CELLS

PROKARYOTIC
CELLS
Introduction
• Prokaryotic organisms include bacteria,
cyanobacteria (gram-negative bacteria that
obtain energy via photosynthesis) &
archaebacterial (belong to kingdom monera but
classified as bacteria; single celled & lack
nucleus).
• Bacteria are most frequently found form of life.
• Some are pathogens but majority do no harm &
are beneficial;
• ….e.g. gut bacteria.
Bacteria
 Bacteria have common structures
• All are single-celled organisms & have common
features that vary between species.

• All bacteria have a cell wall which prevents

swelling & bursting of cell.

• Some other common structures found in

bacterial cell include, cell surface membrane,
DNA, capsule/slime layer, ribosomes &
flagellum.
Structure of bacteria
 Cell surface membrane
• Bacteria cells are prokaryotic, they lack
membrane bound organelles e.g. nucleus,
mitochondria.

• All bacteria have a cell surface membrane, similar

in structure & function to that of eukaryotic cells.

• The cell membrane is also the site of some of

respiratory enzymes.

• In some bacteria e.g. Bacillus subtilis found in soil,

membrane shows infoldings called mesosomes.
 Folded Cell surface membrane
- mesosomes
• Some scientists think that mesosomes are
artifacts due to bacterial cell preparation
for electron microscopy.
• Others think mesosomes are associated
with enzyme activity during DNA
separation & formation of cross-walls
during replication.
• Functions of mesosomes are not clear.
 Bacterial capsule
• Absent in some bacteria.
• Called slime layer if very thin & diffuse.
• May be formed from starch, gelatin,
protein or glycolipid.
• Functions:
• ....protects bacteria against
phagocytosis by white blood cells.
• ....also covers cell markers on cell
membrane which identify cell.
 Bacterial capsule
• Functions:
• ....makes it easier for bacterium to be pathogenic
as it helps it to evade host cell immune systems.

• Capsules may be found in bacteria such as those

that cause pneumonia (Streptococcus
pneumoniae), meningitis (e.g. E coli), TB
(Mycobacterium tuberclosis) & septicaemia
(blood poisoning).
• BUT, many capsulated bacteria do not cause
disease, just have ability to survive very dry
conditions.
Pili or fimbriae
• Some bacteria such as E. coli &
Salmonella spp have them as 1 to hundred
thread-like protein projections on their
surface.
• Functions:
• (1) to attach to host cells.
• (2) used in reproduction.
• Disadvantage: they make bacteria more
vulnerable to viral infections.
• Bacteriophages can infect bacteria via
pili.
 Flagella
• Whip-like structure that projects from
surface of bacterial cell.

• In some bacteria, it aids locomotion.

• Made-up of globular protein flagellin.

Bacterial genetic material - DNA

• Consists of single circular strand of DNA lying

free in cytoplasm nucleoid region;
• .....this is called chromosomal DNA.
• Nucleoid: area where DNA tangle is found; in
E coli, it occupies half of cytoplasm.
• In addition to chromosomal DNA, bacteria also
have smaller circles of DNA called plasmids.
• Plasmids can reproduce themselves
independently of nucleoid & can be transfered
from one bacterium to another in form of sexual
reproduction using pili.
 Bacteria Plasmid DNA is extra
chromosomal genetic material
• Plasmid DNA codes for several genes
that code for special phenotypes.

• These include:

• (i) resistance against certain antibiotics,

• (ii) production of certain toxins,

• (iii) sexual reproduction.

Ribosomes
• Are the only organelles shared with
eukaryotes.
• These here are 70S, thus smaller than
80S in eukaryotes.
• Smaller subunit is 30S & larger is 50S.

• They are involved in protein synthesis in

a similar way to eukaryotic ribosomes.
Bacterial Cell Walls
• Due to the hypertonic nature of bacterial
cell relative to their environments, cell wall
prevents swelling & bursting of cell due to
osmosis.
1.It also maintains cell shape & gives
support & protection to contents of cell.
2.The cell wall is also important for
classification of bacteria.
Bacterial cell walls

• Consist of a layer of
polysaccharide chain
called peptidoglycan
(also known as
murein);
• .....made-up of many
parallel
polysaccharide
chains linked by short
peptide cross-
linkages, which form
an enormous molecule
with net-like structure.
Peptidoglycan
• Polysaccharide made of two glucose
derivatives, N-acetylglucosamine (NAG)
& N-acetylmuramic acid (NAM),
alternated in long chains;
• .....chains cross-linked to one another by
a tetrapeptide [L-alanine, D-glutamine, L-
lysine or meso-diaminopimelic acid (DPA),
& D-alanine] that extends off NAM unit,
allowing lattice-like structure to form.
Gram Staining & Bacterial cell walls
• Two different types of bacterial cell walls:
1. Gram positive cell walled bacteria.
2. Gram negative cell walled bacteria.

• These can be distinguished by Gram staining.

• Different types of disease causing bacteria are

vulnerable to different types of antibiotics, &
type of cell wall they have is one of factors that
affects how vulnerable they are.
Gram staining technique
• Developed by Christian Gram in 1884.
• Before staining, bacterial cells are colorless.
• Cell walls of Gram-positive bacteria have a
thick peptodoglycan layer, which contains
chemicals such as teichoic acid within its net-
like structure.
• Crystal violet/iodine complex in Gram stain is
trapped in thick peptidoglycan layer & resists
decolouring when bacteria are dehydrated
using alcohol; so bacteria do not pick up red
safranin counterstain & appear purple/blue.
Gram-positive bacterium

• Example of Gram-positive bacterium is

Staphylococcus aureus, which is
resistant to drug methicillin & is thus
called methicillin resistant Staphylococcus
aureus = MRSA;
• .....this resistance is linked to presence
of thick peptidoglycan layer.
Gram staining
procedure
Gram stain imaging: immersion oil

• Place a drop of immersion oil in stained

smear and examine directly under a
microscope using an immersion objective.
• Crystal violet binds to teichoic acid
resisting decoloring in rest of process.
• This leaves positive purple/blue colour.
This is a positive test colour for gram stain
Gram-negative bacteria
• These have a thinner layer of
peptidoglycan & do not have teichoic
acid between 2 layers of membranes.
• They have an outer membrane-like layer
made of lipopolysaccharides.
• Crystal violet does not bind due to
absence of teichioic acids, therefore it is
readily decolorized & replaced with red
safranine in Gram stain. They appear
pink/red.
Gram staining procedure
 Gram-negative & Positive
bacteria
• Gram-negative cells appear red.
Gram-negative
bacteria

Gram-positive bacteria
Classifying bacteria
1. Bacteria can be classified based on
Gram stain;
• .....can be Gram-negative or Gram
positive.
2. Bacteria can also be classified based on
cell shape.
3. Bacteria can also be classified according
to their respiratory needs.
Classification based on shape
1. Cocci: spherical shaped. e.g.
Staphylococcus aureus.
2. Bacilli: rod shaped. Example is gut
bacterium called Escherichia coli.
3. Spiral: have twisted structure e.g.
spirrilum.
4. Vibrios: shaped like a comma. e.g.
Vibrio cholerae.
Classification of bacteria by shape
Eukaryotic Cells:
Main Cellular Structures
Introduction
• Eukaryotic organisms include animals,
plants & fungi;
• …..in these groups there is wide range of
different types of cells, with different
function.
• …..make up majority of living things.
• …..name is derived from Greek words, eu
= good & karyon = nucleus.
Introduction…….
• Nucleus surrounded by nuclear
membrane.
• Other membrane-bound organelles such
as mitochondria & chloroplasts are present
in cytoplasm.
• Generally, are larger than prokaryotic cells
(30 -150 μm).
Main Cellular Structures

Example of eukaryotic animal cell

Main Cellular Structures
• Structure & functions:
• 1. Cell wall (plant cell)
• Structure found on exterior of plant cell.
• Has cellulose, which is a polysaccharide.
• Primary cell wall made up of bundles of cellulose molecules called
microfibrils;
• When many more layers of cellulose are laid on primary cell wall,
they form secondary cell wall, which has tightly packed
microfibrils.
• Secondary cell walls may also consist of lignin to give cell greater
strength.
Example of eukaryotic plant cell (Leaf cell – palisade)
 Main Cellular Structures...
• 2. Nucleus (plant & animal cell)
• Largest organelle within eukaryotic cell (10 – 20 μm).
• Surrounded by double membrane, which is perforated
with pores.
• Encloses chromosomes (threadlike structures of
nucleic acids & proteins) that carry genetic or
hereditary information.
• Controls cell activity by regulation of protein &
enzyme synthesis.
• Consists of 50% protein, 20% DNA & 20% RNA of its
dry mass.
 Main Cellular Structures...
• 3. Mitochondria (plant & animal cell)
• Tiny rod-like structures (1 μm wide x 10 μm long).
• Have outer & inner membrane; inner membrane is
folded to form cristae, which give it large surface area.
• Involved in respiration that produces of ATP;
• …..cells that have few mitochondria require little
energy (e.g. white fat storage cells).
• …..cells with energy-demanding functions have many
mitochondria (e.g. muscle cells or liver cells).
• Contain their own genetic material; thus mitochondria
replicate themselves & this is controlled by nucleus;
• Mitochondrial DNA is part of whole genome of
organism.
3D structure of mitochondria
Main Cellular Structures...
• 4. Ribosomes (plant & animal cells)
• Made from ribosomal RNA & protein.
• Sites of protein synthesis.
• Found freely in cytoplasm.
• Ribosomes that are bound to endoplasmic
reticulum produce proteins that are secreted
to outside.
Main Cellular Structures...
• 5. Lysosomes (animal cell)
• Organelles in cells of eukaryotic organism.
• Dark, spherical bodies in cytoplasm & contain mix of
digestive enzymes.
• Generally destroy cells that are worn out.
• …..enzymes break down contents into molecules that
can be reused.
• Old cells need to be removed during development or
have are removed if there is mutation or under stress.
• Plant cells rarely have lysosomes because vacuole
handles most of cell’s waste products.
Main Cellular Structures...
• 6. Vacuole (plant cell)
• Fluid filled cavity surrounded by single membrane.
• Formed by either infolding or pinching off part of cell
membrane or by enlargement of vesicle cut off from
Golgi apparatus.
• Young cells contain numerous small vacuoles, which
unite to form large permanent vacuole in mature cells.
• Generally, filled with fluid called cell sap that contain
dissolved food, ions, pigments & waste products.
• Vacuoles in some animal cells are smaller & not
permanent & contain fluids or solids that may have been
engulfed.
Main Cellular Structures...
• 7. Centrioles
• Present in each cell (a pair) near nucleus;
• Each consists of bundle of nine sets of tubules
& is about 0.5 um long & 0.2 um wide;
• Are involved in cell division; when cells divide,
centrioles pull apart to produce spindle of
microtubules that are involved in movement of
chromosomes.
Main Cellular Structures...
• 8. Cell Membrane
• Form boundary of cells.
• Acts as barrier that controls what enters
& leaves cells.
• About 7nm thick.
• Made of phospholipid bilayer.
Phospholipid Structure
Main Cellular Structures...
• A simple bilayer allows fat soluble
organic molecules to pass through it.
• Many vital chemicals such as ions may
not pass through bilayer even though they
are soluble in water;
• …..they enter because membranes
contain proteins & other molecules in
addition to lipids.
• Integral membrane proteins may form pores or
channels through membranes.
• …..some are permanent while others are temporal.
Main Cellular Structures...
• Functions of membrane proteins include:
• 1. Receptor proteins. e.g. proteins that make
cells sensitive and respond to hormones.
• 2. Channel proteins & carriers involved in
selective transport of polar molecules & ions in
facilitated diffusion.
• 3. Gated channels can be open or shut
depending on physiological conditions of cells.
Main Cellular Structures...
• 4. Some of protein pores are active
carrier systems, which use energy in
form of ATP to move molecules into & out
of cells.
• 5. Other proteins are simply gaps in lipid
bilayer & allow ionic substances to move
through membrane in both directions.
• 6. Enzymes e.g. involved in digestion of
molecules found in cell membranes.
Main Cellular Structures...
• 7. Antigens: membrane proteins that act
as identity markers; these are
glycoproteins & help cells recognise
foreign cells.
• 8. Energy transfer proteins: found in
membranes of mitochondrion where they
are involved in respiration & in
chloroplast membranes where they are
involved in photosynthetic reactions.
 Main Cellular Structures...
• 9. Endoplasmic Reticulum (ER)
• Forms major part of transport system of cell.
• Composed of tubes & vesicles, sac-like structures
that branch off & form a network of ER.
• Its surface membranes are sites for chemical
reactions such as protein synthesis.
• ER is connected to nucleus membrane.
• Two types ER:
 Rough ER & Smooth ER
Main Cellular Structures...
 Main Cellular Structures...
• Rough endoplasmic reticulum (RER)
• …..has rough appearance under an electron
microscope because numerous ribosomes are
present on surface membrane of RER.
• …..consists of tune-like & sac-like structures.
• …..RER membrane is connected to outer
membrane of nucleus.
 Main Cellular Structures...
• Functions of RER
• Collection of proteins produced on ribosomes.
• Isolates & transports proteins produced to their site
of need;
• …..including to Golgi apparatus, plasma membrane
& lysosomes, or where necessary vacuole.
 Main Cellular Structures...
• Smooth ER

• Ribosomes are absent.

• The functions include;

1. Synthesis & transport of lipids & steroids.
2. Breakdown of lipid-soluble toxins in liver
cells.
3. It also regulates calcium release during
muscle cell contraction.
Smooth ER

Rough ER
Main Cellular Structures...
• 10. Golgi body (also Golgi apparatus)
• Appears as dense area in cytoplasm under
light microscope;
• Made up of stacks of parallel, flattened
membrane pockets known as cisternae.
• Cisternae are formed when vesicles that have
pinched off from endoplasmic reticulum
fuse;
• …..thus Golgi body has strong link to ER but is
NOT physically linked to it.
Proteins brought to GB in vesicles are pinched
off from RER where they are made

Vesicles fuse with membrane sacs of GB & proteins

enters Golgi stacks.
 Main Cellular Structures...
• Protein transport & Golgi body
• As these proteins move through the GB stacks they are modified in
various ways which include
– addition of sugars some proteins to form glycoproteins such as
mucus. The GB also is involved in synthesis of ,materials for plant
cell walls and insect cuticle.
– Membrane proteins that need to be on the outer side of the
cell membrane eg receptor binding protein are oriented in
the GB so they are inserted in the correct direction when
they arrive at surface.
 Main Cellular Structures...

• Protein transport & Golgi body

• Some proteins in GB are digestive enzymes. These may
be transported by enclosing them in organelles called
lysosomes or
• transported through the GB and then in vesicles to the
surface membrane where the vesicles fuse with the cell
membrane to release there contents as is the case for
digestive enzymes.
Main Cellular Structures...
Protein transport & Golgi body

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