Biology Bmat Section 2
Biology Bmat Section 2
Biology Bmat Section 2
All living things are made up of one or more units called cells. Cells are microscopic.
Eukaryotes are organisms made up of a cell or cells containing DNA inside a recognisable nucleus. The DNA
is present in the form of one or more linear chromosomes within the nucleus.
A unicellular organism or single-celled organism is an organism that consists of only one cell. Multicellular
organisms are made up of many cells. These may be specialised to perform particular functions. The size and
shape of these cells depend on the role they perform. Plant cells tend to be larger than animal cells.
Structure and function of the main sub-cellular components of animal and plant eukaryotic cells
The features present in cells are known as their sub-cellular components. All eukaryotic cells have a cell
membrane and cytoplasm. Mitochondria (singular: mitochondrion) are present in the cytoplasm, but these
are not visible using a light microscope. Most eukaryotic cells have a nucleus. Exceptions are mature red
blood cells in mammals.
Plant cells also have a cell wall and a sap vacuole. In addition, photosynthetic cells contain chloroplasts.
This table summarises the sub-cellular components of animal and plant cells:
Cell membrane A partially permeable layer that forms a Contains the cell contents and
boundary around the cytoplasm of the cell. controls the movement of some
substances into and out of the
cell. It allows water, oxygen and
nutrients to enter and allows
waste products (e.g. carbon
dioxide) to leave.
Chromosomal DNA One or more linear pieces of double-stranded Stores the genetic material
DNA. required for the various cell
processes.
Cytoplasm A jelly-like region, surrounded by the cell Is the site of chemical reactions
membrane. Salt ions and sugar molecules are and contains enzymes that
dissolved in it. Fat molecules and proteins, e.g. control these reactions. It holds
enzymes, are suspended in it. It also contains the cell organelles.
food reserves (e.g. glycogen in some animal
cells, starch in some plant cells) and organelles
such as the nucleus and
mitochondria. Chloroplasts are present in the
cytoplasm of photosynthetic plant cells.
Cell wall (plant only) A tough, rigid layer surrounding the cell Provides a rigid external coat to
membrane made primarily of cellulose. It is plant cells, providing mechanical
freely permeable to water and salts. strength which allows cells to
resist bursting when the cell is
turgid.
Chloroplast (plant only) A small organelle with its own double Traps (absorbs) light energy and
membrane, found in the cytoplasm of converts it to chemical energy by
photosynthetic plants, containing chlorophyll the process of photosynthesis.
and other pigments.
Vacuole (plant only) A fluid-filled space surrounded by a membrane Stores water-soluble chemicals
in plant cells. It is found inside the cytoplasm. and ions and helps to keep plant
The fluid is called sap, which is a watery cells and tissues firm.
solution of sugars and salts. In some cells, e.g.
rhubarb, it is also coloured.
QUESTIONS:
a) Name three cell parts which are common to both a liver cell and a palisade mesophyll cell.
Cell membrane, cytoplasm, nucleus, cell wall, chloroplast and sap vacuole are present in plant cells.
However, animals do not have a cell wall, chloroplasts or a sap vacuole.
b) Which plant cell part is not found in a root hair cell? Explain your answer.
Chloroplasts are not found in root hair cells because these cells are not exposed to light. Their function is the
absorption of mineral ions and water, as well as anchorage, and they are adapted to do this by having an
elongation to the cell – the root hair.
Match the cell parts to their descriptions. There is only one description for each, and one match has been
completed for you.
Structure and function of the main sub-cellular components of prokaryotic cells (bacteria)
Prokaryotic cells are typically smaller and simpler in organisation than plant or animal cells. They have some
features in common with eukaryotic cells.
Cell membrane A partially permeable layer that forms a Contains the cell contents and
boundary between the cytoplasm and the controls the movement of some
cell wall. Functions as a boundary to restrict molecules into and out of the cell.
materials moving between the inside and
outside of a cell.
Cytoplasm A jelly-like region, surrounded by the cell Is the site of chemical reactions
membrane. It may contain glycogen and contains enzymes that control
granules, fats (lipids) and other food these reactions. It holds the cell’s
molecules. DNA.
Cell wall A tough, rigid external coat that surrounds Provides structural support and
the cell membrane. It is made of a complex protection to bacteria. It is freely
mixture of proteins, lipids (fats) and sugars. permeable to small molecules, so
This makes it different from plant cell walls, does not control the intake or loss
which are made of cellulose. Some bacterial of materials.
cells have a slime capsule surrounding the
cell wall.
Chromosomal DNA A large closed circular coiled molecule of Carries genetic information that
(bacteria) double-stranded strand DNA located within regulates most bacterial cell
the bacterial cytoplasm (no nucleus or processes.
enclosing membrane).
Plasmid DNA A small molecule of closed double-stranded Carries genetic information for
circular DNA, usually present in multiple specialist cell functions (like
copies per cell. antibiotic resistance). It can
replicate and operate
independently from the
chromosomal DNA. Can be readily
moved between different bacterial
cells, including for genetic
engineering (biotechnology)
purposes.
QUESTION:
With reference to DNA, outline how an animal cell and a prokaryotic cell are different.
-An animal cell has a nucleus which contains DNA, and a prokaryotic cell has DNA but no nucleus. Animal
cells usually contain several large chromosomes, but a prokaryotic cell usually only contains one per cell.
DNA in prokaryotic cells is circular with no free ends, but the DNA of eukaryotic cells forms large linear
chromosomes with free ends
-In an animal cell the DNA is surrounded by membrane, but in a prokaryotic cell the DNA there is no nuclear
membrane. A prokaryotic cell contains a plasmid but an animal cell does not. The DNA of a prokaryotic cell
contains fewer genes than the DNA of an animal cell.
Levels of organisation
Most eukaryotic cells become specialised when they have finished dividing and growing. This process is
called differentiation.
Specialised cells tend to carry out one particular function (job) and have certain features which allow them
to do this:
they have a distinct shape
they undertake specific chemical reactions and processes in their cytoplasm.
Examples of specialised cells are ciliated cells, red blood cells, nerve cells (neurones), sperm cells, root hair
cells and palisade mesophyll cells.
Animals have many structures containing ciliated cells. For example, ciliated cells form the lining of the
trachea.
Function: moving mucus to protect the lungs from infection and irritants.
Nerve cells are often elongated to conduct nerve impulses from one part of the body to another. Chemical
reactions cause the impulses to travel along the fibre.
Function: conducting nerve impulses.
Palisade mesophyll cells are columnar (elongated) and the cytoplasm is packed with chloroplasts to trap
(absorb) sunlight.
Function: to make food for the plant through the process of photosynthesis.
Cells in multicellular organisms tend to work together in groups to carry out their functions
effectively.
A tissue is made up of one or a few different cell types, working together to perform a
shared function. Examples include muscle (contractile tissue) associated with the skeleton,
blood circulating around the body, xylem vessels in a plant stem and palisade mesophyll of
a leaf.
An organ is a structure made up of a group of tissues, working together to perform a
specific function. Examples include the heart (pumps blood) and a leaf (harvests energy).
An organ system is a group of organs with related functions, working together to perform a
body function. Examples include the circulatory system, made up of blood, the vessels and
the heart; and the shoot of a plant, made up of the stem, leaves and buds.
Examples of tissues
Muscle tissue
Diagram showing the relationship between cells, tissues and organs in the human respiratory
system
QUESTIONS:
Any four organ systems, and two organs for each system, from:
Organ system Organ 1 Organ 2
circulatory heart artery
digestive stomach small intestine
excretory kidney urethra
nervous brain nerve
reproductive testes penis
respiratory lungs diaphragm
skeletal skull femur
Other organs for each of the organ systems would also be acceptable answers.
For example:
circulatory – vein, capillary
digestive – oesophagus, rectum
excretory – ureter, bladder, lungs
nervous – spinal cord, eye
reproductive – ovary, uterus
respiratory – trachea, ribcage
skeletal – any other named bones
c. REPRODUCTIVE SYSTEM
Q: Place the following levels of organisation for a multicellular animal in order of size from
smallest to largest: cell, organ, sub-cellular component, organism, organ system, tissue.
ANSWER: Smallest to largest: sub-cellular component, cell, tissue, organ, organ system,
organism.
Sub-cellular components are the smallest structures as they are found within cells. A tissue
is a group of one or a few cell types, performing a shared function. An organ is a structure
made up of a group of tissues working together to perform a shared function. An organ
system is a group of organs with related functions, working together to perform a body
function. An organism is a whole living being capable of reproduction. In the example of a
multicellular animal, it is made up of the levels of organisation named in this question.
MOVEMENT ACROSS MEMBRANES
Know and understand the processes of diffusion, osmosis and active transport, including
examples in living and non-living systems.
A membrane is a selective barrier. All cells have a cell membrane which forms a highly
flexible barrier to stop the cell contents from escaping and control which substances enter
and leave the cell.
Processes occurring in cells require certain substances to move across the cell membrane.
Raw materials, like water, nutrients and ions must cross the membrane to enter cells. Cells
also need to get rid of waste substances that have accumulated, as these may be toxic or
interfere with essential chemical reactions. Prokaryotes must acquire nitrogen, carbon, salts
and water from their surroundings in order to grow. Plant cells also need carbon dioxide for
chloroplasts in their photosynthetic cells to generate energy from the sun. Animal cells also
need oxygen for their mitochondria to generate energy from food molecules with high
efficiency.
To enter or exit the cell, nutrients and other substances must cross the cell membrane. This
can occur by a passive process that does not require energy (diffusion or osmosis) or an
active process which requires the cell to use energy (active transport).
Particles in liquids and gases move about randomly. The rate of movement is affected by
factors such as concentration gradients and temperature. Over time, any uneven distribution
of particles will become even as a result of this random motion.
Diffusion
Diffusion is the net movement of molecules and ions from a region of their higher
concentration to a region of their lower concentration down a concentration gradient, as a
result of their random movement. This continues until there is no net movement.
Factors affecting the rate of diffusion:
Temperature – if a system is hot, the particles have more kinetic energy so they
move faster.
Distance – the further the particles have to travel, the longer it takes the molecules
to diffuse.
Size – the smaller the particles, the faster they can diffuse.
Surface area – the larger the surface area, the faster the molecules will diffuse.
If a small amount of a substance such as a single crystal of sugar is put into water, the sugar
molecules will dissolve in the water. Initially the sugar will have highest concentration near
where the crystal is placed. Over time, the kinetic energy of the water molecules will cause
the dissolved sugar molecules to diffuse. Eventually the sugar molecules will become
evenly distributed in the water.
Diffusion
A mixture of glucose and starch dissolved in water is sealed inside a section of dialysis
tubing. The starch molecules are too large to pass through the tubing into the water but the
glucose is small enough to freely diffuse.
Kidney dialysis machine using diffusion through a membrane to regulate the composition
of the patient’s blood
Using the process of diffusion through the dialysis tubing, urea, uric acid and excess salts are
removed from the blood as it passes along the dialysis tubing.
QUESTION
A mixture of glucose and starch dissolved in water is sealed inside a section of dialysis
tubing and the tubing placed into a test tube containing only water. How would the rate of
diffusion of glucose change in the following circumstances:
a) The temperature of the bathing solution was increased?
b) The concentration of glucose was decreased?
c) The dialysis tubing used had a thicker wall?
Asnwers
a) The rate would increase.
Reason: At the higher temperature the glucose molecules have more kinetic energy. So they
would be able to move faster from inside the dialysis tubing into the surrounding water.
b) The rate would decrease.
Reason: The concentration gradient between the inside of the tubing and the water
surrounding the tubing would be lower.
c) The rate would decrease.
Reason: There would be a greater distance for the glucose molecules to travel between the
inside of the dialysis tubing and the surrounding water.
Osmosis
Osmosis is the movement of water from a region of higher water potential (dilute solution)
to a region of lower water potential (concentrated solution) through a partially permeable
membrane.
The model below demonstrates the process of osmosis. A U-shaped tube is divided in half
by a semi-permeable membrane. At the start of the experiment, equal volumes of salt water
and pure water are placed into the two halves.
As time progresses, the water molecules in the pure water (which have the higher water
potential) show a net movement through the membrane into the salt water (which has a
lower water potential). The salt particles are too large to pass through this membrane, so
they don’t move through it. Gradually the height of the salt water column rises and the pure
water column falls as a result of osmosis.
Osmosis
A plant cell contains water in the sap vacuole and cytoplasm in the form of solutions. The
cell wall is freely permeable to water.
An animal cell contains water in the cytoplasm, in which a variety of solutes are dissolved.
Cells in water
When a plant cell is placed into pure water, there is a large difference in water potential
between the outside of the cell (higher water potential), and the inside (lower water
potential). As a result, water will flow across the membrane into the cell. This is called
osmosis.
In a plant cell, the sap vacuole fills up and exerts pressure on the cytoplasm, which presses
against the cell wall. The cell becomes turgid, but the cell wall prevents the cell from
bursting.
When an animal cell is placed in water, osmosis will cause water to enter the cell. The
volume of the cytoplasm will increase and the cell membrane will stretch. Eventually the
membrane will burst because animal cells do not have a cell wall and the membrane is not
strong enough to withstand the pressure of the extra fluid in the cell.
The cells are turgid: in each cell the cytoplasm is pressing against the cell wall, making the
wall bulge outwards.
The cells are flaccid: in each cell the cytoplasm is shrinking away from the cell wall.
Cores of raw potato in the form of cylinders or chips are cut to the same length.
Samples are placed in solutions of a range of concentrations.
After an hour, these are removed and re-measured or weighed.
The potato pieces in pure water get longer and gain mass.
Those in concentrated solutions get shorter and lose mass.
Examples of osmosis
Plants rely on osmosis to obtain water through their roots. Water is transferred from
cell to cell by osmosis.
When plant cells are turgid, their rigidity can keep the whole plant firm and upright.
Leaves can be held in the best position possible to trap (absorb) sunlight for
photosynthesis.
When plant cells are flaccid, they lose their rigidity and the whole plant can wilt.
If animal cells are exposed to pure water, they can swell up and burst (in a red blood
cell this is called haemolysis). Red blood cells, for example, would not then be able to
carry oxygen.
If animal cells lose water, they become flaccid. Red blood cells, for example, would
be less efficient at carrying oxygen.
Water is absorbed by osmosis from the ileum and colon as food passes along the
alimentary canal.
QUESTIONS
a) With reference to osmosis, explain why a cylinder of raw potato cut from a slice of
uniform thickness:
i) gets longer and gains mass when placed in pure water
ii) gets shorter and loses mass when placed in a concentrated sugar solution.
b) Explain why cells in the potato cylinder do not burst when placed in pure water.
ANSWRS
i) The water surrounding the potato cylinder has a higher water potential than the fluid in the
cytoplasm and sap vacuole of the potato cells. Water moves into the potato cells by osmosis, from
the higher water potential to the lower water potential. The cells become turgid, making the cell
walls bulge outwards. Each of the cells gets longer and heavier, so the cylinder gains length and
mass.
ii) The water surrounding the potato cylinder has a lower water potential than the fluid in the
cytoplasm and sap vacuole of the potato cells. Water moves out of the potato cells by osmosis, from
the higher water potential to the lower water potential. The cells become flaccid, removing pressure
on the cell walls. The cells get shorter and lighter, so the cylinder loses length and mass.
b) Potato cells are plant cells, so they have cell walls. Plant cell walls are made of cellulose, which
provides rigidity and prevents the cells bursting when they are turgid.
Active transport
Active transport is the movement of particles through a cell membrane from a region of
lower concentration to a region of higher concentration using the energy from respiration.
Movement of the particles is in the opposite direction to diffusion (moving up a gradient
instead of down a gradient). Cells have to provide the energy to achieve this, through
respiration using ATP. Mitochondria control the energy release, so cells involved with
active transport tend to have large numbers of mitochondria in their cytoplasm. The
chemical energy from respiration is converted to kinetic energy for movement of the
particles. Anything which interferes with respiration, e.g. toxins or lack of oxygen, prevents
active transport from taking place. Active transport is thought to be achieved by carrier
proteins embedded in the membrane. They move the particles from one side of the
membrane to the other.
Examples of active transport
Plant root hair cells use active transport to move mineral salts from the soil into the
root. These salts are commonly in lower concentrations in soil than in the plant root
cells, so diffusion is not adequate to absorb them.
Glucose is moved from the small intestine into the blood stream of mammals by
active transport. Absorption of glucose by diffusion would stop once the concentration
in the blood reached that of the intestine.
QUESTIONS
a) Explain why the rate of respiration in root hair cells of plants may increase when they are
taking in mineral salts.
b) Explain why the uptake of the mineral salts slows down when the root hairs are exposed
to a respiratory poison.
c) Suggest why uptake of the mineral salts does not completely stop after exposure to the
respiratory poison.
ANSWERS
a) The concentration of salts surrounding the root hair cell may be higher than inside. This
would mean that diffusion would not move salts into the cells (it may even cause the cells
to lose salts). Active transport is needed. This process requires energy, which is supplied by
the process of respiration.
b) Active transport requires energy to move the mineral salts. The energy is provided by
respiration. If respiration is inhibited by a poison, little energy is available, so active
transport will stop. However, some salts may still move by diffusion, so movement of salts
may not stop completely.
c) Although active transport will not be available to move salts into the root hair cells, if
there is a higher concentration of salts in the soil around the root hair cells, salts will still
move into the cells by diffusion, which does not require an energy source.