2. 2
Comparing Prokaryotic and Eukaryotic Cells: Overview
– Prokaryote comes from the Greek words for pre-nucleus.
– Eukaryote comes from the Greek words for true nucleus.
Prokaryotes and Eukaryotes are chemically similar, in the sense that they
both contain nucleic acids, proteins, lipids, and carbohydrates. They use the
same kinds of chemical reactions to metabolize food and build protein.
• One circular chromosome, not in a
membrane
• No histones
• No organelles
• Cell walls always contain the complex
polysaccharides peptidoglycan.
• Usually division by binary fission.
The DNA copied and the cell splits
into two cells. It involves fewer
structures and processes than
eukaryotic cell division.
Prokaryote Eukaryote
• Paired chromosomes, in
nuclear membrane
• Histones
• Organelles
• Cell walls, when present,
are chemically simple.
• Usually divide by mitosis.
This process is guided by
mitotic spindle.
4. 4
The Prokaryotic cell
• Average size: 0.2-2.0 µm in
diameter and from 2- 8 µm
in length
• Basic shapes:
Spherical coccus
Rod-shaped bacillus
Spiral
5. 5
• Unusual shapes
– Star-shaped cells ( genus Stella: a recently described genus of Gram
–ve bacteria found in fresh water, sewage and soil)
– Rectangular cells (genus Haloarcula, a genus of halophilic archae)
• Most bacteria are monomorphic which means that they maintain a single
shape.
• A few bacteria (such as Rhizobuim: a genus of Gram –ve bacteria found
in soil and Corynebacterium a genus of Gram +ve bacteria found in
vegetable and soil) are pleomorphic which means that they can have
many shapes, not just one.
7. 7
Simplified Bacterial Cell
Cell-wall
Nucleus
Plasmid
Cell-membrane
Fimbrae
Flagellum
Capsule
• Pairs of cocci are called Diplococci such as Neisseria meningitidis, the
agent of spinal meningitis.
• Chains of cocci are called streptococci such as Streptococcus lactis, a
comonly found in milk.
• irregular cluster if cocci in a grapelike pattern is called staphylococcus.
•Spiral bacteria such as Vibrio cholera, the agent of cholera.
8. 8
Structure external to the cell wall
Glycocalyx
Glycocalyx (meaning sugar coat) is general term used for substances that
surrounded cells.
• Certain species of bacteria are able to form a sticky, gelatinous layer of
polysaccharides and proteins known as capsule. A capsule is neatly organized
and can find in pathogenic species such as Streptococcus pneumoniae which
causes pneumonia only when the cells are protected by a polysaccharide
capsule. Unencapsulated S. pneumoniae cells cannot cause pneumonia and are
readily phagocytized.
The polysaccharide capsule of Klebsiella also prevents phagocytosis and allows
the bacterium to adhere to and colonize the respiratory tract.
• The sugar of glycocalyx is called extracellular polysaccharide (EPS) which
enables bacterium to attach to various surfaces in its natural environment in
order to survive. Through attachment bacteria can grow on rocks, fast-moving
stream, plants root, human teeth, water pipes ,etc.
Streptoccous mutans, an important cause of dental caries, attaches itself to
the surface of teeth by glycocalyx.
If the substance is less, more flowering or
unorganized, and loosely attached to the cell wall,
the Glycocalyx is described as a slime layer.
9. 9
Flagella
Some prokaryotic cells have flagella, which are long filamentous appendages
that propel bacteria.
• Made of chains of flagellin.
• They are long and thin and cannot be seen by the light microscope unless
stained.
• Attached to a protein hook
• Anchored to the cell wall and membrane by the basal body.
• Bacterial cells have four arrangements of flagella:
– Monotrichous: single flagellum
– Amphitrichous: have a flgellum at each end of the cell.
– Lophotrichous: have multiple flagella at the ends of the cells.
– Peritrichous: have flagella distributed over the entire body of the cell.
10. 10
Bacterial cells can alter the speed and directions of rotation of flagella
and thus are capable of various patterns of motility, the ability of an
organism to move by itself.
• The advantage of motility is that it enables a bacterium to move toward
a favorable environment or away from an adverse one.
• The movement of bacterium toward or away from a particular stimulus is
called taxis. Such stimuli include:
– chemical (chemotaxis)
– light (phototaxis).
• Motile bacteria contain receptor in or just under the cell wall. These
receptors pick up chemical stimuli, such as oxygen, ribose, galactose
• Rotate flagella to run or tumble (changes in direction)
• The flagellar protein called H antigens is useful for distinguishing among
serovars (serotypes), or variations within species, of gram-negative
bacteria. (e.g., there are at least 50 different H antigen for E. coli ).
Motile Cells
11. 11
Figure 4.9
Fig 4.9, Flagella and bacterial
motility: A bacterium running and
tumbling.
Proteus cell in swarming stage may have
more than 1000 peritrichous flagella.
Proteus mirabilis and P. vulgaris occur e.g
in soil, polluted water, intestines of
healthy man and animals.
P. mirabilis, in particular is an important
opportunistic pathogen, causing
pneumonia, septicemia, urinary tract
infection.
12. 12
• Spirochetes are a group of bacteria
that have unique structure and
motility.
• One of the best studied spirochetes
is Treponema pallidium (Gram –ve), the
causative agent of syphilis.
• The movement is by axial filaments or
endoflagella, bundles of fibrils that
arise at ends of the cell beneath
outer sheath and spiral around the
cell.
• Axial filaments anchored at one end
of a cell, have a structure similar to
that of flagella.
• Rotation (corkscrew motion) causes
cell to move
Axial Filaments
13. 13
Fimbriae and Pili
Many gram-negative bacteria contain short, hairlike appendages
that are shorter, straighter, and thinner than flagella and are used
for attachment rather than for motility.
• These structure consists of protein called pilin.
• They are divided into two types: fimbriae (few to several hundreds
per cell) and pili (one to two per cell).
– Fimbriae are distributed over the entire surface of the cell and allow
attachment for example, the fimbriae of Neisseria gonorrhoeae, the
causative agent of gonorrhea, help the microbe colonize mucous
membrane, so the bacteria cause the disease.
– Pili are used to transfer DNA from one cell to another, that is why
sometimes they are called sex pili.
14. 14
Cell Wall
• Outside cell membrane; not an organelle
• Maintains cell shape;
• Prevents osmotic lysis (cell bursting)
• It contributes to the ability of some species to cause disease
• It is the site of action of some antibiotics.
• The components of the cell wall is used to differentiate major types
of bacteria.
• The bacterial cell wall contains a large molecule called Peptidoglycan
(murein).
• The only bacteria that have no cell wall are the mycoplasmas such as
Mycoplasma pneumoniae, the causative agent of atypical pneumonia.
15. 15
Peptidoglycan
• Peptidoglycan (murein) is – (in
bacteria only);
– Polysaccharides containing N
acetylglucosamine & N
acetylmuramic acid linked by
numerous chain of amino acid
(polypeptides: tetrapeptide
side chain and peptide cross
chain).
Antibiotic (penicillin
interference)
17. 17
Gram-positive cell walls `
• The cell wall of Gram-positive bacteria consisting of many layers of
peptidoglycan forming a thick, rigid structure together with another organic
substance called teichoic acid.
Teichoic acids (consists primarily of an alcohol and phosphate (negative charge )
– Lipoteichoic acid which spans the peptidoglycan layer and is linked to the
cytoplasmic membrane.
– Wall teichoic acid, which is linked to the peptidoglycan layer.
Teichoic acids
– may bind and regulate the movement of cations (positive ions) into and out
of the cell.
– Play a role in cell growth, preventing extensive wall breakdown and possible
cell lysis.
– Provide much of wall’s antigenic specificity and thus make it possible to
identify bacteria by certain laboratory test.
Thick peptidoglycan layer and teichoic
acids ( act as attachment sites for
viruses such as bacteriophages)
Gram stain: purple; so it retains crystal
violet color( primary stain)
18. 18
Gram-positive cell walls `
• The cell wall of Gram-positive streptococci are covered with various
polysaccharides that allow them to be grouped into medically significant types.
• The cell walls of acid-fast bacteria, such as Mycobacterium, consists of much
as 60% mycolic acid, a waxy lipid, whereas the rest is peptidoglycan. These
bacteria can be stained with Gram stain and are considered gram positive.
19. 19
Gram-negative cell walls
• The cell wall of Gram-negative bacteria consists of a thin peptidoglycan
(one or a few layers) and an outer membrane.
The peptidoglycan is bonded to a lipoprotein and is in periplasm (fluid-filled
space contains a high concentration of degradative enzymes and transport
proteins).
• No teichoic acids
• Because of the small amount of peptidoglycan, G-ve are more susceptible
to mechanical breakage.
The outer membrane: consists
Lipopolysaccharides (LPS), lipoproteins,
phospholipids.
• Protect the cell from phagocytes,
antibiotics, detergents, digestive enzymes
and certain dye.
• Dose not provide a barrier to all
substance.
• Part of the permeability of the outer
membrane is due to Porins (proteins) that
form channels through membrane to
permit the passage of molecules
(nucleotides, disaccharides, peptides,
amino acids, iron, vitamin).
Porins - proteins that allow small
molecules to cross membrane.
20. 20
LPS provides 2 important characteristics:
Polysaccharides portion is composed of sugars called, O polysaccharides, that
function as antigens and are useful for distinguishing species. e.g., pathogen E.
coli O157:H7. (comparable to teichoic acid in G+ve bacteria).
Lipid portion called lipid A, called endotoxin, and is toxic when in host’s blood
stream or gastrointestinal track. It causes fever.
21. 21
The mechanism is based on differences in the structure of the cell walls
of gram-positive and gram-negative bacteria.
• Crystal violet (CV) primary stains both type of cells and the cells becomes
purple.
• When iodine (I) mordant added, it forms large crystals with the dye that
are too large to escape through the cell wall.
• Gram-positive
– Alcohol dehydrates peptidoglycan and make it more impermeable to
CV-I crystals do not leave
• Gram-negative
– Alcohol dissolves outer membrane and leaves holes in peptidoglycan
through which CV-I washes out or diffuse.
• Because gram-ve bacteria are colorless after the alcohol wash, the addition
of safranin turns the cell pink.
Gram Stain Mechanism
22. 22
Atypical Cell Walls
• Mycoplasmas (prokaryotes)
– Lack cell walls
– Their plasma membrane are
unique among bacteria in
having lipids called sterols
which are thought to help
protect them from lysis
(rupture).
• Archaea
– Wall-less, or
– Walls composed of
polysaccharides and proteins
but not peptidoglycan. These
walls do contain substance
similar to peptidoglycan called
pseudomurein.
– Pseudomurein conatins N-
acetyltalosaminuronic acid
instead of NAM and lack D
amino acids.
23. 23
Damage to Cell Walls
• Some genera such as Proteus (G-ve), can loose its cell
wall and swell into irregular shapes are called L forms.
They may form spontaneously or develop in response
to Penicillin which inhibits peptide bridges in
peptidoglycan (so inhibit cell wall formation), or
lyzozyme.
L form can live and divide repeatedly or return to the
walled state.
For lysozyme to exert its effect on G-ve cells, the cells
are first treated with EDTA. EDTA weakens ionic
bonds in the outer membrane and thereby damages it,
giving the lysozyme access to the peptidoglycan.
Lysozyme digests disaccharide in peptidoglycan. This enzyme occurs naturally in
some eukaryotic cells and is a constituent of tears, mucus, and saliva.
Lyzozyme can completely destroy the cell wall of gram +ve cell. If lysis do
not occur and the cellular contents remained intact, this wall-less cell is
called protoplast (spherical and still capable of carrying on metabolism).
Spheroplast is called for the gram -ve cells that looses its cell wall and
keeps the outer membrane.
Protoplasts and spheroplasts are susceptible to osmotic lysis.
24. 24
Structures Internal to the cell wall
Cytoplasmic or Plasma membrane
• This thin barrier, 8 nm thick, controls traffic into and out of the cell.
• Like other membranes, the plasma membrane is selectively permeable,
allowing some molecules and ions to pass through the membrane, but others
prevented from passing through it.
• The main macromolecules in membranes are lipids and proteins, but include
some carbohydrates and sterols, such as cholesterol in eukaryotic cells.
Bacteria have no sterols , except Mycoplasmas - no cell wall and have
cholesterol to add rigidity.
The carbohydrates in
eukaryotic membranes
serves as receptor sites
(cell-cell recognition)
The carbohydrates in
prokaryotic membrane
provide attachment sites.
25. 25
• The most abundant lipids are phospholipids. The phospholipid molecules are
arranged in two parallel rows, called phospholipid bilayer.
• Each phospholipid molecule contains a polar head, composed of phosphate
group and glycerol that is hydrophilic (water loving) and soluble in water, and
non polar tails, composed of fatty acids that are hydrophobic (water
fearing) and insoluble in water.
• Phospholipids and most other membrane constituents are amphipathic
molecules.
– Amphipathic molecules have both hydrophobic (water fearing) regions
and hydrophilic (water loving) regions.
26. 26
Proteins: carriers, channels pores, enzymes
• Proteins determine most of the membrane’s specific functions.
There are two populations of membrane proteins.
• Peripheral proteins are not embedded in the lipid bilayer at all. Instead,
they are loosely bounded to the surface of the protein, often connected to
the other population of membrane proteins.
They may function:
− as enzymes that catalyze chemical reactions,
− as a “scaffold” for support,
− and as a mediators of changes in membrane shape during movement.
Integral proteins:
• some penetrate the membrane
completely, and are called a
transmembrane protein.
• Some integral proteins are channels
that have a pore through which
substances enter and exist the cell.
The phospholipids and proteins in
membranes create a unique physical
environment, described by the fluid
mosaic model.
27. 27
Inclusions
With the cytoplasm of prokaryotic cells are several kind s of reserve deposits, known as
inclusions. Some inclusions are common to a wide variety of bacteria, whereas others are
limited to a small number of species and therefore serve as basis for identification. Cells
may accumulate certain nutrients in inclusions when they are plentiful and use them when
the environment is deficient to avoid the increase in osmotic pressure that would result if
the molecules were dispersed in the cytoplasm.
Metachromatic granules (volutin)
Corynebacterium diphtheriae (also bacteria)
Phosphate reserves (can be used to synthesis
ATP), Algae, fungi, protozoa
Polysaccharide granules (consists of glycogen and
starch)
Energy reserves
Lipid inclusions Mycobacterium, Bacillus,
Azotobacter, Spirillum
Energy reserves
Sulfur granules Genus Thiobacillus Energy reserves
Carboxysomes
Bacteria that use CO2 as their sole source of
carbon. Cynobacteria, Thiobacillus
Contain Ribulose 1,5-diphosphate carboxylase
which is used for CO2 fixation during
photosynthesis
Gas vacuoles
Found in many aquatic prokaryotes including
Cyanobacteria, halobacteria
Each contain rows of several individual gas vesicles
to maintain buoyancy so that the cells can remain
at the depth in the water appropriate for them to
receive sufficient amount of O2, light and
nutrients.
Magnetosomes
Formed by several Gram –ve such as Aquaspirillum
magnetosomes
Contain iron oxide used as magnet so bacteria can
move downward until they reach suitable
attachment site, (destroys H2O2)
28. 28
Internal Structures in Eukaryotic cells
Ribosomes
– free in cytoplasm or attached to ER
• Endoplasmic Reticulum
– Smooth : no ribosomes; makes lipids & membranes
– Rough : ribosomes; makes proteins for use outside of cell
• Lysosomes:
– digestive enzymes
• Peroxisomes:
– organelle – converts hydrogen peroxide to water + oxygen
• Vacuoles:
– stores materials – starch, glycogen, fat
• Cytoskeleton:
– protein fibers to give support, add rigidity, shape to cell
External structure:
Flagella and Cilia
• Few and long projections called Flagella.
• Numerous and short projections called cilia.
Cell Wall and Glycocalyx (meaning sugar goat)
• Most cells have cell walls (Algae, fungi, plant)
• Some cells, the plasma membrane is covered by glycocalyx.
29. 29
Fig 12.8 Two ways in which a sorting signal can be built into a protein. A. The
signal resides in a single discrete stretch of amino acid sequence, called a signal
sequence, that is exposed in the folded protein. Signal sequence often occur at
the end of the polypeptide chain, but the can also located internally. B. A signal
patch can be formed by the juxtaposition of amino acid.
30. 30
Fig 12.26 Protein import by mitochondria. The N-terminal signal
sequence of the precursor protein is recognized by receptors of the
TOM complex. The protein is thought to be translocated across both
mitochondrial membranes at or near special contact sites. The signal
sequences is cleaved off by a signal peptidase in the matrix to form
the mature protein. The free signal sequence is rapidly degraded.
31. 31
Fig 12.7 Vesicle budding
and fusion during vesicular
transport.
In this process, soluble
components (red spots) are
transferred from lumen to
lumen. The membrane is
also transferred and the
original orientation of both
proteins and lipids in the
donor-compartment
membrane is preserved in
the target-compartment
membrane.
32. 32
Fig 12.6 Simplified “roadmap” of
protein traffic. Proteins can move
from one compartment to another
by gated transport,
transmembrane protein, or
vesicular transport. The signals
that direct a given protein’s
movement through the system,
and thereby determine its
eventual location in the cell, are
contained in each proteins amino
acid sequence. The journey begins
with the synthesis of a protein
on a ribosome in the cytosol or
on a ribosome of the ER and
terminates when the final
destination is reached. At each
intermediate station (boxes), a
decision is made as to whether
the protein is to be retained in
that compartment or transported
further. In principle, a signal
could be required for either
retention in or exit from a
compartment.
33. 33
Endospores
• When essential nutrients are depleted,
certain gram +ve cells, form specialized
resting cells called endospores.
• Bacillus, Clostridium
• Resistant to heat, radiation, acids, drying,
chemicals
• The process of endospore formation within a
vegetative cell takes several hours (8 h) and
is known as sporulation.
• Germination: Return to vegetative state.
Germination occurs, under favorable
conditions, in a matter of minutes.
• How long can spores survive?
It has been reported that 250 million year
old spores have been revived
These spores were preserved in salt crystals
of Permian age.
34. 34
• In spore formation, the DNA of the cell and a small
amount of cytoplasm gather at one region of the cell.
See slide 35.
• Depending on the species, the endospore might be
located terminally (at one end) such as clostridium
species, or centrally inside the vegetative cell such as
Bacillus.
• Endospores can survive in boiling water for several
hours (19 hours) or more whereas most vegetative
cells can be killed at temperature 70ºC.