The ability of an organism to resist infection may be naturally acquired or artificially induced.
Natural immunity is immunity which is either inherited, or acquired as part of normal life
processes, e.g. as a result of having had a disease. Artificial immunity is the immunity acquired as a
result of deliberate exposure of the body to antibodies or antigens in nan-natural circumstances e.g.
vaccines. Both natural and artificial immunity may be passively or actively acquired.
Types of natural immunity
a. Natural passive immunity
This involves passing antibodies in the body of an organism into the body of another organism of
the same species e.g. from the mother to the foetus via to the placenta to defend the body against
disease and also via the first milk called colostrum to the child. This type of immunity is
temporary.
b. Natural active immunity
This is the immunity that involves formation of antibodies by the body of an organism in the
presence of certain antigens.
This type of immunity is permanent because during the immune response, memory B-cells are
produced which recognize the microbes on reinfection (second infection) and then stimulate the
rapid production of large amounts of antibodies to curb down the microbes before causing
significant damage. Memory B-cells stay for long in blood. It is for this reason that many people
suffer diseases such as measles only once in a life time.
Artificial immunity
There are two types of acquired immunity namely:
a. Artificial active immunity depends on the response of a person’s own immune system. Here the
individual organism produces antibodies using the B-lymphocytes against the infectious agent.
Active immunity is naturally acquired but it can also be artificially acquired by vaccination.
b. Artificial passive immunity. Occurs when antibodies from another individual are injected as in
the treatment of tetanus.
NB. Passive immunity can also be transferred artificially by introducing antibodies from an animal
or human who is already immune to the disease e.g. rabies is treated in humans by injecting
antibodies from people who have been vaccinated against rabies. This produces an immediate
immunity which is important because rabies progress rapidly and the response to vaccination would
take too long.
EXPLANATION OF HOW THE KEY FEATURES OF AN IMMUNE SYSTEM ARE

REALIZED DURING THE SPECIFIC DEFENCE MECHANISM.
SPECIFITY AND DIVERSITY

Immunological specificity and diversity is based on clonal selection of lymphocytes if the antigen
enters the body and binds to receptors on the specific lymphocytes, the nasal those lymphocytes are
Page 297 of 447

activated to mount an immune response. The selected cells proliferate by cell division and develop
into a large number of identical effector cells known a clone. This clone of cells combat the very
antigen that provoked the response e.g. plasma cells that develop from that function as the antigen
receptor on the original B-cell. Which first encountered the antigen. The antigen specific selection
and cloning of lymphocytes is called clonal selection.
In clonal selection, each antigen by binding to specific receptors selectively activate a tiny traction of
cells from the body’s diverse pool of lymphocytes. These relatively small numbers of selected cells,
all dedicated to eliminating the specific antigen that stimulated the humoral or cell mediated immune
response.
N.B. Antigens are molecules (usually proteins, polysaccharides or glycoproteins carried on the
surface of cells which cause antibody formation. All cells have antigen makers on their cell surface
membranes but the body can distinguish between its own antigen (self) and foreign antigen (non self)
Self and non-self-recognition

Here, molecular markers on cell surface, function in self and non-self-recognition. The antigen
receptors on the surface of lymphocytes are responsible for detecting molecules that enter the body.
Normally, there are no lymphocytes that are reactive against the body’s own molecules. Self-
tolerance begins to develop as T and B lymphocytes bearing antigen receptors mature in the thymus
and bone marrow and continues to develop with receptors for molecules present in the body are
destroyed or rendered passive (non-functional) leaving only lymphocytes that are reactive against
foreign molecules tolerated by an individual’s immune system, are a collection of molecules encoded
by a family of genes called the Major Histocompatibility complex (MHC) two main classes of MHC
molecules mark cells as self. Class 2 MHC molecules are restricted to a few specialised cell types of
the body’s defence system e.g. macrophages, B-cells and activated T-cells.
NB. Class 2 MHC molecules play an important role in interaction between cells of the immune
system.
ABNORMAL IMMUNE FUNCTION

Sometimes, the immune system fails to defend the animal against intruders instead turns against the
components of the body which leads to certain disease. Conditions immune system abnormalities
include;
1. Auto immune disease.
2. Allergy.
3. Immune deficiency.
Acquired Immunodeficiency Syndrome (AIDS). AIDS is a disease caused by a virus called the
Human Immunodeficiency Virus (HIV). HIV infects certain T cells, including helper T-cells,
which carry a receptor called CD4 on their surface. Other cells with CD4 receptors include
macrophages and some B lymphocytes. Glycoproteins on the HIV envelope bind specifically to this
receptor. Following attachment, the virus enters the cells and disintegrates, releasing RNA and an
enzyme called reverse transcriptase. The enzyme causes the cell to translate the viral RNA into
DNA. The viral DNA enters the nucleus and is incorporated into the cell’s own DNA. Thus a gene
representing the HIV becomes a permanent part of the infected person’s CD4 lymphocyte cells.
Newly formed viruses bud from the host cell, circulate, and infect other cells. The infected cells may
Page 298 of 447

produce new viruses by exocytosis for an extended time or may be killed quickly, either by the virus
or by the response of the immune system. HIV may also remain latent for many years as a provirus
assimilated into the genome of an infected cell. When the infected cell divides, it also makes a copy
of the viral DNA. The provirus is invisible to the immune system because it does not produce viral
proteins and infects the cells of the immune system itself, so impairing its ability to respond i.e.
cannot be destroyed by circulating antibodies.
The ability of HIV to remain latent is one of the reasons why anti-HIV antibodies fail to eradicate the
disease. Probably more important, however, are the extremely rapid mutational changes in antigens
the virus undergoes during the infection. Indeed, every HIV probably differs in atleast one small way
from its parent. The immune system responds effectively against HIV infection at first, but it is
eventually overwhelmed by the accumulation of more resistant variants. In the figure below notice
that the number of viruses gradually increase as the helper T-cell population (and hence the body’s
protection,) decreases. When the damage to the immune system reaches a certain point, cell-
mediated immunity collapses and secondary infections (opportunistic infections) e.g. Kaposi’s
sarcoma and pneumonia are established in the patient. [Remember that the major destructive cells of
the immune system, T-cytotic and B cells, depend on stimulation by the T helper cells for their
activity, inactivation of T helper cells knocks out the whole immune system]. Such infections are
established in the late stages of HIV infection and are defined by a specified reduction of T- cells.
These infections are called AIDS. The time from infection to AIDS varies, but it averages about 10
years. Death usually results from the opportunistic infections
The progress of the disease
from HIV infection to AIDS
can be categorised into four
phases.
(a) First phase
Most individuals have no
symptoms, although some may
have flu-like symptoms, skin
rash and swollen lymph glands.
(b) Second phase
Production of anti-HIV rises in the
blood stream. Although the level
of HIV in the blood falls, HIV
replication continues in the lymph
nodes. This phase may last from a
few weeks to 13 or more years.
(c) Third phase
AIDS-related complex refers to the many opportunistic infections which affect the patient. These
include common bacterial, fungal and viral infections such as oral and genital herpes and athlete’s
foot. The patient may lose weight and there is a significant drop I the number of T-helpers cells.
(d) Fourth phase

More opportunistic infections and the development of secondary cancers e.g. Kaposi’s sarcoma. By
this time, there is almost total loss of cellular immunity.
Page 299 of 447

Note:
1. HIV can only survive in body fluids such as semen or blood
2. Individuals who have been exposed to HIV have circulating antibodies against the virus, and
detection of these antibodies is the most common method for identifying infected individuals
who are said to be HIV-positive.
3. HIV is not transmitted by causal contact or even kissing
4. At this time, AIDS is incurable.
5. For the present, the best approach for slowing the spread of AIDS seems to be to educate
people about the practices that transmit HIV, such as unprotected sex (without a condom) and
sharing needles. Anyone who has sex – vaginal, oral, or anal- with a partner who may have
had unprotected sex with another individual during the past 15 years risks exposure to HIV.
6. Breast milk has transmitted the disease from mother to nursing infants.
7. We should avoid discriminating those of HIV i.e. stigmatism, rather we should offer help to
them in any way that you can.
THE LYMPHATIC SYSTEM

The lymphatic system returns tissue fluid to the blood and also plays a role in the body defence. As
blood passes through the capillaries, there is accumulative loss of fluid which is effected by ultra-
filtration of blood and this forms tissues fluid that bathes cells. The lost fluid is similar to blood in
composition except that of lacks blood plasma proteins and cells. The lost fluid returns the blood via
the lymphatic system. It enters the system by diffusion into tiny lymph capillaries which are
intermingled among the capillaries of the cardio vascular system. Once inside the lymphatic system,
Page 300 of 447

the fluid is called lymph. The lymphatic system drains into the circulator system near the shoulders
where it pours its contents on the subclavian vein that leads to the anterior vena cava.
Along the lymph vessels are specialized swellings called lymph nodes. These filter the lymph and
attack bacteria, virus infected cells and other antigens using the lymphocytes in them.
When the body is infected by an antigen the cells in the lymph nodes multiply rapidly and the lymph
nodes become swollen and tender. Like the veins of the cardio vascular system lymph vessels have
valves which prevent back flow of fluids towards the capillaries. In the same way, lymph vessels
depend on the movement of skeletal muscles to squeeze the fluid along the vessel.
N.B the lymphatic system serves to;

a) Defend the body against infection.
b) Maintains the level of interstitial fluid (tissue fluid).
c) Transports fats from the digestive tract to the circulatory system (the lymph capillaries called
lacteals) penetrate the villi of the small intestine which absorb the fatty acids and glycerol.
Whenever the interstitial fluid accumulates rather than being returned to the blood by lymphatic
system, the tissues and body cavities become swollen a condition known as oedema.
Vaccines
Vaccines are toxic chemicals or killed or attenuated (weakened) microbes introduced into the body
of an organism to make it produce very many antibodies against a certain pathogen.
The killed microbes are usually viruses and bacteria. The attenuated microbes are living microbes
which are inactivated and they lack powers to infect the body due to the chemical or temperature
treatment given to them.
Note; toxins are toxic chemicals produced by microbes and therefore can work as antigens
BLOOD TRANSFUSION
This is the transfer of compatible blood from the donor to the recipient.
Blood transfusion based on the ABO system of grouping blood
Blood group A has antigen A on the surface of its red blood cells and antibody b in the blood plasma
of that person. Blood group B has antigen B on the surface of its red blood cells and antibody a in the
blood plasma of that person. Blood group AB has antigen B and A on the surface of its red blood cells
and no antibody in the blood plasma of that person. Blood group O has no antigen on the surface of its
red blood cells and both antibody b and a in the blood plasma of that person.
Blood plasma permanently contains
Blood Antigen on the red Antibody in
antibodies depending on a particular blood
group blood cell membrane plasma
group. However these antibodies do not
A A b
correspond to a specific antigen, if they
B B a
correspond then agglutination occurs
AB A and B Lacks antibodies
(precipitation of blood).
O No antigens a and b
That is why an individual with blood A having antigen b cannot donate blood to an individual with
blood group B having antibody a in the plasma which corresponds to antigen A to cause agglutination.
Similarly, blood groups A and B cannot donate blood to an individual of blood group O because
Page 301 of 447

antigen A will be attacked by antibody a in blood group O and antigen B will be attacked by antibody
b in blood group O to precipitate the recipient’s blood. The table below summarizes the possible blood
transfusions and the impossible ones.
Individuals with blood group AB posses Blood group compatibilities
antigen B which stimulates blood group B of Recipient Donor’s blood group
the recipient to produce antibody a that reacts
Blood Antibody in A B AB O
with antigen A in the donor’s blood to cause
group plasma
agglutination and therefore this transfusion
A b  X X 
from AB to B is impossible. Similarly blood
B a X  X 
group O individuals can donate blood to
AB None    
blood group A because the donor’s blood has
O a and b X X X 
no antigens which would react with antigen A
 = compatible with recipients blood
in the recipient’s blood and therefore
X = Incompatible with recipient i.e.
agglutination is impossible.
agglutination occurs
Individuals with blood group O are called universal donors because they lack antigens which would
react with the corresponding antibodies in the recipient’s blood. Individuals with blood group AB are
called universal recipients because they lack antibodies in their blood plasma which would have
reacted with the corresponding antigens in the donor’s blood.
NOTE; the recipient’s antibody is the one expected to attack and react with the corresponding antigen
in the donor’s blood. Whenever the antigen of the donor corresponds with the antibody of the
recipient’s blood group, an antibody-antigen reaction occurs, leading to agglutination (precipitation or
clotting of blood)
RHESUS FACTOR (D-Antigens)
These are antigens which were first observed in the bodies of the Rhesus monkeys. These antigens are
also carried on the surface of the erythrocytes of some human beings. Those people with D-antigens
on the surface of their red blood cells are called Rhesus positive (Rh+) while individuals missing such
D-antigens are called Rhesus negative (Rh-).
The bodies of individuals do not have already manufactured antibodies against the D-antigens. When
an expectant mother who is Rh- bears the foetus with which is Rh+, some foetal erythrocytes with D-
antigens will cross the placenta and enter into the blood circulation of the Rh- mother towards the end
of the gestation period (pregnancy). It is also possible for the blood of the foetus to mix with that of
the mother during birth so that the mother gets Rh+ by getting the D-antigens from the child.
The D-antigens that have entered the mother’s blood circulation stimulate the maternal body to
manufacture corresponding antibodies (antibody-d or anti-D antibodies) which attack and react with
the D-antigens in the mother. Some formed antibodies-d can also pass via the placenta and enter the
foetal blood circulation where they attack and react with the D-antigens which results into clumping
together and bursting of the foetal red blood cells, a condition called erythroblastosis foetalis
(Haemolytic disease of the new born). This disease results into acute anaemia which can lead to death
of the feotus.
The first born rarely dies because the time is too short for the mother to produce enough antibodies
that can pass to the foetus to cause death but subsequent Rh+ foetus can die due to the many antibodies
of the mother entering its circulation to cause agglutination.
Page 302 of 447

To prevent this disease, pregnant mothers are always given anti-D chemicals 72hours to delivery, to
render her immune system insensitive towards the D-antigen i.e. the mother may be infected with
antibody-d within 70-72hours to delivery or within 72 hours after her first born. Also the blood of the
foetus can be transfused with normal blood to dilute antibody-D so as to save the child.
NOTE: if a rhesus negative mother of blood group O is carrying a rhesus positive child of any blood
group other than O, the problem will not arise. This is because if fetal cells enter the mother’s
circulation, the mother’s a and b antibodies will destroy the blood cells before the mother has time to
manufacture anti-rhesus antibodies.
Page 303 of 447

UPTAKE AND TRANSPORT IN PLANTS

Water and mineral salts are necessary for photosynthetic reactions and other metabolic processes;
hence they must be absorbed in sufficient quantities by using the root system and transporting them
through the xylem to the mesophyll cells of leaves where photosynthesis takes place. Leaves have a
large surface area to absorb light, and stomata to allow adequate inward diffusion of carbon dioxide,
both result in an immerse loss of water.
Water is lost from the mesophyll cells into sub-stomatal air chambers and then eventually lost into
the atmosphere of water vapour through tiny pores called “stomata” by a process known as
transpiration.
TRANSPIRATION
This is the process of water loss inform of water vapour to the atmosphere from the plant mainly
through the stomata pores.
Types of transpiration
There are three types of transpiration which include the following (a) Stomatal transpiration (b)
Cuticular transpiration & (c) Lenticular transpiration
Stomatal transpiration
This is the loss of water vapour to the atmosphere through the stomatal pores of the leaves. This
contributes 90% of the total water loss from a leafy shoot. This is because leaves contain a large number
of stomata for gaseous exchange where this water vapour can pass and also there’s little resistance to
the movement of water vapour through the stomatal pores. In addition, leaves also have a large surface
area over which water vapour can evaporate rapidly to the atmosphere.
Cuticular transpiration
This is the loss of water vapour to the atmosphere directly through the epidermis coated with a cuticle
layer. It contributes 5% to the total water loss from the leafy shoot. This is because the cuticle is hard,
waxy and less permeable to most diffusing molecules including water vapour molecules.
Lenticular transpiration
This is the loss of water vapour through a mass of loosely packed cells known as lenticels found
scattered on the stems. It also contributes 5% of the total water loss to the atmosphere in a leafy shoot.
It is because the lenticels are usually few in number and not directly exposed to environmental
conditions. Lenticular transpiration is the main source of water loss from deciduous plants after
shading off their leaves. Because there are more stomata on the leaves than elsewhere in the shoot
system, it is evidence that most of the water vapour is lost from the leaves.
In order to establish that transpiration occurs mostly in the leaves, an experiment using absorptive
paper, dipped Cobalt II Chloride solution or Cobalt II thiocynate solution is carried out. The paper is
covered on the surface of both sides of the leaves and then clamped with glass slides. After some time,
the blue cobalt thiocynate paper changes to pink, indicating the evaporation of water molecules from
the leaf by transpiration. The rate of change from blue to pink is higher at the lower epidermis than the
upper epidermis. This is because structurally there are more stomata on the lower epidermis to prevent
excessive loss of water by transpiration due to direct solar radiation
Page 304 of 447

Measuring the rate of transpiration

The rate of transpiration can be measured by either determining the rate of transpiration at which the
plant loses mass due to water loss or the rate at which the plant takes in water (water uptake), using
an instrument called a potometer.
Determining the rate of transpiration using
a. the weighing method
The rate of mass loss by the plant can be determined by using the potted plant placed on an
automatic weighing balance whereby the change in mass is noted over a given period of time. Using
this method, it is assumed that the mass loss is only due to water loss by transpiration. However, the
whole pot must be enclosed in a polythene bag to prevent water from evaporating from the soil. In
addition, the soil must be well watered before the beginning of the experiment so that the plant has
enough water throughout the experiment. The rate of transpiration is then expressed in terms of mass
lost per unit time
b. the potometer
The potometer is used to measure the rate (Kent Fig 2 pg 276, Soper Fig 13.10 pg 439, Toole fig
of water uptake by the shoot of the leafy 22.12 pg 457)
plant.
However, since most of the water taken
up is lost by transpiration, it is assumed
that water uptake ≈ water loss. The leafy
shoot is cut under water to prevent the air
bubbles from entering and blocking the
xylem vessels. The cut leafy shoot is
immediately fixed in the sealed vessel and
connected to the capillary tube. The rate
of water uptake is then measured by
introducing an air bubble at the end of the
graduated capillary tube and the distance
moved by the air bubble per unit time is
noted.
To drive the air bubble back to the original position, water is introduced into the capillary tube from
the reservoir by opening the tap on the reservoir.
The leafy area is also established by tracing the outline of the leaves on a squared graph paper and
then counting the number of complete and incomplete squares enclosed in the outline
Number of incomplete
Total area of Number of 1
leaves = complete squares + squares x
2
The rate of transpiration is therefore expressed in terms of the volume of water taken up by the leafy
shoot per unit time per unit leaf area. The structure of a potometer is shown in the diagram above.
Precautions taken when using a potometer
a. The leafy shoot used should have a significant water loss by having very many leaves
b. The stem of the leaf shoot must be cut under water to prevent air from entering and blocking the
xylem vessels
c. The setup must have plenty of water
Page 305 of 447
d. Ensure that only one bubble is present in the capillary tube

e. A well graduated scale must be used e.g. a ruler, so that clear readings are taken
f. The air bubble should always be reset to zero mark before the potometer is used again under
different conditions
g. The water reservoir should be filled with water when setting the air bubble at the zero mark
h. The cut leafy shoot must be in contact with water in the sealed vessel
How to use a potometer
a. The leafy shoot is cut under water to prevent air bubbles from entering and blocking the xylem
vessels. The cut leafy shoot is immediately fixed in the sealed vessel of water connected to a
capillary tube. Allow time (5 minutes) for the apparatus to equilibrate. The rate of water uptake is
measured by introducing the air bubble at the end of the graduated capillary tube and the distance
moved by the air bubble per unit time is noted.
b. To drive the air bubble back to the original point, water is introduced into the capillary tube from
the reservoir by opening the tap.
c. The leafy area is then established by tracing the outline of the leaves on squared papers and then
counting the number of complete and incomplete squares in the outline of the leaves.
d. The rate of transpiration is therefore expressed in terms of the volume of water taken up by the
leafy shoot per unit time per leafy area.
NOTE; since most of the water taken up by the potometer is lost by transpiration, it is assumed that
water uptake = water loss.
Advantages of transpiration
a. It allows the uptake of water from the roots to leaves in form of a transpiration stream. This is due
to a transpiration pull created in the leaves. This ensures proper distribution of water throughout
the plant to keep it alive.
b. It facilitates the uptake of the absorbed mineral salts within the xylem vessels from roots to leaves
c. It brings about the cooling of the plant since as water evaporates to the atmosphere, excessive heat
is also lost as heat of vaporization, which results into the cooling of the plant
d. It brings about mechanical support in non-woody or herbaceous plants, due to water uptake which
provides turgidity to the parenchyma cells of the stem and leaves
e. It is important for cloud formation via evapotranspiration hence resulting into rainfall
Disadvantages of transpiration
a. It causes wilting of plants in case of excessive transpiration
b. It may eventually cause death of the plant, when the plant loses water excessively due to excessive
transpiration
NOTE: wilting is the drooping of leaves and stems as a result of plant cells losing water osmotically
and becoming flaccid. Evaporation occurs at rate greater than that at which it is absorbed, resulting
into reduction in turgor pressure and dropping of the plant. It always takes place in hot and dry areas.
Wilting also results into the closure of the stomata which cuts off gaseous exchange and therefore may
cause death if it persists.
FACTORS AFFECTING TRANSPIRATION

The potometer may be used to investigate the effect of environmental factors on the rate of
transpiration i.e. it can be moved to a windy place or a place which is dark. Transpiration is affected
by both environmental and non-environmental factors.
Page 306 of 447
ENVIRONMENTAL FACTORS
1. Humidity
The humidity of the atmosphere affects the gradient of water vapour between the sub-stomatal air
chamber and the atmosphere around the leaf i.e. it affects the rate of diffusion of water vapour.
Low humidity (low water vapour pressure) outside the leaf increases the rate of transpiration because
it makes the diffusion gradient of water vapour from the moist sub-stomatal air chamber to external
atmosphere steeper.
When humidity is high in the atmosphere, the diffusion gradient or the water vapour pressure gradient
is greatly reduced between the sub-stomatal air chamber and the atmosphere which results into
reduction in the rate of transpiration.
In areas where humidity is too high, plants loose liquid water from their leaves via structures/glands
on their leaf margins known as hydathodes, a process known as guttation. Guttation is the loss of
liquid water from plant leaves (exudation) through hydathodes due to excessive humidity in the
atmosphere. Guttation is common in young grass seedlings and rain forest plants due to the dim light
and high humidity.
2. Temperature
Increase in temperature increases the rate of water loss by the leaves via transpiration. A decrease in
temperature lowers the rate of water loss by the plant leaves via transpiration.
This is because (a) increase in temperature increases
the heat energy which provides the latent heat of
vaporization of water molecules hence the water
molecules evaporate rapidly to the sub-stomatal
chambers and eventually to the atmosphere via the
stomata (b) increase in temperature also lowers
humidity outside the leaf by increasing the random
thermal movement of molecules in the water vapour
which further increases the rate of transpiration.
In extremely hot conditions, the stomata of some
plants close, an adaptation to prevent water loss by
transpiration.
3. Air movements
In still air (no wind), layers of highly saturated vapour build up around the stomatal pores of the leaf
and reduces diffusion gradient between the stomatal air chamber and the external atmosphere,
thereby reducing the rate of diffusion of water vapour from the leaf.
The layers of highly saturated water vapour which (Soper fig 13.9 pg 439)
build up around the stomatal pores of the leaf are
called diffusion shells.
Windy conditions result in increased transpiration
rates because the wind sweeps away the diffusion
shells around the leaf, thereby creating a steep
diffusion gradient which leads to a high the
transpiration rate
Page 307 of 447

4. Atmospheric pressure
Water vapour and the atmospheric pressure decreases with increasing altitude.
The lower the atmospheric pressure the greater the rate of evaporation of water from the sub-
stomatal air chamber. This implies that plants growing on a mountain have a higher rate of
transpiration than those growing in low land areas.
However, when the atmospheric pressure is high e.g. in the lowland areas, the evaporation of water
vapour from the sub-stomatal air chamber to the atmosphere decreases, thereby decreasing the rate of
transpiration.
5. Water availability
For water vapour to diffuse out of the sub-stomatal
air chamber to the atmosphere, the mesophyll cells
must be thoroughly wet. Shortage of water in the
soil or any mechanism which hinders the uptake of
water by the plant leads to wilting of the plant hence
the closure of the stomata.
When water is supplied in large amounts, too much
water evaporates to the atmosphere and therefore a
high rate of transpiration. However, when the water
supply to the mesophyll cells is low, less water
evaporates from the sub-stomatal to the atmosphere, QN. What is the relationship between
hence a low rate of evaporation. transpiration and water up take?
6. Light intensity
It affects transpiration indirectly by affecting the closure and
opening of the stomata, which usually opens in bright sunlight
to allow evaporation of water to the atmosphere. Therefore
sunlight increases the rate of transpiration.
At night and in darkness, the stomata close and therefore there
is no evaporation of water from the sub-stomatal air spaces to
the atmosphere. This greatly lowers the rate of transpiration in
the plant.
Differences between transpiration and guttation

Transpiration Guttation
1. Water is lost in form of vapour 1. Water is lost in form of droplets
2. Occurs through the stomata, cuticle and 2. Occurs through the hydathodes
lenticels 3. Occurs during dim light and low
3. Occurs during bright light and high temperatures
temperatures
4. Enhanced by low humidity 4. Enhanced by high humidity
5. Water lost is pure without mineral salts 5. Water lost is mineral water with sugars,
salts and amino acids
6. Increased transpiration causes wilting 6. Increased guttation does not cause wilting
Page 308 of 447

NON-ENVIRONMENTAL FACTORS
Leaf area Cuticle
The larger the leaf surface area on the The thinner the cuticle, the higher the rate of water
plant, the higher the rate of water loss by loss by transpiration and the thicker the cuticle, the
transpiration. In addition, broad leaves lower the rate of water loss from the plant to the
provide a large surface area over which atmosphere by transpiration. This is because this offers
water vapour diffuses to the atmosphere a significant resistance towards the diffusion of water
as compared to the narrow leaves. vapour from the plant to the atmosphere
Number of stomata
The larger the number of stomata
on the plant, the higher rate of
water loss by transpiration and the
lower the number of stomata, the
lower the rate of transpiration.
However, a very large number of
stomata so close to each other may
instead reduce the rate of
transpiration especially in still air
due to the accumulation of water
vapour around the whole stomata
pore.
Distribution of stomata
The upper surface is more exposed
to environmental factors that
increase the rate of transpiration.
Xerophytic adaptations of leaves that reduce transpiration

Xerophytes (xero = dry, phyte = plant) are plants that are adapted to living in areas where water
losses due to transpiration may exceed their water uptake. Similar adaptations may also be seen in
plants found in dry, windy places, where rainfall is high and temperature relatively low. As the vast
majority of transpiration occurs through the leaves, it is these organs that show most modifications.
Examples include;
a) Having a thick cuticle. The thicker the cuticle, the less water can escape by this means.
b) Curling up of leaves. A region of still air is trapped within the curled leaf. This region becomes
saturated with water vapour and so there is no water potential gradient between the sub-
stomatal air space and the outside, and so transpiration is considerably reduced.
c) Having hairy leaves. A thick layer of hair, especially on the lower epidermis, traps moist air
next to the leaf surface which reduces the water potential gradient between the inside and the
outside of the leaf, therefore less water is lost.
d) Having stomata in pits or grooves. These trap moist air next to the leaf and reduce the water
potential gradient.
e) Reducing the surface area to volume ratio e.g. the cacti leaves are in form of thorns
Page 309 of 447

f) Closing stomata when transpiration rate is very high. This is done by the release of abscisic
acid.
STOMATA
In terrestrial plants, gaseous exchange takes place predominantly in the leaves. The epidermis of the
leaves contains small pores called stomata (singular. stoma). Through stomata, gaseous exchange
between the inside of the leaf and the outside air takes place by diffusion.
The broad leafed shape of the leaf offers a large surface for diffusion of gases, its thinness reduces the
distances over which diffusion of gases from the atmosphere to the inner most cells.
In most terrestrial plants, stomata are more abundant on the lower side than the upper surface of the
leaf. This reduces water loss through transpiration since the upper surface is exposed to direct sunlight.
The number of stomata in leaves vary from one plant species to another. They are normally absent in
submerged leaves of water plants.
Structure of the stoma
Each stoma consists of a stomatal pore, bordered by a pair of crescent or bean-shaped cells called
guard cells. Unlike epidermal cells, guard cells contain chlorophyll. The inner cell wall of guard cells
is thicker and less elastic than the outer wall. Microfibrils are radially orientated in the cell wall and
the guard cells are joined at the ends. The epidermal cells surrounding the guard cells are subsidiary
cells.
(Toole fig 22.7a pg 452) (Toole fig 22.7b pg 452)
(Soper fig 13.15 pg 444)
Page 310 of 447

Ventilation (opening and closing of stomata)

The opening and closing of stomata occurs as a result of changes in the shape of the guard cells.
When guard cells take in water by osmosis, they expand and become turgid. However, they do not
expand uniformly in all directions. The thick inelastic inner wall makes the guard cells to curve away
from each other, opening the stoma. When the guard cells lose water, they become flaccid and
collapse, closing the stomata.
The closing and opening is controlled mainly by the intensity of light. They are normally open
during daylight and closed during the night.
Several theories have been put forward to explain how the light intensity influences the opening and
closing of stomata.
a. Starch sugar inter conversion
This is one of the earliest theories that attempted to explain the control of stomata closure.
- Photosynthesis by mesophyll cells during daylight would remove carbon dioxide from air
spaces within the leaf
- Since carbondioxide is an acidic gas, removal of carbon diode raises the pH of the guard cells
- Starch hydrolyzing enzymes in the guard cells work better in alkaline conditions, and they
convert starch to sugar
- Accumulation of sugar makes the water potential of the guard cells more negative, causing
a net influx of water into the guard cells and opening the stomata.
Note: a starch hydrolyzing enzyme, starch phosphorylase that is affected by pH was found
but some plants e.g. the onion do not form starch at all.
b. Photosynthetic product theory
- Guard cells have chloroplast.
- During day light, they carry out photosynthesis producing sugar.
Page 311 of 447

- The sugar increases the osmotic pressure of the cell sap. This causes water to move into the guard
cells from neighbouring epidermal cells by osmosis. The result is an expansion and increase in
turgidity of the guard cells containing the stomata to open.
- In darkness, photosynthesis stops and the sugar in the guard cells is converted to starch. This
lowers the osmotic pressure of guard cells causing them to lose water to neighboring cells by
osmosis.
- The guard cells become flaccid and the stomata close.
Note; this theory does not explain how the low rate of glucose formation can account for the rapid
opening of stomata
c. Potassium ion (K+) mechanism (mineral ion concentration)
- When guard cells are exposed to light, the light energy activates the ATPase enzyme, hence their
chloroplasts manufacture ATP.
- The ATP drives a K+ - pump on the cell membrane of the guard cells. This causes an active
uptake of K+ ions in the guard cells from the surrounding epidermal cells.
- Accumulation of K+ in the guard cells increases the osmotic pressure of their cell sap. This
causes water to move into the guard cells from neighboring epidermal cells by osmosis. The
result is an expansion and increase in turgidity of the guard cells causing the stomata to open
because when they become turgid, they expend but not uniformly since the inner wall is inelastic,
making the guard cells curve away from each other.
- At the onset of darkness, ATP concentration in guard cells falls rapidly stopping the K+ pump. K+
migrates from the guard cells to neighboring epidermal cells by diffusion. This lowers the
osmotic pressure of guard cells causing them to lose water to neighboring cells by osmosis.
- The guard cells become flaccid and the stomata close.
Note; the above theory is the most widely accepted theory today. It is supported by the fact that the
opening of stomata is prevented by metabolic poisons which inhibit active transport.
(Toole fig 22.8 pg 452 OR Kent fig 3 pg 281)
The above theories can be summarised into a single mechanism of stomata opening and closing as
described below;
Page 312 of 447

Stomata opening
a. Stomata opening is promoted by high light intensity and low mesophyll carbon dioxide levels.
Guard cells generate ATP by photophosphorylation during photosynthesis. .
b. Blue light is absorbed by blue-light photoreceptors which activate a proton-pump (H+-ATPase) in
the cell membrane of the guard cell
c. ATPs generated by the light-dependent reaction of photosynthesis are hydrolysed to provide
energy to drive the proton-pump. As protons (H+) are pumped out of the guard cells, the cells
become increasingly negatively charged. Potassium channels are activated and K+ ions diffuse
from subsidiary cells through the channels down this electrochemical gradient into guard cells.
Chloride ions (Cl-) then enter to balance the charge.
d. In some plants the starch is converted to malate.
e. The accumulation of K+ (and malate ions) causes the water potential in the guard cells to become
more negative. Water enters by osmosis from the neighbouring subsidiary cells into the guard cells.
The guard cells become turgid.
f. The outer wall of the guard cells is thinner and more elastic than the thicker inner wall. There are
cellulose micro fibrils which are radially arranged around the cell wall and the ends of the two
guard cells are joined
g. The increased turgor pressure therefore causes the guard cells to curve outward and the stoma
opens
Stomata closure
a. Stomata closure can be triggered by water stress, high temperature, increasing carbon dioxide
levels in the leaf mesophyll and low light intensity (night time)
b. The hormone abscisic acid (ABA) is secreted by plant cells when transpiration rate is high and soil
water is low.
c. ABA binds to receptors at the cell membrane of the guard cells. This increase the permeability of
calcium channels in the cell membrane. Calcium ions (Ca+) enter into the guard cell. The influx of
calcium ions also triggers the release of Ca+ from the cell vacuole into the cytosol.
d. Potassium ions (K+) move out of the guard cells into the subsidiary cells
e. In some plants (Cl-) and certain organic ions e.g. malate ions also move out of the guard cells
f. The water potential in the guard cells increase. Water diffuses out to neighbouring subsidiary cells
by osmosis. The turgor pressure in the guard cells decreases, the cells become flaccid and the stoma
closes.
g. At night the chloroplasts in the guard cells do not photosynthesise, less ATP is produced and there’s
no active uptake of K+ ions. Instead, the K+ ions diffuse out of the guard cells. The cells become
flaccid and the stoma closes.
Importance of stomata
1. Stomata allow gaseous exchange of carbon dioxide (for photosynthesis) and oxygen (for
respiration) between the plant and the surrounding
2. Stomata regulate the rate of transpiration and control water loss by the plant
3. When water is lost through the stomata, it creates a transpiration pull which can pull the water
and mineral salts from the roots to the higher parts of the plant. Transpiration also has a cooling
effect on the plant.
Page 313 of 447
LENTICELS
A small extent of gaseous exchange takes place Structure of the lenticel
in the stem through structures called lenticels.
The small gaps in the stem, usually circular or
oval slightly raised on the bark surface. The
cells in this area are thin walled and loosely
parked, leaving air spaces which communicate
with air spaces in the cortex. Here oxygen for
respiration is taken up and carbon dioxide is
given out.
ROOT EPIDERMAL CELLS

Root cells can also take in oxygen for respiration and give out carbon dioxide. Gaseous exchange takes
place by diffusion between the epidermal cells of roots and the air spaces in the soil. Most of the
exchange takes place at the root hairs which provide a large surface area.
Water logged soils have their air spaces occupied by water, thereby reducing respiration in the roots
which may subsequently die. This would obviously kill the whole plant.
Some aquatic plants, like pond weeds and multi cellular algae are completely submerged in water.
These obtain their gaseous requirements by diffusion from the surrounding water. Epidermal cells of
such plants have no cuticle and gasses diffuse directly across it.
Others like rice and water lilies are partially submerged in water. Their aerial parts obtain carbon
dioxide and oxygen in the same manner as terrestrial plants. The submerged parts may face the
problems of obtaining adequate oxygen for their respiratory requirement. However such plants have
large air spaces in their stems and roots which store oxygen obtained from the aerial parts and that
formed during photosynthesis. Floating leaves of such plants have stomata on the upper surfaces only.
In swampy environments, root systems give rise to breathing roots or pneumatophores. These grow
out of the water and op into the air. Oxygen diffuses into them and aerates the submerged parts of the
root system.
EXPERIMENT TO OBSERVE STOMATA

Obtain a leaf a leaf of comelina. Hold it in such a way that the lower surface is facing you. Slowly tear
the leaf as you would tear a piece of paper by moving the right hand towards the body. This produces
a thin, transparent membrane-like tissue along the edge of the tear on the part of the leaf in the left
hand. This is the lower epidermis. Using forceps, remove a small section of the epidermis and mount
it in a drop of water on a slide and cover it with a cover slip. Observe under low power and then under
the high power of a microscope. Identify the guard cells and the normal epidermal cells. Observe a
closed stoma and an open stoma under low and high power. Draw each of these
Page 314 of 447

WATER UPTAKE BY THE ROOTS

Internal structure of the root
The root consists of various tissues which occur in concentric layers. The cells at the surface of the
young root forming the peliferous layer are so called because it is by the root hairs. As the roots get
older, they increase in girth (thickness or diameter) and the peliferous layer (breaks) raptures and peels
off leaving the outer most layer of cells known as epiblem, to become the functional outer layer.
Next to the epiblem is the thicker layer of loosely packed parenchyma cells, known as cortex. Adjacent
to the cortex is a layer of cells known as endodermis.
The endodermal cells have their radial and horizontal walls coated with a corky band called casparian
strip. This strip is made up of a substance called suberin. The Casparian strip is impermeable to water
and solutes due to the suberin that it contains and therefore prevents water and solutes to pass through
the cell walls to the endodermis. The endodermis also contains starch grains.
Next to the endodermis is another layer of cells known as pericycle from which lateral roots develop.
The pericycle, that is made up of parenchyma cells which encloses the vascular bundles (xylem and
phloem) in the centre of the root.
Diagram showing the internal structure of the root
(Toole fig 22.13a pg 462)
Page 315 of 447

(Toole fig 22.13b pg 462)
Longitudinal section through a root
Mechanism of water uptake by the roots

For water to be transported up to the leaves through the stem, it must be absorbed from the soil by the
tiny root hairs. Water absorption into the root hairs occurs by osmosis. This is due to the water potential
of the cell sap of the root hairs being lower than that of the soil solution (water content) due to the very
low concentration of mineral ions in water and the soil particles. The cells of the root hairs have a
relatively higher concentration of ions, sugars and organic acids within their vacuoles and cytoplasm
hence a lower water potential (more negative).
When the root hair absorbs water, its water potential increases and becomes higher than that of the
adjacent cells of the root. This facilitates the flow of water from the root hairs to the endodermal cells
across a water potential gradient.
The water flow is also due to the root pressure developed by the cell cortex and endodermis which
ensures that water flows from the root hairs to the xylem vessels and upwards to the leaves.
Water flows by osmosis form the root hairs to the endodermal cells using three pathways, namely; (a)
Apoplast (cell wall) pathway, (b) Symplast (cytoplasm) pathway & (c) Vacuolar pathway
Apoplast pathway
This is the pathway in which water moves through the spaces between the cellulose fibres in the cell
wall of one cell to the cell wall of the adjacent cells. The cohesive forces between the water molecules
enable the stream of water to be pulled along the apoplast pathway.
Page 316 of 447

However, this movement does not occur within the endodermal cells because they possess the
impermeable water proof band of suberin that makes up the casparian strip. The casparian strip
prevents water and solutes flow through the cell walls of the endodermal cells. This means that water
and solutes flow through the cell walls of the endodermal cells via the Symplast and the vacuolar
pathways only.
The significance of this casparian strip is that endodermal cells actively pump salts (ions) from the
cytoplasm into the xylem vessels which creates a high solute concentration in the xylem, thereby
greatly lowering the water potential in the xylem than in the endodermis. This makes the water
potential of the xylem vessels more negative (very low) and results into rapid osmotic flow of water
from the endodermal cells to the xylem vessels, due to the steep water potential gradient between the
endodermal cells and the xylem vessels. This positive hydrostatic pressure is known as the root
pressure.
The casparian strip facilitates the pushing of water upwards through the xylem vessels by root pressure
up to the leaves due to its active pumping of the salts. In addition, this active pumping of the salts into
the xylem vessels prevents leakage of salts (ions) out of the xylem vessels so as to maintain a low
water potential in this vessel.
Note, in some short herbaceous plants, root pressure is strong at times e.g. at night when humidity is
high to cause exudation of water droplets from hydathodes at the edges of leaves. This is known as
guttation.
Symplast pathway Diagram showing the three pathways of
This is the movement of water through the water in the root
cytoplasm of one cell to the cytoplasm of the (Soper fig 13.18a pg 448)
adjacent cell via plasmodesmata along a water
potential gradient. Water leaving the pericycle cells
to enter the xylem causes the water potential of
these cells to become more negative (more dilute).
This facilitates the flow of water by osmosis from
the adjacent cells into these cells. In this way the
water potential gradient from the root hairs to the
xylem is established and maintained across the root.
This pathway offers a significant resistance to the
flow of water unlike the apoplast pathway.
Vacuolar pathway
This is the movement of water from the sap vacuole of one cell to the sap vacuole of the adjacent cell
through the cytoplasm, vacuoles as well as the cell wall following a water potential gradient.
This is achieved by maintaining a steep water potential gradient. However, this also offers a reasonable
level of resistance towards water flow in comparison to the Symplast pathway.
Note; the apoplast is the most appropriate pathway in plants because it provides less resistance to water
flow in the plant.
Page 317 of 447

To ensure maximum absorption of water, the root hairs have the following adaptations
a. They are numerous in number so as to provide a large surface area for the maximum absorption of
water by osmosis.
b. They are slender and flexible for easy penetration between the soil particles so as to absorb water.
c. The lack a cuticle and this enhances the passive osmotic absorption of water without any resistance
d. They have a thin and permeable membrane which allows the absorption of water by osmosis.
e. They have a water potential lower than that of the soil solution which facilitates a net osmotic flow
of water from the soil
f.
Vascular tissues
The transport system in plants is made up of the xylem (which carries water from roots, up the plant
from the aerial parts) and phloem (which carries sugars produced by leaves to other parts of the plant).
The two tissues occur together throughout the plant, sometimes associated tissues, such as
sclerechyma fibres, to form discrete areas, known as vascular bundles.
a) Distribution of vascular tissues in a leaf
The vascular tissues in a dicotyledonous leaf form a network of tiny vascular bundles
throughout the blade, or lamina, of the leaf. These tiny bundles fuse to give a series of side
veins that run parallel with one another. These side veins then merge into a central main vein.
The main vein runs along the centre of the leaf, increasing in diameter towards the petiole, or
leaf stalk. Within in each, or vascular bundle, there is an area of xylem towards the upper
surface of the leaf and an area of phloem towards the lower surface.
b) Distribution of vascular tissues in a stem
The xylem and phloem in a dicotyledonous stem form vascular bundles that are arranged
towards the outside of the stem. The reason for this is that the vascular bundles, along with
associated sclerechyma fibres, not only transport materials but also provide support in
Page 318 of 447

herbaceous plants. The main forces acting on stems are lateral one caused by the action of
wind on them. Such forces are best resisted by an outer cylinder of supporting tissue. Hence
the vascular bundles form a discontinuous ring towards the edge of them. Being discontinuous,
this ring of supporting tissues allows the stems to be flexible and to bend in wind. Within the
vascular bundles, the xylem is to the inside of the stem and the phloem towards the outside.
Between the two is a thin layer of dividing cells called cambium, which give rise to both phloem
and xylem.
c) Distribution of vascular tissues in a root
The vascular tissue in the root of a dicotyledonous plant is situated centrally rather than towards
the outer edge, as in a stem. This is because roots are subject to pulling forces in a vertical
direction, rather than in a lateral direction, as experienced by stems. Vertical forces are better
resisted by a central column of supporting tissues, such as xylem, rather than an outer cylinder
of tissue. The xylem is typically arranged in a star-shaped block of tissue at the centre of the
root, with the phloem situated in separate groups between each of the points of the star-shaped
xylem. Around both is the pericycle and endodermis.
The xylem.
The xylem is the principle water-conducting tissue in vascular plants. It also provides support for
plants. The sclerechyma fibres in the xylem contribute to support whereas vessels and tracheids have
both support and transport roles.
a) xylem fibres are elongated sclerechyma cells with walls that are thickened with lignin; these
features suit them for their role of support
b) vessels vary in structure, depending on the type and amount of thickening of their cell walls,
but are all hollow and elongated. As they mature, their walls become impregnated with lignin,
which causes them to die. The end walls breakdown to form a perforation plate which allows
the cells form a continuous tube. The lignin maybe spiral/network/reticulate or annular/ring in
form; these arrangements are better than continuous thickening, because allows elongation of
vessels as the plant grows. There are areas of the lignified wall where lignin is absent, called
pits. They are not completely open as there is still a cellulose cell wall across them. Pits allows
lateral (sideways) movement of water. In angiosperms, vessels are the structures through which
the vast majority of water is transported.
c)
Adaptations of the xylem structure to its function
 the cells are long and arranged end to end to form a continuous column
 the cell contents die when mature, which means that:
- there is no nucleus or cytoplasm to prevent water flow
- the end walls break down, so that there is no barrier to water flow between adjacent
cells
 cells are thickened with lignin, which
- makes them more rigid and therefore less likely to collapse under the tension created
by the transpiration pull
- increases the adhesion of water molecules, enabling them to rise by capillarity
Page 319 of 447

 annular, reticulate and spiral thickening allow xylem vessels to elongate during growth, and
make them more flexible, so that branches can bend in the wind
 there are pits throughout the cells, to allow lateral movement of water
 the large lumen of the vessels allows a large volume of the water to be transported
Other xylem tissues
a) Xylem parenchyma is composed of unspecialized cells that act as packing tissue around the
other components of the xylem. They are roughly spherical in shape, but when they are turgid
they press upon and flatten each other in places. In this way they provide support.
b) Tracheids have similar structures to vessels except that they are longer and thinner, and have
tapering ends. They, too, are thickened with lignin and therefore die when mature. As with
vessels, the end walls break down, and their side walls possess pits which allow lateral
movement of water between adjacent cells. Tracheids are found in all plants and are the major
conduction tissues in ferns and conifers.
ROOT PRESSURE
Root pressure is the force developed by cells of the roots which forces water from the endodermal cells
into the xylem vessels of the root and constantly forces water upwards through the stem to leaves. This
process is active and involves utilization of many ATP molecules. Root pressure occurs as a result of
endodermal cells actively secreting salts into the xylem sap from their cytoplasm, which greatly lowers
the water potential in the xylem.
In some plants, root pressure maybe large enough to force liquid water through pores called hydathodes
of the leaves in a process called guttation
The following is the evidence to show that water

moves by pressure in a plant.
a. When the stem of a plant is cut water
continues to exude from the xylem vessels of
the plant stem. The continuous exudation of
water from the xylem vessels of the cut stem
is due to root pressure because the leafy
shoot is cut off, meaning that water not only
moves upwards by transpiration pull, but
also due to pressure and other forces.
b. Root pressure can be measured using a
mercury manometer whose diagram is
shown on the right
c. Though it is true that water moves from the roots through the stem to the leaves by transpiration
pull, root pressure partly contributes towards the movement of water from the parenchyma cells to
the xylem of the root, to the stem and eventually up to the leaves
Page 320 of 447

The following is the evidence to support the mechanism of water uptake from the endodermis into the
xylem vessel as an active process
a. There are numerous starch grains in endodermal cells which could act as an energy source for
active transport.
b. Lowering the temperature reduces the rate of water exudation (given out) from the cut stem as it
prevents root pressure, an active process.
c. Treating the roots with metabolic poisons e.g. potassium cyanide also prevents water from being
exuded from the cut stems. This is because the poisons kill the cells thereby preventing aerobic
respiration, a source of ATP molecules.
d. Depriving roots of oxygen prevents water from being exuded from the cut stems. This shows that
water was being pushed upwards in the cut stem by root pressure, an active pressure.
THE UPTAKE OF WATER FROM THE ROOTS TO THE LEAVES

The movement of water from the roots to the leaves, via the stem, is by combination of different forces
which include the following; (1) Root pressure, (2) Transpiration pull (cohesion force)
& (3) Capillarity
Root pressure
This enables movement of water from the parenchyma cells of the main root into the xylem tissue due
to the active pumping of cells from endodermal cells into the xylem tissue.
Root pressure also ensures upward movement of water through the xylem tissues to the leaves.
Transpiration pull (cohesive force/cohesion-tension theory of water uptake)
This offers an explanation for the continuous flow of water upwards through the xylem of the plant
i.e. from the root xylem to the stem xylem and finally to the leaf xylem. Water is removed from the
plant leaves by transpiration which creates a tension within the leaf xylem vessels that pulls water in
the xylem tubes upwards in a single unbroken column or string held together by the cohesive forces of
attraction between water molecules.
According to the cohesion-tension theory, evaporation of water from the mesophyll cells of the leaf to
the sub-stomatal air chamber and eventually to the atmosphere via the stomata by transpiration, is
responsible for the rising of water from the roots to the leaves. This is because the evaporated water
molecules get replaced by neighbouring water molecules which in turn attract their other neighbours
and this attraction continues until the root is reached.
Evaporation of water results in a reduced water potential in the cells next to the leaf xylem. Water
therefore enters these mesophyll cells by osmosis from the xylem sap which has the higher water
potential. Once in the mesophyll cells water moves using the three pathways namely; apoplast,
Symplast and vacuolar pathways from one cell to another by osmosis across a water gradient.
When water leaves the leaf xylem to the mesophyll cells by osmosis, a tension is developed within the
xylem tubes of water which is transmitted to the roots by cohesive forces of water molecules. The
tension develops in the xylem vessels and builds up to a force capable of pulling the whole column of
water molecules upwards by means of mass flow and water enters the base of these columns from
neighbouring root cells. Because such a force is due to water loss by osmosis by transpiration, it is
referred to as transpiration pull.
The upward movement of water through the xylem tissue from the roots to leaves is also facilitated by
the cohesive forces of attraction which holds the water molecules firmly together, due to the hydrogen
Page 321 of 447
bonds which exist between them. This enables water to have a high tensile strength which enables it
to move upwards in a continuous stream without breaking. In addition, the upward movement of water
from roots to leaves is also facilitated by adhesive forces which hold the water molecules on the xylem
walls so that it continues moving upwards.
Capillarity
Since the water rises upwards through narrow leaves, it is also facilitated by capillarity through the
stem. This is because the xylem vessels are too narrow and the flow of water is maintained without
breaking by both the cohesive forces (between the water molecules) and adhesive forces (between the
water molecules and the hydrophilic surface of the walls of the xylem).
NOTE
The continuous mass flow of water through the xylem vessels from the roots to the leaves in a stream
without breaking, due to the transpiration pull is called the transpiration stream
Adhesion is the force of attraction between molecules of different substances while cohesion is the
force of attraction between molecules of the same substance
The diagram below shows the upward movement of water from the soil up to the leaves.
Page 322 of 447

In summary
Movement of water across the leaf Movement of water up the stem in the
The humidity of the atmosphere is usually less than xylem
that of the sub-stomatal air-space and so, provided that The main mechanism by which water
the stomata are open, water diffuses out of the air- moves up the xylem is known as the
spaces into the surrounding air. Water lost from the air- cohesion-tension theory. It operates as
follows;
spaces is replaced by water vapour evaporating from
 Water evaporates from leaves as a
the cell wall of the surrounding spongy mesophyll
result of transpiration
cells. By changing the size of the stomatal pores,  Water molecules form hydrogen
plants can control water loss. Water vapour bonds between one another and
evaporating from the cell walls of spongy cells is hence tend to stick together i.e.
replaced by water reaching them from xylem by either cohesion
the apoplast or symplast pathways. In case of the  Water forms a continuous, unbroken
symplast pathway, the water movement occurs path across the mesophyll cells and
down the xylem
because, once the spongy mesophyll cells have lost
 As water evaporates from mesophyll
water to the sub-stomatal air-space, they have a lower cells in the leaf into the sub-stomatal
(more negative) water potential. Water therefore air space, more molecules of water
enters by osmosis from the adjacent cells. The loss of are drawn up behind it as a result of
water from these adjacent cells cause them to have a this cohesion
lower water potential and so they, in turn, take in water  Water is hence pulled up the xylem as
from the neighbours by osmosis. In this way, a water a result of transpiration. This is called
potential gradient is established that pulls water from the transpiration pull.
 The transpiration pull puts the xylem
the xylem, across the leaf mesophyll, and finally out
under tension, i.e. there is a negative
into atmosphere. pressure within the xylem.
Evidence for the cohesion-tension theory
a) The changes which occur in the diameter of trees according to the rate of transpiration. During
the day, when transpiration is at its greatest, there is more tension (more negative pressure) in
the xylem. This causes the trunk to shrink in diameter. At night, when transpiration is at its
lowest, there is less tension in the xylem and so the diameter of the trunk increases
b) When a xylem vessel is broken, water does not leak out which would be the case if it were
under pressure, but rather air is pulled in, which is consistent with it being under tension.
UPTAKE AND TRANSLOCATION OF MINERAL IONS

Translocation is the movement of mineral salts and chemical compounds within a plant.
There are two main processes of translocation which include;
a. The uptake of soluble minerals from the soil and their passage upwards from the roots to the
various organs via the xylem tubes.
b. The transfer of organic compounds synthesized by the leaves both upwards and downwards to
various organs via the phloem tubes
Mechanism of mineral ion uptake

Minerals such as nitrates, phosphates, sulphates e.t.c. may be absorbed either actively or passively.
Active absorption of minerals
Page 323 of 447

Most minerals are absorbed from the soil solution having the less mineral concentration into the root
hairs with the higher mineral concentration, selectively by using active transport which uses a lot of
energy.
The rate of active absorption of minerals into the root hairs depends on the rate of root respiration.
Factors such as oxygen supply and temperature will affect the rate of ion uptake. The addition of
respiratory poison has shown to inhibit uptake of mineral ions.
(Soper Fig 13.19 pg 449) (Soper Fig 13.20 pg 450)
Passive absorption
If the concentration of a mineral in a soil solution is greater than its concentration in the root hair
cell, the mineral may enter the root hair cell by diffusion.
Mass flow or diffusion occurs once the minerals are absorbed by the root hairs so that they move
along cell walls (apoplast pathway).
In mass flow, the mineral ions are carried along in solution by water being pulled upwards in the
plant in the transpiration stream, due to the transpiration pull i.e. the mineral ions dissolve in water
and move within the water columns being pulled upwards.
The mineral ions can also move from one cell of the root to another against the concentration
gradient by using energy inform of ATP. This is achieved by the use of special carrier proteins.
Two important mineral ions in plants are nitrates and magnesium. Nitrate ions provide nitrogen is a
component of:
 amino acids (make up proteins)
 nucleotides (make up DNA and RNA)
 auxins (are plant growth factors)
Magnesium is a component of chlorophyll and activator of ATPase enzyme.
The mineral ions can also move through the Symplast pathway i.e. from one cell cytoplasm to another.
When the minerals reach the endodermis of the root, the Casparian strip prevents their further
movement along the cell walls (apoplast pathway). Instead the mineral ions enter the cytoplasm of the
cell (Symplast pathway) where they are mainly pumped by active transport into the xylem tissues and
also by diffusion to the xylem tissues.
Page 324 of 447

Once in the xylem, the minerals are carried up the plant by means of mass flow of the transpiration
stream. From the xylem tissues, minerals reach the places where they are utilised called sinks by
diffusion and active transport i.e. the minerals move laterally (sideways) through pits in the xylem
tissue to the sinks by diffusion and active transport.
The following is the evidence to show that most mineral ions are absorbed actively by the root
hairs
1. Increase in temperature around the plant increases the rate of mineral ion uptake from the soil as it
increases respiration that can provide energy for active transport
2. Treating the root with respiratory inhibitors such as potassium cyanide prevents active mineral ion
uptake leaving only absorption by diffusion. This is because the rate of mineral ion uptake greatly
reduces when potassium cyanide is applied to the plant.
3. Depriving the root hairs of oxygen prevents active uptake of minerals by the roots and as a result
very few ions enter the plant by diffusion.
The following is the evidence for supporting the role of the xylem in transporting minerals
1. The presence of mineral ions in the xylem sap i.e. many mineral ions have been found to be present
in the xylem sap.
2. There’s a similarity between the rate of mineral ion transport and the rate of transpiration i.e. if
there’s no transpiration, then there’s no mineral ion transport and if transpiration increases, the rate
of mineral ion transport also increases.
Page 325 of 447

3. There’s evidence that other solutes e.g. the dye, eosin, when applied to the plant roots, it is carried
in the xylem vessels
4. By using radioactive tracers e.g. phosphorous-32. When a plant is grown into a culture solution
containing radioactive phosphorous-32, phosphorous -32 is found to have reached all the xylem
vessels but not the phloem tubes.
(The interpretation of these elements is that where lateral transfer of minerals can take place minerals
pass from the xylem to the phloem and where lateral transfer is prevented, the transport of minerals
takes place in the xylem)
NOTE; Some plants absorb mineral salts by using mutualistic associations between their roots and
other organisms e.g. the association between the fungus and the higher plant roots called mycorrhiza.
TRASLOCATION OF ORGANIC MOLECULES

(Food molecules in the phloem)
The organic materials produced as a result of photosynthesis; need to be transported to other regions
of the plant where they are used for growth or storage. This movement takes place in the phloem
tissue particularly in the sieve tubes.
Structure of the phloem
The phloem of composed of a number of cell types:
 Sieve tube elements are elongated structures that are joined end to end to form long tubes.
The cells are living and retain a thin layer of cytoplasm within their cell surface membrane,
which lie against the cellulose cell wall. Within the cytoplasm are mitochondria and a
modified form of endoplasmic reticulum. However, unlike most cells, there is no nucleus or
Golgi apparatus and there are no ribosomes. These are broken down in order to make the
sieve tubes more hollow and so reduce resistance to the flow of liquid within them. The end
walls of the sieve tubes are perforated by large pores, to form sieve plates. The central space
within the sieve tube is called the lumen.
 Companion cells are always associated with sieve elements and both come from the same
cell division. As the sieve tube elements lack structures such as a nucleus, Golgi apparatus
and ribosomes, they are unable to carry out many of the metabolic processes essential for
their survival. The companion cells are the sites for these processes. Materials can easily pass
through the many plasmodesmata that link the two types of cells. At the tips of veins in the
leaf, companion cells have very folded cell walls and cell surface membranes. These special
types of companion cells are called transfer cells and their large surface area increases the
rate of transfer of sucrose into sieve tube elements. (fig 11.19 page 238 Clegg and Mackean)
Page 326 of 447

How structure phloem is related to its function

1. Sieve tube elements are elongated and arranged end to end to form a continuous column
2. The nucleus and many of the organelles are located in the companion cells, leaving the lumen of
the sieve tube elements more open so reducing resistance to the flow of liquid
3. Sieve plates are perforated with sieve pores, reducing resistance to liquid flow
4. Sieve tubes hold the walls of sieve tube elements together and prevent them from bursting
5. The walls contain cellulose microfibrils that run around cells, giving strength and preventing the
tubes bursting under pressure
6. The walls are thin to allow easy entry of water at the source so as to build up pressure
7. Companion cells have many mitochondria to release the ATP needed for translocation of organic
materials
8. Companion cells have numerous ingrowths of their cell walls to increase the surface area for
active uptake of solutes .i.e. the transfer cells
9. Plasmodesmata allow easy movement of substances to and from companion cells
10.
Evidence to support that organic molecules of photosynthesis are transported in the phloem
1. When the phloem is cut, the sap which exudes out of it is rich in organic food materials
especially sucrose and amino acids.
2. Removal of a complete ring of phloem
around the phloem causes an accumulation
of sugar around the ring, which results into
the swelling of the stem above the ring. This
indicates that the downward movement of
the sugars has been interrupted and results
into the part below the ring failing to grow
and may dry out. This is called the ringing
experiment.
3. The sugar content of the phloem varies in relation to environmental conditions. When the
conditions favor photosynthesis, the concentration of the sugar in the phloem increases and when
they not favor photosynthesis and concentration of the sugar in the phloem reduces
4. The use of radioactive tracers. If radioactive carbon dioxide-14 is given to plants as a
photosynthetic substrate, the sugars later found in the phloem contain carbon-14. When the
phloem and the xylem are separated by waxed paper, the carbon-14 is found to be almost entirely
in the phloem.
5. Aphids have needle like proboscis with which they penetrate the phloem so as to suck the sugars.
If a feeding aphid is anaesthetized using carbon dioxide or any other chemical e.g. chloroform
and then its mouth parts cut from the main body, some tiny tubes called the proboscis remain
fixed within the phloem sieve tubes from which samples of the phloem content exudes
6. When the contents of the phloem are analyzed, they are confirmed to be containing
carbohydrates, amino acids, vitamins e.t.c. which further confirms that the phloem transports
manufactured foods
7. When small sections of the pierced stems are cut following the proboscis penetration, the tips of
the proboscis are found within the phloem sieve tubes.
Page 327 of 447

MECHANISM OF TRANSLOCATION IN THE PHLOEM

It was found out that organic materials do not move through the phloem sieve tubes by diffusion
because the rate of flow of these materials is too fast for diffusion to be the cause. The mechanism of
translocation of food in the phloem is explained by the following theories or hypothesis; (a) The
mass flow or pressure flow hypothesis (i.e. Much’s hypothesis), (b) Electro-osmosis & (c)
Cytoplasmic streaming
Mass flow or pressure flow hypothesis

Mass flow is the movement of large quantities of water and solutes in the same directions.
According to this theory, photosynthesis forms soluble carbohydrates like sucrose in the leaves. The
photosynthesizing cells in the leaf therefore have their water potential lowered due to the accumulation
of this sucrose. Sucrose diffuses down a concentration gradient from the photosynthesising cells into
the companion cells. Hydrogen ions are actively pumped from companion cells into the apoplast
(spaces within cell walls) using ATP. These hydrogen ions then flow down a concentration gradient
through carrier proteins into the sieve tube elements. Sucrose molecules are transported along with the
hydrogen ions, a process called co-transport. The carrier proteins are therefore also known as co-
transport proteins.
Sucrose is actively pumped into the phloem
sieve cells of the leaf via transfer cells. As a
result, water which has been transported up to
the stem xylem enters these mesophyll cells by
osmosis due to the accumulation of sucrose.
This causes an increase in the pressure potential
of the leaf cells including the leaf sieve tube
elements more than that in the cells in the sink
i.e. the mesophyll cells where the sugars are
manufactured are referred to as the source
while the other parts of the plant such as the
roots where food is utilized are referred to as
the sink.
A diagram showing movement of the products of photosynthesis by mass flow
(Toole fig 22.23 pg 470, Kent fig 2 pg 286)
Page 328 of 447

The food solution in the sieve tubes then moves from a region of higher pressure potential in the leaves
to that of lower pressure potential in the sink such as roots following a hydrostatic pressure gradient.
At the other parts of the plant which form the sink e.g. the roots, sucrose is either being utilized as a
respiratory substrate or it is being converted into insoluble starch for storage, after being actively
removed from the sieve tubes and channeled into the tissues where they are required. The soluble
content of the sink cells therefore is low and this gives them a higher water potential and consequently
lower pressure potential exists between the source (leaves) and the sink such as roots and other tissues
The sink and the source are linked by the phloem sieve tubes and as a result the solution flows from
the leaves to other tissues (sinks) along the sieve tube elements
Evidence supporting the mass flow
theory
1. When the phloem is cut, the sap
exudes out of it by mass flow
2. There’s rapid and confirmed
exudation of the phloem’s sap from
the cut mouth parts of the aphids
which shows that the content of the
sieve tubes move out at high pressure.
3. Most researchers have observed mass
flow in microscopic sections of the
sieve tube elements.
4. There’s some evidence of
concentration gradient of sucrose and
other materials with high
concentration in the leaves and lower
concentration in the roots.
5. Any process that can reduce the rate of photosynthesis indirectly reduces the rate of translocation
of food.
6. Certain viruses are removed from the phloem in the phloem translocation stream indicating that
mass flow rather than diffusion, since the virus is incapable of locomotion.
Criticism of mass flow

1. By this method all organic solutes would be expected to move in the same direction and at the
same speed. It was however observed that the organic solutes move in different directions and at
different speeds.
2. The phloem has a relatively high rate of oxygen consumption which this theory does not explain.
3. When a metabolic poison such as potassium cyanide enters the phloem, the rate of translocation is
greatly reduced, implying that translocation is not a passive process, but an active one.
4. The mass flow hypothesis does not mention any translocation of solutes with influence of transfer
cells and Indole Acetic Acid (IAA) hormone that loads the sugars or solutes into the sieve tubes
and also unload it into the cells of the sink.
Page 329 of 447

5. The sieve plates offer a resistance which is greater than what could be overcome by the pressure
potential of the phloem sap. This implies that the pressure would sweep away the sieve plates
during this transport.
6. Higher pressure potential is required to squeeze the sap through the partially blocked pores in the
sieve plates than the pressure which has been found in the sieve tubes
NOTE: the mass flow theory is considered to be the most probable theory in conjunction with electro-
osmosis
Electro-Osmosis
This is the passage of water across a charged membrane.
This membrane is charged because positively charged ions e.g. K+ , actively pumped by the companion
cells across the sieve plate into the sieve tube element using energy from ATP of the companion cells.
Potassium ions accumulate on the upper side of the sieve plate thereby making it positively charged.
Negatively charged ions accumulate on the lower sides of the sieve plate thereby making it negatively
charged.
The positive potential above the sieve plate is (Clegg fig 16.39b pg 341)
further increased by hydrogen ions, actively
pumped from the wall to the upper sieve tube
element into its cytoplasm.
Organic solutes such as sucrose are transported
across the sieve plates due to an electrical
potential difference between the upper and the
lower side of the sieve plate whereby the lower
side is more negative than the upper side i.e.
solutes move from the upper sieve tube element
which is positively charged to the lower sieve
element which is negatively charged.
The electrical potential difference is maintained across the plate by active pumping of positive ions,
mainly potassium ions, in an upward direction. The energy used is produced by the companion cells.
The movement of K+ ions through the pores of the sieve plates rapidly draws molecules of water and
dissolved solutes through the sieve pores, to enter the lower cell.
Evidence to support the electro-osmosis theory

1. K+ ions stimulate the loading of the phloem in the leaves with sugars during photosynthesis.
2. Numerous mitochondria produce a lot of energy for translocation, an indicator that translocation
is an active process. If however, the phloem tissues are treated with a metabolic poison, the rate
of translocation reduces.
Cytoplasmic streaming theory

This suggests that the protoplasm circulates using energy from sieve tubes elements or companion
cells through the sieve tube elements from cell to cell via the sieve pores of the sieve plates.
As the protoplasm circulates, it carries the whole range of the transported organic materials with it.
The solutes are moved in both directions along the trans-cellular strands by peristaltic waves of
contraction, such that they move from one sieve tube element to another using energy in from of
Page 330 of 447
ATP. The proteins in the strands contract in a wave form, pushing the solutes from one sieve tube
element to another, using energy in form of ATP.
Evidence supporting the cytoplasmic Diagram showing Cytoplasmic streaming
streaming theory (Kent fig 3 pg 287)
1. It has been found that the solute materials
move in both directions in the phloem tissue
2. The theory explains the existence of the
trans-cellular strands in the phloem tissue as
well as many mitochondria in the
companion cells
3. Presence of a sieve plate where a potential
difference can be developed across the plate
4. Criticism of the Cytoplasmic Streaming
Theory
5. Cytoplasmic streaming has not been
reported in mature sieve tube elements but
only in young sieve tubes.
6. The rate at which the protoplasm streams is
far slower than the rate of translocation
SAMPLE QUESTIONS
1. Distinguish between the terms immunity and autoimmunity (02 marks)
(b) Suggest three key roles played by the body’s immune system (03 marks)
(c) State three ways body openings are protected from entry of pathogens (03 marks)
(d) State two human diseases resulting from autoimmune disorders (02 marks).
2. The figure shows the changes in the cardiac output of two individual Mammals and A and B of
different sizes, determined from 6:00a.m up to 4:00p.m in the evening when the mammals were given
a hot drink.
i. Compare the cardiac output of both mammals. (04marks)
Page 331 of 447

ii. Explain the effect of day time on the cardiac output of both mammals. (08marks)
iii. Comment on the difference in the cardiac output both mammals. (04marks)
iv. Suggest factors that are likely to affect the cardiac output of a mammal. (03marks)
b) The table below shows the volume of blood flowing from the left vertical side of the heart of
various parts of the body in one minute at rest and during a heavy exercise.
Organ Volume of blood/cm3
Rest Exercise
Brain 750 750
Hear Muscle 250 750
Skeletal muscle 1,200 1,250
Skin 500 1,900
Kidney 1,100 600
Other organs 2,000 1,000
i. Calculate the percentage increase in blood flow from rest to exercise in skeletal muscle.(03 mark
ii. Give three ways in which the increase in b(i) is achieved. (03 marks)
iii. Explain the changes in volume of blood flow rest to exercise to various parts of the body. (11 ma
iv. Suggest with reasons the likely changes in composition of blood as it flows through the
kidney.(04
3. In an investigation pea plants were dug up from the field and washed thoroughly. The nodules were
removed surface sterilized and transferred aseptically to a sterile liquid culture medium. After two weeks
incubation, small samples of culture media were removed and added to trays each containing a batch of
pea plants growing in an inert medium. Each batch was watered regularly with a nutrient solution
containing a particular concentration of sodium nitrate for four weeks, at the end of four weeks the mean
number of root nodules and biomass were obtained from the investigation are shown in the table below.
Nitrate concentration of nutrient Mean number of nodules Biomass of pea plants
solution (arbitrary units ) per plant /gm-2
0 82 140
1 70 200
2 68 230
3 40 350
3.5 20 400
4 10 460
5 0 440
5.5 0 400
6 0 350
(f) Represent the results of the table above graphically (08 marks)
(g) Explain the changes in mean number of nodules per plant and changes in the biomass of pea plants
with increasing nitrate concentration of nutrient solution (20 marks)
(h) How was accuracy of results to be obtained ensured throughout the experiment (05 marks)
(i) (i) on the graph draw a graph to represent the plot for biomass you would expect if the experiment
was repeated and in this case the sample culture medium was not added to the trays containing pea
plants (02 marks)
(ii) Suggest reason(s) for the appearance of the graph drawn in d (i) above (03 marks)
(j) How can the information from the investigation be beneficial in crop production? (02 marks
Page 332 of 447

4. In an experiment to investigate the effect of light intensity on the rate of transpiration and stomatal
opening a leafy herbaceous plant was used. A potometer with the stem of a herbaceous plant was
placed in an open grassland, the following results were obtained.
Light intensity in Number of open stomata per Rate of transpiration in
µm branch mgm-2h-1
10 30 28
30 50 36
40 62 41
60 90 50
80 51 33
90 28 20
100 0 7
(a) Represent the above results graphically on the same axes (09 marks)
(b) Compare the effect of light intensity on the rate of transpiration and the number of open stomata
(06 marks)
(c) Explain the effect of light intensity on the number of open stomata (14 marks)
(d) (i) State the relationship between the number of open stomata and the
rate of transpiration (02 marks)
(ii) Explain the relationship stated in (c) (i) above (06 marks)
(e) Explain the results obtained at 100µm of light intensity (03 marks)
5. (a) The human immune deficiency Virus (HIV) is a retrovirus that suppresses the immune system
resulting into Acquired Immune Deficiency Syndrome (AIDS). Figure 1 below shows the development
of an infection with HIV over a period of 10 years and the changes in the number of T-lymphocytes
that activate other cells of the immune system. Use this information and figure 1 to answer the questions
that follows
Page Fig
3331 of 447
(i) Describe the Variation in the number of HIV particles and T-lymphocytes for the period of ten (10)
years. (05 marks)
(ii) Explain the relationship between number of HIV particles and T-lymphocytes for the period shown.
(09 marks)
(iii) From the figure; what evidence shows that HIV suppresses the immune system. (03 marks)
(iv) Predict with a reason what would happen if the development of an infection continued for another five years.
(03 marks)
(v) Suggest a reason why it has taken long to obtain a vaccine for HIV. (04 marks)
(b) Figure 2 below shows the comparison of antibodies produced to the same antigen during primary and
secondary response.
Time days
(i) Compare the primary and secondary response (03 marks)

(ii) Explain how each response is being stimulated. (08 marks)
(iii)From figure 2, what is the significance of a secondary response in the immune system of an
individual. (03 marks)
(iv) Suggest other ways in which the body defends its self against diseases causing organisms.
(02 marks)
Page 334 of 447

6. Differentiate between natural active immunity and artificial active immunity. (02 mark)
(b) What are the different ways in which the mammalian body naturally defends itself against pathogens?
[12 marks]
(c) Explain how artificial active immunity occurs. [06 marks]
7. An experiment was carried out to investigate the rate of water loss by three groups of leafy plants
under different conditions. Twelve leafy plants of approximately the same age, leaf surface area and
of the same species were used in the experiment. Four plants were placed in each group and treated
simultaneously as follows:
Group 1: Plants completely covered with transparent polythene bags.

Group 2: Plants fanned with an electric fan.
Group 3: Plants placed in still air in the open
The figure below shows the results of the experiments and the mean volume in cubic centimetres of
water lost through evaporation over the leaf surfaces of groups of plants recorded. Each group of plants
is represented as A, B and C in the figure 1 below
(a) Compare the volume of water lost by the leaves of different groups of plants shown in figure 1 above. (12
m)
(b) (i) From the curves drawn, identify the experimental conditions to which each group of plants A, B and
C were placed. (03 marks)
Page 335 of 447

(ii) With respect to group of plants A and B, suggest reasons for the observed difference in the two
curves drawn. (07 marks)
(c) Why were the plants of the same age, leaf surface area and same species used in the experiment? (05
marks) Suggest
(i) a hypothesis which this experiment was designed to test. (01 mark)
(ii) the name of the apparatus commonly used in this type of experiment. (01 mark)
(d) (i) Calculate the rate of water loss over the leaf surfaces by evaporation in group C between the time of
the day 12:00 – 14:00 hours and 16:00 – 18 hour (03 marks)
(ii) Explain the difference in the rate of water loss by the same group of plants at various times of
the day. (08 marks)
8. The figure below shows the amount of oxygen carried by haemoglobin in three different mammals during
the course of the day.
Gorilla
Amount of oxygen carried by
Rat
haemoglobin (a.u)
6am 8am 10am 12 2pm 4pm 6pm

Time of the day (hours)
(i) Outline the differences in the amount of oxygen carried by haemoglobin of a rat with that of a
human. (04marks)
(ii) Explain the trend of oxygen carried by haemoglobin for the;
 Gorilla (05 marks)
 Human (06 marks)
 Rat (06 marks)
b) Table 1 below shows the data obtained during an investigation on the effect of altitude on the amount of
oxygen carried by haemoglobin and the rate of oxygen delivery to body tissues, for a person with sickle cell
trait.
Altitude Amount of oxygen carried by Rate of oxygen delivery to blood

(metres) haemoglobin/cm3 tissues (cm3/minute)
Page 336 of 447

0 85 20
100 80 5
200 73 11
300 48 20
400 32 45
500 20 60
600 15 75
700 13 85
800 11 88
900 8 90
1000 7 93
(i) Represent the above information on the graph paper. (06 marks)
(ii) Explain the relationship between altitude and the;
 the rate of oxygen delivery to body tissues. (05 marks)
 amount of oxygen carried by haemoglobin (06 marks)
c) What possible conclusion can be made from figure 1 and the graph plotted? (02 marks)
9. (a) What is meant by the term human specific defence system? (02 marks)
(b) Describe the role played by the thymus glands in the human specific defence system (12 m
(c) Of what importance is memory and diversity to a defence system? (06 marks)
10. Differentiate between Natural active immunity and Artificial active immunity (02 marks)
b) State the different ways in which the mammalian body naturally prevents pathogens from accessing its
internal environment (11 marks)
c) What is the significance of the high body temperature experienced when the mammalian body is
attacked by Plasmodium Spp? (07 marks)
a) What is meant by the term chloride shift? (03 marks)
11. Account for the relative position of the oxygen dissociation curves of the human and rat haemoglobin
(b)Explain the rapid dissociation of oxyhaemoglobin of a rat during a vigorous activity (07 marks)
(c) Describe the events which occur during the heart beat (16 marks)
(d)Outline the features which ensure efficient flow of blood within the mammalian body (04 marks)
.
12. What are the essential features of the immune system in mammals?
b) (i) Give an account of the ABO blood group system in humans, and explain how certain ABO
group donations cause agglutinations with the recipients, while others do not.
(ii) Besides blood, other tissues can be transplanted from one individual to another. Mention problems
associated with them, and steps taken to minimize the transplant failure
Page 337 of 447

13. (a) What is the physiological significance of the Bohr effect in animals? (08 marks)
(b) Discuss the factors that may alter the rate of heart beat in mammals (12 marks
14. Figure 3 shows the union of oxygen and haemoglobin in three different physiological conditions
The straight line near the bottom of the graph shows the uptake of oxygen by a solution when hemoglobin
is not present while the dotted curves on either side of the solid curve shows the formation of
oxyhaemoglobin under two different levels of carbon dioxide
(a) Label the curves of blood in
(i) veins and muscles and
(ii) arteries and lungs (02 marks).
(b) Explain the importance of the positions suggested above in the physiology of the animal (04 marks).
(c) Explain the difference in the variation of the oxygen content of normal and physiological solutions(03
ma
15. Give an account of the structures involved in the translocation of organic solutes between the different
parts of a flowering plant.
(b) Briefly describe how dissolved blood carbon dioxide is expelled in gaseous form by
the lungs.
16. In fish, oxygen is transported in the blood in the form of oxyhaemoglobin. The table below shows the
percentage saturation of blood with oxygen of a teleost (bony) fish after equilibrating with oxygen of
different partial pressures. The experiment was carried out at two different partial pressures of carbon
dioxide.
Page 338 of 447

Percentage saturation of blood with oxygen

Partial pressure of Partial pressure of Partial pressure of
oxygen in Pa carbon dioxide at 500 carbon dioxide at 2600
Pa Pa
500 30 5
1000 70 13
2000 90 24
3000 96 33
4000 98 41
5000 99 48
7000 100 60
9000 100 69
11000 100 76
13000 100 81
(a) Present the data in a suitable graphical form.
(b) Calculate the difference of percentage saturation of blood with oxygen at the two different partial
pressures of carbon dioxide at oxygen partial pressures of 500 Pa.
(c) With reference to the graph, describe the effects of different partial pressure of carbon dioxide on the
percentage saturation of blood with oxygen.
(d) Explain how changes in oxygen content of blood at different partial pressure of carbon dioxide are
important in the release of oxygen to the tissues of fish.
(e) What information do such experiments give about the environmental conditions in which fish would
maintain a high level of growth as required in commercial fish farming?
(f) Explain how the properties of haemoglobin molecule are affected by changes in the oxygen and
carbon dioxide partial pressures.
17. The linear velocity of flow of sap through the xylem of a tree was measured in 𝑚ℎ−1 in the trunk and
in one of the small branches at the top of the tree. Measurements were taken at two-hourly intervals
during a hot day. The results are shown in the below.
(i) Compare the flow velocity in the trunk and branch with time.
Page 339 of 447

(07 marks)
(ii) Explain the difference obtained in the flow velocity for the trunk and branch at 14:00
hours. (04 marks)
(iii) Briefly explain the difference would you expect in the circumference of the trunk
measured at 14:00 hour when compared with that measures at 18:00 hrs?(04 marks)
(iv) Explain how the results would change if the experiment was carried out on a cold day
(03 days)
(b) Table 1 shows the relative number of stomata and relative rate of transpiration, in your
difference plant species.
Table 1
Plant Species A B C D
Relative number of stomata 𝑚𝑚−2 of leaf (upper 5:30 0.80 10:15 0.50
: lower surface)
Relative transpiration rate 10:12 0:4 15:30 20:50
(upper: lower surface)
(i) Comment on the distribution of stomata in the four species (06 marks)
(ii) Explain the relationship between the distribution of stomata and the rate of
transpiration in;
 Species B. (04 marks)
 Species D. (03 marks)
(iii) From the data, what conclusions can be drawn about the difference between the upper
leaf surface of species B and D.? (03 marks)
(c) (i) Describe how cohesive and adhesive forces ensure a continuous water column up the
xylem vessels. (03 marks)
(ii) How does bulk flow in the xylem differ from diffusion (03 marks)
18. Figure 1 below shows the diurnal variation in the sugar content of leaves and the phloem in
stems.
(i) Compare the trend in sucrose concentration in the leaves and the stem. (08 marks)
(ii) Account for changes in sucrose concentration between 00.00hrs and 16:00 hr (10 ma
(iii) Describe the relationship between sucrose content of leaves and sucrose content of
phloem in the stem. (03 marks)
(iv) Explain the relationship described in (a) (iii) above. (06 marks)
Page 340 of 447

(b) In an experiment, a scientist used very sensitive recording equipment to observe the diameter
of certain very large trunks.
Figure 2 shows the results obtained when changes the trunk diameter of one of the trees was
measured for a period of about four days
(i) Explain the effect of time of day on the diameter of the trunk between 16th and 17th of
March. (08 days)
(ii) Explain the effect of steady rain on the diameter of tree as seen on the 18th day.
(05 days)
19. The figure below shows changes in the blood pressure in the aorta and the left ventricle during two
complete cardiac cycles.
20
Pressure 15 Aortic pressure

/ kPa
10
5
Left
ventricular
0
pressure
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
Time /s
(a) On the graph, draw an arrow to show when the left atrioventricular (mitral) valve closes.(01 mark
(b) Use the information in the graph to calculate the heart rate. Show your working. (02 marks
(c) During the cardiac cycle, the pressure in the left ventricle falls to a much lower level than in the aorta.
Suggest an explanation for this difference. (03 marks)
(d) During the cardiac cycle, the pressure in the right ventricle rises to a maximum of about 3.3 KPa.
Suggest reasons for the difference between this pressure and the maximum pressure in the left
ventricle.(03 marks)
Page 341 of 447

20. Blood that is fully saturated with oxygen carries 105cm3 of oxygen in 1 dm3(liter) of blood
(a) Calculate the volume of oxygen released from 1 dm3 of blood when blood that has become 90%
saturated at 380C reaches a part of the body where the partial pressure is 18% (03 marks)
The figure below shows the oxygen dissociation curve of hemoglobin from a mammal at 380C.
% Saturation of Haemoglobin
PO2/KPa
(b) Draw the curve of hemoglobin when the body temperature is raised to 430C(01 mark)
(c) Name one change in the conditions in the tissues which has the same effect on the oxygen dissociation
curve as change in temperature (01 mark)
(d) Explain the effect of increased body temperature on the oxygen dissociation curve for hemoglobin in
mammals(03 marks)
(e) State how this effect of temperature on the oxygen dissociation curve of hemoglobin might be
advantageous to the mammal (03 marks)
21. The table below shows the results of an experiment on the rate of absorption of sugars by a
mammalian intestine. Study it carefully and answer the questions that follow.
Sugar Relative rates of absorption taking normal glucose uptake as 100
By living intestine By intestine poisoned with cyanide
Hexose Glucose 100 30
sugars Galactose 106 35
Pentose Xylose 32 32
sugars Arabinose 30 31
(f) Suggest a reason for the difference between the rates of absorption of hexose and pentose sugars in
the living intestines (03 marks)
1
(g) Mention the mechanism by which hexose sugars are absorbed by living intestines (0 mark)
2
(h) What is the advantage to the individual of having hexose sugars absorbed in the way mentioned
above?
(i) What could be the effect of cyanide on the mechanism of hexose absorption? (02 marks)
(j) In an intact mammal, absorption of fatty acids is drastically curtailed by any clinical condition which
leads to a reduction in bile salt excretion or release. Explain why this is so.
(03 marks)
22. (a) What is meant by the term Bohr’s effect? (02 marks)
(b) Briefly explain the following observations;
The oxygen dissociation curve of,
Page 342 of 447
i. man shifts to the right during exercise (03 marks)

ii. the elephant is on th e left of the oxygen dissociation curve of a mouse? (03 marks)
iii. the lungworm is on the left of that of man? (03 marks)
c) Explain three factors that influence the affinity of haemoglobin for oxygen (06 marks)
23. The table below shows the difference in percentage saturation in blood with oxygen at varying partial
pressure of oxygen between a pregnant woman and that of the fetus developing in her uterus.
Partial pressure Percentage saturation of blood with oxygen
of oxygen
/mmHg mother Fetus
1.3 8 10
2.7 20 30
3.9 40 60
5.3 65 77
6.6 77 85
8.0 84 90
9.3 90 92
10.6 92 92
i) Plot the results in a suitable graphical form. (08 marks)

ii) Compare the percentage saturation of blood for the mother and that of the fetus.(04 marks)
iii) State and explain the shape of the curve for the mother. ( 07 marks)
iv) Explain the physiological significance of the position of the fetal curve (05 marks)
b) Explain what is meant by: (06 Marks)
i) Bohr’s effect iii) Un loading tension
ii) Loading tension
24. What fundamental physical constraints necessitate a circulatory system in large organisms? (02
b) State one advantage and one disadvantage of a closed circulatory systems (02 marks)
c) State two physiological advantages of separate pulmonary and systemic circuits in a
mammalian circulatory system (02 marks)
d) Figure 6 shows the interrelationship of blood flow velocity, cross sectional area of
Explain the relationship between area and velocity in the arteries
Page 343 of 447

REFERENCES
8. D.T.Taylor, N.P.O. Green, G.W. Stout and R. Soper. Biological Science, 3rd edition, Cambridge
University Press
10. C.J.Clegg with D.G.Mackean, ADVANCED BIOLOGY PRICIPLES AND APPLICATIONS, 2 nd
thornes
Page 344 of 447

TOPIC 8: EVOLUTION
SYLLABUS EXTRACT
Specific objectives: The learner should Content

be able to;
1.1Origin of life 
 Explain the theories related on the  Theories of the origin of life: special creation spontaneous
origin of life generation, biochemical evolution
1.2 Mechanism of evolution 
 Discuss Lamarck’s theory of  Lamarck’s theory of evolution of acquired character through use and
evolution disuse.
 Explain Darwin’s theory of natural  Darwin’s theory of natural selection: observations and deductions.
selection  Importance of variation in evolution.
 Explain the importance of variation in  Neo-Darwinism (present day theory of evolution)
evolution.  Causes of present day evolution: competition changes in the
 Discuss Neo-Darwinism environment, sexual reproduction, mutations , gene recombination,
 Explain causes of present day industrialization, effects of drug/ chemical resistance, artificial
evolution selection , polyploidy.
1.3 Evidence of evolution 
 Discuss evidence of evolution  Evolution evidence based on : fossilization , comparative study of
 Explain the emergence of variations anatomy, embryology, cytology, biochemistry, taxonomy,
among organisms geographical distribution, vestigial structures, analogous
structures, homologous structure.
 Emergence of variations among organisms.
1.4 Selection and speciation
1 Explain natural selection and 1 Natural selection
artificial selection 2 Definition of speciation
2 Define speciation 3 Allopatric speciation and sympatric speciation.
3 Describe allopatric speciation and 4 Role of natural selection and artificial selection in speciation.
sympatric speciation. 5 Mechanisms related to speciation : continental drift, migration,
4 State the roles of natural selection and adaptive radiation, divergent and convergent evolution , isolation
artificial selection in speciation. i.e. Ecological reproductive and genetic.
5 Describe mechanisms related to 6 Extinction: meaning causes and effects.
speciation of organisms
6 Explain extinction
1.5 Population genetics
1 Explain gene frequency in the gene 1 Gene frequency in the gene pool of a population.
pool of a population 2 Hardy- Weinberg equilibrium: natural selection, no random mating
2 State the hardy Weinberg mutation, migration small population size.
equilibrium
3 Explain how different factors affect
the gene frequency in a population
EVOLUTION
This is a gradual process by which new species are formed from pre-existing less differentiated species over a
period of time due to changes in the prevailing environmental conditions. It also be defined as change, over a
long time, in the genetic composition of a population which leads to the emergence of new species.
During this process organisms undergo various structural and physiological modifications in order to fit in the
prevailing environmental conditions which are genetically transferred to subsequent generations thereby
forming new species. This has led to diversification of life forms since they are varying environmental
conditions in which organisms have to adopt.
Page 345 of 447

THEORIES FOR THE ORIGIN OF LIFE

1. SPECIAL CREATION
It is believed that living organisms were created by the almighty God in the very forms they exist today. This
implies that there has not been any evolution or change of life form.
2. SPONTANEOUS GENERATION
This theory suggests that living organisms emerged from non-living forms spontaneously (suddenly).
i. This is supported by the fact that dead decomposing materials may lead to formation of
maggots.
ii. A newly constructed pond of water may eventually contain certain organisms such as fish
iii. Dirt may lead to the emergence of lice in the hair.
However, this theory was rejected by scientists who believe that living organism must originate from the
already existing living organisms of their kind (Pre-existing life).
3. ORGANIC EVOLUTION THEORY

This theory suggests that life did not start from planet earth but instead from other planets. According to this
theory, life came into planet earth in form of meteorites (fragments) from other planets which contained
organic materials. These fragments later joined to form living organisms. This theory therefore presupposes
that a living organism must give rise to new individuals of its own origin.
4. THE SYNTHETIC THEORY

It suggests that inorganic elements such as oxygen, ammonia, water, carbon dioxide e.t.c. Inorganic
compounds joined together to form organic material from which the living organisms emerged. It was
believed that forces from the earth crust facilitated the joining together of these materials to form living
organisms. This is supported by the fact that inorganic compounds like water and carbon dioxide can be used
to synthesise organic compounds under sunlight energy by green plants
5. STEADY STATE THEORY (COSMOZOAN OR PANSPERMIA) THEORY

This theory suggests that life has no origin. It has been available and there has never been any change and it
will continue to be available. According to this theory life could have arisen once or several times in various
parts of the universe and then remained the way it was.
THEORIES TO EXPLAIN THE MECHANISM OF EVOLUTION

DARWINISM
This theory was proposed by Charles Darwin from his voyage around the world which took him to many
places particularly Galapagos Islands of South America. His theory states that change in environmental
conditions make organisms which are better adapted to survive and reproduce and transmit their alleles at
the expense of the poorly adapted organisms which are gradually eliminated before reaching the reproductive
age, thereby leading to the formation of new species from the pre-existing species.
Darwin’s theory is based on three observations and two deductions. In this theory Charles Darwin observed
that;
1. Organisms within a population tend to produce far more off springs than what the environment can
support.
2. Despite the tendency to increase the numbers of the species due to over production of off springs,
populations actually maintain relatively constant numbers.
3. He also observed that different individuals show variations advantageous to their environment (i.e. the
fit) while other individuals show variations disadvantageous to their environment (i.e. the unfit)
Page 346 of 447

From the observations he concluded that there’s struggle for existence within a population and so many
individuals fail to survive or reproduce. Organisms always compete for the limited resources with each other
in an effort to survive. This competition is based on the adaptations of organisms such that when variations in
the environment factors exerts a selection pressure, individuals with best suited characters (variations) to the
new prevailing environment survive and reproduce at the expense of those individuals who are poorly adapted
that are eventually eliminated from the population, thereby liming population size.
From these observations Darwin also concluded that in the struggle for existence individuals showing
variations better adopted to their environment (the fit) survive and reproduce more off springs for the next
generation that the least adapted organisms (un fit) which die before the reproductive age and fail to pass on
their characteristics to the next generation. Individuals which survive pass on their favorable characteristics to
the next generation. Individuals with unfavorable variations are eventually eliminated in the struggle for their
existence. Nature therefore allows the survival of those organisms whose characteristics fit them in the
prevailing environmental conditions and eliminates those with poor characteristics so that they are not given
chance to survive and reproduce. This is called survival for the fittest by natural selection. Organisms which
survive to reproduce are likely to produce off springs similar to themselves i.e. the like produces the like. This
leads to the emergence of new species under the constantly changing environmental conditions.
DARWIN’S FINCHES
The Galapagos finches show an example of adaptive
radiation. It is assumed that a stock of ancestral finches
reached the islands from the mainland and then, in the absence
of competition, evolved to fill all the empty ecological niches
occupied by other species on the mainland. The large ground
finch, the closest to the mainland finch in form and function,
has a typical finch-like beak for crushing seeds. The cactus
ground finches have a long straight beak and split tongue for
getting nectar out of the flowers of the prickly pear cactus.
The vegetarian tree finch has a curved parrot-like beak with
which it feeds on fruits and buds. The insectivorous tree
finches have a similar beak which they use to feed on beetles
and other smaller insects. The Warbler finch uses its slender
beak to feed on small insects which it catches on the wings.
The woodpecker finch, lacks a long tongue, therefore it uses
its beak to pick up a stick which it uses to poke it a hole full of
insects. When the insects emerge, the bird drops the stick and
devours the insects (this tool handling is only thought to be in
man and monkeys).
LAMARCKCISM
According to Lamarck organisms acquire certain structural and physiological characteristics according to the
environmental need for survival. These characteristics acquired are then passed on to the offsprings of the
organisms genetically. Gradually a group of organisms better adapted to the environment are produced and
therefore evolution occurs. Lamarck concluded that characteristics acquired through an organism’s interaction
with the environment can be inherited by the offsprings i.e. inheritance of acquired characteristics.
According to Lamarck when an organism constantly uses part of its body that part develops greatly to better
fit in the environment. However, the part which is not constantly used begins to degenerate (Becomes
vestigial).
Lamarck referred to this as the principle of use and disuse. For example, Lamarck speculated that earlier
giraffes had short necks and time came when they over produced and competed for the existing vegetation.
This made them to stretch their neck heights to eat the tall vegetation and eventually the necks became longer.
Page 347 of 447

However, Lamarck was not scientific in his explanation because acquired characteristics cannot be passed on
to the next generation. The use and disuse of somatic cells that make up body parts have no influence on
gamete formation and cannot be inherited to cause evolution.
NOTE;
A. Natural selection is a process by which organisms that are better adapted to their environment survive
while those that are less adapted are eliminated.
B. Natural selection promotes speciation (emergence of new species ) as follows;
Individuals with unfavorable characteristics are less likely to survive long enough to reproduce as nature
causes their death unlike those with favorable characteristics which survive long enough and reproduce.
Over very many generations, their numbers in the population will decrease while for those with favorable
characteristics their numbers will increase. Individuals with favorable characteristics breed with
consequent increase in their numbers with in the population due to the development of a number of
favorable variations in them.
The development of a number of favorable variations in these individuals over many generations greatly
leads to the emergency of new species.
C. According to Darwin the original giraffes had variations in the length of their necks varying from short to
long. When they over produced there was competition for the ground vegetation during which the short
necked giraffes were eliminated while the long necked giraffes which could reach the leaves of the trees
survived and therefore passed on their genes to the next generation which came with only long necks
Assignment;
Read and make briefs notes on
a. disappearance of dinosaurs
NEODARWINISM (Modern synthetic theory of organic evolution)

This theory was improved from the views of Darwin by including in Mendel’s genetic principles, ethology,
paleontology, molecular biology and ecology.
This theory therefore suggests that in sexually reproducing organisms, there is always genetic variation which
occurs during gamete formation as a result of crossing over, mutations, random fertilization and cross
Page 348 of 447

breeding. This provides a gene pool from which natural selection occurs to eliminate the unfit individuals
from the population.
Consequently the prevailing environmental conditions will favour and allow propagation of the fit individuals
by eliminating individuals with unfavourable genes from the population according to variation of inherited
characteristics
According to this theory, a population rather than an individual organism evolves and therefore it is the gene
pool that evolves. Therefore, Neo-Darwinism theory is the theory of organic evolution by natural selection of
inherited characteristics. Genetic recombination between sexually reproducing organisms produce most of the
variations in the characteristics that make adaptations possible. New combinations of genes produce unique
genotypes according to this theory whose phenotypes undergo environmental selection pressures which
continually select and determine which genes pass on to the next generation.
Therefore phenotypic characteristics (variations) are determined by both genotypes of the organisms and
environmental factors, upon which natural selection acts to give rise to new species.
NOTE:
a. A gene pool is the total variety of genes and alleles present in a sexually reproducing population.
b. A selection pressure is any environmental resistance factor that can increase or decrease the
frequency of an allele within the gene pool through eliminating the unfit organisms from the fits ones
thereby leading to an evolutionary change. A selection pressure may be predation, disease outbreak or
competition.
c. Microevolution is the change in the genetic makeup of a population or gene pool over many
generations
EVIDENCE FOR EVOLUTION

The proof that evolution occurs or occurred includes the following;
1. Comparative embryology 6. taxonomy
2. comparative anatomy 7. biogeography
3. Palaeontology 8. Industrial melanism.
4. comparative biochemistry 9. artificial selection
5. cell biology 10. Resistance to drugs
COMPARATIVE EMBRYOLOGY
Embryology refers to the study of the developmental stages of embryos.
Critical observations of these embryos during various stages of growth such as cleavage, gastrulation and
differentiation indicate that embryos tend to go through or repeat the developmental stages of their ancestors
and this is called ontogeny recapitulates phylogeny i.e. the developmental stages (ontogeny) repeats
(recapitulates) the evolutionary history of the ancestors (phylogeny).
The developmental stages of embryo reveal striking similarities between embryos of different species which
shows that these species evolved from the same ancestor. The more closely related the species are the more
they go through similar developmental stages due to having a common ancestor therefore organisms tend to
show developmental patterns that their ancestors went through which proves that evolution occurs.
For example, all vertebrate embryos begin as one cell which undergoes cleavage (rapid cell division) to form
many cells which later leads to the formation of organs (gastrulation) and later the whole/entire organism is
formed through differentiation of the tissues to form systems.
Page 349 of 447

However, some organisms remain as one cell an indicator that multi-cellular organisms evolved from
unicellular organisms.
OTHER EXAMPLES
a. Mammalian embryos grow within amniotic fluid, which confirms that the ancestral organisms lived in
water.
b. Mammalian embryo grows within the amniotic fluid confirms that the ancestral organisms lived in
water
c. At comparable stages of growth e.g. 1 month of growth, all vertebrate embryos possess the following
features; A single blood circulation with a two chambered heart showing no separation between left
and right halves a situation retained only in fish the ancestor of other vertebrates. Others include
possession of a tail, visceral clefts and the gill pouches. In human embryos the gill pouches disappear
leaving the Eustachian tube of the ear.
d. A human embryo and a tadpole of amphibians possess a tail which later breaks. This indicates that
amphibians and humans share the same ancestry.
e. The larvae of a sea squirt and amphioxus possess a notochord like vertebrae embryo which is later
replaced by the vertebral bones excepting amphioxus. This shows that all vertebrates have common
ancestors.
PALEONTOLOGY (The study of fossils)

Fossils are preserved remains of an organism that lived long ago in sedimentary rocks. Paleontology the study
of fossils of organisms that lived many years ago in form of impressions or imprints. Oldest fossils are found
in bottoms strata of sedimentary rock and possess simpler strictures while present day fossils (recent fossils)
are found in young top strata of sedimentary rocks and containing a variety of many complex organisms. This
indicates a progressive change from simpler to complex forms which suggests that present organisms could
have evolved from ancient organisms e.g. evidence also shows that the climatic conditions have been varied
through the earth’s crust, which explains differences in structure of fossils of common ancestry but located in
different parts of the world due to sudden environmental changes. Organisms have been undergoing adaptive
radiation to survive in these changes that bring about natural selection which progressively eliminate the unfit
organisms leaving the fit.
When fossils are compared with the present day organisms, there are striking similarities and differences
which is an indicator that ancient species are ancestors of the present species. This proves that the present day
species have gradually evolved from ancient species and are not a result of sudden appearance
Fossils document the existence of now extinct species, showing that different organisms have lived on earth
during different periods of the planet history. Fossils indicate the time at which species originated and became
extinct (geological time scale) via carbon dating. This supports the disappearance of organisms due to natural
selection of the fit against the unfit organisms.
The differences in fossils and living species also reveal the evolutionary trends that these organisms went
through in the course of evolution to become more modified and complex i.e. they reveal that organisms
change gradually in a course of evolution. Indeed most fossils found so far can be classified in to the same
taxonomic groups with a present day living organisms.
Example
Fossil records indicate that amphibians evolved from fish and then gave rise to reptiles which finally evolved
into birds and mammals.
Birds
Fish (ancestral group) Amphibians Reptiles
Page 350 of 447 Mammals (most advanced)
Such an evolutionary trend is an indication that organisms undergo modifications so as to fit in the prevailing
environmental conditions by acquiring special adaptations for such environments. Paleontology shows that
there’s progressive increase in diversity and complexity of species as the old sedimentary rocks in bottom
layers contain fewer and primitive forms while young rocks contain many and advanced organisms. This
supports a theory of progressive increase in complexity or organisms,
Fossils also indicate the time at which species originated and became extinct i.e. geological time scale. This is
because fossils, can be excavated from underground and then their age determined by carbon dating. From
carbon dating an evolutionary trend the organisms, supplemented with similarities in structure, is established
where by a group of organisms have better modified structures than another group.
Paleontology also shows that geographical regions and climatic conditions have varied throughout the earth`s
history and since organisms are adapted to particular environmental conditions the constantly changing
conditions in the world may have created progressive changes in the structures of organisms as shown by the
fossil records.
Fossils provide information about which taxon of organisms appeared first, survive for hundreds of years and
then disappear later as more advanced forms of organisms appear, which shows emergence of advanced
species and extinction of the primitive species. Thus fossils indicate times at which species originated and
became extinct.
Note.
Paleontology is limited because the fossil record is incomplete so that few fossil forms are represented among
organisms living today, as not all fossils have been dug up and not all life has been fossilised e.g. for
invertebrates the whole body may decompose to live an impression or a mould this is because which animal is
fossilised and is discovered is a matter of chance. In addition, fossils are usually broken down by forces of
nature and therefore paleontology gives incomplete information. However, paleontologists have constructed
geological periods calculating the ages of the discovered fossils which are a strong evidence for evolution.
It should be noted that paleontology is limited as an evidence for evolution because extinction is a frequent
event, so that only very few fossils are represented among organisms living today.
CELL BIOLOGY
The study of cell structure and physiology reveals a lot of evidence for evolution e.g. the presence of common
cell component in different species serving the same function is a clear indicator that organisms having them
have a common ancestor. Such structures include, mitochondria, ribosomes, endoplasmic reticulum, Golgi
body, nucleus e.t.c.
TAXONOMY (CLASSIFICATION)
This is the grouping of organisms using their similarities and differences particularly in their structures.
Classification is based on the presence of common homologous structure such as the pentadactyl limb.
Page 351 of 447

Organisms with the same homologous structure are put under the same taxonomic group e.g. same phylum,
class, order, e.t.c. which indicate that they evolved from a common ancestor e.g. all organisms under phylum
chordata have a notochord and a post anal tail at least one time in their life time. This indicates that chordates
(vertebrates) evolved from the same ancestor since the genes for these characteristics are inherited from
generation to generation.
COMPARATIVE ANATOMY
The study of comparative anatomy brings out similarities in the structures which indicate a common ancestor
for different species. Comparative anatomy proves that evolution occurred in organisms using homologous
structures, where two or more species share a unique physical feature such as a complex bone structure or a
body plan, which they may all have inherited from a common ancestor.
When homologous structures of organisms of different species are compared and found to be basically
similar they indicate a common ancestor or an evolutionary trend of organisms. This is because homologous
structure arises through adaptive radiations, due to the different environmental conditions in their habitat
which make organisms possessing these structures become different species due to the differences in the
environmental conditions.
Presence of homologous structures in different species of
organisms is an indicator of evolution through adaptive
radiation i.e. specialisation of homologous structures to serve
different functions in different environment in apparently
similar organisms e.g. the mouth parts of butterfly are modified
for sucking while that of cockroaches are modified for biting
and chewing. The hind legs of ducks are modified for
swimming by being webbed while those of rats are modified
for hopping to bring about fast locomotion.
Homologous structures are built on the same basic plan in
different species of the same ancestral origin but are modified
to perform different functions in different species due to
adaptive radiation e.g. pentadactyl limb system in vertebrates.
This similarity in basic plan suggests that organisms possessing
similar homologous structures have a common ancestor
In addition, vestigial organs possessed by some organisms give evidence of changes from ancestral
conditions to the present conditions and seem to represent a revolutionary link with the previous species. The
vestigial organs include the vestigial tail and appendix of humans which suggests that they were well
developed in the ancestors of man but later degenerated and became functionless in man. This shows that
many organs have changed in the course of evolution e.g. the salivary glands of the snake have been modified
into poison glands.
Comparative anatomy proves that divergent evolution occurs arising through great modifications of
homologous structures due to adaptive radiation and also proves that convergent evolution occurs in species of
different ancestral origin making them have similarities due to natural selection and adaptation to the same
ecological conditions, making their analogous structures perform the same function.
Comparative anatomy therefore confirms that the present organisms are descendants of the ancient organisms
through change of structures by adaptive radiation.
NOTE
a) Adaptation refers to the structural and physiological modifications of an organism brought about by
evolution to enable the organism survive in its environment.
b) Homologous structure and physiological functions reveal divergent evolution i.e. the evolution of
organisms from a common ancestor through great modifications of their homologous structure to serve
different functions in different environments due to adaptive evolution.
Page 352 of 447

c) Homologous structures are those structures that have the same basic plan in different species of
organisms having the same ancestral origin but modified to serve different functions in different
environments due to adaptive radiation e.g. the pentadactyl limb of vertebrates which is modified in
different species of vertebrates for different functions, the wings of birds and legs of horses as well as the
limbs of man are all similar in number of arrangement of bones because they have evidently evolved from
the same type of ancestral appendage.
d) The modifications of homologous structures include the following
i. Monkeys
The pentadactyl limb is modified for grasping tree branches by being highly flexible while in man it is
modified as a tool of manipulation.
ii. In pigs and dogs this limb is modified for walking by having hooves which are hard enough to support
the process of walking. While in rodents it is modified for digging tunnels by having highly flexible toes
and blunt claws.
e) Presence of homologous structures in different species of organisms is an indicator of divergent

evolution from common ancestor as they indicate adaptive radiations. Adaptive radiation is
important because it enables organisms with the same basic structure (same homologous structures) to
exploit different ecological niches thereby decreasing competition for resources.
f) Analogous structures and convergent evolution
These are structure having different basic plan in different organisms but serving the same functions
in organisms of different ancestral origin e.g. the wings of bats and insects (both are used for flight
but have different structures and are in organisms of different ancestral origins, the hind legs of a frog
and grasshopper, the eyes of octopus and vertebrates, fins in fish and the penguins, storage organs of
the potatoes and the sweet potatoes e.t.c.
These analogous structure and physiological functions reveal convergent evolution i.e. the type of
evolution which creates similarities in organisms of different ancestral origins due to natural selection
and adaptation to the same environmental conditions making such organs perform the same functions.
Page 353 of 447

Convergent evolution suggests that organisms from different ancestral origins tend to evolve along the
same lines.
COMPARATIVE BIOCHEMISTRY AND PHYSIOLOGY

This proves evolution in that similarity in chemical composition and metabolic properties of all cells suggests
a common ancestor to living organisms e.g. similarity in protoplasmic chemical contents like enzymes in
different species. At the most basic level, all living organisms share;
 the same genetic material (DNA)
 same or highly similar genetic codes
 same basic process of gene expression like transcription and translation.
 Same molecular building blocks such as amino acids for proteins.
These shared features suggest that all living things descended from a common ancestor that had DNA as its
genetic material, used the genetic code and expressed its genes by transpiration and translation. Other
evidence under comparative biochemistry include;
A. SEROLOGY (serum studies)

Blood tests are based on antigen antibody reactions, which antigens and antibodies are synthesised according
to the genetic makeup of the organism. Blood tests indicate that all primates have the ABO blood group
system and the rhesus blood group system. This indicates that they possibly originated from the same
ancestors and that man could have evolved from primitive primates such as monkeys, baboons, gorillas e.t.c.
Since blood tests are based on antigen antibody reactions, when serum from different species is mixed this
reaction occurs if the species are related leading to the formation of a precipitate. The larger the precipitate
formed, the more closely related the two species are, by having similar antigens and antibodies that can easily
react, indicating that the two species are closely related as they have similar protein structure.
B. AMINO ACID SEQUENCE
Proteins have similar amino acid sequences in different species which shows that these species have a similar
ancestral origin e.g. chemical analysis have shown that cytochrome C has 35 amino acids in humans, rabbits,
fish, snakes e.t.c arranged in the same sequence. This reveals the common ancestor for vertebrates since the
synthesis of cytochrome C is the base of the gene frequency of organisms controlled DNA.
C. DNA HYBRIDISATION
This is where DNA molecules of the different organisms which are assumed to be related are separated into
individual strands. When these DNA strands are mixed together they may show complementary base pairing
to reform the DNA molecules of different species whose strands were mixed earlier depending on how closely
related these species are. Pairing of the separated DNA bases of different species indicates that the organism
have a common ancestor and are therefore closely related.
D. BLOOD PIGMENTS
These provide evidence to evolution because some organisms have similar blood pigments and therefore
believed to be closely related as they belong to different species e.g. haemoglobin exists in all vertebrates and
also in some few invertebrates such as the earthworm an indicator that possibly vertebrates evolved from
invertebrates. Other blood pigments used as evidence for evolution include the following; Chlorocruorin
(made of iron) found in some lower organisms called Polychaetae e.g. the lugworm. Haemocyanin (made of
copper) found in molluses and arthropods such as crustaceans.
These pigments indicate the common ancestor of organisms having them
E. CHROMOSOME STRUCTURE
Some organisms of different species have the same chromosome structure and the same chromosome number
which is evidence of evolution since the chromosomes in their cells are an indicator of the same ancestral
origin. For example humans, gorillas and chimpanzees have 46 chromosomes.
Page 354 of 447

BIOGEOGRAPHY (SPECIES DISTRIBUTION/GEOGRAPHICAL ISOLATION)

In the past, the present day continents were all joined together to form a single land mass known as Pangaea
which floated on the denser molten core of the earth. During this period all animals were mainly found in the
northern hemisphere particularly North America, from where they used to migrate to other areas and later
returned to the same northern hemisphere. However, due to the pressure in the earth crust (plate tectonics) the
Pangaea split up into the current continents which became separated by water bodies immediately.
N.B. Continental drift is the movement apart of land masses (continents) due to plate tectonics.
The splitting of the Pangaea into the current continents resulted in geographical isolation of animals which got
trapped in these new continents and therefore in the new environmental conditions since they could not cross
the large water bodies. This isolation made animals in the new continents to undergo adaptive radiations so as
to survive in the new environmental conditions permanently which resulted into the formation of many new
species and therefore evolution of new species.
The existence of closely related species occupying similar ecological niches in different continents together
with the fact that some species exist in only one particular continent supports the origin of organisms from the
common ancestor mainly from North America. In addition, the absence of more advanced forms of organisms
from a region usually indicates the prior geographical separation of that region from the Pangaea or from other
regions of the world where these organisms originated.
Groups of organisms that already had evolved before splitting of Pangaea are distributed worldwide in similar
ecological niches in different continents. In contrast, groups of organisms that evolved after breakup of
Pangaea into current continents appear uniquely in smaller regions of the world such unique allopatric species
on islands like Galapagos Fiches. Marsupials (and not placental mammals) evolved in Australia because
Australia was geographically isolated by water for millions of years which made those species able to evolve
without competition from advanced placental mammals elsewhere in the world.
Indeed the distribution of fossils and the present day organisms can be precisely related to the positions of
continents in the past before the Pangaea split up. For example the Bison of North America has similar
adaptations to the Zebra of Africa, an indicator of the same ancestral origin. The lung fish species are found
separately in tropical areas of South America (lepidosiren), Africa (protopterus), Australia (neoceratodus).
These species acquired different structures in different tropical continents by adaptive radiation which made
them become separate species.
Page 355 of 447

Biogeography also shows that in evolution, the more adapted organisms survive at the expense of the poorly
adapted ones due to changes in environmental conditions e.g. when placental mammals (eutherian mammals)
evolved in the world, Australia had been geographically isolated from the Pangaea and therefore placental
mammals never migrated into Australia.
In other parts of the world, these mammals eliminated the more primitive marsupials e.g. the kangaroo and
monotremes e.g. the dark billed platypus from their ecological niches except Australia which earlier broke
Page 356 of 447

away. This is supported by the many fossils of these primitive mammals that are found in most of other
continents.
INDUSTRIAL MELANISM
This supports evolution because industrial activity changes the background of an area from white to black due
to soot (smoke) from these industries, which contains sulphur dioxide gas that kills the lichens thereby
changing the back ground to black. This environmental change affects the predation selection pressure by
promoting camouflage of the melanic organisms against predators on the black background. Greater
predation of the non-melanic forms of organisms that are easily seen on the black background compared to the
inconspicuous melanic forms, selects the few melanic forms to survive, reproduce and greatly increase in
number as the non-melanic organisms greatly decrease in number.
For example, before industrialisation in Britain, the majority of the peppered moth, Biston betularia were
white due to their camouflage against the predatory birds on the white background made of mainly lichens and
very few moths were black but during industrialisation, sulphur dioxide killed the lichens, thereby changing
the background from white to black, which allowed the back mutant peppered moths to survive and greatly
reproduce to increase their numbers as the white peppered moths were decreasing in numbers.
The camouflage provided by the black environment sets up a basis for natural selection that allowed the
black forms of moth to survive and reproduce rapidly while the white forms of moth were being selected
against as they became conspicuous to predators. This confirms that evolution occurs via natural selection
when the environment changes. This confirms that evolution occurs via natural selection when the
environment changes
CROSS BREEDING/ ARTIFICIAL SELECTION
Artificial selection is a type of selection where humans eliminate organisms with undesirable characteristics,
leaving only those with desirable characteristics which may become new species after several generations.
Humans ensure that the reproductive potential of species with desirable characteristics increases so that their
alleles are passed on to the next generation. Those without the desirable characters have their reproductive
potential decreased and their alleles eliminated through segregation, extermination and sterilization. This
makes the population more divergent (vary greatly) from the original population due to hybridisation.
When closely related different species of organisms are allowed to cross breed together, through selective
mating, selective propagation or selective pollination, they can develop a new breed (hybrid vigour). Hybrids
have better characteristics like increased size, high yields, quick maturity and increased resistance to diseases.
Selection for these advantageous characteristics over several generations may lead to emergence of a new
species with better characteristics than the parental organisms (ancestral organisms) due to increase in allele
frequency of the hybrid characteristics in the population caused by a directional selection pressure exerted
on populations by superior characteristics.
Continued selective breeding by humans has produced the varieties of domestic animals and plants of
agricultural importance seen today. Therefore from cross breeding new species can be easily evolved through
human activities. Therefore humans are preserving animal and plant genes which are considered to be
desirable and eliminating those genes which are undesirable via extermination, castration or isolation of
organisms with inferior features. Selection for the desirable characteristics over several generations leads to
the formation of new species. This replicates how evolution could have taken place in the past.
RESISTANCE TO DRUGS AND PESTICIDES
When these are first administered a large population of a non-mutant strains die leaving very few mutant
strains. The few mutant resistant strains reproduce greatly and pass on their mutant gene for resistance against
the drug or pesticide to the next generation. Eventually the resistant strains of the pest or bacteria become the
majority in the population and after several generations may become new species. In addition some plant
species grow on the heap of highly poisonous metals such as gold, copper, lead, silver, e.t.c because such
plants have mutated and developed the ability to tolerate such poisonous heavy metals.
Page 357 of 447

SELECTION
There are 4 major types of selection as a key factor for evolution and these include the following;
A. Natural selection
B. Sexual selection
C. Kin and group selection
D. Artificial selection
NATURAL SELECTION
This is a process by which organisms that are better adapted to their environment survive and reproduce while
those that are poorly adapted to their environment are eliminated.
Natural selection promotes speciation as described below; Natural selection occurs because different
individuals in the population show variations advantageous to the environment while others show variations
disadvantageous to their environment. Individuals with favorable variations in the population survive, grow
and reproduce thereby passing on their favorable characteristics to the next generation while those with
unfavorable variations are less likely to survive long enough to reproduce and are therefore eliminated in the
struggle for existence by the various selection pressures as nature selects against them.
A selection pressure is any environmental resistance factor that can increase or decrease the frequency of an
allele within the gene pool by eliminating the unfit individuals from the population and leaving the fit to
reproduce, thereby leading to evolutionary change.
A selection pressure may be predation, disease outbreak or competition.
Environment influences natural selections in that it exerts a selection pressure on organisms’ population,
which eliminates those with unfavorable variations from the population.
Natural selection is important in the following ways;
i. It promotes the emergence of new species under the constantly changing environmental conditions.
ii. It enables organisms which are best adapted for a particular environment (the fit) to survive and
reproduce there by passing on their genes to the next generation.
iii. It ensures that undesirable genes are eliminated from the population as it causes elimination of the unfit
organisms.
iv. It ensures that the population size is supported by the given environment as it maintains the carrying
capacity of the habitat
There are 3 types of natural selection and these include the following
I. Stabilising selection
II. Directional selection
III. Disruptive selection
I. STABILISING SELECTION
It is defined as the selection which occurs when selection acts to eliminate both extremes from the phenotypes
of the population resulting into increase in the frequency of the already common intermediate phenotype
If there is a little change in the environmental conditions and little competition within the population
individuals which do not show adaptation to extreme conditions survive and reproduce while those which
show adaptations to extreme conditions die out without reaching reproductive age i.e. both extremes are
removed from the population leaving only organisms with intermediate phenotypes.
Examples of the extreme conditions include very hot or very cold climate. Organisms which are adapted to
such extreme conditions die because little change in the environment does not result into such extreme
conditions. This stabilises the population as it removes extremes within the population hence reducing chances
of evolution.
Page 358 of 447

Stabilising selection therefore occurs when optimum environmental conditions do not favour organisms with
extreme characteristics but instead favour those with intermediate phenotypes within a population. The
extreme phenotypes are eliminated by natural selection because they are adapted to extreme conditions.
TRAIT
Examples include;
i. Wing length in hawks. wing spans larger or smaller than the optimum wing length will have
reduced breeding potential which eliminates those birds with wings larger or smaller than the
optimum.
ii. .Similarly children born with extreme weights (very small or very large weight) die while those
born with intermediate weight (3 to 4 Kgs) have the highest chances to survive.
iii. In ducks and chicken, the eggs laid with intermediate weight have the highest chances of hatching
compared to the ones laid with extreme weights.
II. DIRECTIONAL SELECTION

This is the type of selection which acts to eliminate one of the extreme phenotypes from the population
thereby increasing greatly the frequency of the other extreme phenotypes in the population.
Directional selection occurs when gradual changes in environmental conditions favour a particular extreme
characteristic in the population to survive more as the intermediate and the other extreme characteristic are
less favoured which lowers their population.
If there is a gradual change in environmental conditions organisms that are best adapted to the new
environmental conditions survive and breed at the expense of the poorly adopted ones which get eliminated.
This selection occurs so as to eliminate one extreme character from a group of phenotypes there by making
the genes promoting this extreme phenotype become less frequent in the population as compared to the
frequency of the genes promoting another extreme phenotype in the population with beneficial characteristics
it therefore reduces the frequency of organisms in the population with poor characteristics.
This kind of selection brings about evolutionary change by producing a selection pressure which favours
increase in frequency of the new alleles in the population usually of the extreme characteristic. Therefore it
exerts a selection pressure which moves the mean phenotype towards one extreme phenotype.
Page 359 of 447

It is the commonest type of selection. And examples include industrial melanism of the peppered moth (B.
betularia) in England whereby the black peppered moths are the majority and the white peppered moths are
extremely very rare. It also includes artificial selection whereby humans promote the survival of organisms
with beneficial characteristics and cause those with poor characteristics to become less frequent in the
population.
III. DISRUPTIVE SELECTION OR DIVERSIFYING SELECTION

Disruptive selection is defined as the selection within a population which occurs when two contrasting
environmental conditions favour the survival of extreme phenotype within the population and does not favour
individuals with intermediate phenotypes resulting into two discrete forms of the species (or polymorphs)
each adapted to one set of extreme conditions.
If the environment takes a number of distinct/discrete forms in its variations e.g. very hot climate or
temperature which suddenly changes to rapidly very cold with no intermediate temperatures, selection may
occur to favour two discrete forms in the population with each form of the species adapted to one set of the
extreme conditions. This selection rapidly causes evolutionary change by splitting the population into two
demes (subpopulations) which after several generations may become new species if gene flow between the
demes is prevented for many generations. This reduces intraspecific competition between the demes and so
resources are made available for the two demes which increases their survival. A deme is a genetically
isolated population that forms a new species.
Examples of disruptive selection include organisms adapting to very cold conditions and very hot conditions
but not to intermediate conditions, the different beak sizes of the African seed cracker finch bird (Pyrenestes
astrinus). Populations of these birds contain individuals with large and small beaks but very few individuals
with intermediate sized beaks. The large beaks are used for opening very tough seed coats while the small
beaks are used for opening very small seeds. The intermediate beaks are unable to open both large and small
seeds and are therefore selected against by lack of food. Other examples include; A, B, O blood group system
and red green color blindness in humans.
Disruptive selection occurs due to selection pressures acting from within a population usually as a result of
increased competition for resources which may push the phenotypes away from the population mean towards
the extremes thereby leading to the formation of new species. This can split the population into two new
demes. If gene flow between the demes is prevented, each deme may give rise to a new species, due to the
different selection pressures exerted on each deme in the different extreme conditions. Gene flow is the
movement of alleles from one population to another mainly through breeding. The genetically isolated
population usually undergoes selection to form a new species.
Page 360 of 447

Disruptive selection may result into appearance of different phenotypes within the population, a phenomenon
called polymorphism. Polymorphism is a phenomenon whereby forms of organisms of a given species
show differences in body structure and reproductive potential which determines their roles in the habitat.
There are 2 types of polymorphism namely;
i. Stable or balanced polymorphism.
ii. Unstable or transient polymorphism.
i. STABLE OR BALANCED POLYMORPHISM

This is a form of polymorphism in which different forms of a species co-exist within the same population in a
stable environment and their genotype frequencies show equilibrium as each form has a selective advantage
of equal intensity.
This type of polymorphism selects for the heterozygotes and gives them an advantage over the homozygotes.
In humans, sickle cell trait individuals (heterozygotes) do not suffer from malarias as compared to the
homozygous individuals.
Examples include;
 A, B and O blood group system.  Workers, drones and queen bees.

 Red-green colour blindness.  Workers, queen and soldier termites.
ii. UNSTABLE OR TRANSIENT POLYMORPHISM

This is a form of polymorphism which arises when different forms of species exist in a population undergoing
a strong selection pressure which results into one form having a higher genotypic and phenotypic frequency
from another e.g. the melanic and non-melanic forms of peppered moth (B. betularia) in England where the
melanic form has a higher a frequency in industrial areas and a very low frequency in the rural non-
industrialised areas.
Polymorphism increases adaptation of organisms to their environment which sets up natural selection in the
population hence allowing evolution to occur.
Page 361 of 447

ARTIFICIAL SELECTION
This is where humans deliberately allow some organisms with desirable characteristic to survive, reproduce
and pass on their genes to the next generations while preventing other organisms with undesirable
characteristics to survive and reproduce. This eventually leads to the formation of new species having
beneficial characteristics.
SEXUAL SELECTION
This is where organisms’ mating success is selected for or against depending on the organisms’ sexual
behaviours and sexual structural dimorphism.
Sexual dimorphism is a situation where by different sexes have different or unique morphological makeup’s
for sexual attraction (secondary sexual characteristics) to the members of the opposite sex e.g. the male birds
have bright plumage (feathers) so to attract the female while the feathers have dull plumage purposely for
camouflage during incubation of their eggs.
Sexual selection brings about preservation of species because different species have specific reproductive
behaviors and genitalia. This brings about sexual selection which contributes to evolution because different
species adopt unique reproductive behaviors and genital organs during the course of evolution which restricts
reproduction within a particular species only, thereby preventing inter-breeding within different species.
Organisms which are sexually selected for reproduction, reproduce and pass on their genes to the next
generation, which causes variations in the population by bringing about natural selection and formation of
new species. Organisms which are selected against reproduction are described as genetically dead
organisms such as the sterile organisms. Genetic death is the failure of an organism to reproduce and pass on
its genes to the next generation which may disappear from the gene pool upon death of such an organism.
Sexual selection is important because it brings about preservation of the species, as different species have
specific reproductive behaviours and genitalia which allow reproduction to occur only within that particular
population. This enables sexual reproduction to contribute to evolution because different organisms have
unique reproductive behaviours and genital organs which restricts reproduction within one particular species
only thereby preventing interbreeding between different species. Sexual selection leads to evolution
NOTE:
i. Unrandom mating. This is the selective choosing of the sexually fit individuals in a
population for mating. This ensures that some individuals in a population that are sexually fit
have an increased reproductive potential than others that are sexually unfit. The alleles of the
sexually fit individuals are likely to be inherited in subsequent generations.
Individuals that are sexually unfit for mating may not be selected for mating hence their alleles
are less likely to be inherited in subsequent generations and such alleles may be eliminated
from the population over several generations. This results into a divergent population from the
parental population that is better adapted for reproduction and survival. Over several
generations, this divergent population may form new species.
ii. Genetic load: is the existence within the population of disadvantageous alleles in the
heterozygous genotypes which enable them to be easily transmitted by carriers to the next
generation e.g. the existence of the sickle cell trait individuals in human population.
Genetic load can also be defined as a condition when a population harbors disadvantageous
alleles in its gene pool due to the heterozygous advantage of the carriers.
Genetic load is advantageous because it confers the selective advantage of the phenotype in
certain environmental conditions e.g. a sickle cell trait individuals are highly resistant to
malaria.
Page 362 of 447

KIN AND GROUP SELECTION

Kin selection is a form of selection where an animal tries to save the lives of other related animals but usually
risks its life for others e.g. the worker bees sting the enemy that has attracted the bee hive but accidentally if
their sting breaks the worker bees dies at the expense of other bees in the bee hive.
Group selection is where an animal tries to guard other members of the species against danger although not
related to them e.g. some birds try to alert chicken of the presence of eagles, cattle egrets alert cattle and
buffaloes of the incoming danger by suddenly flying away from the animals.
This kind of behaviour where an individual animal sacrifices its own life purposely to save some other
individuals against enemy is known as altruistic behaviour.
SPECIATION
This is the process by which one species splits into two or more sub-species which gradually develop into
different genetic lineages to form new species.
A species is a population whose members can interbreed and produce viable fertile offsprings, but are
unable to produce viable fertile offsprings with members of other populations.
A single species may give rise to new species i.e. intraspecific speciation e.g. breeding organisms that
are pure breeds but one having better characters than another which results in the formation of a hybrid, with
hybrid vigor, that doesn’t resemble any of the parents also two different species may interbreed and give give
rise to one new species i.e. interspecific hybridisation
INTERSPECIFIC HYBRIDISATION
This is the form of sympatric speciation which occurs when a new specie is produced by the crossing of
individuals from two unrelated species.
A Fertile hybrid usually appears and only increases the chances of chromosome mutations to occur in that
hybrid which makes it betted adapted to the environment. Such hybrids are normally formed by alloploidy
e.g. a cross between cabbage and radish.
An allopolyploid is a fertile individual that has more than two chromosome sets as a result of two different
species interbreeding and combining their chromosomes to form infertile F1 hybrids, but due to non-
disjunction the F1 hybrids form fertile F2 hybrids.
As formed in the above cross, during meiosis to the form F1 hybrid, the chromosomes from each parent cannot
pair together within the gametes to form homologous chromosomes and this makes the F1 hybrids sterile.
However, non-disjunction of F1 hybrids produces gametes within diploid sets of chromosomes (2n=18) to
form tetraploids (F2 hybrids) which are fertile. Homologous pairing can occur in meiosis and F2 hybrids since
the two sets of parental chromosomes are present.
Non-disjunction is an error in meiosis or mitosis in which members of a pair of homologous chromosomes or
a pair of sister chromatids fail to separate properly from each other.
F1 X F1
2n=18 2n = 18
F2 hybrids (4n=36)
Page 363 of 447

MECHANISM OF SPECIATION
An isolation mechanism is the one that tries to maintain the gene pool of a genetically isolated population by
preventing successful inter-breeding with members of different species but restricting reproduction within the
members of the same species.
For the development of a new species to occur the gene flow within the isolated populations (sub-species or
demes) must be interrupted or stopped by isolating mechanisms to ensure isolated populations stop inter-
breeding. The various isolating mechanisms isolating the two species may be geographical, reproductive or
genetic.
The process of speciation, is initiated by the isolation of species using isolation mechanisms which occurs for
a long period of time. This results into the splitting of one population into two or more separate sub-
populations or demes for a long time each of which will remain with its own gene pool so that any new
variations that arise within these sub-populations will not flow up to the other populations. This makes these
sub-populations to become independent populations after a long time of separation. The demes must be
isolated from one another to avoid exchange of genes between them in a single population. This encourages
gene differentiation which results into the formation of the new species. Gene differentiation refers to the
adaptation of genes to particular environmental conditions. In addition mutation and selection can take place
independently in the two demes, causing each of the demes to develop into a distinct species. This is because
when a deme is isolated, new variations which arise in each deme via mutation will not flow to other demes
and so, mutation, natural selection and gene differentiation occur differently. This makes these demes to
become independent populations or new species after a long time of separation by geographical, genetic or
reproductive isolation. Changes in allele and genotype frequencies within the isolated populations due to the
effects of natural selection on the range of phenotypes produced by mutation and sexual recombination lead to
the formation of the sub-populations which become new species after several generations. If isolation persists
for a long period of time and the sub-species come together to occupy the same area they may not inter-breed
and therefore they will be described as new species.
NOTE; if the new species formed from each deme come together to occupy the same area, they cannot
interbreed successfully but competition between the different species would occur. The less adapted species
would be eliminated leading to the reduction in the number of species. Alternatively, the species may occupy
different ecological niches thereby avoiding direct competition between the species hence all the spies would
survive.
Isolation mechanisms are significant in evolution because they permit stabilisation of the population,
generation after generation, and also they enable organisms to evolve along different lines as they prevent
inter-breeding. This isolation mechanism is possible as a result of various isolation mechanisms described
below;
A. GEOGRAPHICAL ISOLATION
This is isolation whereby the population splits into sub-populations or demes due to separation by
geographical barriers such as larger water bodies, mountains and deserts which prevent the sub-populations
from inter-breeding to exchange their demes. This may occur when some members migrate or are dispersed,
or when the geography changes catastrophically e.g. earthquakes, floods, volcano eruptions or gradually e.g.
erosion, continental drift. This may lead to evolution in that when species become separated geographically
they undergo adaptive evolution because the y experience different selection pressures due to the different
abiotic and biotic factors within their environments, which brings about divergent evolution. Even if the
abiotic and biotic factors are the same, the populations may change by random genetic drift, especially if the
populations are small.
Page 364 of 447

If the separated population come together after many generations each may have changed physiologically or
genetically making inter-breeding difficult and will therefore be called new species. The new two populations
formed therefore adapt to the different environmental conditions as they are separated by a geographical
barriers and such species are called allopatric species.
Allopatric speciation is therefore the evolution of new species due to separation by geographical barriers e.g.
evolution of Darwin’s finches of Galapagos Islands.
B. REPRODUCTIVE ISOLATION
This is the existence of biological factors (barriers) that impede members of two species from producing
viable, fertile offsprings.
It involves the stopping of free inter-breeding by individuals of a population due to differences in courtship
behavior and a non-correspondence of the genitalia even if they occupy the same habitat. This divides the
population into two or more categories and genitalia within the same area.
Reproductive isolation arises because during evolution, different species acquire unique sexual behaviours,
thereby making them stay together in the same area. This brings about sympatric speciation i.e. the evolution
of new species due to genetic or reproductive isolation in the same geographical area. The species formed are
called sympatric species e.g. the evolution of a lion and hyena that live in the same habitat.
Reproductive isolation brings about speciation in that it leads to differential reproduction occurs only within a
few individuals of the population that have similar courtship behaviours and genitalia. Reproduction therefore
occurs in a few individuals who are better adapted for reproduction and therefore mutation, natural selection
and gene differentiation occur only in these few individuals. This causes new characteristics to accumulate
within only this interbreeding sector which may become a new species. This reproductive isolation occurs
through the following isolation mechanisms;
 mechanical isolation
 seasonal isolation
 behavioral isolation
 habitat isolation
These isolation mechanisms work in such a way that organisms in the same geographical area belonging to
different species fail to interbreed (premating isolation mechanism)
i. MECHANICAL ISOLATION
This is whereby the genitalia of the two organisms of the opposite sex of different species are
incompatible (non-correspondent) in that mating and fertilization cannot occur.
ii. SEASONAL ISOLATION (TEMPORAL ISOLATION)
This is where the reproductive organs of organisms of different species of opposite sex mature at the
different times or seasons of the year thereby resulting into fertilisation failure. The mating seasons
may not overlap therefore between different species e.g. the stamens and pistil of flowers of different
Page 365 of 447

plants mature at different times of the year. Many animals usually have different reproduction
systems i.e. protandry and protagyny
iii. BEHAVIOURAL ISOLATION
This is where organisms show differences in courtship behaviours such that there is no attractiveness
that can lead to mating. Courtship rituals/activities that attract species for mating are unique to a
species and are very effective reproductive barriers even between closely related species.
iv. HABITAT (ECOLOGICAL) ISOLATION
This is where habitat preferences keep members of different species or demes apart though living in
the same area therefore preventing mating and fertilisation between them.
NOTE: another form of reproductive isolation occurs through unrandom mating i.e. selective choosing of
individuals in the population for mating. Unrandom mating ensures that some individuals in the population
have increased reproductive potential than others and therefore their alleles are likely to be inherited in
subsequent generations. Individuals with unfavorable characteristics for reproduction may not be selected for
mating and therefore their alleles are less likely to be inherited in subsequent generations which alleles may be
eliminated from the population over several generations. This results into a divergent population from the
parental population i.e. a new population different from the parental population due to new variations
acquired. This divergent population is better adapted for reproduction and survival and over several
generations may completely become a new species.
C. GENETIC ISOLATION (post mating isolation mechanisms)

This is where mating between two sub-populations or demes may occur but fertilisation is prevented due to
the fundamental differences in the genetic constitution of the different organisms resulting into failure of the
male gametes to survive into the female reproductive system e.g. in plants the pollen tube may not grow down
to the style to cause fertilisation while in animals the sperms may not survive in the female genetalia.
The fusion of gametes may not take place due to different genetic make-up of the gametes, the production of
infertile hybrids (hybrid sterility) and the failure of the hybrid to grow and develop up to maturity (hybrid
inviability).
The organism formed may fail to adapt to the environmental conditions or they may be inferior sterile
offsprings. Therefore the two isolated populations (demes) only reproduce successfully by breeding
independently within the demes, thereby resulting into the formation of new species.
Genetic isolation may also arise through reduced hybrid fertility. In this case, even if hybrids are viable,
they may be sterile especially if the chromosomes of the two parent species differ in number or structure,
which leads to failure of meiosis in the hybrids, so that they cannot produce normal gametes. Since the
infertile hybrids cannot produce offsprings when they mate with either the parental species or between
themselves, then genes cannot flow freely between the species.
Genetic isolation may also occur through the following;
a. Polyploidy
b. Hybrid breakdown.
c. Hybrid sterility.
POLYPLOIDY
This is a chromosomal alteration in which the organism possesses more than two complete chromosome sets.
It is a result of an accident of cell division.
This brings about hybrid sterility due to the odd number of chromosomes that cause sterility. Even numbered
polyploids are fertile and polyploidy therefore divide the population into two groups i.e. one group being
Page 366 of 447

fertile and another group being infertile which is genetically isolated as a result. This restricts breeding in only
polyploidy species with even numbered chromosomes.
Polyploidy therefore results into the formation of new species having very good characteristics such as greater
resistance to diseases and adverse environmental conditions, high yields, early maturity e.t.c. such
characteristics arise mainly due to gene mixing during cross breeding to form the hybrid vigour organisms,
especially in plants. Selection for these characteristics in nature leads to the formation of new species.
HYBRID BREAKDOWN
This is a situation where the first generation hybrids (F1 hybrids) are fertile but later generations (F2 hybrids)
hybrids are infertile.
HYBRID STERILITY
This is a condition where by hybrids fail to produce functional gametes and this leads to genetic isolation of
such organisms i.e. it usually occurs when two species mate or flower at different times of the year or when
the species have different genetic composition. For example, the formation of the mule (2n=63) from the cross
between the horse (2n=60) and donkey (2n=66).
NOTE: In addition to the isolation mechanisms mentioned above, new species may be formed from the
following;
a. Genetic drift and founder`s effect
b. Adverse environmental effects like drought
c. Drug and pesticide resistance
d. Mutations
e. Migrations
f. Predations
These selection mechanisms may be directional or disruptive.
GENETIC DRIFT AND FOUNDER`S EFFECT
Genetic drift causes speciation in that loss of individuals before reproduction from a small population may
lead to loss of an allele from the small population thereby greatly changing the allele frequency of the gene
pool e.g. premature accidental death of organisms prior to mating or death of an organism which is the sole
possessor of that particular allele would result in the elimination of that allele from the population. It is also
possible for an allele to increase to a higher frequency by chance.
These changes in allele frequency in a small
population lead to a divergent population
from the parental population that may form
new species over several generations.
Genetic drift is not common in large
populations because the changes in allele
frequency are usually buffered by the large
population and so, they become negligible.
Related to genetic drift is a condition known
as founder`s effect or founder`s principle.
This principle refers to one or few
individuals of the parental population
moving away to another place and then
establishing a new small isolated population
at some distance away from the original
location. The new small population has a
Page 367 of 447gene pool which is not reflective of the
original population.
ADVERSE ENVIROMENTAL EFFECTS

Adverse effects such as floods, bush burning, excessive heat e.t.c. may lead to the formation of new species.
The survivors of such an occurrence may be few and there genotypes may not resemble those of the original
population. Mutations that may happen as a result of such events leading to speciation. Breeding may also be
of closely related homozygous members only, resulting into heterozygotes that may emerge into new species.
MUTATIONS
New species may form through mutations because mutations may bring about a new allele in the population
thereby introducing a new variation within the population. If the new variation confers an advantage on the
mutants, especially when environmental conditions drastically change, the mutant is favoured to survive and
reproduce to increase its numbers. When there is no gene flow between the mutants and the non-mutants,
further increase in genetic differences of the two gene pools occurs hence leading to the formation of new
species.
MIGRATIONS
These result into the transfer of genes from one population into another. This reduces the size of the gene
pools if all the genes are taken away from the population and the demes may eventually become non-identical
thereby resulting into the formation of new species. Migrations also encourage the spread of mutant genes in
the population hence leading to a change in allele frequency which may lead to the formation of new species.
PREDATION
This forms new species because it puts up a selection pressure of nature among the predators. As the predator
population increases the prey population decreases and competition arises among the many predators for the
few remaining prey. The well adapted predators to compete for the prey are naturally favoured to survive.
While the poorly adapted predators to compete for the prey die or migrate away. The well adapted predators
reproduce and pass on their adaptive characteristics to their offsprings. The offsprings eventually undergo
adaptive radiation and form new species of the predator.
Selection mechanisms may also favour the formation of new species as they determine alleles to be passed on
to the next generation and they also determine the way alleles spread in the gene pool. This may lead to the
formation of new species as it results into changes in allele frequency in the new population thereby leading
into an evolutionary change that results into speciation.
EXTINCTION OF SPECIES
Extinction is the termination of a genealogical lineage i.e. total disappearance of all organisms of a gove
species. The term is used most frequently in the context of species, but applicable also to populations and taxa
higher than species.
A specie may become extinct if it fails to adapt to its environment and as a result it is at a selective
disadvantage. This selection pressure can include excessive/massive predation, whereby organisms of a
species are preyed upon up to the level where there is no existing member of that kind in the wilderness.
Other selection pressures include the following;
I. Lack of the basic nutrients for the survival of the organisms or elimination of a link in a food chain
which cause starvation of the organisms that used to survive on that link. This may seriously affect the
availability of the species hence leading to their extinction.
II. An organism can become extinct through excessive competition from development of new species
which redder the less adapted species extinct due to failure of such species to adapt to the highly
competitive environment. As its population declines due to the competition, gene flow from within
the species is interrupted hence leading to the extinction.
Page 368 of 447

III. Species may become extinct due to the destruction of their habitats which exposes them to their
predators that eventually wipe them out, thereby leading to extinction. This may be due to human
activities during exploitation of natural resources, introduction of domestic animals, environmental
pollution or direct destruction through hunting.
IV. It may be due to climatic changes which result into the change in the average conditions of the
habitat that drastically affect the population hence leading to extinction.
V. It may be due to pests and diseases in that the pathogen micro-organism which attacks the population
may kill the organism may kill them in large numbers as these organism fail to develop resistance
quickly enough. This may lead to extinction of the species as they may take long to become resistant
and even the young ones that would reproduce may be killed before reaching the reproductive stage.
VI. Other factors include drastic environmental changes such as drought, volcanic eruptions, fire, floods
e.t.c. that create new selection pressures such that species which cant adapt die or fail to reproduce
ALLELE/ GENE FREQUENCY

This refers to the total number of individuals in the population baring a particular gene. It can be affected by
the following factors in addition to what has already been described;
a) Closeness of the population d) Genetic drift
b) Mutation e) Migrations
c) Selection
CLOSENESS OF THE POPULATION

This allows free movement between individuals in the population and therefore increases the chances of
random mating which results into gene mixing, from which hybrid individuals with altered gene frequencies
arise. Two neighbouring populations interact whereby organisms of one population migrate into another
population, where it may interbreed. This makes the allele frequency of the donor population to decrease and
that of the recipient population to increase due to introduction of new alleles.
MUTATION
Mutations change the allele frequency by bringing about a new allele in the population which can form a basis
for natural selection by conferring an advantage onto the mutants, especially when the environment changes,
thereby leading to the formation of new species. Mutations can also confer disadvantages to the mutants
which decreases the allele frequency as some of the mutants may die or reduce their reproductive potential.
SELECTION
Selection favours certain alleles in the population and eliminates others in a certain environment and in so
doing, the alleles increase or decrease respectively. The more favoured individuals with increased
reproductive potential are able to pass on their alleles to the next generation while the less favoured
individuals usually have decreased reproductive potential which reduces their chances of passing on their
alleles to the next generation.
GENETIC DRIFT
This refers to random changes in allele frequency of a small population occurring by chance alone rather than
natural selection. In a small population, not all the alleles that represent a gene pool of that species population
are present e.g. some may be lost through premature accidental death prior to mating of an organism which
happens to be the sole possessor of that allele from the population which will decrease that allele frequency. It
affects the amount of genetic variations within very small populations as explained below;
The effect of a few individuals within a small population not contributing their alleles to the next generation
can have a great effect on allele frequencies. This is because certain alleles may become lost as the only
alleles for the genes present may be eliminated if the individual with the alleles dies.
Page 369 of 447

In a small breeding population, the fluctuations in allele frequency are more severe because random changes
in a few alleles cause a greater percentage change in allele frequency i.e. genetic drift occurs.
In a large breeding population, the fluctuations in allele frequency are minimal because the large number
buffers the population against the random loss of alleles therefore on average losses for each allele type in a
large population will be similar in frequency and therefore little changes occur in such a large population
Variations in genes within a small population can occur by chance rather than by natural selection.
THE HARDY-WEINBERG PRINCIPLE

This states that the frequency of the dominant and recessive alleles in the population remains constant
generation after generation as long as environmental conditions do not change i.e. genetic equilibrium
However, in nature, this principle is never realised as organisms always vary from each other and therefore
adapt differently to the environmental conditions which causes natural selection so that evolution occurs.
This principle is used to determine the gene frequencies in a given population.
CONDITIONS FOR THE HARDY-WEINBERG PRINCIPLE TO BE HELD TRUE
 When the population is large.
 When there are no mutations in the population.
 When mating is random.
 When there is no natural selection
 When there’s no genetic drift
 When generations do not overlap.
THE HARDY-WEINBERG EQUATION
This equation is used to determine the genotype and allele frequencies in the population.
If a gene controlling a particular character A and a with allele frequencies p and q respectively, then p + q =
1.0 or p + q = 100%.
Where P = frequency of the dominant allele.
Q = frequency of the recessive allele.
In this population, individuals will have the following possible genotypes; AA, Aa and aa where A is
dominant over a.
If heterozygous individuals in the first cross are crossed, the genotypes and frequencies in the F2 generation
will be as follows;
Aa x Aa
From the Punnet square
All A a a) Genotype frequency of homozygous

dominant individuals = p2.
Punnet square to show the fusion of gametes b) Genotype frequency of heterozygote’s
= 2pq.
A (p) a (q) c) Genotype frequency of recessive
individuals = q2.
In this population therefore, the total genotype
A (p) AA (p2) Aa (pq)
frequency of genes if the heterozygous individuals
are selfed is 100% and this is obtained using the
Aa (q2) hardy-Weinberg equation shown below;
a (q) Aa (pq)
Page 370 of 447

p2 +2pq + q2 = 100% OR p2 +2pq + q2 = 1.
Example
In a population of 5,000 individuals, 84% of them are non-albinos and 16% are albinos. Using the hardy
Weinberg formula determine the number of individuals who are;
a) Heterozygous normal for melanin formation.

b) Homozygous normal for melanin formation.
c) Albino
SOLUTION
Hardy-Weinberg formula; p2+2pq+q2 =1.0
Where p = frequency of dominant allele.
q = frequency of recessive allele.
a.Heterozygotes are represented by 2pq

q2 = 0.16
q = √0.16
q= 0.4
But p + q =1.0
p = 1.0 – q
p = 1.0- 0.4
p = 0.6
Therefore 2pq = 2 X 0.6 X 0.4 = 0.48 or 48%
48
Heterozygotes = 100 x 5,000 = 2,400 individuals
b. Homozygous dominant individuals are represented by P2

p2+2pq+q2 =1.0
p2 + 0.48 + 0.16 =1.0
P2 = 1.0 – 0.64
P2 = 0.36
= 0.36 X 100 = 36%
36
Homozygotes = 100 x 5,000 = 1,800 individuals
c. Albinos
16
= 100 x 5,000 = 800 individual
Page 371 of 447

P530 (2020) Ecology by Nakapanka Jude Mayanja 0704716641
SAMPLE QUESTIONS
1. (a) What is meant by the term extinction? (01 mark)
b) State one natural cause of extinction. (01 mark)
b) State the ways human activity has accelerated the rate of extinction in present times(03 marks)
c) Suggest measures that can be put in place to prevent extinction (03 marks)
d) Explain why large predators e.g. birds of prey are more prone to extinction than herbivorous birds
2. (a) Describe the main features of Neo-Darwinism theory of evolution (10 marks)
(b) Explain the evidence from the geographical distribution of organisms that supports the theory of
evolution described in (a) above. (10 marks)
3. The figure below shows changes in the number of peppered moths in area in which industries were
established after 3 years. Peppered moths are expressed as percentage number before and during the
industrial revolution.
figure 7
100
90 Dark form
Population size of moths (%)
80
70
60
50
40
30
Grey form
20
10
0
0 1 2 3 4 5 6 7 8
Time in years
a) (i) What was the effect of the industries on the number of peppered moths in the area?
(ii) Explain the effect above? (04 marks)
b) Three populations of the peppered moths were anlaysed and the results below were obtained
Genotypes
AA Aa Aa
Population 1 0.430 0.4810 0.890
Population 2 0.4225 0.4550 0.1225
Population 3 0.0025 0.1970 0.8005
i. Which of the following populations is in the Hardy-Weinberg equilibrium? (02 marks)

ii. Determine the frequency of A allele in the population stated above? (02 marks)
4. (a) Explain the modern theory of evolution by natural selection.(10 marks)

(b) How does each of the following affect gene frequency within a population?
(i) Non-random mating. (03 marks)
(ii) Disruptive selection. (03 marks)
Page 372 of 447

(iii) Mutation. (04 marks)

5. The figure below is a graph that shows the extent of precipitation that occurs when serum from different
mammals mixed with sensitized rabbit serum
120
amount of precipitation in percentage

100
80
60
40
20
0
Human Chimpanzee Baboon Lemur
Mammals
a) (i) Describe the trend of precipitation of serum from human to lemur (02 marks)
(ii) Explain how precipitates are formed when sensitized rabbit serum is mixed with any
mammal’s serum (03 marks)
b) Explain the difference in the amount of precipitate formed between Chimpanzee and Lemur.
c) State one evolutionary conclusion about the relationship between human beings and
i. Chimpanzee (01 mark)
ii. Lemur (01 mark)
6. The table below shows the amount of antibiotics used (in kilograms) in treating bacterial diseases and the
percentage of bacterial strains which have become resistant to the antibiotics used in one of the hospitals
in Uganda over twenty years.
Time /years 2 4 6 8 10 12 14 16 18 20
Amount of antibiotics/Kg 0.5 2.5 6.4 3.0 3.7 3.1 2.8 0.5 0.8 0.5
% of bacterial strains resistant to 3 4 2 18 22 15 17 5 5 4
antibiotics
a) (i) By using appropriate scales, represent the above data graphically

(ii) Describe how the resistance of the bacteria varied with the amount of antibiotics administered
b) Briefly explain how each of the following could have contributed to the rapid emergence of
resistant strains;
i. DNA in a bacterium is haploid
ii. Bacteria usually reproduce asexually by fission
iii. Fission may take place as often as every 3o minutes
c) The graph may be used to demonstrate evolution in action. Explain fully how
7. A factory emitting smog containing sulphur dioxide and carbon dioxide was cited in a rural district. The
table below gives distance and directions of :
i. Number of moths and
ii. Concentration of sulphur dioxide in smog in different directions from the factory chimney
Table 1
Page 373 of 447

Distance from factory in a south-south West 1 2 4 8 12 16 28

direction (miles)
Number of moths species 0 1 2 3 7 9 12
Sulphur dioxide concentration/parts per million 28 27 26 23 19.5 16 2
Table 2
Distance from factory in a north north east direction 1 2 4 8 12 16 28
(miles)
Sulphur dioxide concentration/parts per million 27 26.5 25 24 23 22 19
a) Plot the information to show the relationship between the moth species distribution and the
sulphur dioxide concentration the same X-axis and Y-axis. (12 marks)
b) Explain the difference in results between obtained for the south-south west direction and those
obtained for the north-north east direction (02 marks)
c) Fully explain why the number of moths increase with increasing distance from the factory (04)
d) The results obtained give evidence of present day evolution. Explain fully this evidence and its
significance in evolution (10 marks)
e) What are the environment effects of sulphur dioxide and carbon dioxide? (08 marks)
8. (a) Give the characteristics of predator-prey interactions in nature (05 marks)
(b) In what ways do the predator-prey interactions compare with parasite-host interactions (07 m)
(c) Explain how predator-prey interactions influence the formation of a new species (08
9. (a) What is meant by gene reshuffling (04 marks)
(b) Explain how;
i. Reshuffling of genes may contribute to genetic variation (06 marks)
ii. Interactions at the gene locus affect phenotypic variation (10 marks)
10. (a) Describe the structural evidence of evolution in animals (10marks)
(b) Explain the role of the following in evolution.
i. Non-random mating (5marks)
ii. Gene flow (5marks)
11. a) Describe the evidence of evolution based on palaeontology? (09marks)
b) Explain how the following may lead to evolution of a new species;
(i) selective breeding (05marks)
(ii) increased population size (06marks)
12. a). Differentiate between discontinuous and continuous variation. [05marks]
b). What is the role of each of the following in evolution?
i). Meiosis. [05marks]
ii). Mutation. [05marks]
c). How does biogeography show that evolution occurs? [05marks]
13. (a) Describe how Darwin explains evolution of a new species by natural selection. (10
(b) Explain how the following may lead to evolution of a new species.
(i) Selective breeding. (05 marks)
(ii) Increased population size. (05 marks)
14. (a) Explain the main features of Darwin’s theory of evolution. (12 marks)
(b) The beetles belonging to the genus Colophon are unable to fly and are found on hilltops in three areas
in South Africa. Suggest an evolutionary explanation for each of the following statements.
(i) All of these beetles are of very similar general appearance. (02 marks)
(ii) There are slight differences between the species of Colophon found in the three areas.
(iii) The fact that the beetles of the genus Colophon are unable to fly has been important in the
evolution of twelve different species of the genus in a small area of South Africa. (03 marks
15. (a) Explain the various forms of isolating mechanisms
(b) Describe the factors that lead to species extinction
16. a) Define the following terms:-
i. A deme (03 marks)
ii. Genetic equilibrium (02 marks)
Page 374 of 447

iii. Reproductive isolation (03 marks)

b) Describe how genetic equilibrium can be upset in a deme/population. (07 marks)
c) Given an account of how reproductive isolation is brought about in a population (05 marks)
17. (a) What is variation? (02 marks)
b) Giving examples, explain the various ways in which variation may arise (12 marks)
c) What is the role of natural selection in the evolution of organisms making a population 06 m
18. (a) Explain the significance of the following in evolutionary study
i. Comparative anatomy (07 marks)
ii. Comparative serology (03 marks
(b) How does the concept of development and distribution of Darwin’s finches at the Galapagos
Islands explain evolution? (10 marks)
19. (a) What is meant by the following as it relates to the survival of living organism?
i. Adaptation (03 marks)
ii. Selection pressure (03 marks)
(b) Explain how the following are considered to be selection pressures
i. Insecticides (07 marks)
ii. Artificial selection (07 marks)
20. (a) Explain how industrial melanism shows that evolution occurs (10 marks)
(b) How does Meiosis bring about variation in organisms? (10 marks)
21. (a). Describe the contribution of meiosis towards variation. [5mks]
(b). Describe the role played by the following in the formation of new species.
(i). Polyploidy. [7mks]
(ii). Artificial selection. [4mks]
(iii). Genetic drift. [4mks]
22. a) How does Darwin’s theory of evolution explain the existence of Finches in the Galapagos Islands?
b) Explain how industrial melanism and use of insecticides prove that evolution occurs
23. (a) Discuss the various forms of isolating mechanisms
(b) Distinguish between stable and unstable polymorphism
24. (a) What is meant by the term genetic equilibrium? (02 marks)
(b) Suggest three reasons why populations in genetic equilibrium fail to undergo evolution (03 marks)
(c) State two instances where the evolution of one species has been influenced by evolution of another
(d) Explain how extinction of species is an important aspect of speciation (03 marks)
25. (a) Describe the different evidence for the occurrence of organic evolution (10 marks)
b) Explain how gene reshuffling brings about speciation (10 marks)
END
Page 375 of 447

TOPIC 9: ECOLOGY
SYLLABUS EXTRACT
Specific objectives; The learner should be Content
able to:
1.1 Components of environment  A biotic components: air, water, soil and factor:
light, temperature, humidity atmospheric pressure
 Describe the abiotic and biotic components
rainfall, edaphic factors (PH, moisture) and biotic
and factors.
components living things and factors: Competition,
 Explain how the components and predation biological associations.
environmental factors influence the
 Influence of a biotic and biotic components and
distribution and abundance of organisms in
factors of the environment on distribution and
an ecosystem.
abundance of organisms.
 Collect data from field studies  Data ecological components and factors of an
 Analyses and interpret data or literature ecosystem.
on ecological principles  Data or literature on ecological principles
4.1 Concept of ecosystem  The ecosystem defining types quarter time limit
 Describe an ecosystem proper
 State the type and properties of an  Types and properties of an ecosystem: aquatic and
ecosystem. terrestrial ecosystems.
 Explain changes in an ecosystem  Ecological factors influencing the life of organisms
 Describe feeding relations in an ecosystem. in an ecosystem: abiotic, biotic, and edaphic.
 Explain energy flow and recycling of  Changes in an ecosystem (ecosystem productivity
nutrients in an ecosystem. succession and climax).
 Describe biogeochemical cycles.  Feeding relations: Food chains, food webs,
recycling of nutrients and energy flow in
ecosystems.
 Biogeochemical cycles (nitrogen, Carbon, water).
1.3 Population and natural resources  Population characteristic: density, age structure, sex
growth pattern, birth rate, death rate.
 State population density
 Population density dependent factors and density
 Describe methods or techniques of
independent factors.
estimating population
 Method or techniques of measuring and estimating
 Explain population growth patterns
 Population growth patterns
 Explain the terms renewable and non-
 Natural resources: type renewable and non-
renewable resources.
renewable
 Discuss environmental resistance and
 Environment resistance: density dependent
“balance of nature”.
factors affecting “ balance of nature”
 1.4 Population and natural resources practical
 Demonstrate the methods used in  Methods of estimating population: quadrant, line
estimating populations transect, capture – recapture.
1.5 Interdependence  Interactions among organisms and their effects
 Explain the various interactions of interspecific and intraspecific relationships
organisms in nature. between organisms
Page 376 of 447

 State the significance of organisms’  (Competition, Parasitism, predation saprophytes,

interactions in nature. Mutualism) commensalism.
4.6 Effects of human activities on ecosystem  Significance of organism interactions nature.
 Impact of human activities on an ecosystem
 Discuss the impact of human activities on
 Effects of human activities on ecosystem
an ecosystem.
components and factors in a habitat interruption
 Explain the influence and effects of
of biogeochemical cycles, natural resources
human activities on ecological
imbalances, population imbalances soil erosion,
components and factors in a habitat.
soil exhaustion, extinction, pollution speciation
 Discuss natural resources utilization
 Natural resources utilization and sustainable
development.
 Natural resource conservation practices: mulching,
terracing, crop rotation afforestation. Mixed farming
agro forestry, wise use of resource organic manure.
 Natural resource utilization and sustainable
development.
Page 377 of 447

Introduction
Ecology is the scientific study of the relationship between living organisms and the environment or surrounding.
The living organisms are the flora (plants) and fauna (animals). Ecology lays a foundation for the understanding
agriculture, forestry, fisheries, conservation, impact of human activities on the ecosystem and how to remedy
these impacts.
Ecological studies can be directly towards a particular organism or a single species, communities or an
ecosystem. The word ecology originates from a Greek word ‘oikos’ meaning a home. Two types of ecological
studies namely autecology and synecology are commonly carried out.
Autecology is the study of the relationship between a single species and the environment in relation to its
environment.
Synecology is the study of the relationship between natural communities or different populations of organisms
in a given environment i.e. the study of the relationship between all plants and animals in a particular area to
the environment.
Description of ecological terms
Ecosystem Species
An ecosystem refers to the interaction between living A group of organisms showing resemblance
organisms and non-living components of the among themselves in appearance, behaviour,
environment or habitat to form a natural self-supporting chemistry and genetic makeup for sexually
system e.g. in a pond or aquatic ecosystem. It consists of reproducing organisms, individuals are capable
phytoplanktons, saprophytes, zooplanktons as the biotic of interbreeding to produce fertile off springs
component (living) whose interaction with dead decaying Native species
organic matter or recycle for self-sustainability. Species that normally survive and shrive in a
Habitat particular ecosystem
This is a place or physical area where the organism or Non-native/alien/exotic species
species lives in an ecosystem. Species that migrate into the ecosystem or are
Microhabitat deliberately or accidentally introduced into an
Small locality within the habitat with particular ecosystem by humans e.g. crops and game
conditions (microclimate) that support specific species
organisms e.g. mosses can grow at the upper side of a Indicator species
fallen log, forests have more micro-habitats e.g. upper Species that serve as early warnings of damage
and lower leaf surfaces to a community or an ecosystem
Population Keystone species
This is a group of organisms or individuals of the same Species that play more important roles than
species which occupy a particular area or habitat at the others in maintaining the structure and function
same point and time e.g. toads in a pond of ecosystems of which they are a part i.e. it is a
Community. This refers to all populations that occupy a dominant species that dictates community
well-defined area at a given time. This implies that all structure by affecting abundances of other
plants, animals and fungi in a particular area form a species. E.g. elephants uproot and break trees,
community therefore a community is a group of plants creating forest openings in the savanna
and animals of different species living together in a grasslands and woodlands, which promotes
certain environment i.e. plant and animal community. growth of grasses for grazers and also
Biosphere accelerates nutrient recycling.
This is part of the earth inhabited by living organisms. Note: all species play some role in their
The biosphere comprises of terrestrial and aquatic ecosystems and thus are important, therefore the
ecosystem. assertion that some species are more important
The biosphere is subdivided into bio-geographical than others remains controversial
regions each inhabited by destructive species of plants
and animals that are favored by unique conditions of such Niche /Ecological niche
areas. Bio-geographical regions are also subdivided into The role an organism plays in the habitat, and its
particular areas called ‘biomass/biomes’. interactions with other organisms i.e. the sum of
Biome refers to a large recognizable community formed all environmental factors that influence the
as a result of interaction between regional climates with growth, survival and reproduction of a species.
A niche is like the “profession” of an organism
Page 378 of 447

regional/biotic e.g. tropical rain forests, tropical savanna, Fundamental niche

desert, temperate region. The biome is divided into zones The physical conditions under which a species
e.g. forests biome forms the ground level and canopy. might live, in the absence of interactions with
The lake forms the limnetic zone, littoral zone, benthos other species
and profundal zone, each zone supporting a particular Realised niche
type of organisms. The role an organism plays in the habitat, and its
Ecotones are boundary zone between two biomes or interactions with other organisms in the presence
ecosystem where one merges into the other. of competition and other constraining factors i.e.
is the set of conditions under which an organism
exists in nature.
1.0 COMPONENTS OF THE ENVIRONMENT (structure of the ecosystem)

It consists of:
Biotic component: Abiotic factors (density independent factors)
These are the components which interact These are factors affecting the population regardless of the
between the different living organisms number of individuals within e.g. temperature changes, natural
e.g. competition, predation, symbiosis. catastrophes like foods, storms, volcanicity, fire, earth quake,
Also they are called density dependent drought, etc.
factors. Edaphic factors
The biotic part includes producers, The soil directly influences plant growth and indirectly the animal
primary consumers (first level population e.g. soil texture, soil pH, air, humus, salts, water, etc.
carnivores), tertiary consumers (higher Climatic factors (density dependent)
carnivores), decomposers & detritivores, They include light, water/rain fall, wind/air, relative humidity,
e.t.c. temperature
THE PHYSICAL ENVIRONMENT

The conditions in which organisms live i.e. physical (abiotic) environmet and biotic environment, determine
the distribution of the organism. The abiotic environmet embraces everything that is not associated directly
with the presence of other organisms.
(i) Temperature. Enzymes work within a narrow (iii) Water. The need for water depends on its
range of temperature Organisms have requirement and the animal’s ability to
physiological and / or behavioural adaptations to conserve water in adverse conditions.
avoid extremes of environmental temperature. (iv) Humidity. Humidity affects the rate of eater
(ii) Light. This is essential for all green plants and loss from the surface of an organism, which in
photosynthetic bacteria, and for all animals turn influences its ability to withstand drought.
dependent on plants.
(v) Wind and air currents. Wind is instrumental is dispersal of seeds and fruits. Only plants with strong
root systems and tough stems can live in exposed areas.
Edaphic factors are as a result of the climate and they also have an influence on the distribution of organisms
(vi) pH. Most plants are highly sensitive to changes in (x) Water currents, in streams and rivers.
pH and their distribution in soil and fresh water Animals which cannot actively swim live under
ponds depends on the pH. stones or in burrows and crevices in the bank.
(vii) Mineral salts and trace elements. This (xi) Wave action. Animals that live in the intertidal
affects plat distribution in soil and plants that can zone have special adaptations e.g. sessile habit
survive in soil deficient in a given mineral have of animals like sea anemones, burrowing by
special methods of obtaining that mineral. These shrimps and sand hoppers, and firm attachment
methods include carnivorous habits and harboring to rocks and general toughness of sea weeds
of nitrogen-fixing bacteria e.g. Fucus.
(viii) Salinity. Influences the distribution of estuarine (xii) Topography leads to differences in
animals, fresh water animals and marine animals. illumination, temperature and moisture all of
(ix) Background. Animals which depend on camouflage which influences the distribution of animals
for survive will be influenced by the appearances of and plants.
the background e.g. black and white peppered moth.
Page 379 of 447

How biotic factors affect the distribution and abundance of organisms

Of all living organisms, humans exert most influence on the distribution and survival of other species through
a multitude of activities like pollution, deforestation, farming, construction, e.t.c.
Other factors which affect the distribution and abundance of organisms include:
a) Predation. In predation, members of one
species (the predator) feed on all or part of a
living organism of another species (the prey).
Therefore, predators are only found where
there is prey e.g. herbivores are found where
there is suitable plant material, carnivorous
plants where there is suitable insects,
predators where there is suitable prey
b) Competition.
Intraspecific competition is the competition
between members of the same species for the
same resources, while Interspecific
competition is the competition between
members of two or more different species for
food, space, good hiding place, water,
sunlight, nesting sites or any other limited
resource.
Competition is very intense when there is significant overlap of niches, and in this case, one of the competing
species must (i) migrate to another area if possible (ii) shift its feeding habits or behaviour through natural
selection and evolution (iii) suffer a sharp population decline or (iv) become extinct in that area, otherwise
two species can never occupy exactly the same ecological niche i.e. the Gaussian competitive exclusion
principle
c) In instances where one organism uses another as host, their distribution is related e.g. in parasitism one
species (parasite) lives on or in another organism (host), in mutualism two species interact in ways that
benefit both e.g. nitrogen fixing bacteria and legumes, in commensalism one species neither harms nor
helps the other species much it interacts with e.g. There is no harm caused to large trees when epiphytic
plants like orchids attach on the branches to get support and be elevated to access sunlight and water
vapour in air.
d) The pollination and dispersal relationship between flowering plants and animals such as insects, birds
and bats may be highly elaborate and species-specific. This Co-evolution ensures that the distribution of
the plants and their pollinators or agents of dispersal is related e.g. arum lily flower is pollinated by dung
flies.
What is Co-evolution? i) Many features of flowering plants have evolved in relation
This is the long term evolutionary to the dispersal of the plant’s gametes by animals, especially
adjustment of two or more groups or insects. The animals have in turn evolved a number of
organisms that facilitate those special traits that enable them to obtain nectar.
organisms living with one another. ii) Grasses have evolved the ability to deposit silica in their
Examples: leaves and stems to reduce their risks of being grazed. In
turn, large herbivores have evolved complex molars with
enamel ridges for grinding up grass.
Page 380 of 447

e) Some species gain protection to avoid predation by mimicking (looking and acting like) other species that
are distasteful to the predator. This ensures that the distribution of the mimic and mimicked species is
related e.g. the non-poisonous viceroy butterfly mimics the poisonous monarch butterfly.
f) In the course of evolution preyed upon species have evolved deceptive looks to avoid predation e.g. span
worms have shapes that look like twigs, some insect pupae may look like tree thorns. Therefore such
animals are distributed where there are plants that ensure their survival.(Roberts pg 524 fig 32.5)
g) Antibiosis. This is the secretion of chemicals, by organisms, that may be repellant to members of the
same species or different species e.g. penicillium (a fungus) secretes antibiotics that inhibit bacterial
growth, ants release pheromones to warn off other members of a species in case of danger.
Fire as an ecological factor
Factors that control the effectiveness of fire
i) Kind and amount of fuel: ii) Weather conditions:
Tall grasses produce much fire more than heavily During the rainy season fires do not spread very
grazed areas. However, forest fires are more vigorous far and become wild but in a dry season fires are
than grass fires and they cause much more more wild, strong and destructive e.g the
destruction. This is due to the amount of fuel that Australia fires
takes time to be completely burned.
iii) Topography: iv) Frequency of burning:
Fires are fastest uphill and slowest downhill therefore Continued burning has a more permanent destructive
the effect of fire on soil is greatest on fires downhill effect. It does not only destroy vegetation cover but
rather than uphill. kills soil and fauna.
v) Direction of fire:
Back fire burning against the wind direction is more severe on the soil than forward fire burning with the
wind direction.
Advantages of fires Disadvantages of fire

 It breaks seed dormancy due to hard seed coat  It destroys the habitat of animals which may
leading to fast germination. cause extinction of some animals.
 It increases recycling of nutrients in an  It causes air pollution
ecosystem.  It destroys green plants which are producers of
 It is used in selective weeding. the community.
 It controls pests and diseases.  It destroys animals in the ecosystem.
 It improves on herbage (pasture, herbs) in an area.  It increases predation due to improved
 It improves on light penetration leading to rapid visibility.
under growth in the forest.  It leads to loss of some nutrients from the soil
 It improves on the visibility of the prey to by decomposition e.g. humus and nitrates.
predators by burning the vegetation cover down.
2.0 CONCEPT OF THE ECOSYSTEM

An ecosystem is a natural system consisting of ecological communities of living organisms (biological
communities) interacting with the abiotic (non-living) components of the environment to form a stable and
equilibrium system which is self-sustaining.
The dynamics of an ecosystem can be disturbed if there are changes in the biotic or/and abiotic environmet.
Examples of ecosystems are the pond, lake, ocean, grassland and tropical rainforest.
Functions taking place in the ecosystem
 Recycling of matter i.e. nitrogen cycle, carbon cycle, e.t.c.
 Energy flow/transfer from producers, consumers and decomposers.
 Food interactions/food chain and water.
Page 381 of 447

 Population control/dynamics/cybernetic of the population.

 Succession.
 Development and evolution of species of organisms (death due to competition and resistance due to
competition/survival for the fittest).
Types of ecosystems
There are two major types of ecosystem, namely;
1. Terrestrial/land ecosystem
2. Aquatic ecosystem
Each of the two can further be grouped into several habitats.
Aquatic ecosystems
Aquatic ecosystems support a great diversity of life forms. Water occupies 50% of the earth’s surface. Water
provides a more constant and protective environment than land (desiccation, less affected by sudden and drastic
changes in physical and chemical conditions, some change due to climatic or seasonal variation). It provides
support and dissolved oxygen and nutrients to aquatic organisms.
Aquatic ecosystems are classified as the following depending on the concentration of salts they contain;
 Fresh water ecosystem
 Marine ecosystems
 Estuarine ecosystem
Fresh water ecosystem
Fresh water habitats occupy a small portion of the earth’s surface as compared to marine and terrestrial habitats.
However, fresh water habitats are of great importance to man for the following reasons:
 Cheapest source of water for domestic and industrial use
 Provide the cheapest waste disposal systems
 Habour various animals
Fresh water habitats can be classified into:
1) Lotic (running water bodies) e.g. rivers and streams
2) Lentic (standing water bodies) e.g. pond, lake and swamps
Structure of a lake/pond (Lake Zonation)
The lake environment (lake zonation) is
generally classified on the basis of three
physical criteria: light penetration
(photic and aphotic zones), distance
from shore and water depth (littoral and
limnetic zones), and whether it is open
water (pelagic zone) or bottom (benthic
zone).
i) Littoral zone: shallow water region with high light penetration. It has the highest productivity due to high
carbon dioxide/oxygen and suitable temperatures.
ii) Limnetic zone: it’s the open water zone to the depth of effective light penetration. The community here
includes phytoplankton, floating insects and algae. Like littoral zone, productivity/net productivity is
Page 382 of 447

highest because of high effective light penetration, more dissolved gases, high temperatures at the surface
and turbulence due to the high air content/wind so high photosynthesis. Dissolved nitrogen is fixed by
nitrogen fixing bacteria and blue-green algae to make proteins. Dissolved carbon dioxide formed carbonic
acid which results in formation of H+, HCO3- and CO32-.
iii) Profundal zone: receives little or no light. Light penetration decreases with depth and also net productivity
decreases with depth.
iv) Benthic zone: this is the bottom most, receives no light at all, no dissolved gases, aerobic bacteria exists
so little productivity. The productivity is due to water currents which tend to mix the upper layers with
bottom layer and photosynthesis and chemosynthesis bacteria exist.
Ecological classification of fresh water organisms

Organisms in water can be classified depending on their life form Three zones are generally evident
which is based on their mode of life. The following terms are used: (refer to the figure on page 8);
1. Neuston: i) Littoral zone:
These are organisms resting or swimming on the surface of water. Such This is the shallow-water region
organisms may be supported by the surface film or cling to the surface with light penetration to the bottom.
film from beneath or swim in the upper waters. Examples include pond Such a zone is typically occupied by
skaters, air breathing diving beetles, water boat men, floating plants plants in natural ponds and lakes.
like duck weed, bladder work, e.t.c. ii) Limnetic zone:
2. Plankton (floating): This is the open water zone to the
This is a mass of floating small plants (phytoplankton) and animals depth of effective light penetration.
(zooplankton) whose movements and distribution are more or less The community in this zone is
dependent on currents. Their powers of locomotion are restricted to composed of plankton, nekton and
small vertical movements or to catching prey. Examples include arolia, sometimes Neuston. In shallow
Pistoia, water burg, tadpole, e.t.c. ponds, this zone is absent. The total
3. Nekton: illuminated depth including the
These are free-swimming organisms that can swim against water littoral and limnetic zone is referred
currents. Some of them are small e.g. swimming insects while others to as the euphotic zone.
are large e.g. bony fish, amphibians, e.t.c. iii) Profundal:
4. Benthos: This is the bottom and deep water
These are organisms attached or resting on the bottom or living in the area which is beyond the depth of
bottom sediments. Most of them feed on fresh water organisms in
effective light penetration. This
ponds and lakes. They may also be classified depending on the sub
habitat they occupy. zone is often absent in ponds.
Factors affecting productivity of the lake

 Temperature  Water current
 Nutrient availability  Pollution
 Salinity
Warm temperature provide optimum medium for aquatic organisms distribution as well as enzymes involved in
photosynthesis.
Cool temperature of bottom water inactivate enzyme and affect distribution of phytoplankton thus reduced
productivity.
Availability of nutrients in water due to decomposition of organic matter like sewage, dead organisms and
fertilizers washed off from farm and water would lead to algal blooming or eutrophication of phytoplanktons.
This would instead increase productivity since phytoplanktons are many.
Man’s activities that harm the environment. With the recent increase in the human population, there has been
over exploitation of natural resources.
Page 383 of 447

Limiting factors in fresh water ecosystems

Limiting factors restrict the distribution of living organisms hence preventing the colonization of otherwise
favourable environment. The most important limiting factors in fresh waters are:
1. Temperature:
Water has several unique thermal properties. Although temperature is less variable, it is a major limiting
factor. Aquatic organisms have narrow tolerance. Temperature changes produce characteristic patterns of
circulation which greatly influence aquatic life.
2. Light penetration:
Penetration of light is often limited by suspended materials (turbidity). This restricts the photosynthesis
zone. Plants cannot survive below the compensation level. Light penetration can be measured using a Secchi
disc. It consists of a white disc that is lowered from the surface until it just disappears from view. This
ranges to about 40 cm in very clear waters.
3. Water currents:
Currents determine the distribution of vital gases, salts and small organisms. Water current is a limiting
factor in fast flowing streams and on shores when it prevents colonization by weak swimming organisms.
4. Dissolved gasses:
Gases from the atmosphere dissolve in water at the surface. However, some gases are more soluble than the
others. E.g. oxygen is 30 times less abundant in water than in air.
This limits the distribution of living organisms. The diffusion of dissolved gases through deep layers of
water is very slow. In some places currents and wave action aid the diffusion, but in still waters, very little
oxygen is transferred to lower levels. Once the little oxygen available is used up by decomposers, the effects
may be disastrous to the whole community.
Dissolved nitrogen is used by nitrogen fixing bacteria and blue-green algae in the manufacture of proteins.
Effects of carbon dioxide are complex due to the formation of carbonic acid to form H + HCO3- and CO32-
ions. These combine with other dissolved substances in the water.
5. Dissolved salts:
Fresh water ecosystems show a considerable variation in salt content. This depends on the minerals present
in drainage water from the surrounding land mass and activities of living organisms. Deposition of nutrients
in water is known as eutrophication.
Nitrate and phosphate are the most limiting factors in fresh water ecosystems e.g. phosphorous is a limiting
factor because the ratio of P to other elements in organisms is greater than the ratio in the primary sources
of the biological elements. K, Ca, S and Mg may also act as limiting factors.
Biological classification of lakes

Biological classification of lake ecosystems depends on the circulation rates of inorganic plant nutrients in the
lake. Three major types are recognized:
1. Eutrophic lakes:
These are with waters relatively rich in plant nutrients.
Characteristics
 Have high surface area to volume ratio hence easy circulation
 They are relatively shallow with gentle slopping banks which can support wide belts of marginal
vegetation (wide littoral zone).
 Have relatively high phosphates and nitrates, i.e. they are very productive.
 Due to emergent and submerged plants plus a lot of phytoplanktons, upper layers are rich in oxygen.
The bottom layers are low in oxygen concentration since it is continuously being used for bacterial
decomposition e.g. Lake Kyoga.
2. Oligotrophic lakes:
These are with low plant nutrients and they are highly oxygenated.
Page 384 of 447

Characteristics
 They have low surface area to volume ratio, hence limited circulation.
 They are deep with steep rock sides.
 Waters are low in plant nutrient but highly oxygenated.
 Neither have extensive marginal vegetation nor organic bottom deposits which results in their low
productivity e.g. Lake Tanganyika.
3. Dystrophic lakes
These have brown water where the bottom deposits of such lakes consist of unrotten organic matter which
accumulates as heat. Productivity of such lakes is very low.
The pond ecosystem

The pond ecosystem is complex and is affected by several environmental conditions. The living organism and
the nonliving environment are inseparable and the following can be recognized:
i) Abiotic substances:
These include basic inorganic and organic compounds e.g. water, CO2, O2, Ca, N, P, soil, etc. A small portion
of the vital nutrients is in soil and available to organisms but much larger portion is held in the bottom sediments
and in the organisms themselves. The rate of release of nutrients from the solids, solar input and other
environmental factors determine the productivity of the entire ecosystem.
ii) Producer organisms:
There are two major types only; Rooted or large floating plants growing in shallow water e.g. papyrus and
Phytoplankton distributed throughout the pond as deep as limnetic zone.
Note: in deep ponds and lakes, phytoplankton is much more important than rooted vegetable in the production
of the basic food from the ecosystem (algal blooms)
iii) Macro organisms:
These include animals like insect larvae, crustacea, fish, etc. primary consumers feed on plants or plant remains
e.g. zooplankton and benthos (molasses).
Secondary consumers e.g. predaceous insects and fish feed on primary or secondary consumers.
Detritivores e.g. worms, larvae and rotifers consume organic matter from upper layers.
iv) Saprotrophic organisms:
Aquatic bacteria, flagellates and fungi are distributed throughout the ponds, but are abundant at the bottom
where plant and animal organic matter accumulates.
Dead organisms are rapidly broken down by detritus feeding organisms and microorganisms and their nutrients
are released for re-use.
How thermal stratification occurs in lakes
In warm weather the surface of a lake is heated by the sun. The warmed surface becomes less dense, and so
remains at the surface, floating on the colder water beneath. The surface continues to gain heat from the sun,
while the bottom water remains cold. If this surface heating continues for some days, without storm winds to
stir the lake, a marked temperature difference can develop between the top and bottom water and the
following compartments are recognised:
a) Epilimnion: upper warmer water, usually well oxygenated.
b) Metalimnion (thermocline): middle portion between Epilimnion and Hypolimnion where the rate of
temperature change with depth is rapid.
c) Hypolimnion: the deepest portion, with denser and cooler water, usually with low oxygen concentration.
Seasonal changes that occur in temperate lake

Winter Late spring/early summer Late summer Autumn
During the winter, In the spring, sunshine and There is maximum As the lake surface
water is evenly warm air temperature melt the thickness of epilimnion cools by evaporation,
mixed and tends ice cover and bring the upper and maximum convection and
to have the same layers of the lake to the same temperature difference radiation, the stability of
temperature and temperature as the lower, about between epilimnion and the stratified system
chemical 40C, enabling strong winds to hypolimnion. reduces.
composition at all mix the waters of the lake
depths. completely (spring overturn)
Page 385 of 447

If the lake freezes, The lake stratifies with a warm Epilimnion may be Strong winds mix the
ice of 00C floats epilimnion floating over a cold depleted of nutrients and lake water (fall
over warmer hypolimnion and a thermocline Hypolimnion may be overturn), causing
water of 40C. between. depleted of oxygen. uniform temperatures
and chemical
composition throughout
the water column
The nutrient and Phytoplankton bloom in the In very fertile lakes the Bottom mud and water
oxygen surface water. hypolimnion may are resupplied with
concentrations of Nutrient concentrations begin become anoxic and the oxygen.
the water are high, to fall in the epilimnion and sediments of the
but oxygen can be oxygen concentrations begin to mud/water interface may
depleted under ice fall in the hypolimnion. be chemically reduced.
in shallow lakes
Sediment at the Sediment at the mud/water Waters of the Surface water is
mud/water interface is still oxidised. Hypolimnion may be resupplied with
interface usually nutrient-rich both from nutrients, which may
will be oxidized. solution of particles result in blooming of
settling down from the phytoplankton at the
surface water and from surface.
solution of nutrients from
reduced bottom mud.
B.O.D (Biological oxygen demand)
Mass of oxygen consumed by microorganisms in a sample of water in a given time - usually measured as the
mass (in mg) of oxygen used by 1dm3 of water stored in darkness at 200C for 5 days.
B.O.D indicates the oxygen not available to more advanced organisms. Therefore a high B.O.D indicates
anaerobic conditions (low oxygen availability).
 Productivity then falls during the summer,
despite the fact that this is the warmest time
of the year with long days. This is because
the short-lived algae of the spring bloom
are not replaced and the standing crop is
lowered, ending the bloom.
 But productivity goes up again with the
coming of autumn, despite lowered
temperatures. His is because of the fall
overturn, which brings to the surface
nutrient-charged water formerly held in the
hypolimnion
Explanation for the observed changes
 Productivity is low in winter due to short days and low temperatures.
 Productivity is high in spring due to proliferation of algae causing blooming because of the warming that
precedes summer.
 But the lake then stratifies and the process of exporting nutrients to the hypolimnion via precipitating
organic matter proceeds to lower nutrient concentrations in the Epilimnion.
Oxygen and carbondioxide availability in water

Oxygen enters an aquatic system from the atmosphere and through photosynthesis by aquatic producers, and
is removed by aerobic respiration of plants, animals and decomposers.
Carbondioxide enters an aquatic system from the atmosphere and through aerobic respiration by plants,
animals and decomposers, and is removed by photosynthesising plants. Some dissolved CO2 forms carbonate
ions (CO32-), which are stored as calcium carbonate for long periods in sediments, minerals and shells and
skeletons of aquatic animals.
Page 386 of 447

Stratification in tropical lakes

In tropical lakes, the range of temperature from top to bottom of the columns of water is not great because the
density of water changes more rapidly at higher temperatures.
The hypolimnion in a lowland tropical lake is nearly always without oxygen due to;
i) high productivity caused by high temperature
ii) biological decomposition of the large inputs of organic matter from productive tropical ecosystems on
the banks.
The typical jungle lake therefore, is turbid, murky and opaque with biological activity.
Terrestrial ecosystems
Regional climates interact with regional biota and substrate to produce large recognizable community units
called biomass. A biome is identical with a major ‘plant formation’ but it is a total community unit in which
both animals and plants are considered. The six major biomass of Africa include:
 Tropical rain forest  Sahel region (semi-desert)
 Tropical savanna and grass land  Mountain forests
 Desert  Temperate region
The above form the major terrestrial ecosystems.
Tropical rain forest ecosystem

This is characterized by high temperatures of 250C and 350C and a high monthly rain fall distributed over 10
months of the year i.e. 200 and 400 cm3 of rain fall annually. They are dominated by broad leaved evergreen
trees which occupy low altitude zones near the equator (amazon, Congo, Malaysia, etc.).
Seasonal changes in breeding and other activities of plants and animals in a tropical rain forest are largely related
to variations in rain fall and to a certain extent temperature.
Forest communities are well structured and contain specific plant and animal populations that interact in a
complex fashion.
Trees in the forest form three layers (stratification):
1. Emergent layer: 2. Canopy layer:
This consists of scattered, very tall This forms a continuous evergreen carpet 50-80m tall. The
emergent trees (80-100m) that project crowns of such trees are small compared to the emergent and
above the general level. They have buttresses are narrow.
wide spread, umbrella shaped crowns 3. Undertone layer:
and huge buttresses. Examples are the This includes relatively short trees 1-1, 20-40m tall and young
Chlorophora excelsa (Mvule), trees of the emergent and canopy layers. Ferns e.g. platycerium
mahogany, mbizia, e.t.c. spp is common as an epiphyte high on trees. Other epiphytic
plants include figs and orchids.
Ground layer:
 This includes shrubs, herbs, lianas, shade loving plants with broad leaves and thallophytes e.g.
lichen, mosses, liverworts and shade loving animals.
 A much large proportion of animals live in the upper layers of the vegetation. These include birds,
mammals, amphibians and others. Some animals are ground dwellers e.g. ants, butterflies, moths,
snakes and other reptiles.
 Tropical rain forests are rich in flora and fauna species e.g. a six square mile area can contain
20,000 species of insects. A tropical rain forest is the only major vegetation type which does not
burn i.e. fire is not an ecological factor.
 Variation in environmental factors (temperature, light, moisture) caused by the stratifications
creates micro-habitat conditions.
Page 387 of 447

P530 (2020) ECOLOGY By Nakapanka Jude Mayanja 0704716641
 The ground layer receives light of low intensity approximately 10% of the total value received by
the emergent. Ground layer plants are therefore adapted to such conditions.
 The shade effect of the canopy layer cuts off the sun’s rays, thus relatively lower temperatures are
experienced in the lower layers.
 Moisture is influenced by temperature as it increases rates of evaporation and transpiration.
Underground plants are in a region of lower rates of evaporation and transpiration than those above
them.
 Crowded leaves on the upper layer of tree branches act as wind breaks so the interior of the forest
is not windy. The relative humidity inside is relatively constant to the upper layers.
Adaptations
 Emergent and canopy layer trees prevent excessive transpiration by having leathery surface and adequate
deposits of cuticle.
 Plants of the undergrowth have large thin leaves.
 Animals on the ground use the soil for protection against extreme condition.
 Arboreal animals possess special features that enable them to climb e.g. specialized feet in squirrels and the
monkeys’ prehensile tails.
 Some animals use camouflage for protection against predators.
Grass land ecosystem

 Tropical savanna (grassland with scattered trees or clumps of trees) forms the grassland ecosystem
in Africa. Grasslands are characterized by hot weather with a moderate temperature range. Rainfall
is about 120cm3 per annum which falls in one period, followed by a long period of drought.
 Grassland ecosystems are dominated by grasses such as guinea grass, elephant grass, spear grass,
and palms.
 Animals include a variety of numerous hooved mammals e.g. antelopes, elephants, zebra, giraffes
which graze or browse on the vegetation. Others include predators like lions, cheetahs, scavengers
like hyenas, jackals and culture insects most abundant during the dry season which include
grasshoppers, termites, ants and locusts. Reptiles are abundant during the dry season and these
include snakes, lizards, chameleons, tortoise, etc.
 In the savanna grassland ecosystem, seasons are determined by rainfall. Other two factors include
herbivore and fire. Trees and grass present must be resistant to drought and fire. This explains
why the number of species in the vegetation is not large.
 Grazing mammals are important in determining the flora composition of the community. Some
species of grasses and other plants are more sensitive to grazing pressure than others.
 During the dry season, fire is a major ecological factor. It destroys non-resistant plant species like
grasses but it also stimulates those with underground parts to grow. Trees develop a dense and
shady canopy and grasses grow to high heights during the short rainy season.
Adaptations
 Savanna trees grow long tap roots and develop thick barks which enable them to survive the long dry season
and resists fires. They have umbrella shaped canopies which shade the ground and limit loss of soil moisture.
The leaves have thick surfaces which minimize the loss of water by transpiration.
 Grasses have durable roots which remain underground when the tops have been burnt away after a fire.
They sprout again with the onset of the first rains in the following year.
 Animals usually migrate and hibernate.
Page 388 of 447

ECOLOGICAL SUCCESSIONS
 Ecological succession is a gradual change in community composition from the initial colonization
of an area/habitat to establishing a relatively stable community. Or
 Ecological succession is a fairly orderly process of changes of communities in a region or an area.
It involves replacement in the course of time of the dominant species within a given area by other
species. Or
 It’s the establishment of a sequence of different communities in a particular area over a period of
time.
 A community is a group of interacting populations living in a given area and represents the living
part of an ecosystem. Its functions are energy flow and cycling of nutrients. The structure of a
community is always built up over a period of time until a stable climax community is established.
Ecosystems are dynamic, constantly changing in response to both physical and biological factors.
 Communities succeed each other in an orderly sequence in which such successive stage i.e. it’s
dependent on the one that precede it.
 Succession progresses gradually from a small number of colonizing species known as seres or
seral stages (i.e. communities that replace one another in a given area are called seres. These
temporary consists the seral stages/seral communities).
 Each sere has its own community of organisms until the terminal relatively stable and final stage
community called climax community.
 The climax community comprises of dominant or several co-dominant species which refers to
species with the greatest collective biomass/productivity and physical size of individuals in a given
area after some time (years).
 At climax community the net productivity/biomass tends to remain constant but dependent on
species number and population size.
Types of succession
1. Primary succession 2. Secondary succession
Primary succession
It occurs during the colonization of uninhabited area or where no new life previously existed e.g. volcanic
islands, bare rocks, sand dunes, lake shore, river banks, bare pavements, bare soil surface, dry area devoid of
vegetation, ponds, swamps.
An example of succession on a rock:
(i) On a bare rock/bare pavement several seral stages are identified, lichens (algae and fungi) are the
pioneer community to be established first. They are able to utilize the low moisture, nutrients, and
ions on rock surface. The hyphae of the fungi penetrate the tiny pores on the rock providing a firm
attachment and absorbing inorganic nutrients from the rock while the algae provide food since it
is photosynthetic. Bacteria and fungi also aided by weathering loosen rock surface by the process
of rock decay. Their decaying bodies (algae/fungi and bacteria) add humus to the loosen rocks to
form sedimentary soils.
(ii) The loosen rock is now able to be inhabitable by the drought resistance second colonizers to support
plant life of rhizoids on humus/traps the tiny organic and inorganic debris and water/moisture and
further loosen the rock surfaces. Also death of some moss plants add nutrients to the soil due to
decay by saprophytic organisms, more soil is formed to support the germination of seeds/grass of
the large colonizing angiosperms/vascular plants. Small animals like insects, molasses, earth
worms, and rodents break down rocks. The herb seeds germinate to replace proceeding growths
Page 389 of 447

and they in turn provide suitable conditions for large woody shrubs to begin to grow in the newly
fertile formed soils.
(iii)Eventually as a thicker layer of soils develops, shrubs get replaced by deciduous trees with deeper
roots that penetrate crevices/cracks. The seeds of the trees become germinated/grow in the created
suitable conditions by their parents’ previous plants and animal colonizers and the mature forest
community develops which becomes self-sustaining.
Note:
As the number of tree species increase, there is increased modification of the micro climates in the habitat
e.g. shade increases, making light demanding shrubs to disappear and are replaced by light tolerant species
of trees.The tolerant species of trees finally form the climax vegetation. The savanna grass land and forest
ecosystems are the dominant terrestrial ecosystems.
Summary:
Pioneer species animals (mosses) herbaceous perennials (herbs) shrubs tree forest
Secondary succession
This is the establishment of communities on areas/habitats previously occupied by developed communities but
has been disrupted in some ways such as burnt farm, playground fire cleared, forests destroyed by natural
disaster like hurricanes, drought, volcanic eruption, floods, human activities like fires, cultivation, fire,
overgrazing.
Such areas have seeds/spores, organs of vegetative reproduction/propagation rhizomes and abundant nutrients
in soil to support life. The successions are called secondary seres. E.g. fires from lightening burn plants stable
community living a bare ground. The ground layer plants are killed, the heat destroys hollow roots/seeds and
animals burnt in soil.
Often the first green plants on a burnt wood ash are the mosses that form an extensive green carpet. Within
carpet the seeds of herbaceous and woody plants germinate. A new herbaceous layer grows, forming the grasses
and followed by shrubs and trees. Each dominant plant community has associated dominant animal population
within it. The climax community persists for a long time until when factors that favour that favour invasion of
better adapted forms of organisms set in.
The climax vegetation makes efficient use of resources of the community ensuring indefinite self-sufficiency
i.e. a community maintaining itself.
A similar secondary succession takes a short time to reach climax community. This is because the soil is already
formed and supports growth of a wide range of plants immediately.
Note:
Both primary and secondary succession is affected by the animal (fauna) and flora (plants) of the surrounding
environment/areas through dispersal and migration
Page 390 of 447

Characteristics of a succession process Factors affecting the number and diversity of

 A pioneer community which is quite simple in species reaching an area/colonization
biomass content and composition.  Geographical barriers like mountain ranges/
 A series of intermediate stages/seres river/lake/rift valley.
 Increasing biomass/productivity and species  Ecological barriers like unfavourable habitats
biodiversity separating areas of favourable habitats.
 Ends into a stable community which is in  Distance over which dispersal must operate
equilibrium with its environment called the  Size and nature of invasion areas
climax community.
.
Characteristics of the stages of primary succession
Early succession Late succession
 Species grow very close to the ground and have low  Plants are of large size and complex.
biomass.  Species diversity is high.
 Species have short lifespan.  The community is a mixture of
 Species are simple and small sized. producers, consumers and
 Species diversity is very low. decomposers.
 The community is open i.e. allows space for further  Biomass is high.
colonizers.  Net productivity is low.
 Species may show symbiotic relationships to aid their  Community takes a long time to
establishment. establish.
 Species are poor competitors and hence get replaced by  The climax community is often
higher, more demanding plants like grasses, shrubs and determined by one dominant species.
eventually trees.  There is increased soil depth and
 Species can establish large populations quickly under harsh nutrients.
conditions like lack of moisture and soil nutrients, hot and  Interspecific competition is very high.
cold temperature extremes.  There is little space for new species.
 The community is mostly composed of producers and a few  The climax community is stable and
decomposers. is in equilibrium with its
 Net productivity is high. environment.
 Feeding relationships are simple, mostly herbivores feeding  Feeding relationships are complex,
on plant with few decomposers. dominated by decomposers.
Productivity and biomass

Biomass refers to the dry weight of organism(s) at a trophic level. The biomass at the time of sampling or given
moment in time is called the standing biomass or standing crop biomass.
Ecosystem productivity increases until climax community but there is a decrease in gross productivity
associated with the climax community. This could be due to an accumulation of nutrients in the increasing
standing crop biomass may lead to a reduction in nutrient recycling. Reduction in vigour as the average age of
the individuals in the community increases to a constant point would cause a reduction in productivity.
Climax communities lead to maximum accumulation of biomass. An upper limit of biomass is reached when
total respiratory losses from the system are almost equal gross primary productivity i.e. the ratio of productivity
to respiration = 1.
During succession, more and more of the available nutrients become locked up in the biomass of the community
with subsequent decrease in nutrients in the abiotic component of the ecosystem (such as soil and water). The
Amount of detritus produced also increases and detritus feeders take over from grazers as the main primary
consumers. Appropriate changes food webs occur and detritus becomes the main source of nutrients.
Page 391 of 447

The process or trend of succession on a bare rock or bare pavement or bare soil surface or dry area is devoid of
vegetation. On bare rock the first organisms to colonize the area are lichen/bacteria/fungi aided by weathering
loosens rock by the process of rock decay. Their dead bodies add humus to the loosen rock enabling algae
growth. The invertebrates invade and feed on them. When these organisms die and decompose and their
metabolic wastes cause rock weathering leading to soil formation. Mosses/liverwort would then come in
including insects that feed on them. Other plants with better roots like ferns and animals like earth worms,
molluscs, amphibians, birds, reptiles and mammals comes in. Evergreen plants with deeper roots like vascular
grasses, shrubs and trees and then come in animals which finally form a climax community.
Note:
The biomass of the climax community is higher also in cleared forest than in developing community, e.g. algae
growing on a concrete. This is so because a formerly cleared forest has the soil substratum rich in organic
matter/nutrients on which woody species can grow very fast accumulating organic matter. However on a
rock/concrete/non-decomposable blocks little or no nutrients are obtained slow growth occurs so less
accumulation of organic matter or biomass occurs. The algae are small in size contributing less organic matter.
Therefore trees have a higher biomass since they accumulate it over a long time period compared to the small
algae.
Energy flow through an ecosystem
The first thermodynamic law states that energy cannot be created or destroyed but can be transferred from
one form to another. ]
The second thermodynamic law states that when energy is transformed from one form to another, there is
loss of energy through the release of heat.
The fate of solar energy reaching the earth
Because of the small size, the earth receives only about one-billionth of the sun’s output of energy, much of
the energy being either reflected away or absorbed by chemicals in the atmosphere.
 Most of the energy that reaches the atmosphere is: (i) visible light (ii) infrared radiation-heat (iii) ultra
violet radiation that not absorbed by ozone.
 The incoming energy (i) warms the troposphere and land (ii) evaporates water and cycles it through the
biosphere (iii) generates winds (iv) is captured by green plants, algae and bacteria to fuel photosynthesis
and make the organic compounds that most forms of life need to survive.
Factors that sustains life on earth How the sun sustains life on
Three interconnected factors sustain life on earth: the earth
a) The one-way flow of high quality energy from the sun:  Lights and warms the planet.
i) Through materials and living organisms in their interactions  Supports photosynthesis in
ii) Into the environment as low quality energy – mostly heat plants and some bacteria.
dispersed into the air or water molecules at low temperature.  Powers the cycling of matter.
iii) Eventually back into space as heat.  Drives the climate and
b) The cycling of matter through parts of the biosphere weather systems that
c) Gravity, which: distribute heat and fresh
i) Allows the planet to hold on to its atmosphere. water over the earth’s
ii) Causes the downward movement of chemicals in the matter surface
cycles.
Food chain
This is a simplified sequence illustrating the flow of energy from one organism to another in a community.
Grazing food chains start with green plants while detritus food chains begin with dead organic matter e.g. in
temperate forests
Primary primary secondary tertiary quaternary
producer consumer consumer consumer consumer
(1st trophic (2nd trophic (3rd trophic (4th trophic (5th trophic
level) level) level) level) level)
Page 392 of 447

Example of food chain:

Algae mosquito larvae Tilapia Nile perch Human being
Grass rabbit snake Eagle
Detritivore food chain A food web is shown below;

Leaf litter earthworm chicken
Hawk
Dead animal maggots frog snake
Food web
This is a complex nutritional relationship showing Chameleon grasshopper praying mantis
alternative sources of food for each organism in a
food chain i.e. a complex network of food chains
linked to one another. Caterpillar Butterfly
Plant
An outline of the process of energy flow through the ecosystem

The primary source of energy is the sun. Of the energy received by the earth, less than 0.1% is fixed by green
plants as chemical energy in biomass in organic substances during photosynthesis (Gross Primary
Production); the rest is reflected by the atmosphere, heats the earth’s surface or causes evaporation of water.
Part of gross primary production is used by green plants for aerobic respiration while what remains (Net
Primary Production) is used as food by primary consumers, during which only about 5 – 10% of the energy
is transferred from producer to primary consumer (a loss of 90 – 95% occurs) because much of plant biomass
is indigestible to herbivores. The energy obtained by herbivores through feeding on producers is less than
what producers get from the sun because some is lost in egestion, excretion, and not all parts are eaten.
When carnivores eat herbivores still the energy they obtain is less than what herbivores obtain from feeding
on producers, and the trend is maintained even as top carnivores feed on secondary consumers.
Therefore, with each energy transfer some usable energy is degraded and lost to the environment as low
quality heat, thus the energy available to each successive trophic level declines, and the more trophic levels in
a food chain or web, the greater the cumulative loss of energy during its flow through the various feeding
levels. This limits the number of feeding levels to 3 or 4, or very rarely 5 or 6.
Note: carnivores are more efficient, transferring about 20% of the energy available from their prey into their
own bodies. It is the relative inefficiency of energy transfer between trophic levels that explains why;
 Most food chains have only four or five trophic levels
Page 393 of 447

 The biomass of organisms is less at higher trophic levels

 The total energy stored is less at each trophic level as one moves up a food chain
Energy budgets
An energy budget shows the percentage allocation of energy consumed by an individual organism to the
various processes in the body such as respiration, growth and reproduction.
Terms associated with energy budgets
Term (Definition and explanation) Extra facts
Gross primary productivity (GPP) GPP is greatest: (i) in shallow waters near continents
 It is the rate at which producers convert solar (ii) along coral reefs where abundant light, heat and
energy into chemical energy stored in organic nutrients stimulate the growth of algae. (iii) Where
substances. upwelling currents bring nitrogen and phosphorus from
 It is the total amount of energy fixed by the ocean bottom to the surface.
producers per unit area of photosynthetic GPP is lowest in: (i) deserts due to low precipitation
surface per unit time. (rainfall, hail, snow, sleet) and intense heat (ii) the open
 Productivity may be expressed as units of ocean due to lack of nutrients and sunlight except near
energy (e.g. kJm-2yr-1 or kCal m-2yr-1), or units the surface.
of mass (e.g. kg m-2yr-1)
Net primary productivity (NPP) NPP most productive ecosystems are: (i) Estuaries (ii)
 It is the rate at which energy for use by Swamps and marshes (iii) Tropical rainforests
heterotrophs or consumers is stored in new NPP least productive ecosystems are: (i) Open ocean
organic substances. (ii) Tundra – arctic and alpine grasslands (iii) Desert.
 NPP is the energy that remains to be used by Despite its low net productivity, the open ocean
consumers after producers have used part of produces more of the earth’s NPP per year than any
GPP for their own respiration. other ecosystem because of its large size
NPP = GPP – (respiration + metabolism)
Secondary production This the energy remaining in heterotrophs available for
It is the rate which energy is used to make new production (growth, repair and reproduction) after
biomass in consumers losses through egestion, excretion and respiration
Biomass Biomass is expressed as g/m2
It is the dry weight of all organic matter
contained in organisms per unit area of ground or
water
Standing biomass (Standing crop biomass)
It is the dry weight of all organic matter contained in organisms per unit area of ground or water at a given
moment in time
Trophic efficiency (Ecological efficiency) Trophic efficiencies range from less than 1% (e.g.
It is the percentage of energy at one trophic level herbivores eating plant material) to over 40% (e.g.
that is converted into organic substances at the zooplanktons feeding on phytoplanktons)
next trophic level
NOTE:
a) Of the energy received by the earth, averagely less than 3% is fixed by green plants.
b) Energy transfer from producer to primary consumer is typically in the order of 5 – 10% of NPP (a loss of
90 – 95% occurs) because:
i) Much of plant biomass (NPP) is indigestible to herbivores e.g. no animal enzymes can digest lignin
and cellulose
ii) An individual herbivore may not eat much of the plant biomass e.g. roots may be inaccessible.
c) Energy transfer from primary consumers (herbivore) to secondary consumers (carnivores) is typically 10
– 20% of herbivore mass (a loss of 80 – 90% occurs).
This more efficient than in (b) above because:
i) Animal tissue is more digestible than plant tissue
ii) Animal tissue has a higher energy value
iii) Carnivores may be extremely specialized for prey consumption.
But still less than 100% because:
i) Some animal tissue e.g. bones, hooves, hides is not readily digestible
Page 394 of 447

ii) Feeding is not 100% efficient – much digestible material e.g. blood and food fragments may be lost to
the environment.
d) The number of trophic
levels (feeding levels)
rarely exceeds five because:
The more trophic levels in a
food chain or web, the
greater the cumulative loss
of usable energy as it flows
through the various trophic
levels, leaving very little
energy to support organisms
feeding at the high trophic
levels.
This explains why:
i) There are so few top
carnivores e.g. eagles,
hawks, tigers, white
sharks
ii) Such species are first to
suffer when the systems
that support them are
disrupted
iii) These species are so
vulnerable to
extinction.
The longest food chains can
only be supported by an
enormous producer biomass
e.g. in oceans.
ECOLOGICAL PYRAMIDS
These are histograms that provide information about trophic levels in ecosystems.
Pyramid of numbers Disadvantages:
It is a histogramatic representation of the i) Drawing the pyramid accurately to scale may be very
numbers of different organisms at each difficult where the range of numbers is large e.g. a
trophic level in an ecosystem at any one million grass plants may only support a single top
time. carnivore.
Note: ii) Pyramids may be inverted; particularly if the producer
i) The number of organisms at any trophic is very large e.g. an oak tree or parasites feed on the
level is represented by the length (or consumers e.g. fleas on a dog (B & C)
area) of a rectangle. iii) The trophic level of an organism may be difficult to
ii) Generally, as the pyramid is ascended, ascertain.
the number of organisms decreases, but iv) The young forms of a species may have a different diet
the size of each individual increases. from adults, yet they are considered together.
[Toole pg. 338 fig. 17.3a] [Toole pg. 338 fig. 17.3b]
Page 395 of 447

Pyramid of biomass [Pickering p. 54]

It is a histogramatic representation of the biomass
(number of individuals x mass of each individual) at
each trophic level in an ecosystem at any one time.
 Biomass is expressed as g/m2
Advantage:
It eliminates scale problems encountered when
constructing a pyramid of numbers.
Disadvantages: The sampling situation at any one time deals with
i) Biomass may change with time standing crop biomass (amount of material present
ii) It does not show productivity at a particular instant) rather than with productivity
iii) Problems encountered in determining the (the capacity of any trophic level to produce
biomass since it involves killing and drying the biomass over a period of time) e.g. zooplankton in a
organisms marine ecosystem.
iv) An inverted pyramid of biomass can occur if the producer level includes organisms with a high
turnover rate (rapid reproduction) so that they have a high productivity over a period of time e.g. in
open water of oceans, the zooplankton biomass can exceed that of phytoplankton because the former
eat the latter almost as fast as they are produced, so the producer population is never very large
Pyramid of energy flow Advantage:
It is a histogramatic representation of the flow of energy i) It compares productivity because a time
through each level of an ecosystem during a fixed time factor is incorporated.
period (usually one year, to account for seasonal ii) Biomass may not be equivalent to energy
effects). value, e.g. 1g of fat has many more kJ than
 Energy values may be expressed variously e.g. kJm- 1g of cellulose or lignin.
2 -1 -2 -1 iii) No inverted pyramids are obtained because
yr or kCal m yr
[Pickering p. 54] of the automatic degradation of energy
Note: quality.
i) Because such pyramids represent energy flows, not iv) The solar input of energy may be included
energy storage, they should not be called pyramids of as an extra rectangle at the base.
energy (a common error in some books) Disadvantage:
ii) Energy flow pyramids explain why the earth can Obtaining the necessary data required in
support more people if they eat at lower trophic levels constructing pyramids of energy flow is
by consuming grains, vegetables and fruits directly difficult.
rather than passing such crops through another trophic [Soper Fig. 10.9 pg307]
level and eating grain eaters.
BIOLOGICAL AND GEOCHEMICAL CYCLING (NUTRIENT CYCLING)

This is the process by which Reservoir A Reservoir B
chemical compounds of a Organic materials Fossilization Organic materials
particular element that available as nutrients unavailable as nutrients
constitutes living matter are Coal oil and peat
Living organisms, detritus
transferred between living
organisms (biotic phase) and Respiration
non-living environment Assimilation Burning of fossils fuels
decomposition
(abiotic phase). Because photosynthesis
excretion
nutrient cycles involve both
biotic and abiotic components, Erosion and Reservoir D
they are called biogeochemical Reservoir C
Inorganic materials weathering Inorganic materials
cycles. Page 396 of 447 unavailable as nutrients
A general model of nutrient available as nutrients
Formation of
cycling is shown on the right Atmosphere, soil, sedimentary rocks Minerals in rocks
water
BS page 301 fig 10.3 These cycles driven directly or

indirectly by incoming solar energy
and gravity include the carbon,
nitrogen, phosphorus, oxygen, Sulphur
and hydrological (water) cycles, but a
few have been considered below.
The earth’s chemical cycles also
connect past, present and future forms
of life. Just imagine:
i) Some of the carbon atoms in your
skin may once have been part of a leaf.
ii) Some of the oxygen molecules you
just inhaled may have been inhaled by
a person at Jesus’ time!
1. Hydrological (water) cycle

The water cycle is powered by energy from the sun and by gravity, and it involves the following main
processes:
a) Evaporation (conversion of water into water vapour)
b) Transpiration (evaporation from leaves of the water extracted from soil by roots and transported
throughout the plant)
c) Condensation (conversion of water vapour into droplets of liquid water)
d) Precipitation (rain, hail, snow and sleet)
e) Infiltration (movement of water into soil)
f) Percolation (downward flow of water through soil and permeable rocks to ground storage areas called
aquifers)
g) Runoff (downslope surface movement back to the sea to resume the cycle)
2. Carbon cycle
 The carbon cycle is based on carbondioxide gas, which makes up 0.036% of the volume of the
troposphere and is also dissolved in water.
 Carbon fixation involves the reduction of carbondioxide to large organic molecules during photosynthesis
and chemosynthesis.
 During aerobic respiration, the carbon in glucose and other complex organic compounds is converted to
carbondioxide into the atmosphere or dissolves in water.
 Over millions of years, buried deposits of dead plant debris and bacteria are compressed between layers of
sediment to form the carbon-containing fossil fuels e.g. coal, oil and natural gas, which when burnt release
carbondioxide into air. In natural ecosystems the carbon in the carbon-containing fuels would be lost from
the cycle. It should be noted that the production of fossil fuels is a very slow process and there is a limit to
the rate at which man draw upon them. The so-called energy crisis results from this obvious ecological
fact.
 In aquatic ecosystems, carbondioxide may
i) remain dissolved
ii) be utilised in photosynthesis
iii) react with water to form carbonate ions (CO32-) and bicarbonate ions (HCO3-).
 As water warms, more dissolved carbondioxide returns to the atmosphere.
 In marine ecosystems, some organisms take up dissolved carbondioxide molecules, carbonate ions (CO32-)
and bicarbonate ions (HCO3-) and these ions react with calcium ions (Ca2+) to form calcium carbonate
(CaCO3) to build their shells and skeletons.
 When the animals with calcium in shells and skeletons die and drift into deep bottom sediments of oceans,
immense pressure causes limestone and chalk to form after a very long period of time.
 Weathering processes release a small percentage of carbondioxide from limestone into the atmosphere
Page 397 of 447

[Roberts pg 532 fig 32.12]
How human activities affect the carbon cycle

i) Clearing of trees and other plants that absorb CO2 through photosynthesis results in its increased
concentration.
ii) Burning of fossil fuels and wood adds large amounts of CO2 into the troposphere.
3. Nitrogen cycle
 Nitrogen is the atmosphere’s most abundant element, with chemically unreactive nitrogen gas making
up 78% of the volume of the troposphere. However, nitrogen gas cannot be absorbed and metabolized
directly by multicellular plants and animals.
 Atmospheric electrical discharges in the form of lightning causes nitrogen and oxygen in the
atmosphere to react and produce oxides of nitrogen, which dissolve in rainwater and fall to the ground
as weak acidic solutions e.g. nitric acid.
 Nitrogen fixation occurs when the nitrogen in soil is reduced to ammonium ions, catalysed by
nitrogen-fixing bacteria which may be free-living e.g. Azotobacter, symbiotic in root nodules e.g.
Rhizobium or cyanobacteria e.g. Nostoc or by nitrogen-fixing blue-green algae in water bodies.
 Nitrification occurs when ammonium compounds in soil are converted first to nitrite ions by
Nitrosomonas bacteria and later to nitrate ions by Nitrobacter bacteria.
 Ammonification (putrefaction) occurs when decomposers e.g. some bacteria and fungi convert
nitrogen-rich organic compounds, wastes like urea and dead bodies of organisms into ammonia and
ammonium ion-containing salts.
 Assimilation occurs when inorganic ammonia, ammonium and nitrate ions are absorbed by plant roots
to make DNA, amino acids and protein.
Page 398 of 447

 Denitrification occurs when mostly anaerobic bacteria e.g. Pseudomonas denitrificans and
Thiobacillus denitrificans in water logged soil and deep in ocean, lake and swamp bottoms convert
ammonia and ammonium ions back into nitrite and nitrate ions, and then into nitrogen gas and
oxygen. Nitrogen gas is released into the atmosphere while oxygen is used for the respiration of these
bacteria.
Soper page 311 fig 10.11
How human activities affect the nitrogen cycle

1. Burning of fuels forms nitric oxide, which 3. Nitrogen is removed from top soil when we;
reacts with atmospheric oxygen to form a) harvest nitrogen-rich crops
nitrogen dioxide gas that reacts with water b) irrigate crops
vapour to form acid rain containing nitric c) burn or clear grasslands and forests before
acid. Nitric acid together with other air planting crops
pollutants; 4. Adding nitrogen compounds to aquatic ecosystems
a) damages trees e.g. sewage algal blooming, which upon death, their
b) corrodes metals decomposition causes oxygen shortage resulting into
c) upsets aquatic ecosystems. death of aerobic organisms e.g. some fish.
2. The inorganic fertilizers applied to soil are 5. The accelerated deposition of acidic nitrogen
acted upon by anaerobic bacteria to release containing compounds e.g. NO2 and HNO3 onto
nitrous oxide into the stratosphere, where it; terrestrial ecosystems stimulates growth of weeds,
a) contributes to ozone depletion which outcompete other plants that cannot take up
b) contributes to greenhouse effect. nitrogen as efficiently.
QN. Describe the role of microorganisms in the nitrogen cycle.
Page 399 of 447

3.0 POPULATION AND NATURAL RESOURCES

Population, population growth and population growth curves
Population density is the number of organisms per unit of space/area.
Population size: is the number of organisms of the same species sharing the same habitat at a certain time.
Population size change as a result of four factors natality (birth), mortality (death), migration in
(immigration) and out (emigration) of the population.
Types of population Biotic potential.
(a) Open population The maximum rate at which a population can
This is the one in which density changes as a increase under ideal conditions i.e. when population
result of the interaction of mortality, natality, density is low and resources are plentiful.
migration and emigration. It occurs in a natural The biotic potential depends on the age structure
environment. and male: female ratio existing in the population. It
(b) Closed population/cultured populations is also influenced by the age at which the individual
This is one in which density changes are the first reproduces, the frequency at which
result of natality and mortality with neither food reproduction occurs, the reproductive life span and
nor wastes being allowed to enter or leave the the number of the offsprings the individual is
given environment. It occurs in laboratory capable of producing.
settings and game reserves/cultured populations
Factors that tend to increase or decrease population the size of a population
Factors that cause a population to grow Factors that cause population size to decrease
(Biotic potential) (Environmental resistance)
i) Favourable light – mostly for plants. i) Too much or too little light, mostly for plants.
ii) Favourable temperature. ii) Too much or too little temperature.
iii) Favourable chemical environment iii) Unfavourable chemical environment (too much or too
(optimal level of critical nutrients and toxic little of critical nutrients and high waste accumulation).
wastes).
iv) High reproductive rate. iv) Low reproductive rate.
v) Adequate food supply v) Inadequate food supply
vi) Ability to compete for resources. vi) Too many competitors for resources.
vii) Ability to hide from or defend against vii) Insufficient ability to hide from or defend against
predators. predators.
viii) Ability to resist disease and parasites. viii) Inability to resist disease and parasites.
ix) Ability to adapt to environmental changes. ix) Inability to adapt to environmental changes.
x) Ability to migrate and live n other habitats. x) Inability to migrate and live n other habitats.
xi) Suitable habitat. xi) Unsuitable or destroyed habitat.
xii) Generalised niche xii) Specialised niche
Population Histograms
Population growth curves only show how populations change over time but don’t tell or show the age
distribution of the members. The population histograms show or represent population of an organism in terms
of its age structure and the proportion of males and females at a specific instant in time (sex ratios).
Note, the study of vital statistics of populations and how they change over time is called demography. Such
statistics include birth rate and how they vary among individuals and death rate
Age distribution/structures
It’s the proportion of the individuals of different ages in their population. It is an important factor because it
influences mortality and natality. It’s determined by:
i) Observing the teeth and bones of organisms.
Page 400 of 447

ii) Observing horns, claws, rings and scales, etc. depending on different types of animals or organisms or
plants. E.g. some animals or organisms show annual increment in rings e.g. scales in fish and horns in
cattle.
iii) In invertebrates and some vertebrates, weight and size are used to determine the age of an individual.
Types of ecological ages
1. Pre-reproductive age; represent organisms that are below the reproductive age (between 1-14 years).
2. Reproductive age; shows organisms of the population able to mate or reproduce.
3. Post-reproductive age; represent members that are old enough to reproduce e.g. 65+ years in humans.
The relation duration/time of each one age varies with different species. Age structure is studied using the
age sex graph or population pyramids. It deals with relationships in number between males and females of
age groupings.
Population growth
Natural populations start with small size and gradually increases to a climax/carrying capacity where it is no
longer growing/increasing. At this point the population undergoes a number of changes as a result of the
changes in the environmental factors.
Carrying capacity of a population refers to the maximum number of the individuals of a population which
the resources in a particular environment can support maximally at a given time. At carrying capacity,
changes in environmental factors such as food supply decline/reduced rainfall fluctuation in temperature or an
outbreak of epidemics, temperature, etc. results in an increased death rate which over powers the birth rate
hence leading to a fall in the population. This is known as a decline phase
Population growth curve of organism in a given Lag phase:
habitat (sigmoid curve) This is the period of low growth rate because the
reproducing organisms are few and the members
are still adjusting to the environmental conditions.
There is plenty supply of nutrients, space, oxygen
and low or few wastes. At this point the decrease in
the population is directly proportional to the group
members/individuals that are reproducing.
Exponential/log phase:
This is the phase of fast increase in the
population/increased rate of growth because the
individuals are used to the environment, majority
have reached their reproductive potential and there
is no limiting factors such as food, space, oxygen
hence the organisms are able to grow and reproduce
at fast rate i.e. non environmental resistance.
Stationery phase:
This is also called the equilibrium stage. This occurs as a result of low growth rate. The birth rate decreases
while the death rate increases as a result of shortage of food nutrients, over-crowding, accumulation of toxic
waste products, predation and competition for the above resources amongst the death rate and birth rate are
equal and the population size becomes stable or attains its climax which is called the stationery phase or the
carrying capacity of the population i.e. environmental resistance is evident.
Environmental resistance
Refers to the sum total of limiting factors, both biotic and abiotic which affect together to prevent the biotic
potential from being obtained or all the factors that tend to reduce population numbers, such as predation, food
supply, heat, light, space, regulatory mechanisms like intraspecific competition and behavioral adaptation.
Mac Arthur and Wilson (1967) estimated population growth using the logistics equation i.e.
Page 401 of 447

For sigmoid growth curve

Where:
t- Time
r- biotic potential/maximum rate of increase of
the population
N- No. of individuals in the population
K- Carrying capacity
I- population change
Where the population is low, the I value is close to
NѴR
When the population increase and resources become
depleted, I decrease sharply to zero where N=K.
What happens when population size exceeds the carrying capacity?

The population suffers a dieback or crash, unless the excess individuals switch to new resources or move to an
area with more favourable conditions.
Note. When a population reaches equilibrium it does not remain absolutely constant but fluctuates because of
variations in the environmet resistance. For a give species in a particular environmental situation there is a
certain optimum population which the environment can support. This can be called the norm or set point. If
the population rise above the set point, competition or predation takes place to such an extent that the
population falls. If it falls below the set point environmental resistance if temporary relieved so that the
population rises again. In the normal course of events populations fluctuate on either side of the set point, but
the fluctuations are not excessive a clear case of negative feedback.
Control of the human population

The homeostatic control of the population breaks down if some factor of the environmet is suddenly changed.
Such factors could be removal of the predators for a given prey or decrease in competition for food, this would
shift the set point which would increase the birth rate, reduction in the death rate. All this leads to an exponential
increase in the population.
In the case of man, the environmental changes responsible for raising the set point have been created by man
himself, and inevitably brings in problems such as food shortage and sheer physical over-crowding. Man’s
Page 402 of 447

population has exploded due to (1) improvement in hunting and food gathering techniques i.e. tool making
revolution (2) improvements in agriculture i.e. agricultural revolution (3) improvement in food production,
industry and medicine i.e. the scientific-industrial revolution.
The only humane way of curbing the exponential increase in the human population is by birth control. This
includes behavioural means such as abstinence, the rhythm method, coitus interuptus, contraceptives e.g.
condoms, hormonal methods e.g. the contraceptive pill, various intra-uterine devices (IUDs), sterilization of
male and female and abortion.
Types of growth curves
There are two basic forms, i.e. J-shaped growth curve and S-shaped (sigmoid) growth curve.
The above curves where there is a log phase, exponential, stationery and declining phase describes a sigmoid
curve or logistic population growth or S-shaped curve as a result of changes in both density dependent and
density independent factors.
Population growth starts out slowly and then proceeds
faster to a maximum (carrying capacity) and then levels
off. Population then fluctuates slightly above and below the
carrying capacity with time.
The population stabilises at or near the carrying capacity (K)
of its environment due to environmental resistance
Exponential population growth (J-shaped curve)
This describes a situation in which after the lag
phase/population growth continues in an exponential form
‘boom’ until when it stops abruptly and due to
environmental resistance. The crash ‘bust’ (abrupt
B.S pg 323 fig 10.19 (b)
stoppage) may be caused by factors like seasonality i.e. end
of breeding season of the organism or of prey species. The
crash may also be due to human interaction like application
of insecticides to control pests, herbicides to control weeds.
Growth is density independent.
It occurs when resources are unlimited and the population
can grow at its intrinsic rate of growth. However this is rare
in nature because of limiting factors (environmental
resistance). E.g. (1) Algal blooms (2) some insect species
e.g. long horned grasshoppers (3) Bidens pilosa (black
jack) (4) If cats totally fed on prey/rats. The removal of
prey results in the crash of the predator/cat population.
Factors that influence population growth size

Collectively the factors which limit population growth are termed as environmental resistance and are grouped
into two.
1. Density dependent (biotic) factors
These include diseases, competition for food, light, shelter, etc. assume there is a fixed food supply available
for the organisms, the larger the population, the less the food available for each individual hence the slower
the growth rate of the organism. However, a small population can expand more rapidly or exponentially.
2. Density independent (abiotic) factors
These affect the population regardless of the number of individuals within the community such as
temperature, natural calamities, e.t.c.
Page 403 of 447

a. Climatic factors b. Edaphic factors

(1) Light: this affects the population growth and These are soil factors such as;
the distribution of organisms move so the plants. (1)Texture:
It is essential for (i) photosynthesis (ii) This is the proportion of soil particles (sand, silt, clay).
chlorophyll formation (iii) stomatal opening, It influences the water holding capacity of the soil
hence gaseous exchange (iv) phototropism (v) which in turn influences the number of individuals in a
flowering stimulants (vi) seed germination (vii) population since plants need water for growth.
broadness of leaves and (viii) ripening of fruits. (2) Humus:
(ix) induces flowering in long-day plants e.g. These are the dead remains and decaying organic
barley, but (x) inhibits flowering in short day matter of living organisms. Humus acts as sponge in
plants. In animals light enables (xi) hibernation, retaining water as so to reduce soil erosion, humus also
(xii) vision and (xiii) migration (xiv) Courtship; holds soil particles together, further reducing on the
with some animals preferring light so as to carry rate of soil erosion. Humus decomposes to release
out courtship while others prefer darkness (xv) minerals which add nutrients to the soil enabling the
Stimulates synthesis of vitamin D in mammals plant number to increase. Humus is a habitat for some
(2) Water/rain fall. soil organisms. Its dark colour retains heat in the
(i) Habitat for many aquatic organisms e.g frogs, ground.
fish e.t.c. (ii) Raw material for photosynthesis (3) Air:
High thermal capacities (iii) cooling agent for Oxygen is required by roots in the soil and most soil
terrestrial organisms (iv) Agent for fruit, seed, animals. Therefore well aerated soils support a number
spore, larva and gamete dispersal (v) Condition population/community. Water logged soils are due to
for germination (vi) Highly transparent; compacted soil particles like clay has less air which
therefore allowing light to reach aquatic leads to anaerobic conditions and hence cause death of
organisms, for photosynthesis; and aquatic most aerobic plants, animals and fungi hence reducing
predators to locate their prey (vii) Important their population.
factor in decay and decomposition ; therefore (4) Water:
increases in recycling of nutrients in an Water is an important metabolic and a medium of
ecosystem. transfer of gametes/dispersal of fruits/seeds thus its
(3) Air and wind: wind brings about changes in availability supports the growth of populations. This
the weather and it directly affects the organisms. explains the presence of xerophytes and halophytes
(i) wind enables seed and spore dispersal (ii)wind (normal soils) and hydrophytes (aquatic).
affects the rate of transpiration (iii) determines (5) Mineral/inorganic salts:
the distribution of many plant species (iv) Different species of organisms (plants) require different
Because of wind, plants have strong deep roots mineral salts and their qualities and so the distribution
(v) Wind also enables mixing of nutrients and and number of any population is pH influence the type
pollutants in water. of plants in a particular area. It also affects the physical
(4) Humidity: affects transpiration rates in properties and uptake of the mineral salts hence affects
plants, number of stomata in the leaves and rate growth. Some plants require alkaline soils yet others
of evaporation in animals more so the small acidic soils.
invertebrates like earth worms, snails hence (6) Soil organisms:
distribution of plant and animal populations. These are the macro-organisms/population of
(5) Temperature: (i) temperature is vital organisms which influence aeration, drainage tunnels
because it affects the enzymes in plants and and also soften soil thus exposing the mineral content
animals (ii) temperature also enables fruit to plants.
ripening (iii) transpiration rate, wilting of leaves. Saprophytes/bacteria/fungi break down plant tissue and
(iv) Low temperatures break dormancy of some dead animals to release humus or mineral salts.
plants (v) stimulate flowering in some plants e.g The symbiotic association in the soil such as root
cabbage (Vernalisation) (vi) In animals it affects nodule bacteria and plant roots enable fixation of
humidity and available water in the habitat which gases/nitrogen in the soil for proper plant growth.
factors affect the distribution of organisms.
Other factors include:
 Man’s activities like immigration, pollution, drainage.
 Predators.  Epidemic out break/diseases.
 Accumulation of toxic substances.  Migrations in and out.
Page 404 of 447

c. Biotic factors (interactions within the population)

This is the way how organisms deal with each other in their habitats and the relationship influence their
distribution and abundance in the habitat. The interactions could be positive of negative.
The negative interactions enables the growth rate of individuals affected by the presence of related
organisms or species to decline as a result of over predation, competition, parasitism, symbiosis,
mutualism, commensalism.
Population growth and survivorship curves
Population grows and declines in size. The size of population increase is determined by the reproductive
potential of the concerned organism and by environmental resistance.
The biotic potential/reproductive potential is the maximum number of off springs that can be produced by a
species under ideal conditions or is the rate of reproduction given unlimited environmental resources.
Factors affecting biotic potential Factors hindering biotic potential
 Off spring; the maximum number of off springs per  Loss of food.
birth.  Increased predator population.
 Capacity for survival; the chances the organisms’ off  High pollution in the environment.
springs will reach reproductive age.  Fire out break; destroys organisms,
 Procreation; the number of time per year the breeding sites, nest, eggs, slow moving
organisms reproduces. organisms.
 Maturity; the age at which reproduction begins.  Man’s activities of man e.g. encroaching
 Male to female ratios in the population. on swamps, wet lands, forests, road
 Age structure; age at which reproduction is high e.g. construction(separates ecosystems)
in man is 45, chances of producing become minimal.  Diseases, parasites and pests
K and r population strategies
Most natural population fall between two extremes called r-selected and K-selected population as were used in
the equation for population growth.
Note: r-species/r-strategists species produce rapidly and have a high value of r.
Characteristics of r-selected populations
 They are found in habitat/environments which undergo many changes.
 The individuals are small in size.
 Have a short life span i.e. they attain reproductive potential very early.
 Have a high mortality rate not density dependent.
 Reproduce at a high rate.
 Off springs grow rapidly with little parental care provided.
 Favourable conditions favour rapid explosion of population growth hence no or less competition. Thus
selection pressure in such species favours high reproduction rate and short generation.
 A sudden environmental change results in a massive number of deaths. But their rapid birth rate and short
life span favour the ability to adapt to a changing habitat e.g. insects, seeds, spores, bacteria, annual
plants, paramecium.
 They are opportunist pioneer species of new and disturbed habitats. Migration and dispersal are key
factors of their strategy.

K-selected populations
These are associated with specific habitat conditions or fairly stable environmental conditions with fewer
fluctuations, relatively undistributed habitats and ever the changes of the seasons are regular and
predictable and where competitive ability rather than reproductive speed is a major survival
attribute/factor.
Page 405 of 447

They tend to be more typical of the later stages of succession and such species are not very adapted to
recover from population densities significantly below their equilibrium level (K-value or carrying
capacity).
Characteristics of K-selected population
 Reproduce slowly (low fecundity, long generation time) therefore low value of r.
 Reproduction rate is sensitive to population density, rising rapidly if density falls.
 Population size stays close to equilibrium level determined by K.
 Species are persistent in a given area.
 Disperse slowly
 Large in size e.g. woody stems and large roots if plants.
 Individuals live long
 Habitats stable and long lived (forests for monkeys).
 Good competitors
 Many become dominant.
 Less resistant to changes in environmental conditions e.g. butterflies, birds, humans and trees.
Comparison between r-selected and K-selected (equilibria) populations
Characteristic r-selected species K-selected species
Body size Small Large
Lifespan Short Long
Competitive abilities Poor Good
Defensive strategies Lacking Well developed
Dispersal Disperse or migrate widely and in Disperse slowly. Species generally
large numbers persistent in the habitat
Degree of specialisation to the Poorly specialised (more adaptable to Highly specialised (less adaptable
habitat changes in environmental conditions) to changes in environmental
conditions)
Age at first reproduction Early Late
Number of reproductions per Usually one Several
life time
Number of offsprings per Many; reproductive rate not sensitive Few; reproductive rate is density-
reproductive episode to population density dependent
Size of offspring/egg Small Large
Parental care None Extensive
Mortality rate High Low
Examples Bacteria, aphids, flour beetles, annual Humans, owls, large trees, whales,
plants, weeds, cockroaches large marine birds
Survivorship curves
This is a graph that shows the proportion or numbers of
individuals in a group will still be alive at a given age. For any
population size to remain constant at least two off springs from
each male and female pair on average must survive to
reproductive age.
The percentage of individuals that die before reaching
reproductive age, pre-reproductive mortality (infant mortality), is
a major factor determining population size. Knowledge of
survivorship enables ecologists to study population growth
Page 406 of 447

Survivorship curve & Typical Explanation

examples of organisms
Late loss curves(Type I)  These are organisms with stable populations close to carrying capacity of
the environment (K).
Humans, elephants,
 They produce few young ones which are cared for until reproductive age,
rhinoceroses, mountain
thus reducing juvenile mortality and therefore enabling high survivorship to
sheep
a certain age, then high mortality at later age in life.
Constant loss (Type II)  This is characteristic of species with intermediate reproductive patterns with
a fairly constant rate of mortality in all age classes and thus a steadily
declining survivorship curve.
Many song birds, lizards,
 There is an equal chance of dying at all ages.
small mammals and hydra
 These organisms face a fairly constant threat from starvation, predation and
disease throughout their lives.
Early loss curves (Type  These are organisms with a high intrinsic rate of increase.
III)  They produce many offspring which are poorly cared for resulting into high
juvenile mortality.
Most annual plants, most
 There is high survivorship once the surviving young reach a certain age and
invertebrates and most
bony fish species size.
Question: which population I and III would need the highest reproductive rate to maintain a stable
population?
It is population III because a high percentage of individuals would die before reproductive age is reached.
Population I would have to combine its high survival rate with low reproductive rate to maintain a stable
population size.
Most populations have survivorship curves which are a combination of the three types. For example,
herring gull starts out with survivorship type III (when newly-hatched chicks are most vulnerable) but once
the chicks are independent, the survivorship curve resembles type I.
Determination of population size of organisms

Importance of estimating population size Factors to consider before counting organisms
i) It enables monitoring of population growth. i) The area of land or volume of water or air under
ii) It enables determination of habitat requirements study must be determined.
of a species. ii) The nature of vegetation cover of the habitat.
iii) It enables determination of carrying capacity of iii) Size of the organisms under study.
the area i.e. to determine whether existing iv) Facilitation in terms of equipment to be used.
populations are likely to be sustainable v) Behaviour of the organisms e.g. their hostility
iv) It enables determination of age structure and and level of excitement when disturbed.
sometimes sex ratio of the population. vi) Topography of the area.
v) It enables projection of how population size is vii) Whether the habitat is terrestrial or aquatic.
likely to change with time for proper planning viii) Risks expected to be faced during the
e.g. determining the peak populations of pests exercise.
enables control measures to be prepared. ix) The duration the exercise is expected to last.
Page 407 of 447

Methods of determining population size of organisms

1. Total count/Census 2. Counting by sampling
This is the physical counting of every individual of a This is when the number of organisms is
population in a specified area of ground. Examples: determined in several sample plots that represent a
a. census of people after every ten years known fraction of the total area under investigation
b. aerial counting from a low flying aircraft of large, from which estimation of the total population size
sessile, slow moving animals and mobile but large of the whole area is made by simple calculations.
animals e.g. giraffes, elephants, lions, e.t.c. Sample counting is applied when the number of the
Accurate results obtained from conducting several organisms is large, covers a large area or where the
aerial counts in the area and then the average is behaviour of organisms does not allow easy
computed and expressed as numbers per unit area of contact.
ground.
A survey of sampling methods

Quadrant
Quadrant sampling is suitable for slow moving animals and plants.
A metallic, plastic or wooden frame of known area e.g. 1m2 or 0.25m2 is randomly thrown several times in the
area under investigation and all individuals within the quadrant are counted each time.
Population density is expressed as an average figure per metre squared. Total population is got by multiplying
the average with the total area under investigation.
Advantages: Example:
(1) The method is relatively accurate (2) It enables Three counts of 103, 46 and 20 of a plant species, were
comparison of species density in different areas made using a quadrant of 25cm2. The population density
(3) It provides an absolute measure of abundance. of the plant per m2 is?
103+ 46 +20
Disadvantages: Population density = = 56.3
3
It is: (1) time consuming (2) not suitable for fast 56.3
25cm2 contain 56.3 1cm2 contains 25 plants
moving animals (3) not suitable for large sized
56.3
animals. (4) Grass in tussocks appear 100cm2 contain x 100 = 225 plant
25
indistinguishable and may disturb.
Line transect
A line or string is laid along the ground in a straight line between two poles. Sampling is confined to only those
organisms touching the line. Line transect is useful in studying transition of one community to another
Capture-recapture (Lincoln index) method
This method is used on highly mobile animals like fish, Assumptions which the method bases on:
small mammals e.g. rats, birds, and arthropods e.g.  That organisms mix randomly within the
insects like butterflies, grasshoppers, cockroaches. population.
It involves capturing and counting the organisms (N1),  That the time allowed for random mixing is
enough.
marking them in a way that causes no harm, and
 That changes in population size due to
returning them to the natural environment.
immigration, emigration, death and birth are
After allowing sufficient time for the population to mix negligible.
thoroughly, a second catch (N2) is made and the number  That the movement of organisms is restricted
of marked individuals recaptured is recorded as N3 geographically.
Estimated total population (P) = N1 x N2  That there is even dispersing of organisms
N3 within the study area.
 That the mark does not hinder the movement
of the organisms or make them conspicuous to
predators.
Page 408 of 447

Example: Disadvantages / Limitations:

In estimating the number of fish in a small lake, (1)It is only reliable when the organisms’ range of
625 fish were caught, marked and released. After movement is relatively restricted and defined (2) Animals
one week, 920 fish were caught and of these, 150 often move in groups whose members recognise one
bore marks. What was the estimated size of fish another and avoid mixing with those of other groups (3)
population? Many animals have particular localities (home ranges)
P = N1 x N2 where they confine, so the marked animals may not
N3 spread widely (4) Loss of marked individuals reduces
P = 625 x 920 = 3833 those recaptured and this causes inaccuracy.
150
QN. Suggest and describe the suitable methods for estimating the population size of the organisms below. Give
reasons for your choice of each method and outline the associated limitations.
a) Fish in a pond
b) Terrestrial plants
Page 409 of 447

Estimating population density

Population size can be estimated in various ways which include;
a) Quadrat method
b) Transect method
c) Capture-mark release recapture method (the Lincoln index)
How population density affects population growth
a) Density-dependent population control; involves factors whose effectiveness depends on the number of
individuals present in a unit of space i.e. the more individuals there are in the population, the greater the
percentage of the population that dies or fails to reproduce. Examples: Competition for resources like food,
disease, predation, parasitism (biotic factors mainly)
b) Density-independent population control; involves factors whose effectiveness is not related to the density of
the population. Any change in the factor affects the same proportion of the population regardless of population
density. Examples: temperature, rainfall, light, floods, soil nutrients, pollution, fires, drought, hurricanes, habitat
destruction e.g. clearing a forest or filling in a wetland, pesticide spraying etc. (mainly abiotic factors)
Kinds of population change curves in nature

Causes and Examples
a) Stable population growth curve: c) Irregular population
Trees in a tropical rain forest where there is little variation in temperature growth curve
and rain fall from year to year. This is partly attributed to
b) Irruptive population growth curve: chaos in such systems but
Some algal populations in freshwater habitats, raccoon and feral house causes and interactions are
mouse. This is caused by some factor that temporarily increases the not clearly understood.
carrying capacity e.g. more food, favourable weather, or fewer predators.
d) cyclic population growth
curve
Lemmings (small rodents
whose populations rise and fall
every 3-4 years), grouse, lynx
and snowshoe hare (whose
populations generally rise and
fall on a 10-year cycle). The
actual causes are poorly
understood, although
predators sometimes are
blamed.
Distribution of organisms (population dispersion)

Dispersion refers to the structure/distribution of individuals or organisms within an area
Dispersal is the movement of individuals/organisms or their seeds, parts into or out of the population or
habitat to a different locality/area.
Importance of dispersion in animals
 Individuals acquire a home/nest/habitat within which they can live and breed.
 Individuals are spread out such that resources like food, breeding grounds become enough.
 Chances of obtaining a mate is increased since males attract females into their territories.
 Reduces distances moved away from home to search for food, mate, etc. this saves energy, time and
prevents exposure to predators.
Page 410 of 447

 New or depopulated areas are colonized.

 It increases the rate of gene exchange/gene mixing between populations via mating.
 It may prevent extinction and or speed up population growth.
 Natural barriers like rivers and rift valleys restrict animals in particular areas e.g. bush backs, chimps and
elephants.

Dispersal mechanism of the population supplements natality and mortality in shaping population growth form
and density.
There are 3 forms of dispersion:
1. Uniform/regular distribution 3. Random dispersion/distribution
This occurs where intraspecific competition is This is relatively rare in nature. It occurs where
severe. However, man artificially can induce it the environment is very uniform in terms of
through agricultural practices e.g. planting of seeds resources and there is no tendency of organisms
or birds nesting on a small island to aggregate. There is equal and even distribution
of resources. There is low or no competition. For
example herbaceous plants in a garden
2 Clumped distribution/aggregate/clustered
It’s the naturally occurring type of distribution
where individuals tend to aggregate at a
particular point on the habitat. It’s due to;
Many communities are dominated by clumped
i) Distribution of resources that are not
patterns of distribution for several reasons:
regularly distributed due to climate and soil
factors. i) Effects of parent plant i.e. seeds may not be
ii) Social behaviour like termites and bees have dispersed far from the parent plant hence
division of labour among members, animals plant seedlings are usually found near the
that live in colonies like buffalos, baboons, parent plant.
monkeys, etc. clumps could be irregular or ii) Distribution of environmental factors. These
regularly distributed. are not uniform for all areas.
Regular pattern: iii) Species interrelations i.e. a species may be
depending on another directly e.g. epiphytes.
Animals exhibit dispersion in form of
territorial behaviours. A territory is a defined
area owned by a group of animals/family and
defended against other members of the same
species.
iv) Natural barriers like rivers and rift valley
Irregular pattern or random: restrict animals in particular areas e.g. bush
backs, chimps and elephant
Page 411 of 447

4.0 INTERDEPENDENCE
Interaction within the populations
Competition
Within a population, individuals compete with each other for food, water, mineral salts, territory, shelter, mates,
resting sites, etc. therefore competition is the interaction that occurs between two or more organisms,
populations or species that share resources.
Types of competitions
1. Interspecific competition; is competition among individuals from other species for resources.
2. Intraspecific competition; is competition among individuals of the same species for essential resources.
The closer the ecological niches of the competing organism, the fierce are the competition.
Co-existence between two species which compete is impossible. To avoid severe/stiff competition and
extinction the two different species occupy different ecological niches. This is called competitive exclusion
principle.
It states that, “no two organisms can occupy the same ecological niche when they compete for the same
resources. If they did so, one would become extinct or will be out-competed thus becomes extinct.”
Organisms develop structural features and behavioral patterns to enable them succeed in the exploitation of
natural resources.
The successful organism has a faster rate of reproduction and a higher tolerance to waste materials e.g.
seedlings in forests show rapid growth due to competition to gain access to sunlight for photosynthesis.
Consequences of competition
 Weak competitors are eliminated or extinction of species or migration.
 It results in feeding habits/feed on food nutrients which they used not to feed on.
 It affects pollination between certain plants and specific insects.
 Gene loss or change in gene frequency.
The competitive exclusion principle (Gause’s exclusion principle)

Two species (populations) that require identical resources cannot coexist indefinitely in the same habitat.
G. F Gause (1932), a Russian microbiologist and ecologist was one of the first to make laboratory
investigations on competitive systems
Experiments with Paramecia
Two closely related species of single-celled, bacteria eating Paramecia were grown, first separately and then
together in culture tubes.
Lemna (duckweed) species also show similar results when grown in mixed cultures. The results of this classic
experiment are reflected in the graphs below
Page 412 of 447

The research revealed the following; Other observations

i. Pure population of Paramecium caudatum and i. The maximum population density of P.
Paramecium aurelia grown in full-strength aurelia attained in separate cultures is
concentration of food had a high carrying capacity (of higher than that of mixed cultures.
195). ii. The rate of population density increase
ii. Pure population of Paramecium aurelia grown in half- for both organisms is higher in separate
strength concentration of food had a low carrying cultures than in mixed cultures.
capacity (of 105). iii. The initial population density of P.
iii. Pure population of Paramecium caudatum grown in aurelia in both experiments is lower
half-strength concentration of food had a low carrying than that of Paramecium caudatum.
capacity (of 64). iv. The decrease in population density of P.
iv. Pure population of Paramecium caudatum grown in caudatum is very gradual in separate
full-strength concentration of food showed a high cultures in comparison with mixed
carrying capacity (of 137) culture.
v. When grown together, Paramecium aurelia survived, v. The population density of P. caudatum
while the population of Paramecium caudatum rapidly in mixed cultures decreases to extinction
decreased. but that in separate culture remains
vi. At a full-strength food concentration, the decrease in higher even at carrying capacity.
the Paramecium caudatum population was approaching vi. P. aurelia out-multiplied and eliminated
exclusion by 16 days but exclusion was not complete. P. caudatum when grown together.
vii. At half-strength food concentration, Paramecium
caudatum had been entirely eliminated by day 16.
Observations Explanations
Experiment; each species grown alone
Population density of Paramecium Aurelia There is rapid growth of population densities of
 Increases very gradually for the first 2 days, very both organisms because of:
rapidly to a maximum on the 9th day, remains relatively i. Abundance of food.
constant till the 12th day, then decreases very gradually ii. Absence of interspecific competition for
thereafter. food.
 Grew faster than that of Paramecium caudatum
Population density of Paramecium caudatum: Stable populations are established afterwards for
 Increases gradually but later increases rapidly for the both organisms because of intraspecific
th competition for food.
first 9 days, increases very gradually till the 14 day
and then declines very gradually thereafter.
 Increase less rapidly when compared with P. aurelia. Population density of P. aurelia grew faster
 Remains lower than that of P. aurelia, except for the than that of P. caudatumbecause the former
first three days. used the available food supply more efficiently
than the latter.
Experiment; Species in mixed culture
Population density of Paramecium aurelia: The population density of
 It is lower than that of P. caudatum for the first 3½ days. P. caudatum decreased to
 Increases gradually for the first two days, very rapidly till day 7 and then extinction because of being
gradually thereafter. out-multiplied and
Population density of Paramecium caudatum: outcompeted for food by P.
 Remains higher than that of P. aurelia for the first 3½ days only. aurelia. This is attributed
 Increases rapidly to a maximum in the first 5½ days, decreases gradually till to the smaller size of P.
day 15, and more gradually to extinction at day 17. aurelia.
How species reduce or avoid competition through resource partitioning

Resource partitioning is the dividing up of scarce resources so that species with similar needs use them (i) at
different times (ii) in different ways or (iii) in different places.
Over a time scale long enough for evolution to occur, some species that are in competition for the same
Page 413 of 447

resources evolve adaptations that reduce or avoid competition or an overlap of their fundamental niches.
Resource partitioning decreases competition between two species leading to increased niche specialisation
Examples of resource partitioning
i. When living in the same area, lions prey mostly on larger animals while leopards on smaller ones.
ii. Hawks and owls feed on similar prey, but hawks hunt during the day and owls hunt at night.
iii. Each of the five species of common warblers (insect-eating birds) minimises competition with the others
by;
a) spending atleast half its feeding time in a different part of spruce tree branches e.g. some hunt at the
extreme top, others at the lower portion, some mid-way e.t.c
b) Consuming somewhat different insect species.
iv. Different species of eagles in a forest feed at different times of the day e.g. bald headed eagles are most
active early mornings and evenings while the white-breasted eagles feed vigorously towards noon.
v. Paramecium aurelia and paramecium bursaria can coexist in a tube containing yeast because the former
feeds on yeast suspension in the upper layers of the fluid whereas the latter feeds on the bottom layers.
vi. When three species of ground finches of Galapagos Islands occur on separate islands, their bills tend to be
the same intermediate size, enabling each to feed on a wider range of seeds, but where they co-occur,
there is divergence in beak size to suit each finch species to feeding on seeds of either small, medium or
large size, but not all sizes.
vii. Various bird species in a coastal wetland feed in different ways e.g. flamingos feed on tiny mud
organisms, brown pelicans form air dives for fish, herons wade into water to seize small fish, herring gulls
are tireless scavengers, piping plovers feed on insects and tiny crustaceans on sandy beaches.
viii. In an abandoned field, drought tolerant grasses with shallow, fibrous root system grow near the soil
surface to absorb moisture; plants with a taproot system grow in deeper soil while those with a taproot
system that even branches to the topsoil and below the roots of other species grow where soil is
continuously moist.
ix. Two species of barnacles do not occupy as much area of the intertidal zone as possible. The upper
intertidal zone is the realised niche of the smaller barnacle Chthamalus stellatus since it is drought
resistant while the larger Balanus balanoides being poorly adapted to drought lives on the lower intertidal
zone. Interestingly, C. stellatus can also grow very well in the lower zone only in the absence of B.
balanoides, suggesting that Balanus barnacles when present either forces the smaller Chthamalus
individuals off the rocks or grows over them. This indicates that the entire intertidal zone is the
fundamental niche of Chthamalus, but competition for space restricts it to the upper intertidal zone.
Note:
a) The more two species in the same habitat differ in their use of resources, the more likely they can coexist.
b) Two competing species also may coexist by sharing the same resource in different ways or at different
times
c) The tendency for characteristics to be more divergent when populations belong to the same community
than when they are isolated is termed character displacement.
Predation
Predation is a feeding relationship where one organism of a given species, the predator, hunt, kill and feeds on
another, the prey of another species.
The growth and decline of the population of such organisms depend on the number of each group in an
ecosystem. Initially prey population grows at a faster rate than the predator. The predators feed on the prey, thus
increasing in production.
A reduced prey population triggers off competition for density dependent factors like food, space, mates among
the increased predator population and also increased accumulation of wastes. These will check the increase in
predator population hence predator number will start to decrease in number due to starvation.
Page 414 of 447

When predator populations decrease, prey will Graph showing relationship between predator-prey
reproduce and multiply in number and increase. populations
Therefore, large numbers of preys provide food and
therefore food becomes available. Thus the population
of prey and predator affects each other which bring
about fluctuation in the growth of their populations.
Note: normally the numbers of predators tend to lag
behind than those of prey because predators being
larger have a slower rate of increase.
Importance of predation
a. Predation maintains populations within the carrying capacity of their habitats and lessens the sudden
explosion of prey species within a population.
b. Predation is a mechanism by which excess animal productivity is re distributed by conversion to other
animal tissues at higher trophic levels.
Examples
1. The graph below show the relationship between a predator, Didinium and prey, paramecium in a culture
medium.
Description of the trend for;
 Paramecium
From o days to 1 days the Paramecium population
increases gradually.
From 1 day to 2 days the Paramecium population
increases rapidly to a maximum.
From 2 days to 3 days the Paramecium population
decreases rapidly
1
From 3days to 42days the Paramecium population
Explanation gradually decreases up to 0
Paramecium population rapidly decreased to  Didinium
extermination because it was being preyed From 2 days to 3 days the population of Didinium
upon by the introduced Didinium on day 2. increases gradually
The gradual increase in Didinium population From 3 days to 5 days, the population Didinium remains
was supported by food (Paramecium) almost constant
presence, what resulted was starvation as soon From 5 days to 6 days the population of Didinium
as food depleted. gradually decreases up
Description of the number of individuals for:
From 0 days to 1 day the Paramecium population
1
decreases from 1 day to 52days the Paramecium
population increases to a maximum.
From 0 days to 2 days the Didinium population increases
to a peak
From 2 days to 5 days the Didinium population decreases
to a extinction
Explanation
Both Paramecium and Didinium were introduced on same
day in an oat medium with sediment in which some, but
not all of the Paramecium hid from the predator. The
Paramecium population slightly higher than that of
Didinium.
Explanation. Paramecium population decreases for the first day because all of it that was in the clear-fluid
medium was preyed upon by Didinium, which increases and later starves to death. The increase in
Paramecium population is because of its emergency from the sediment after death of the Didinium
Page 415 of 447

How predators are suited for capturing prey

i) Herbivores simply walk, swim, or fly up to the plants they feed on.
ii) Some carnivore like cheetah and lions have muscular limbs for running very fast to catch prey.
iii) Other carnivores like bald eagles, hawks, lions e.t.c. have keen eyesight for locating prey.
iv) Carnivores like wolves and African lions cooperate in capturing prey by hunting in packs.
v) Other predators have characteristics that enable them to hide and ambush their prey e.g. praying mantises
sit in flowers of similar colour and ambush insects, alligators lie at stream bottom and dangle worm-like
tongue to entice fish into its powerful jaws, a chameleon has a cryptic colouration that enables walk to
prey unnoticed.
vi) Some predators have a highly developed sense of smell for locating prey e.g. meerkat locates insects by
smelling.
vii) Some nocturnal predators like bats and owls have a highly developed sense of detecting sound made by
prey.
viii) Electric fishes such as Malapterurus (cat fish) and electric rays produce high voltage discharge of up
to 350v that stun their prey while the electric fish Gymnarchus produces a low electric potential of less
than 0.1v to detect prey.
ix) Web-spinning spiders use their silky webs to catch small sized ground walking or flying insects.
x) Ant-lions (lacewing fly larvae) lay traps by making pits in the ground where preys fall.
xi) Some snakes e.g. puff adder, cobra, Naja and mamba, Dendroaspis have glands that secrete poison
(venom), which the fangs inject into the prey to immobilise it.
How prey are suited for avoiding predation (strategies of protection by prey against predation):
i) Ability to run, swim or fly fast.
ii) Possession of highly developed sense of sight or smell for alerting the presence of predators.
iii) Possession of protective shells e.g. turtles and snails for rolling into armour-plated ball
iv) Possession of spines (porcupines) or thorns (cacti and rose-bushes) for pricking predators.
v) In some lizards tails break off when attacked, giving the animal enough time to escape.
vi) Some prey camouflage by changing colour e.g. chameleon and cuttlefish, or having deceptive colours that
blend with the background e.g. arctic hare in its winter fur blends into snow.
vii) Some prey species discourage predators with chemicals that are poisonous (e.g. oleander plants), irritating
(e.g. bombardier beetles), foul smelling (e.g. stinkbugs and skunk cabbages) or bad tasting (e.g. monarch
butterflies and buttercups)
viii) Some prey species have evolved warning colouration – contrasting pattern of advertising colours that
enable predators to recognise and avoid such prey e.g. the poisonous frogs, some snakes, monarch
butterflies and some grasshoppers.
ix) Some species gain protection to avoid predation by mimicking (looking and acting like) other species that
are distasteful to the predator e.g. the non-poisonous viceroy butterfly mimics the poisonous monarch
butterfly. Batesian mimicry occurs when the palatable species mimics other distasteful species viceroy
butterfly mimics the poisonous monarch butterfly, the harmless hoverfly mimics the painful stinging wasp
while Mullerian mimicry occurs when both the mimic and mimicked are unpalatable or dangerous e.g.
the five spot Burnet and related moths.
x) Other preys gain some protection by living in large groups e.g. schools of fish, herd of antelope, flocks of
birds.
xi) Some prey scare predators by puffing up e.g. blowfish, or spreading wings e.g. peacock.
xii) Coiling as seen in millipedes
Note: Plants have poor hosts for invertebrate parasites because;
 plants have strong cellulose cell walls so that are too difficult to be penetrated by parasites as
invertebrate parasites lack cellulose enzyme to dissolve such walls
Page 416 of 447

 plants store insoluble food that is undigested e.g. starch yet invertebrates parasites absorb already
digested food and so cannot provide suitable conditions for survival
 plants do not locomote to spread the parasites to other hosts
 plants have limited cavities where the parasites can be sheltered against climatic extremes
 some plants are seasonal which limits the dependence of parasites on them
Note: Benefits of predator-prey relationships to both the prey and the predator
 maintains both populations at carrying capacity as they regulate their number interpedently
 They eliminate the weak and aged predators and preys thereby maintaining a healthy populations
 They allow evolution of better adapted predators and preys via natural selection
 They allow nutrient cycling and energy flow
 These relationships avail resources to both predators and preys by maintaining carrying capacity
Note: precautions taken before the predator is introduced in an area
 Careful marching of climatic conditions to ensure that they favour the survival of the predator
especially when the prey population is at the peak
 Monitoring of interactions of the natural enemy with native species to ensure that the predator is not
preyed upon by other unsuspected organisms. This also allows identification of the prey organisms
which may be presented by the predator instead of the targeted prey
 The predators must be released when the prey populations have reached large numbers to provide
sufficient food for the predators. Otherwise the predator may get wiped out prematurely via starvation
resulting into resurgence of the prey population.
Parasitism
An organism called parasite obtains part or all its nutrients from the body of another organism of different
species called host. The parasite is usually smaller than its host in size. Parasites do not usually kill their
hosts, but the host suffers harm. Many parasites live permanently on (ectoparasites) or in their hosts
(endoparasites) while some visit their hosts only to feed. Some parasites are facultative, live on or in the host
for some time e.g. Pythium (a fungus) that causes damping off seedlings, on killing the seedlings, lives as a
saprophyte on their dead remains and others are obligate (live in or on the parasite for their entire time).
Note: In most cases, the parasites don’t directly kill the host. However, by weakening its host, a parasite rises
the host’s susceptibility to other forms of environmental characteristics e.g. a severe fall in temperature during
winter is more likely to kill parasitized organisms than healthy one, and a predator is more likely to catch
parasitized animal than a healthy one.
Parasites which are introduced into a new habitat can cause disastrous effects on their host population e.g they
can cause death on the host organism. However, given time, the relationship between a parasite and its host
Page 417 of 447

evolves to minimize the harm to the host e.g. when the myxomatosis virus was introduced into Australia, all
rabbits died in a short time of infection; however, after some time, infected rabbits were found to survive longer.
5.0 EFFECTS OF HUMAN ACTIVITIES ON ECOSYSTEMS
POLLUTION
It is the release of substances or energy into the external environment in such quantities and for such duration
that they cause harm to living organisms or their environment.
Pollutants include noise, heat and radiation as different forms of energy, many chemical compounds and
elements and excretory products.
The parts of the external environment affected include air, water and land.
Harm caused by pollutants

 Disruption of life support systems for living organisms
 Damage to wildlife, human health and property.
 Nuisances such as noise and unpleasant smales, tastes and sights
Categorization of pollutants basing on their persistence in the environment

a) Degradable (non-persistent) pollutants:
These are pollutants that are broken down completely or reduced to acceptable levels by natural physical,
chemical and biological processes.
Biodegradation is the breakdown of complex chemical pollutants into simpler chemicals by living organisms
(usually specialised bacteria) e.g. sewage is a biodegradable pollutant.
b) Slowly degradable (persistent pollutants):
Are those that take a longer time to degrade e.g. DDT - an insecticide, and plastics.
c) Non-degradable pollutants:
These cannot be broken down by natural processes e.g. the toxic elements lead, mercury, arsenic, selenium
AIR POLLUTION
Pollutant and its sources Effects on living organisms Control measures
Carbon monoxide [CO] i) Prevents oxygen usage by blood by i) Efficient combustion of
 Motor vehicle exhausts forming carboxy-haemoglobin, which fuels in industry and
 Incomplete combustion of may cause death. homes
fossil fuels ii) Small concentrations cause dizziness and ii) Avoid smocking.
 tobacco smocking headache iii) Vehicle exhausts gas
control e.g. in USA.
Sulphur dioxide [SO2] i) Causes lung diseases, irritation of eye
Combustion of Sulphur surface, and asthma resulting into death i) Use of Sulphur free fuel
containing fuels, oil, coal gas if in high concentrations. e.g. natural gas.
ii) Forms acid rain which increases soil PH. ii) Installation of SO2
iii) Reduces growth of plants and kills extraction units in
lichens. Lichens are indicator species for industrial flues and
SO2 pollution. chimneys.
The presence of many lichen species
indicates low level of SO2 pollution in that
area.
Ozone, O3 Low level (tropospheric) ozone causes
 Motor vehicle exhausts i) Internal damage to leaves hence
 combustion of fossil fuels reducing photosynthesis.
to form nitrogen dioxide ii) Eye, throat and lung irritation which may
which decomposes to form result into death.
oxygen atoms that combine iii) Greenhouse effect by absorbing Vehicle exhausts gas
and radiating heat which raises the control e.g. in USA.
temperature at the earth’s surface.
Page 418 of 447

with oxygen molecules to High level (stratospheric) ozone offers

form ozone protection against excessive solar heat by
absorbing solar ultraviolet radiation which
would reach the earth’s surface
Smoke i) Causes lung diseases when inhaled i) Usage of smokeless
i) House coal, smoke, soot ii) Sunlight barrier, hence reducing fuels
ii) Motor vehicle exhausts photosynthesis. ii) Efficient combustion
iii) Tobacco smocking iii) Stunted growth of plants iii) No smoking
iv)Incomplete combustion of iv) Stomatal blockage hence reducing iv) Vehicle exhausts gas
refuse in incinerators and photosynthesis. control
bonfires
Dust i) Lung diseases i) Installation of dust
Solid fuel ash, soil, quarrying, ii) stomatal blockage precipitators in industrial
mining, e.t.c. iii) Stunted growth of plants. chimneys.
iv) Smog – forms when temperature ii) Efficient combustion.
inversion occurs (layer of warm air traps iii) Wearing of face
cool air containing dust and smoke close masks by factory
to the earth’ surface) workers.
Carbondioxide [CO2] i) Causes Greenhouse effect; warming up
i) Motor vehicle exhausts of the earth’s atmosphere as a result of the Planting more green
ii) combustion of fossil blanket of some atmospheric gases e.g. plants, reduction in
fuels CO2 preventing escape of solar radiation combustion of fossil fuels
higher into space. by relying on alternative
ii) Increased atmospheric CO2 levels results sources of energy e.g.
in global warming – the observed average solar energy
global temperature rise of 0.80C since
1900
Nitrogen oxides Acid rain formation, contribute to
Car exhaust emissions and greenhouse effect. Car exhaust control
industrial flue gases
Chlorofluorocarbons
[CFCs] Enters stratosphere, the chlorine reacts with
Aerosol propellants, ozone hence reducing the ozone layer and
refrigerator and air permitting greater penetration of UV light Ban on the use of CFCs
conditioner coolants, to cause global warming.
expanded plastics. E.g.
bubbles in plastic foam used
for insulation and packaging
Noise Hearing impairment, total deafness, and Effect laws against
Discos, road traffic, engines, nervous disorders. excessive noise, put on ear
machines, aeroplanes, muffs and plugs while in
firearms industry.
Radioactive fallout from Ionizing radiation causes cancer Nuclear power controls
explosion
Nuclear weapons and nuclear
power fuels
GREEN HOUSE EFFECT AND GLOBAL WARMING

Greenhouse effect
This is a description of the condition which results when greenhouse gases i.e. gases in the troposphere
(atmosphere’s inner most layer extending about 17km above sea level) like carbon dioxide, water vapour,
methane and nitrous oxide allow mostly visible light, some infrared radiation and ultraviolet radiation from
the sun to pass through the troposphere to the earth, which transforms this solar energy to longer-wavelengths
- infrared radiation (heat), which then rises into the troposphere. Molecules of greenhouse gases absorb and
Page 419 of 447

emit this heat into the troposphere as even longer-wave length infrared radiation, which causes a warming
effect of the earth’s surface and air. The tropospheric gases act like a glass of large green house surrounding
the earth.
Global warming
This is the observed average global temperature rise of 0.80C since 1900 as a result of the enhanced natural
greenhouse effect.
Origins/sources of greenhouse gases Effects of global warming
i) combustion of fossil fuels by motor engines and i) Rise in sea level due to melting of polar ice
industries releases carbondioxide and methane and thermal expansion of seas.
into the troposphere ii) Altered temperature gradients cause
ii) deforestation and clearing of grasslands reduces cyclones and heavy rains as water
the uptake of carbondioxide in photosynthesis evaporates quicker.
iii) ruminant fermentation produces methane, which iii) Species migrations which are likely to cause
is released into the troposphere pests/diseases to extend their ranges.
iv) use of aerosol propellants, which contain CFCs iv) Reduced crop yields due to drier weather.
that are 105 times worse than carbondioxide as v) Increased crop yields because of more
greenhouse gases rainfall and longer growing seasons in some
v) cultivation of rice in swamps and paddy fields regions.
causes anaerobic fermentation , which produces vi) Flooding low-lying islands and coastal
methane cities.
vi) use of inorganic fertilisers causes the release of vii) Extinction of some animal and plant
nitrous oxide species.
viii) Increased death of the human
population.
ix) Greatly increased wild fires in areas where
the climate becomes drier.
ACID RAIN
Formation
Combustion of fossil fuels releases sulphurdioxide and nitrogen oxides into the atmosphere. Catalysed by
ammonia and unburnt hydrocarbons, these oxides react with water in the clouds to form solutions of sulphuric
acid and nitric acid, which make up acid rain.
Effects
i) Hydrogen ions bound to soil particles are displaced into runoff water by the SO42- ions from sulphuric
acid, causing formation of soft exoskeletons, which results into death of invertebrates
ii) Aluminium ions are displaced from soil by SO42-ions into water where it interferes with gill functioning in
fish causing their death.
iii) Aluminium ions displaced from soil by SO42- ions into water are toxic when absorbed by plants
iv) The leaching action of acid rain removes calcium and magnesium ions from soil causing poor formation
of middle lamella and chlorophyll in leaves.
v) Contributes to human respiratory diseases such as bronchitis and asthma.
vi) Can leach toxic metals such as lead and copper from water pipes into drinking water.
vii) Damages statues and buildings
viii) Decreases atmospheric visibility, mostly because of sulphate particles
ix) Promotes the growth of acid-loving mosses that can kill trees.
x) Loss of fish populations when the PH lowers blow 4.5
Prevention
i) Installation of SO2 extraction units (wet scrubbers) in chimneys of industries.
ii) Cleaning up exhaust emissions by encouraging several pollutants to react with one another to give less
harmful products in catalytic converters.
iii) Reduce coal use.
iv) Increase use of renewable resources.
v) Tax emissions of Sulphurdioxide i.e. “Polluter pays principle” should be adopted everywhere
Page 420 of 447

Why high-altitude lakes quickly become acidic than low-altitude lakes?

Low-altitude lakes are richer than high-altitude lakes in limestone which buffers against the effects of acid
rain, and also the surrounding soils to low-altitude lakes are deeper.
MAJOR CATEGORIES OF WATER POLLUTION

A) Sewage discharge into rivers
Sewage is liquid waste (composed of faeces, urine, water, detergents and other substances) from industries
and or homes carried through pipes called sewers.
Effects of sewage discharge into rivers - changes in abiotic (a and b) and biotic (c and d) conditions
Component description from the point of
discharge and explanation
Part a
Dissolved oxygen and B.O.D (Biological oxygen
demand)
BOD is the mass of oxygen consumed by
microorganisms in a sample of water in a given
time - usually measured as the mass (in mg) of
oxygen used by 1dm3 of water stored in darkness
at 200C for 5 days.
B.O.D indicates the oxygen not available to more
advanced organisms. Therefore a high B.O.D
indicates anaerobic conditions (low oxygen
availability).
Dissolved oxygen level is high in unpolluted
water but decreases rapidly at sewage outfall
(discharge) to the minimum and then increases
gradually downstream, returning to a normal
level further downstream
 B.O.D is very low in unpolluted water,
increases rapidly at outfall then decreases
gradually downstream.
 Decomposition of organic components of
sewage by aerobic bacteria coupled with
reduced photosynthesis because of low
illumination caused by suspended solids in
sewage rapidly reduce oxygen (cause oxygen
sag) and create a high BOD at outfall.
 The gradual increase of oxygen downstream
is because of increased photosynthesis and
dissolution from atmosphere.
 The death of aerobic bacteria due to
reduction in organic substances decreases
BOD down stream
Suspended solids
 Suspended solids are very few before outfall, increase rapidly at the sewage discharge but
progressively decrease downstream
 Sewage discharge adds decomposable organic matter into the water at the point of discharge, the
progressive decrease downstream is due to bacterial consumption and dilution by water.
Part b
NH4+, NO3- and PO43-(Ammonium, nitrate and phosphate ions)
 NH4+, NO3- and PO43-ion concentration is very low before outfall
Page 421 of 447

 NH4+ions increase rapidly at discharge, then very rapidly to a maximum just after outfall, then decreases
first rapidly and later gradually to a very low level downstream.
 NO3- ions first decrease gradually to a minimum concentration after outfall, gradually increase to a
maximum a short distance downstream, then decreases gradually further downstream.
PO43- ion concentration increases (1) rapidly at discharge, (2) gradually just after outfall to a maximum, then
decreases gradually to a very low level downstream.
 Sewage contains NH4+ions. Putrefying (ammonifying) bacteria convert organic nitrogen-containing
compounds in sewage to NH4+ just after outfall. Downstream, NH4+ions are converted to NO3- by
nitrifying bacteria and further downstream there is dilution by water.
 NO3- ions first decrease due to consumption by sewage fungus abundant at outfall, then gradually increase
because NH4+ions are converted to NO3- by nitrifying bacteria, then decrease gradually due to
consumption by plants and algae.
 Sewage contains PO43- ions from (1) detergents and (2) decomposition of organic matter, yet the
consumption by autotrophs is very low at outfall, accounting for the high PO43- ion concentration.
 PO43- ion gradual decline downstream is caused by (1) absorption by the progressively increasing
populations of autotrophs (2) storage in sediments.

Part c
Aerobic bacteria, sewage fungus, algae and higher plants
 Aerobic bacteria are very few before, but very many at outfall, then their population decreases rapidly
immediately and gradually after out fall downstream.
Page 422 of 447

 Sewage fungus is contained in sewage population, increases to a maximum immediately after outfall, but
decreases rapidly downstream to very low level.
 Algae and higher plant populations decrease rapidly to a minimum at outfall but increase rapidly a short
distance downstream and return to normal further downstream.
 Sewage contains aerobic bacteria that feed on organic substances, but population falls as availability of
oxygen and nutrients diminishes.
 Population increases at outfall because the sewage fungus thrives in anaerobic conditions and is very
tolerant of high ammonia concentrations.
 The rapid decrease in populations results from reduced photosynthesis because of the turbidity caused by
suspended solids, the rapid increase is because of the high concentrations of NO3- (nitrate) ions and
increased illumination because suspended solids reduce and water becomes clearer.
Part d
Clean water fauna (e.g. stonefly nymphs, may fly larvae, perch, trout), Asellus (fresh water louse),
Chironomus (bloodworm), rat tailed maggot and Tubifex
(not indicated on the graph but it can be sketched basing on tolerance to pollution)
 The populations of clean water fauna are high before outfall, decrease rapidly to zero at outfall only
appearing and increasing to normal with distance downstream.
 Asellus population decreases rapidly to zero at outfall, only appearing and increasing rapidly to a maximum
a short distance downstream after which it decreases rapidly.
Tubifex population increases rapidly to a maximum at outfall and then decreases rapidly downstream. Rat
tailed maggots’ population increases rapidly to a maximum a short distance after outfall and then decreases
rapidly downstream and Chironomus population increases rapidly to a maximum at a slightly longer distance
from outfall and then decreases rapidly downstream.
 Clean water species cannot tolerate anaerobic conditions at outfall, populations increase downstream
because oxygen and food become available.
 Asellus cannot tolerate anaerobic conditions at outfall and therefore dies and/or migrates to the relatively
less polluted water downstream where it shrives. The decrease thereafter is due to consumption.
Tubifex, rat tailed maggots and Chironomus are (i) relatively inactive to reduce oxygen demand and (ii) have
respiratory pigments with very high affinity for oxygen enabling them to be tolerant to anaerobic conditions.
The increase in their population downstream indicates the level of pollution in the water. Tubifex, is the most
tolerant to anaerobic conditions, followed by rat tailed maggots and Chironomus. The decrease in population
downstream is partly due to predation.
Note:
Flowing rivers naturally undergo self-purification to recover from pollution through a combination of dilution
and biodegradation, but the recovery time and distance depend on;
i) volume of incoming degradable wastes in sewage
ii) flow rate of the river
iii) temperature of the water
iv) PH level of the water
Addition of inorganic chemicals, plant nutrients and sediments into lakes

Pollutant Examples Main human Harmful effects
sources
Plant Nitrate (NO3-), Raw sewage (1) Rapid growth of algae and green protists (algal
nutrients phosphate (PO43- discharge, blooming) (2) reduces light penetration in water
) and ammonium detergents and leading to (3) Death and decay of algae, which
+
(NH4 ) ions. The other chemical depletes water of dissolved oxygen, killing fish and
nutrient release from other aerobic animals. (4) Excessive levels of NO3-
enrichment of industries, if drank in water lowers the oxygen carrying
water bodies is leaching of
Page 423 of 447

termed inorganic capacity of blood and kills unborn children and

eutrophication fertilizers e.g. infants (“blue baby syndrome”)
NPK from
farmland.
Sediment (1) soil (2) silt Land erosion (1) cause turbidity / cloudiness in water and reduce
photosynthesis, (2) settle and destroy feeding and
spawning grounds of fish, (3) clog and fill water
bodies, shortening their lifespan (4) disrupt aquatic
ecosystems (5) carry pesticides, bacteria and other
harmful substances into water.
Inorganic Acids, Surface runoff, (1) Drinking water becomes unusable for drinking
chemicals compounds of industrial and irrigation (2) Lead and Arsenic damage the
toxic metals like effluents and nervous system, liver and kidneys (3) they harm fish
lead (Pb), household and other aquatic life (4) they lower crop yields (5)
mercury (Hg), cleaners. they accelerate corrosion of metals exposed to such
arsenic (As) and water.
selenium (Se)
and (3) salts e.g.
NaCl in ocean
water
Heat (thermal) pollution:

Main human sources
Water cooling of electric power plants and some types of industrial plants
Harmful effects
1) Lowers dissolved oxygen levels since solubility of most gases reduces with temperature.
2) Makes aquatic organisms more vulnerable to disease, parasites, and toxic chemicals
3) When a power plant shuts down for repair or opens, fish and other aquatic organisms adapted to a
particular temperature range can be killed by the abrupt change in water temperature. This is known as
thermal shock.
4) Some aquatic animals may migrate to waters with favourable temperature.
Note:
Effects of eutrophication are more severe in water bodies where thermal pollution occurs because of (1)
increased decomposition of organic matter and metabolism, which raises the demand for oxygen by higher
organisms, (2) reduced dissolved oxygen levels in water.
Chemical pest control

Properties of an ideal pesticide, should;
i) be biodegradable / non-persistent so that toxic products are not left in or on crop plants
ii) be specific so that only pest species is killed.
iii) not accumulate either in specific parts of an organism or as it passes along food chains.
iv) effectively control the pest under field growing conditions.
v) be easy to apply at the correct dosage.
Page 424 of 447

Problems of using insecticides

1. Bioaccumulation (some molecules of the
pesticide may be stored in specific organs
or tissues at levels higher than would be
expected) and biological magnification
(the pesticide may get more concentrated
as it passes along the food chains and
webs) may occur. See the figure on the
right: (Soper page 336 fig 10.32)
2. Many are non-specific, killing non-target
species, particularly natural predators of
the pest species i.e. by killing pests, the
natural enemies of the pests will lack food
hence they too will die.
3. Accidental misuse of toxic chemicals results in death of humans and domestic animal.
4. Effect on the cycling of materials
5. Pest resistance occurs i.e. genetic variation enables a few individuals in the pest population to survive and
may quickly reproduce.
6. Effect on the cycling of materials
7. There is pest replacement i.e. since most crop are susceptible to attack by more than one pest species, and
the pesticide may be more deadly to one species than another, elimination of one species may simply
allow another species to assume major pest proportions.
8. Pest resurgence may occur i.e. non-specific pesticides may kill natural predators as well as pests, and so a
small residual pest population may multiply quickly without being checked.
Page 425 of 447

Dichlorodiphenyltrichloroethane (DDT)
Concentration of DDT in water = 0.000003 parts per million (ppm)
Concentration of DDT in ospreys = 75ppm
DDT concentration in osprey compared to water = 75/0.000003
DDT therefore was magnified by magnified by a factor of 2.5 million times
Effects of DDT Effects of other pesticides in general

i) Inhibits cytochrome oxidase enzyme. i) Inhibition of enzyme activity
ii) Limits reproductive success, especially by ii) Kill organisms.
causing formation of thin eggshells in birds of iii Reduce species diversity
prey. iv) It increase productivity at lower levels of the
iii) Kills organisms. ecosystem
v) It increase productivity at higher levels of the
ecosystem
Fig 32.18 page 540 Roberts
Biological pest control

Control of a pest population using its natural predator, parasite or pathogens
Steps involved in biological control:
a) Identifying the pest and tracing its origins, i.e. where it came from.
b) Investigating the original site of the pest and identifying natural predators, parasites or pathogens of the
pest.
c) Testing the potential control agent under careful quarantine to ensure its specificity.
d) Mass culturing of the control agent.
e) Development of the most effective distribution / release method for the control agent.
f) Use procedures which make it impossible for the pest to complete its life cycle
Several human activities which directly destroy the environments are:

1. Deforestation; forests are important for:
Have most species and diverse wild life communities. Their destruction will lead to extinction of numerous
species and less of genetic variety and potential resources.
Forests protect the soil. Deforestation leads to soil erosion, clear water supply are destroyed and silting of
reservoirs.
Timber harvest, poles, food, fuel, honey, fruits and herbs
Forests catch large amounts of rain and release the water slowly into streams and rivers. Their destruction cause
floods in areas down-hill.
Forests release large amounts of oxygen and absorb carbon dioxide (lock it up) during photosynthesis.
Deforestation has led to increased global carbon dioxide hence causing global warming.
Forests influence the amount and frequency of rain fall received in an area.
Page 426 of 447

2. Poor agricultural methods:

These include Monoculture, shifting cultivation, use of artificial fertilizers and pesticides. Excess fertilizers and
pesticides leads to eutrophication of water bodies giving rise to build up of toxic by-products by leaching and
draining. Pesticides/herbicides/fungicides draining away from fields enter water ways are connected through
the food chain. This leads to poisoning of top carnivores.
3. Poor methods of mining; such as open cast mining destroy habitats. Mineral elements mined cause
destruction of the environment around the mine. Heavy metals like lead and mercury drain into water bodies
causing severe destruction of aquatic fauna, poison water for human consumption; sulphides destroy
vegetation altering the structure of plant and animal communities.
4. Urbanization/human settlement; e.g. aggregation of people, food and water supply, garbage disposal.
5. Fires; burning of fossil fuel
6. Cement manufacture
NATURAL RESOURCES
From a human stand point, a resource is anything obtained from the environment to meet human needs and
wants. Natural resources are those not made by man.
While some resources are directly available for use e.g. solar energy, fresh air, wind, fresh surface water,
fertile soil, wild edible plants others become available after processing has been done e.g. petroleum, metallic
elements like iron, ground water, modern crops.
CLASSIFICATION OF NATURAL RESOURCES

Type of resource Example
Perpetual resources Solar energy, wind, tides.
Resources that are replaced (renewed) continuously on human
time scale
Renewable resources Fresh water, fresh air, fertile soil,
Resources that are replenished (replaced) fairly rapidly (hours to animals and plants (Forests, grasslands)
decades) through natural processes as long as the usage is not
faster than the replacement
Nonrenewable resources Fossil fuels (e.g. coal, oil, natural gas),
Resources that exist in a fixed quantity or stock in the earth’s metallic minerals (e.g. copper, iron,
crust. aluminium), non-metallic minerals (e.g.
On the shorter human time scale, they depleted much faster than salt, clay, sand, phosphates)
they are formed i.e. the energy crisis
Further terms associated with natural resources

Sustainable yield
The highest rate at which a renewable resource can be used indefinitely without reducing its availability
supply.
In spite of the renewability, renewable resources can be depleted or degraded.
Environmental degradation
The process when the resource’s natural replacement rate is exceeded resulting into a decline in its
availability.
Urbanization of productive land, excessive soil erosion, deforestation, ground water depletion, overgrazing of
grasslands by livestock, reduction in the earth’s forms of wildlife by elimination of habitats and species,
pollution, water logging and salt buildup in soil.
Sustainable yield
The highest rate at which a renewable resource can be used indefinitely without reducing its availability
supply.
In spite of the renewability, renewable resources can be depleted or degraded.
Page 427 of 447

Reusing of resources
Using of resources over and over in the same form.
Glass bottles of alcoholic and soft drinks can be collected, washed and refilled many times.
Wildlife
This is includes plants and animals that occur in their natural environment such as forests and wild animals
WHAT IS BIODIVERSITY (BIOLOGICAL DIVERSITY)?

The different life forms and life-sustaining processes that can best survive the variety of conditions currently
on earth.
Kinds of biodiversity
a) Genetic diversity – variety in the genetic makeup among individuals within a species.
b) Species diversity (species richness) – number of species present in a habitat.
c) Ecological diversity – the different biological communities e.g. forests, deserts, lakes etc.
d) Functional diversity – biological and chemical processes or functions such as energy flow and matter
cycling needed for the survival of species and biological communities.
Species abundance refers to the number of individuals of each species
Term Definition/ explanation
Immigration Movement of individuals into a population from neighbhouring populations.
Emigration Departure of individuals from a population.
Rare species Species with small populations either restricted geographically with localized
habitats or with widely scattered individuals.
Endangered Species with low population numbers that are in considerable danger of becoming
species extinct.
Extinct species Species, which cannot be found in areas they previously inhabited nor in other likely
habitats.
Factors that affect species diversity on land and in water

a) Latitude (distance from equator) in terrestrial communities; species diversity decrease steadily with
distance from the equator toward either pole, resulting in the highest species diversity in tropical areas e.g.
tropical rain forests and lowest in polar areas such as arctic tundra. The main effect of latitude is on
temperature, which later affects life.
b) Depth in aquatic systems; in marine communities, species diversity increases from the surface to a depth
of 2,000 metres and then begins to decline with depth until the deep-sea bottom is reached. This change is
attributed to light penetration which affects photosynthesis, availability of oxygen and availability of dead
organisms at the sea bottom.
c) Pollution in aquatic systems; increased pollution kills off or impairs the reproductivity of various aquatic
species hence reducing species diversity and abundance.
d) Increased solar radiation increases species diversity in terrestrial communities.
e) Increased precipitation in terrestrial communities increases species diversity.
f) Increased elevation decreases species diversity.
g) Pronounced seasonal changes increase species diversity.
FACTORS THAT AFFECT SPECIES DIVERSITY IN AN ISLAND ECOSYSTEM

Robert MacArthur and Edward O Wilson (1960s) studied communities on islands after which they proposed
the species equilibrium model or the theory of island biogeography.
 According to this model, the number of species found on an island is determined by a balance between
two factors:
a) The rate at which new species immigrate to the island and
Page 428 of 447

b) The rate at which species become extinct on the island.

 The model predicts that at some point the rates of immigration and extinction will reach an equilibrium
point that determines the island’s average number of different species (species diversity)
 The model also predicts that immigration and extinction rates (and thus species diversity) are affected by
two important features of the island;
a) Size of the island.
b) Distance of the island from the nearest main land.
Size of the island
Larger islands tend to have higher species diversity than Small islands because of two reasons:
i. Small islands generally have lower immigration rates since they are a smaller target for potential
colonizers
ii. Smaller islands should have a higher extinction rate because they generally have fewer resources and
less diverse habitats for colonizing species.
Distance of the island from the nearest main land:
For two islands of about equal size and other factors remaining constant, the island closest to the main land
source of immigration species will have the higher immigration rate and thus a higher species diversity
(assuming that extinction rates on both islands are about the same)
Explanations from the observations made from the graphs

a) Immigration and extinction rates
 The rate of immigration decreases with increase in species number, while the extinction rate increases
with increase in species number on the island.
 The equilibrium number of species on the island is reached when immigration rate and extinction rate
equal.
 Extinction rate increases with increasing species number because of interspecific and intraspecific
competition.
b) Effect of island size on immigration and extinction rates
 The rate of extinction increases with increase in species number on the island on both small and large
islands, but it is higher on small islands than on large islands. The higher extinction rate on small
islands is because of the fewer resources and less diverse habitats for colonizing species.
 The rate of immigration decreases with increase in species number on both small and large islands,
but with a large island having a higher immigration rate than a small island. Small islands have lower
immigration rates because they are a smaller target for potential colonizers.
c) Effect of distance from mainland on immigration and extinction rates
Page 429 of 447

 For both near and far islands, immigration rate decreases with increase in species number, but
immigration rate is higher on near island than on the distant island. The higher immigration rate on
near island is because of the easy reach by organisms enabled by its proximity to the main land.
 Since extinction rate increases with increasing species number that exert interspecific and
intraspecific competition, extinction rate is far higher on small islands due to the fierce competition
caused by the higher immigration rate because of easy reach by organisms enabled by its proximity to
the main land.
Conservation of natural resource

Ecosystems contain natural biological resources which are useful for (1) food (2) medicine (3) raw materials
for industries (4) biological control of pests and diseases (5) biogeochemical cycling (6) pleasure i.e.
ecotourism.
Some of the conservation methods employed include;
(a) restricting urban and industrial development (f) establishing conservation areas such as national
(b) implementing recycling programmes parks, zoos, botanical gardens, nature reserves
(c) legislating the protection of endangered and and sanctuaries
keystone species, and enforcement of the law (g) effective pollution control methods, especially
(d) carrying out breeding programmes and when wildlife species are vulnerable to
establishing sperm/seed banks to maintain high pollutants
biodiversity (h) creating corridors that connect habitat fragments
(e) implementing sustainable development (i) bioremediation i.e. using organisms e.g. fungi
programmes and prokaryotes to detoxify polluted ecosystems
SAMPLE QUESTIONS
1. The graph in figure1 table shows the rate of decomposition of the same plant material with depth below
the soil surface in forest habitat. Study the figure and table and answer the questions that follow.
Page 430 of 447

Depth ( cm) 1 2 3 4 5 6 7 8 9 10
%decomposition 7mm 95 80 65 50 35 20 8 2 0 0
0.5mm 40 35 20 10 6 3 1 0 0 0
a) Using the same axes plot graphs of percentage decomposition against soil depth.
b) Explain the relationship between
i) The mesh size
ii) Soil depth and rate of decomposition of the leaf discs.
c) Copy out figure 1 and extrapolate both graphs in the figure to Show the trend of decomposition of
the leaf discs with time.
d) What is the ecological significance of leaf decomposition in a natural habitat?
2. Analysis of oxygen dissolved and PH in the upper layer of water together with productivity of the entire water body
was carried out in one of Uganda’s most productive lakes. The results of this analysis are shown in the table below.
Study the tables and answer the questions that follow.
Table 1: OXYGEN DISSOLVED AND PH.
Night Day Night

Time/hrs 24:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 24:00
%saturation of 65 55 35 65 85 115 105 85 75
oxygen
PH 6.6 6.5 6.4 6.6 6.8 6.8 7.0 6.8 6.7
TABLE 2: PRODUCTIVITY.
Still air Windy air

Day Dark Net Day Dark Net
Time productivity/ productivity/
Depth/m gm-3 gm-3
Limnetic 100 3.30 1.30 2.00 2.70 0.30 2.40
Littoral 300 7.50 1.50 6.00 7.80 2.00 6.80
Profundal 500 4.70 0.40 4.30 5.30 600 4.70
Benthic 700 0.30 0.10 0.20 0.40 0.20 0.20
(a) Plot a graph of net productivity in the different zones/layers of the lake under still and windy
conditions.
(b) Describe and explain the pattern of variation of productivity in still and windy conditions in the
different layers of the lake.
Page 431 of 447

(c) Explain the pattern of oxygen content dissolved and PH in the upper layer of the lake for a period of 24
hours.
(d) Explain what would happen to the above three factors in the water body if organic fertilizers were added to
the system after a long period of time.
(e) Apart from the factors mentioned above, state any other factors that would determine the percentage
saturation of oxygen in a lake.
3. Figures 1, 2 and 3 show the immigration and extinction of species on different categories of virgin islands.
Figure 1 shows the rate of immigration of new species on an island nearby the shore and one that is far from the
shore.
Figure 2 shows the rate of extinction of species on a large island and on a small island.
Figure 3 shows the rate of immigration and extinction of species on an island.
Study the information and use it to answer the questions that follow.
Rate of extinction of
species on island
large island
Fig. 2 Number of species

Rate of immigration and
extinction on an island

(a) Explain the rate of
Page 432 of 447

(i) immigration of new species on an island that is near to the shore and one that is far from the shore ( figure
1 ). (10 marks)
(ii) extinction of species on a small island and on a large island ( figure 2 ) ( 09 marks)
(iii) immigration and extinction of species on an island. ( figure 3 ) ( 07 marks)
(b) From figures 1, 2 and 3 what conclusions can you draw about what determines the number of species on an island?
( 05 marks)
(c) Describe how factors other than those depicted in the information provided, may affect the immigration of new
species on an island. (04 marks)
(d) Suggest the factors that would cause immigration of new species to a virgin island. 05 marks)
(b) In what ways do the predator-prey interactions compare with parasite-host interactions (07 marks)
(c) Explain how predator-prey interactions influence the formation of a new species (08 marks)
5. Discuss the effects the following might have if
a) Untreated sewage entered a slow flowing river (07 marks)

b) Hot water from a cooling tower of a power station entered a slow flowing river (06 marks
c) A forest is cleared to establish an industrial park (07 marks)
6. (a) Discuss the various forms of population dispersion in ecosystems
(b) Explain the various phases of the following population growth curves in ecosystems
i. Sigmoid curve
ii. Boom and burst curve
iii. T-shaped curve
(c) (i) Sketch a curve to illustrate population changes in island biogeography
(ii) Explain the rate of extinction and immigration curves in a bigger and small island shown in your sketch
(c) Discuss the various methods of estimating population sizes of small animals and plant, stating the advantages
and disadvantages of each method
7. (a) (i) How might thermal pollution cause the death of fish such as tilapia? (02 marks)
(ii) Explain why the addition of nitrogen fertiliser to sea water can accelerate the cleanup of an oil spill (02)
(b) For each of the following agricultural practices, state two benefits and two adverse environmental or human
consequences. (16 marks)
i. Deforestation
ii. Applying nitrogenous fertiliser to crops
iii. Growing crop plants with genetically engineered plants which are resistant to herbicides
iv. Burning agricultural waste such as coffee husks
8. The graph below shows the changes in total net production, standing dry mass of a plant litter and root production in
a freshly planted forest over the years
Page 433 of 447

(a) Compare the total net production with litter production of plants (06 marks)
(b) Describe the changes in the four variables shown in the graph (15 marks)
(c) Account for the changes described in (b) above (12 marks)
(d) Predict the likely changes in each variable if the study is repeated and the climate remains constant
9. (a) What is meant by endangered species? (02 marks)

(b) Describe how organisms become endangered (08 marks)
(c) How would you ensure that organisms that are endangered get conserved (08 marks)
(d) Suggest reasons why large mammals are more prone to extinction than small mammals. (02 marks
10. (a) What is an ecosystem?
(b) Describe the flow of energy and recycling of carbon and nitrogen in any named ecosystem
11. (a) How are the following organisms adapted to their modes of life
(i) Schistosoma mansoni (iv) Puccinia
(ii) Ancylostema duodenale (v) Tick
(iii) Ascaris Lumbricoides (vi) Phytophora infestans
(b) Give a brief description of the major types of interspecific associations in nature
(c) Outline the life cycle of:
(i) Plasmodium (iii) Schistosoma
(ii) Tapeworm (iv) Filarial worm
(d) Rhizobia lives a symbiotic life with legume plants
(i) What is meant by symbiotic life
(ii) What are the benefits of the symbiotic associations between rhizobia and the legume
12. The data below shows the number of larvae in the sprayed and unsprayed areas with varying concentration of DDT
for given months of application. Table 1 shows the changes after three applications of DDT in June, July and
August. Table 2 shows changes after one application of DDT in august.
Table 1
Months June July August

Concentration of DDT (ppm) 3 5 7 9 11 13 15 17 19 21 23
Number of Sprayed 2 4 6 10 16 34 72 104 114 118 120
larvae Unsprayed 6 6 6 8 10 16 30 46 56 60 60
Table 2
Month August
Concentration of DDT (ppm) 20.0 20.8 22.2 23.4 24.6 26.0 28.0 30.0
Number of Sprayed 0 2 10 30 60 92 106 112
larvae Unsprayed 0 6 18 18 78 92 98 100
(a) Using graph papers draw graphs for;
i. One application of DDT in august
ii. Three applications of DDT in June, July and August
(b) Using bar graphs and the graphs you have drawn in (a) (i) and (ii), describe the changes in the number of
larvae for sprayed and unsprayed after
i. One application of DDT in august
ii. Three applications of DDT in June, July and August
(c) (i) Explain the terms non-targets and pesticide resurgence
(ii) Using your knowledge of pesticides, explain the results obtained in the experiments for both one
application and three applications of DDT in the sprayed and unsprayed areas
(d) (i) Why is DDT considered to be a potentially bad pesticide?
(ii) What are the quantities of a good parasite?
13. In shallow water bodies, e.g. ponds and lakes, the water temperature varies with depth. In deep waters of lakes in
temperate regions, a thermocline forms during summer. A thermocline is a middle layer in a lake where temperature
changes rapidly with depth.
The table below shows the seasonal changes in temperature and oxygen content with depth
Depth/m Temperature/oC Oxygen concentration/ppm
Winter Summer Winter Summer
0 0 25 12 15.0
1 2.5 24.5 12 15.0
2 3.5 23.3 12 15.0
Page 434 of 447

3 4.0 23.0 12 14.5

5 4.0 22.0 12 13.0
7 4.0 15.0 12 9.0
9 4.0 15.0 12 3.5
11 4.0 8.0 11.5 3.5
13 4.0 5.0 11.5 0.0
15 4.0 5.0 11.5 0.0
a. Represent the data in a graph form on the same axis

b. From your graph, determine the depth range at which the thermocline forms in summer
c. (i) In summer, organic matter tends to collect, i.e. accumulate at the lake bottom. Explain the role of the
thermocline in this phenomenon
(ii) Where and when the thermocline persists, the warm layers can support a very limited biomass
14. (a) What is meant by the term ecological succession?
(b) Describe the stage involved in succession on an abandoned farm land?
(c) How does biomass, specie number and biodiversity vary with the several stages of succession?
(d)
15. (a) What is meant by;
(i) Greenhouse effect (02 marks)
(ii) Eutrophication (02 marks)
(b) To what extent have human activities contributed to the enhanced greenhouse effect? (10 marks)
(c) Suggest practical remedies to the greenhouse problem (06 marks)
16. Antelopes are common in E. Africa grasslands. Outline the inter-relationship which may exist between the antelopes
and;
a) Other mammals
b) Insects
c) Green plants
17. The table below shows the characteristics of pastures under varying grazing intensities in Rwenzori National Park in
Uganda.
QUALITY OF PASTURE GRAZING INTENSITIES
Under grazed Moderate over grazing Heavily overgrazed
Number of sample surface quality (% of sample) 158 60 110
With vegetation 24.1 31.1 17.6
Bare but protected (with shade) 72.1 15.7 17.0
Completely bare with soil erosion 3.8 52.6 65.4
Composition of vegetation occurrence 0.0 6.1 50.8
Palatable grasses 71.3 36.3 0.4
Unpalatable grasses 5.3 34.5 28.9
Shrubs 0.0 4.7 33.1
Study the table above and answer the questions below;

(a) (i) Describe quality of the surface in Rwenzori National Park
(ii) Account for the quality of the surface from the composition of the vegetation
The figure below illustrates the pattern of changes of flora and mammals faced with increasing levels of overstocking in a
game reserve
Page 435 of 447

A B C
biomass of
mammals
number of plant
species
biomass of
resistant plant
species
number of
resistant
(b) (i) State the level of overstocking in A, B and C.

(ii) Explain the relationship between the number of plant species, number of species of mammals and biomass
of resistant plant species
(iii) State five criteria that can reflect the presence of over grazing
(c) What factors should be taken into account for proper management of range in a game park?
18. (a) What is meant by the term parasite?

(b) With examples, describe the adaptations of parasites to their mode of nutrition
19. In nature, organisms of the same kind rarely occupy the same niches at the same intervals, studies carried our around
Natete Market showed four species of fresh eating birds occupying the same niches. These birds were found when
either packing feathers, resting, soaring or feeding. The feeding patterns of all species of birds were recorded over
time on a daily basis and the average of the numbers feeding in each species was computed for a period of one
month. The results are shown in the table below;
Time 7am 8am 9am 10am 11am 12pm 1pm 2pm 3pm 4pm
Marabou stock 5 6 8 10 9 8 6 5 3 2
Kites 10 11 13 20 15 10 13 12 10 10
Bird species Bald headed eagle 25 27 24 16 11 5 5 18 19 20
White breasted eagle 3 10 18 26 37 47 38 30 21 26
The sizes of these birds from the smallest are arranged as in the table above;
(a) Plot a graph of the diurnal feeding patterns of these birds on the same axes
(b) (i) Describe the nature of the graphs
(ii) Suggest explanations for the patterns described in (b) (i) above
(iii) Identify the curve which behaves abnormally and account for the strange behaviour of the bird it represents
(c) (i) From your graph, suggest how organisms of the same kind can occupy the same niche
(ii) What would happen to the organisms if they do not adopt to the mechanisms in (c) (i) above
(d) (i) What is the importance of vultures in nature?
(ii) What would happen to the habitat if all the vultures were poisoned?
(e) (i) Outline the method which was used to obtain the data above
(ii) Suggest two advantages of the method given in (e) (i) above and give reasons for your answer
(e) How does the relationship in (d) compare with that in (a)
20. A study was carried out on the effect of fire on a savannah grassland. The figure below shows temperature recorded
during a period of 5 minutes while a front of fire passed through a stand of dry grasses.
Page 436 of 447

figure 1
800
700
TEMPERATURE IN 0C
600
0.5m above soil surface
500
400
300
200
soil surface
100 1.5m above soil surface
0
0 1 2 3 4 5 6
TIME IN MINUTES
Table 1 below shows the effect of grass fire upon biomass and numbers of arthropods per 100cm 3 in grassland plots
studied.
Eve of fire One day after fire % reductions One month after fire
Biomass Nos. 60.1 19.5 67.6 19.8
Grasshoppers 135 9 93.3 200
Caterpillars 83 23 72.3 30
Mantids 84 39 53.6 36
Aphids e.t.c. 331 11 96.7 17
Large Hemiptera 201 53 73.6 64
Cockroaches 104 86 17.3 29
Arachnids mostly spiders 1341 1273 5.1 533
Total 2688 1710 36.1 1044
Study figure 1 and table 1 carefully and then answer the questions that follow;
(a) (i) Comment on the temperature changes recorded during the study
(ii) What would have most likely
(b) (i) What was the effect of the grass fire on the arthropods?
(ii) Explain the recovery from the effect of such fire upon other fauna?
(c) Explain the recovery from the effects of fire in the grassland
(d) In what other ways is fire important in a grassland?
(e) Outline the ways in which a grassland community may be modified?
(f) Which arthropods are most affected by fire
i. Immediately after the fire
ii. One month later
(g) How would you explain each of these observations?
(h) Why is grassfire less likely to destroy birds and small animals?
21. (a) Explain the causes and effects of destruction of the stratosphere (07
marks)
(b) Compare oligotrophic and eutrophic lakes (10 marks)
(c) By giving an example, explain the term indicator species (03 marks)
22. (a) (i) How might thermal pollution cause the death of fish such as tilapia? (02 marks)
Page 437 of 447

(ii) Explain why the addition of nitrogen fertiliser to sea water can accelerate the cleanup of an oil spill
(b) For each of the following agricultural practices, state two benefits and two adverse environmental or human
consequences. (16 marks)
v. Deforestation
vi. Applying nitrogenous fertiliser to crops
vii. Growing crop plants with genetically engineered plants which are resistant to herbicides
viii. Burning agricultural waste such as coffee husks
23. (a) Distinguish between primary and secondary succession

(b) Describe the process of primary succession on bare rock
(c) Describe how energy flows in an ecosystem
24. How are the following plants adapted for their habitants
a. Xerophytes
b. Hydrophytes
c. Halophytes
d. Mesophytes
25. Discuss the various methods of estimating population sizes of small animals and plant, stating the advantages and
disadvantages of each method
26. In an ecological study of an aquatic habitat, a researcher carried out experiments on a small stagnant water body
which had been left behind a swamp during a long drought. He observed a surface which contained some
decomposing leaves and grass. After carrying out his experiments, the results on the saturation of oxygen and pH of
water at the various depth of the water body were put in a table as shown below.
Depth below water surface (cm) 00 05 10 20 40 60 80 100 In mud
Oxygen saturation by percentage 86 90 80 40 10 08 04 02 00
pH 8.0 7.6 7.0 6.0 5.8 4.5 3.0 2.7 2.0
(a) Represent the above information on a graph paper

(b) From the graphs above, explain your observations about variation of
i. Oxygen saturation and depth
ii. pH and depth
iii. pH and oxygen saturation
(c) Give an account of the effect of pH and oxygen concentration on aquatic organisms
(d) How the aquatic organisms in (c) above are adapted to their environment
(e) Briefly describe an experiment on how the researcher obtained the pH at various depths
27. The graph below shows the changes in total net production, standing dry mass of a plant litter and root production
in a freshly planted forest over the years
Page 438 of 447

(a) Compare the total net production with litter production of plants (06 marks)
(b) Describe the changes in the four variables shown in the graph (15 marks)
(c) Account for the changes described in (b) above (12 marks)
(d) Predict the likely changes in each variable if the study is repeated and the climate remains constant
(e) What factors can deflect the changes occurring in the forest (05 marks)
Figure 2 shows the change net primary productivity and biomass above the ground in a forest following an ecological fire.
1.4 Productivity
30
1.2
(NP. Per kg dry weight m-2 yr-1)
1.0 Biomass/kg dry

wtm-2
20
0.8
0.6 Biomas
Productivity
0.4 s 10
0.2
0 20 40 60 80 100 120 140

Age of forest in years
(i) Describe the changes in each of the variables shown on figure 2. (05 marks)
(ii) Account for the changes described in b (i) above. (07 marks)
(iii) Sketch a curve on figure 2 to show the productivity; biomass ratio would change with time.
(iv) Suggest an explanation for the curve drawn in (iii) above. (05 marks
28. (a) Outline the major adaptations of plants to survive;

i. In fresh water
ii. On dry land
(b) How do these adaptations compare with those of animals in similar habitats?
29. Echnococcus granulosus is a tapeworm commonly found in domestic animals, and man gets infected when he eats
meat containing viable larval stages of the parasite. An investigation was carried out to determine the incidence of
the parasite at a Kampala City abattoir. Table 1 shows the results of the investigation. Study the table carefully and
then answer the questions that follow
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct
Total slaughtered 1099 1014 947 1201 940 1070 1050 768 1253 1091
No. of infected livers 10 1 8 0 4 7 8 8 8 5
No. of infected lungs 18 18 45 71 59 74 112 65 81 55
Cows % infection 2.7 1.8 5.6 5.9 6.7 7.6 11.4 9.5 7.1 5.5
Total slaughtered 1074 1481 1055 1053 60 1239 1209 1150 851 649
No. of infected livers 5 3 4 0 0 5 12 5 3 2
Goats No. of infected lungs 15 8 39 0 29 42 89 41 37 22
% infection 1.9 0.7 4.1 0.0 4.8 3.8 8.4 4.0 4.7 3.7
(a) What conclusions can you draw from the results in the table?
(b) (i) Plot histograms of percentage infection against time of infection for goats and cows
(ii) What do you observe concerning the patterns of infections of goats and cattle?
(iii) What explanation can you suggest for these patterns?
(c) Suggest three ways by which the risk of infection to man could be reduced
(d) Describe how you would conduct an experiment to determine the incidence of E granulosus in goat populations
in a district in Uganda
Page 439 of 447

30. Describe how you would conduct experiments to determine changes in population of the following organisms:
(a) A species of plant in a grassland
(b) A small mammal such as a rodent also in a grassland area
(c) Protozoa in a water pond
31. (a) Outline the effects of large mammals in a national park

(b) Using a suitable method, describe how their population size may be determined
(c) Explain the management of such mammals for a suitable ecosystem
(d) What are the values of National Parks to Uganda
(e) What problems are often encountered in the management of National Parks?
32. (a) Insects are known to be the most successful animals in colononising a wide area on land. Discuss the adaptations
which have made them successful.
(b) Some insect species are known to be very serious pests and vectors of disease to man and his crops. Outline the
various ways available to control them and their effects.
(c) Describe the systematic operation of an insect’s heart
33. (a) Plants occupy the most important niches in an ecosystem.

(i) Explain the above statement
(ii) How do plants avail energy to the next trophic level
(iii)
(b) Explain why trophic levels do exceed six
(c) Describe the process of primary succession leading to the climax community
(d) Discuss man’s activities that have led to the formation of endangered species
34. (a) Explain the effect of predation on population size of organisms

(b) How has man affected the balance between predator and prey in a negative way?
(c) Giving examples, explain the various ways in which variation may arise
35. An investigation into the composition and number of arthropods found in forest and shrub savannah habitats was
carried out. The table below gives the profile of the results. Study the table and answer the questions that follow;
Type of arthropod Forest habitat Savannah habitat
Arachnids other than mites 25 10
Mites 180 25
Winged insects other than mosquitoes 140 39
Mosquitoes 30 10
Larvae of winged insects 81 12
Millipedes 82 10
Centipedes 11 11
Wood lice 52 10
Beetles 30 130
(a) Using the same axes, represent the information on a graph paper
(b) Comment briefly, on the relative abundance of the arthropods in both the forest and savannah habitats
(c) Account for the habitat preference shown by
i. Beetles
ii. Millipedes
(d) (i) Suggest two suitable methods which could possibly have been used to determine the number of winged insects in
the forest habitat and describe in detail the procedure used for both of them
(iii) State the advantage of the methods used in (d) (i) above
36. (a) (i) Explain the effect of predation on population size of organisms (05 marks)
(ii) How has man affected the balance between predator and prey in a negative way (05 marks)
b) (i) How can the population of tilapia fish in a pond be determined by the Lincoln index (Capture-mark-
recapture) method? (07 marks)
(ii) State the assumptions made in (b) (i) above (03 marks)
(b) In what ways do the predator-prey interactions compare with parasite-host interactions (07 marks)
(c) Explain how predator-prey interactions influence the formation of a new species (08 marks)
Page 440 of 447

38. Discuss the effects the following might have if

d) Untreated sewage entered a slow flowing river (07 marks)
e) Hot water from a cooling tower of a power station entered a slow flowing river (06 marks)
f) A forest is cleared to establish an industrial park (07 marks)
39. (a) Discuss the various forms of population dispersion in ecosystems

(b) Explain the various phases of the following population growth curves in ecosystems
i. Sigmoid curve
ii. Boom and burst curve
(c) (i) Sketch a curve to illustrate population changes in island biogeography
(ii) Explain the rate of extinction and immigration curves in a bigger and small island shown in your sketch
40. A factory emitting smog containing sulphur dioxide and carbon dioxide was cited in a rural district. The table below
gives distance and directions of :
iii. Number of moths and
iv. Concentration of sulphur dioxide in smog in different directions from the factory chimney
Table 1
Distance from factory in a south-south West direction (miles) 1 2 4 8 12 16 28

Sulphur dioxide concentration/parts per million 28 27 26 23 19.5 16 2
Table 2
Distance from factory in a north north east direction (miles) 1 2 4 8 12 16 28

Sulphur dioxide concentration/parts per million 27 26.5 25 24 23 22 19
f) Plot the information to show the relationship between the moth species distribution and the sulphur dioxide
concentration the same X-axis and Y-axis. (12 marks)
g) Explain the difference in results between obtained for the south-south west direction and those obtained for the
north-north east direction (02 mar ks)
h) Fully explain why the number of moths increase with increasing distance from the factory (04 marks)
i) The results obtained give evidence of present day evolution. Explain fully this evidence and its significance in
evolution (10 marks)
j) What are the environment effects of sulphur dioxide and carbon dioxide? (08 marks)
41. The figure below shows some of the effects of sewage and waste copper discharge into a river.
Page 441 of 447

(a) What is meant by the term biochemical oxygen demand (BOD)?

(b) Account for the changes in:
(i) BOD.
(ii) ion concentration
(iii) Population of micro-organisms
(iv) Population of fresh water fish
(c) Compare the populations of sewage fungus and algae.
(d) (i) Using evidence from the diagram, suggest the method by which an organism might be used as a pollution
indicator
(ii) Suppose that the chemical works also discharged thermal pollution, suggest one possible effect on the river’s
chemical content and one possible effect on its biological content.
42. (a) State the advantage of complete metarmorphosis over incomplete metarmorphosis (03 marks)
(b) Account for the physiological changes that occur during pregnancy up to lactation (10 marks)
(c) Explain the effect of photoperiod on metarmorphosis in a named arthropod or amphibian (07 marks)
43. (a) What do you understand by the term global warming?

(b) To what extent global warming affected your country? Discuss with the aid of examples from your country
and the globe at large.
(c) Give ecological impacts of global warming on ecosystems
(d) How can the nations work to alleviate the impact of global warming?
(e) Discuss the impacts of non-degradable pollutants like polythene papers on living organisms
(f) Describe with specific examples how different animals conceal themselves from their enemies
44. In an experiment to study the effect of DDT towards the cabbage pest, Pieris rapae which feeds on cabbage leaves,
two adjacent farm yards were prepared. Pieris was introduced in each of them and left for some time. After spraying
one farm with DDT for three consecutive times, the number of eggs that survived and hatched into larvae at the
sprayed and non-sprayed farm yards were determined as shown in graph A
In another experiment, Pierisrapae was exposed to birds as its control agents and the changes in the population of
both, with time, was determined as indicated by graph B
GRAPH A
120
SPRAYED
100
80
NON-
Percentage survival
SPRAYED
60
40
20
0
0 2 4
Page 442 of
6
447 8 10 12
Time/Weeks
GRAPH B
140
120
TARGET
100
Population
80
60
40
20
AGENT
0
0 2 4 6 8 10 12
Time/weeks
a) (i) Compare the number of eggs of Pieris between the sprayed and non-sprayed farm yards (04 marks)
(ii) Account for the differences in the number of eggs of Pieris at the sprayed and non-sprayed farm yards (08
(iii) Explain any one property of DDT other than the one shown above, which renders it unsuitable for
environmental use ( 08
b) (i) Compare the populations of the target organism and the control agent (06 marks)
(ii) What term is normally given to such controls?
(iii) Describe the changes in the population of the control agent (05 marks)
(iv) Explain the changes in the population of the control agent and the target organism (10 marks)
(v) From the graph, what seems to be the ultimate aim of this type of controls?
(vi) What would happen if the control agent completely eradicates the prey?
(vii) What should be taken into account in selecting such as a controlling agent?
c) Outline any three advantages of the method in Gaph B to that in graph A
45. (a) What is meant by the term ecosystem?

(b) Describe the flow of energy and cycling of carbon and nitrogen in any named ecosystem
(c) Suggest reasons why felling and removal of forest trees result in changes in the levels of nutrients in the soil
(d) In an investigation of a fresh water pond, 35 water bugs (Notonecta) were caught, marked and released. Three
days later 35 water bugs were caught ad 7 were found to be marked.
i. What is the approximate size of population of water bugs in the pond? Show your working.
ii. Give three reasons why capture-recapture is unlikely to be an accurate way of assessing the size of a
water bug population
Page 443 of 447

46. The figure shows the amount of DDT at different levels on a food chain. The figure below represents the amount of
DDT in parts per million (ppm)
Concentration of DDT/ppm Trophic level Example

75 Carnivore 2 Osprey
50 Carnivore 1 Large fish
10 Herbivore Small fish
0.04 Producer Algae
a) Calculate the concentration factor of DDT in passing from into;

i. The primary producer
ii. The carnivore 1
iii. The herbivore
If the concentration of DDT in the water was 0.02ppm (show your working)
b) What conclusions can be drawn from the answers in (a) above?
c) At which trophic level;
i. Is DDT likely to have the most marked effect?
ii. Would DDT be most easily detected?
d) Penguins inhabiting an island far away from the area where DDT was applied were found to contain DDT.
Suggest the way in which the penguins might have come to contain DDT?
e) Analysis of the small fish showed levels of DDT of 1-200ppm in the flesh and 40-200ppm in the fatty
tissue. Suggest a reason why animals die of DDT poisoning in times of food shortage.
f) When DDT was used, the first and second application killed 99% of the pests. Their numbers quickly
recovered and the third application had little effect on the population. Explain why DDT did not eradicate
the pests and why their number recovered quickly
g) Suppose you are to develop a new pesticide, what qualities would you want it to have?
47. The graph below shows the variation of leaf area index for Trifolium fragiforum and Trifolium repens with time
20
18
T. fragiform
16 (grown alone)
14
leaf area index
12 T. repens
(grown alone)
10
8
T. repens (mixed)
6
T.fragiform (mixed)
4
0
0 5 10 15 20 25 30
Weeks after sowing
Page 444 of 447

a) Compare the;
i. Growth of Trifolium species grown separately
ii. Growth of Trifolium species grown together
b) Why did the Trifolium species grown together not behave like the case of beetles above? Give evidence for
your answer from the graph.
c) State other factors that may affect the population of the beetles and clover in an ecosystem
d) Explain the variation in other abiotic factors between the surface water and that at the lake bottom
A scientist carried out a research on two species of flour beetles (Tribolium) and clover (Trifolium). In research, she
grew the beetles in the same medium and different media under different climatic conditions. On the other hand, she
grew the plants together and separately. The following are her findings
Climate Temperature Relative Results of interspecific competition
/oC humidity/% Tribolium castaneum Tribolium confusum
Hot-wet 34 70 100 0
Hot-dry 34 30 10 90
Warm-wet 29 70 86 14
Warm-dry 29 30 13 87
Cool-wet 24 70 31 69
Cool-dry 24 30 0 100
e) Comment on the effect of changing temperature and relative humidity on the population of Tribolium species
f) Explain the observed behaviour of the Tribolium species over time
g) What biological principle is demonstrated by the results in the table above
h) Describe how interspecific competition may lead to speciation?
43. (a) State advantages of using biological agents to control pests over chemical pesticides (02marks)
(b)The figure below shows percentage of leaves of strawberry occupied by two-spotted mites (prey) and predatory mites
over a period of 16weeks.
i) Describe how the percentage of leaves occupied by predatory mites changes during the experiment.
(02 marks)
ii) Explain how the graph supports that the control of two spotted mites by a biological agent was successful
(02 marks)
c) Farmers who grow straw berry might decide not to use predatory mites. Suggest two reasons why. (02 mar
d) If the experiment was to be repeated, but after 10 weeks a chemical pesticide was sprayed on straw berry.
Suggest and explain what would happen after 16 weeks (03 marks)
Page 445 of 447

48. The table below shows characteristics of the principal seral stages in succession from an abandoned farm to mature
forest.
Seral stage Duration (years) Canopy height (m) Biomass (tones)
Farm growth 0-2 1-5 5-20
Secondary thicket 2-10 8-20 15-100
Secondary forest 10-80 15-50 100-150
Mature forest >80 40-60 100-150
(a) (i) What is meant by ecological succession? (2 marks)

(ii) Describe how succession occurs in each seral stage above and give the general life form in each stage.
(13 marks)
(iii) In what ways is the succession above different from that which begins with a bare rock. (5 marks)
(b) Phytoplankton consists of single celled photosynthetic organisms which are suspended in the surface water.
The graphs show the relationship between the biomass of phytoplankton and depth at which it is found in two
different lakes. Lake A has low nutrient concentration. Lake B has a high nutrient concentration.
Low nutrient concentration high nutrient concentration

i) Briefly describe a method by which you can measure the biomass of the phytoplankton. (2 marks)
ii) Describe the effect of different nutrient concentrations on the distribution of phytoplankton in the two lakes.
(04 marks)
iii) Explain why the rate of biomass production at a depth of 5 metres was much higher in the Lake A that in Lake B.
(04 marks)
(c) Phytoplankton organisms do not live for long. After a short time they die and their remains decay.
i) Use this information to explain how application of large amounts of nitrate fertilizer to the land surrounding a
lake might affect other organisms living in the lake. (03 marks)
ii) Suggest why a period of drought might increase the effect described in your answer in C (i) above. (01 mark)
(d)Biological techniques may be used to monitor water pollution such as that caused by the addition of fertilizer. Explain
used to monitor water pollution.
i) The presence or absence of indicator species. (02 marks)
ii) An index of diversity (02 marks)
iii) Biochemical oxygen demand. (02 marks)
49. In an experiment, the relationship between two insect species was studied by comparing their population over a
period of time. Icerya is a pest on citrus fruit and Rodolia is a carnivore. The two species were simultaneously
exposed to an orchard and their populations were determined over time. The results are shown in the table below;
Time/month 0 1 2 3 4 5 6 7
Population in Rodolia 2 70 200 630 300 70 50 50
thousands Icerya 700 690 680 545 193 90 68 67
(a) On the same axis, plot the data in appropriate graph

(b) (i) Describe the population changes in the two species
(ii) Account for the relationship between the two populations of these insect species
(iii) What is the technical term for the kind of relationship between the two populations of these insect
species?
(c) Outline the significance of;
Page 446 of 447

i. The population in the sixth and seventh months

ii. This relationship to humans
(d) Outline the precautions you would take before introducing the two species simultaneously in the
environment
50. The table below shows average mineral contents of sea water, river water and dry matter of marine brown algae.
Study the table and answer the questions that follow.
ELEMENT CONCENTRATION (PPM)

SEA WATER RIVER WATER DRY MATTER
Sodium 10,500 6.3 33,000
Calcium 1.350 4.10 5,200
Potassium 380 2.30 52,000
Strontium 8 0.08 1,400
Iron 0.01 0.67 700
Magnesium 0.002 0.012 53
Silicon 3 6.5 1,000
Carbon 28 11 345,000
Chlorine 19,000 7.8 4,700
Sulphur 885 3.7 12,000
Bromine 65 0.021 740
Boron 4.6 0.013 120
Fluorine 1.3 0.09 4.5
Nitrogen 0.5 0.23 15,000
Phosphorous 0.07 0.005 2,800
(a) Comment briefly on the relative concentration of mineral elements in sea water and river water
(b) Explain the difference in concentrations of elements in the two water bodies
(c) Suggest why some minerals are more concentrated than others in the marine brown algae
(d) Comment briefly on the differences in mineral concentrations between seawater and marine brown algae
(e) (i) What is the source of mineral elements found in sea water?
(ii) Suggest the possible ways by which terrestrial ecosystems obtain minerals
END
Page 447 of 447

Biology 1 and 2

Uploaded by

Copyright:

Available Formats

Biology 1 and 2

Uploaded by

Document Information

Original Description:

Original Title

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Biology 1 and 2

Uploaded by

Copyright:

Available Formats

P530 [2020] By Nakapanka J Mayanja 0704716641

2020 form five biology notes

PROTEIN SYNTHESIS ............................................................................................................................. 96

Microbodies ................................................................................................................................................ 204

TOPIC 6: TRANSPORT IN LIVING ORGANISMS ................................................................................ 256

SAMPLE QUESTIONS ................................................................................................................................ 331

B.O.D (Biological oxygen demand) ...................................................................................................... 386

TOPIC 1: LEVELS OF ORGANISATION AND DIVERSITY OF

 State characteristics of fungi.  Characteristics of fungi (feeding, reproduction)

THE NEED TO CONSERVE BIODIVERSITY

1. Food. 75% of our food supply comes from just 12

1 a)b) Has 8 legs……………………..W

2 a)b) Has long antennae……………..X

3 a)b) Has proboscis………………….Y

a. They possess genetic material

Examples of viral diseases;

Harmful roles: BACTERIA

b. Rod shaped (bacilli, singular = bacillus) Nitrobacter O2+NO2- NO3 + Energy

Classification by staining reaction

Gram positive bacteria; they stain purple with a gram

9. Manufacturing processes e.g. making soap PHYLUM RHIZOPODA

Non-cellular (acellular) Single-celled

Have no metabolism of Have metabolism of their

Do not grow and do not Grow in size and divide

Take no food by any Take food by adsorption

An infected anopheles mosquito bites a person,

 They contain chlorophyll and therefore they are

 It produces agar which is extracted from them for

They are characterized by production of spores that bear

These are referred to as the lower fungi

Spirogyra is a filamentous algae that lives in fresh water

PHYLUM PHAEOPHYTA ECONOMIC IMPORTANCE OF ALGAE

 It is marine  Carbohydrates are stored as glycogen but not as

 They reproduce by spores that lack flagella PHYLUM ASCOMYCOTA

Characteristics of the yeast cell

7. Fungi causes decomposition of stored food and

KINGDOM PLANTAE (plants)

 Their cell walls contain cellulose

PHYLUM BRYOPHYTA (bryophytes)

Bryophytes are the smallest land plants and they are

 They lack vascular tissues

The haploid biflagellate sperms fuse with haploid eggs

The zygote develops into sporophytes which attach and

When mature, the sporophyte produces haploid spores

When the spores land on moisten soils, they germinate

A moss consists of two distinct forms in its lifecycle Characteristics

 Spores are produced in sporangia (singular;

 On rupturing, the ciliated sperms from the

COMPARISON BETWEEN A MOSS AND

It lacks true roots, leaves True roots, stems and

Rhizoids present No rhizoid SIGNIFICANCE OF ALTERNATION OF

Moss Ferns 1. A source of soft wood for timber

Sporophytes grow on the Sporophyte is self- PHYLUM ANGIOSPERMOPHYTA

Definition of terms 3. Colonial level of organisation

1. They require large quantities of food

Significance or importance of possessing a

1. It allows triploblastic organisms to increase in size

1. They have a special way of gaining entry into the