Chapter 2 Cell Biology
2.1 Overview of the structure of microbial cells
2.2 Procaryotic cell wall
2.3 Cytoplasmic membrane
2.4 Cellular genetic information
2.5 Cytoplasmic matrix – Ribosome and Inclusions
2.6 Components external to the cell wall
2.7 Bacterial endospores
2.8 Comparison of the prokaryotic and eukaryotic cell
Chapter outline
A procaryotic cell
A eucaryotic cell
Overview of cell structure
3,Their cell wall almost always contain the complex
polysaccharide peptidoglycan
The prokaryotic cell
1,Their genetic material (DNA) is not enclosed
within a membrane and they lack other membrane
– bounded organelles
2,Their DNA is not associated with histidine
4,They are very small!!
Size,Most bacteria fall within a range from 0.2 to 2.0
um in diameter and from 2 to 8 um in length
A rod-shaped prokaryote
is typically about 1-5
micrometers (μm) long
and about 1 μm wide
Microorganisms in
general are very small
and are completely
invisible to the naked
eye.
A cyanobacterium 8
x 50 um
size comparison of microorganisms
Visibility scaleMeters Relative size of Microbes
Prokaryotes
Eukaryotes
Viruses
Naked eye
Light microscope
Electron
microscope
a cell increases in size,
its surface area – to –
volume ratio decreases
Surface area and volume relationships in cells
spirallum
Shape,Bacteria have a few basic shapes
spherical coccus
Rod-shaped bacillus
? The cell wall of the bacterial cell is a complex,
semi-rigid structure that is responsible for the
characteristic shape of the cell,The cell wall
surrounds the underlying,fragile plasma
(cytoplasmic) membrane and protects it and
internal parts of the cell from adverse changes in
the surrounding environment.
? Almost all prokaryotes have cell walls.
Prokaryotic cell wall
Gram+ Gram-
Schematic diagram of bacterial cell walls
Bacteria can be divided into two major groups,called
gram-positive and gram-negative,The original distinction
between gram-positive and gram-negative was based on a
special staining procedure,the Gram stain
The Gram-positive cell wall has a peptidoglycan layer
that is relatively thick (ca,40 nm) and comprises
approximately 90% of the cell wall,The cell walls of
most Gram-positive eubacteria also have teichoic acids.
Gram-positive cell wall
Structure of the Repeating Unit in Peptidoglycan
These constituents are
connected to form a
repeating structure,the
glycan tetrapeptide.
Peptidoglycan is composed of two sugar derivatives,N-
acetylglucosamine (NAG) and N-acetylmuramic acid
(NAM),and a small group of amino acids consisting of
L-alanine,D-alanine,D-glutamic acid,and either lysine
or diaminopimelic acid (DAP),
Peptide and glycan units are connected
in formation of the peptidoglycan sheet
Gram-positive Bacteria frequently have acidic
polysaccharides called teichoic acids attached to their
cell wall,The term teichoic acids includes all wall,
membrane,or capsular polymers containing
glycerophosphate or ribitol phosphate residues,These
polyalcohols are connected by phosphate esters and
usually have other sugars and D-alanine attached.
Teichoic acids
Teichoic acids and lipoteichoic acids are arranged in
the overall wall structure of gram-positive Bacteria,
Teichoic acidLipoteichoic acid
The Gram-negative cell wall is a thin layer attached to an outer
membrane via lipoproteins,The outer membrane contains
phospholipid on its inner surface and lipopolysaccharide (LPS) on
its outer surface,The space between the outer membrane and the
cytoplasmic membrane is called the periplasmic space.Teichoic
acids do not occur in Gram-negative bacterial cell walls..
O side chain
Core
polysaccharide
Lipid A
Chemical structure of Lipopolysaccharide
Molecular model of E.coli lipopolysaccharide
? The bonds between the carbohydrates in
pseudopeptidoglycan are β 1-3 instead of'β1-4 as
in peptidoglycan.
CELL WALLS OF ARCHAEBACTERIA
? The archaebacteria do not contain peptidoglycan in
their cell walls as occurs in eubacteria.
? N-acetylmuramic acid and D-amino acids are not
found in the cell walls of archaebacteria.
( Differences from eubacteria )
? Some archaebacteria have walls composed of
pseudopeptidoglycan,which resembles the
peptidoglycan of eubacteria but contains N-
acetyltalosaminuronic acid instead of N-
acetylmuramic acid and L.-amino acids instead of
the D-amino acids in eubacterial cell walls.
Protoplast Formation
Peptidoglycancan be destroyed by certain agents for instance
lysozyme,that breaks the 1,4-glycosidic bonds between N-
acetylglucosamine and N-acetylmuramic acid in the molecule,
The difference between gram-positive and gram-negative
bacteria is due to the physical nature of their cell walls,
If the cell wall is removed from gram-positive bacteria,
they become gram negative,
The peptidoglycan seems to act as a permeability
barrier preventing loss of crystal violet,Gram-
negative peptidoglycan is very thin,not as highly
cross-linked,and has larger pores,Alcohol treatment
also may extract enough lipid from the gram negative
wall to further increase its porosity,For these
reasons,alcohol more readily removes the purple
crystal violet-iodine complex from gram-negative
bacteria.
The Mechanism of Gram Staining
Procedures of Gram Staining
Gram positive or Gram negative?
Morphology of a gram-positive bacterial cell
? Structure of cytoplasmic membrane
? Function of cytoplasmic membrane
2.3 Cytoplasmic membrane
A,The typical cytoplasmic membrane of
prokaryotic and eukaryotic cells is a lipid
bilayer,as illustrated here showing the
orientations of the hydrophilic (tan spheres)
and hydrophobic (black) ends of
phospholipids that make up this structure.
B,Colorized electron micrograph of'the
cytoplasmic membrane (CM) of the
bacterium Bacillus subtilis reveals the
characteristic railroad track appearance of
this lipid bilayer.
Structure of cytoplasmic membrane
It is a typical UNIT MEMBRANE !
The cytoplasmic membrane,a highly selective
barrier,is constructed principally of lipid,within
which certain proteins are embedded.
Membranes contain
both lipids and
proteins,although
the exact proportions
of lipid and protein
vary widely.
1,Permeability barrier -
prevents leakage and function
as gate way for transport of
nutrients into and out of the
cell.
2,Protein anchor-
site of many proteins involved
in transport,bioenergetics,
and chemotaxis.
3,Energy conservation- site
of generation and use of the
proton motive force.
Function of membrane
Co m p a r a t i v e p e r m e a b i l i t y o f
m e m b r a n e s t o v a r i o u s m o l e c u l e s
S u b s t a n c e Ra t e o f p e r m e a b i l i t y *
W a t e r
G l y c e r o l
T r y p t o p h a n
G l u c o s e
Ch l o r i d e i o n ( Cl
-
)
P a t a s s i u m i o n ( K
+
)
S o d i u m i o n ( Na
+
)
100
0, 1
0, 0 0 1
0, 0 0 1
0, 0 0 0 0 0 1
0, 0 0 0 0 0 0 1
0, 0 0 0 0 0 0 0 1
* R e l a t iv e r a te – P e r m e a b il i ty w it h r e s p e c t to p e r m e a b il it y o f w a t e r g iv e n a s 1 0 0
Intracellular membrane system
? Bacteria cells don’t contain membrane-
enclosed organelles,However,bacteria may
have specialized invaginations of the
cytoplasmic membrane,
? Their function may be to provide a larger
membrane surface for greater metabolic
activity.
Structure of Mesosome
Mesosome may be involved in wall formation
during division or play a role in chromosome
replication and distribution to daughter cells,It
may also be involved in secretory processes
2.4 Cellular genetic information
? Bacterial Chromosome
1,Supercoiling and chromosome structure
2,Chromosomal copy number
? Plasmids
Micrograph of a bacterium showing the
nucleoid region (green) within the cytoplasm
where the bacterial chromosome occurs
The bacterial chromosome is a circular DNA
macromolecule except in Streptomyces where it is
linear and Rhodobacter sphaffoides,which has two
separate chromosomes.
Bacterial chromosome
The bacterial chromosome is usually a single
covalently closed circular molecule.
The term nucleoid is used to describe aggregated
DNA in the prokaryotic cell.
Range of genome sizes in virous groups of
organisms and the organellesof eukarya
The bacterial chromosome
and supercoiling:
Example of E,coli cell
? There are over 50 supercoiled domain in the E.coli
chromosome.
? The total amount of DNA is about 4600 kb.
? If the total DNA is opened and linearized,it would be
1 mm in length.
? The the cell is only about 2-3 um long,
So to package this much DNA into the cell requires
that the DNA be highly supercoiled.
Electron micrograph of an isolated nucleoid
released from E.coli.
Chromosome copy number
? Bacteria that reproduce asexually are typically
haploid in genetic complement.
? Rapidly growing cells contain more than 1
copy of the chromosome,and only when cell
growth has ceased does the chromosome
number approach one per cell.
Reproduction of
a bacterial cell requires the replication of the
bacterial chromosome,The micrograph shows the
sequence of synthesis of new circular loops of
double helical DNA.
Bacteria normally reproduce
by binary fission,The
inward growth of the septum
divides the parent cell to
produce two equal-sized
progeny cells.
Plasmids don’t contain the
genetic information for the
essential metabolic
activities of the cell,but
they generally do contain
genetic information for
special features.
Plasmid
Prokaryotic cells have small extra-chromosomal
genetic elements called plasmids.
Resistant plasmids
Col plasmids
Conjugative plasmids
Metabolic plasmids
Major types of plasmids
2.5 Cytoplasmic matrix – Ribosome
and Inclusions
All eucaryotic and procaryotic cells
contain ribosomes,which function
as the sites of protein synthesis,
Ribosomes are composed of two
subunits
Procaryotic ribosomes are called
70S ribosomes,and those of
eucaryotic cells are known as 80S
ribosomes
Ribosomes
The letter S refers to Svedberg units,which
indicate the relative rate of sedimentation
during ultra-high-speed centrifugation
Within the cytoplasm of procaryotic (and eucaryotic)
cells are several kinds 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 a
basis for identification,Among the more prominent
bacterial inclusions are the following:
Carbon storage polymers – PHB and glycogen
Phosphate polymers
Sulfur Granules
Gas Vacuoles
INCLUSIONS
Polyhydroxybutyric acid
(PHB)
PHB is a lipidlike compound - one of the most common
inclusion bodies in prokaryotic organisms,
PHB is commonly found as a storage material and unique
to bacteria
Glycogen is a starchlike polymer of glucose subunits,
Glycogen granules are usually smaller than PHB granules.
A Vibrio species
Many microorganisms accumulate granules of
polyphosphate,which are large reserves of
inorganic phosphates that can be used in the
synthesis of ATP
Polyphosphate granule
in a bacterial cell
A Pseudomonas species
The sulfur globules
inside the cells of
purple sulfur bacterium
Chromatium buderi
Some bacteria,including many photosynthetic
bacteria,accumulate elemental sulfur granules
as a result of their metabolism.
Gas vacuoles (blue)
and storage granules (red)
in the cyanobacterium
Microcystis
The formation of gas vacuoles by aquatic bacteria
provides a mechanism for adjusting the buoyancy
of the cell.Many aquatic cyanobacteria use their
gas vacuoles to move up and down in the water
column.
2.6 Components external to the cell wall
? Flagella
? Fimbriae and pili
? Capsules and slime layers
Motility allows the cell to reach different regions of its
environment,In the struggle for survival,movement to a
new location may mean the difference between survival
and death of the cell,But,as in any physical process,cell
movement is closely tied to an energy expenditure,and
the movement of flagella is no exception.
Many prokaryotes are motile,and this
ability to move independently is usually
due to a special structure,the flagellum
(plural,flagella),
Four basic types of flagellar arrangements
a,monotrichous b,amphitrichous
c,lophotrichous
d,peritrichous
Flagella are arranged differently on different bacteria,
In polar flagellation the flagella are attached at one or
both ends of the cell,Occasionally a tuft (group) of
flagella may arise at one end of the cell,an
arrangement called lophotrichous,In peritrichous
flagellation the flagella are inserted at many places
around the cell surface (peri means "around"),The
type of flagellation,polar or peritrichous,is often used
as a characteristic in the classification of bacteria.
The flagellum of a Gram-negative bacterium
The filament of bacterial flagella is composed of
subunits of a protein called flagellin.
The base of the flagellum is different in structure from
that of the filament,There is a wider region at the base
of the flagellum called the hook,The hook consists of a
single type of protein and functions to connect the
filament to the motor portion of the flagellum.
The basal body is anchored in the cytoplasmic
membrane and cell wall,The basal body consists of a
small central rod that passes through a system of rings.
In gram-negative Bacteria,an outer ring is anchored in
the lipopolysaccharide layer and another in the
peptidoglycan layer of the cell wall,and an inner ring is
located within the cytoplasmic membrane.
In gram-positive Bacteria,which lack the outer
lipopolysaccharide layer,only the inner pair of rings is
present,Surrounding the inner ring and anchored in the
cytoplasmic membrane are a pair of proteins called
Mot, These proteins actually drive the flagellar motor
causing a torque that rotates the filament,A final set of
proteins,called the Fli proteins function as the motor
switch,reversing rotation of the flagella in response to
intracellular signals.
The movement of a procaryotic flagellum results from
rotation of its basal body and is similar to the movement
of the shaft of an electric motor,Bacterial cells can alter
the speed and direction of rotation of flagella and thus
are capable of various patterns of motility.
Fimbriae can be evenly distributed over the entire
surface of the cell,They can number anywhere from a few
to several hundred per cell,Fimbriae allow a cell to adhere
to surfaces including the surfaces of other cells,
Pili are usually longer than fimbriae
and number only one or two per cell,
Pili function to join bacterial cells prior
to the transfer of DNA from one cell to
another.
F pilus
Many prokaryotic organisms secrete on their surfaces
slimy or gummy materials,A variety of these structures
consist of polysaccharide,and a few consist of protein.
The terms capsule and slime layer are frequently used to
describe polysaccharide layers.
Demonstration of the
presence of a capsule
is usually by means
of negative staining
Capsules and Slime Layers
Many prokaryotes contain a cell surface layer composed
of a two-dimensional array of protein,These layers are
called S-layers,S-layers have been detected in
representatives of virtually every phylogenetic grouping
of Bacteria and are nearly universal among Archaea,In
some species of Archaea the S-layer is also the cell wall,
The major function of S-layers is unknown,However,as
the interface between the cell and its environment it is
likely that in cells that produce them the S-layer at least
functions as an external permeability barrier,allowing
the passage of low-molecular-weight substances while
excluding large molecules,
Paracrystalline Surface Layers (S-Layers)
2.7 Bacterial endospores
? Certain species of bacteria produce special
structure called endospores.
? They are very resistant to heat and can not
be destroyed easily,even by harsh
chemicals,Endospores are also resistant to
other harmful agents such as drying,
radiation,acids and chemical disinfectants.
Sporulating cell
Central core
Cortex
Spore coat /
membrane
exosporium
Micrograph of a endospore
Vegetative cell
Containing abundant DPA
(dipicolinic acid) which is combined with
calcium ions.
? Lower water content – only 10-30% of the
water content of the vegetative cell.
? Low pH value,and contains small acid-soluble
spore proteins (SASPs)
Structure of DPA
Properties of endospore and its resistance
Sporulation involves a very complex series of events
in cellular differentiation,
Bacterial sporulation does not occur when cells are
dividing exponentially but only when growth ceases
owing to the exhaustion of an essential nutrient,Thus,
cells of Bacillus cease vegetative growth and begin
sporulation when a key nutrient such as the carbon or
nitrogen source becomes limiting.
Endospore formatiom
1,Axial filament formation
2,Septum formatiom
3,Engulfment of forespore
4,Cortex formation
5,Coat synthesis
6,Completion of coat synthesis,
Increase in refractility and heat
resistance
7,Lysis of sporangium,spore
liberation
Stages in endospore formation
Spore germination
1,Activation – Usually results from treatments like heating.
2,Germination – Breaks spore’s dormant state,This process
is characterized by spore swelling,loss of resistance to heat and
other stresses,loss of refractility and increase in metabolic activity.
3,Outgrowth – The spore protoplast makes new components
and develops once more into an active bacterium.
Parasporal Crystal
(Spore-companioned crystal)
Several Bacillus species,most notably
B.popilliae and B,thuringiensis,produce
intracellular crystals of toxic glycoproteins
when they sporulate.
Toxic crystals
Please carefully read and be familiar with
the contents of table 3.3 on the page 100.
Comparison of the prokaryotic and eucaryotic cell
Their DNA is associated
with histone proteins.
membrane-bounded
organelles
9+2 type flagella
Eucaryotic cell
The chief distinguishing characteristics of
procaryotic cells are:
1,Their genetic material (DNA) is not enclosed within a
membrane.
2,They lack other membrane-bounded organelles.
3,Their DNA is not associated with histone proteins.
4,Their cell walls almost always contain the complex
polysaccharide peptidoglycan.
5,They usually divide by binary fission,During this
process,the DNA is copied and the cell splits into two
cells.
Summary
1,Prokaryotic genetic material is located in
an area called the nucleoid and is not
enclosed by a membrane.
2,Most bacteria have a cell wall outside the
plasma membrane to give them shape and
protect them from osmotic lysis.
Bacterial walls are chemically complex and
usually contain peptidoglycan or murein.
3,Bacteria often are classfied as either gram
positive or gram negative based on differences in
cell wall structure and their response to Gram
staining,
Gram-positive walls have thick,homogeneous
layers of peptidoglycan and teichoid acid,Gram-
negative bacteria have a thin peptidoglycan layer
surrounded by a complex outer membrane
containing lipopolysaccharides and other
components
4,Some bacteria are motile,usually by means of
flagella,and bacterial species differ in the number
and distribution of their flagella.
5,Structures such as capsules,fimbriae,and sex
pili are found outside of some bacterial cell wall.
6,Some bacteria survive adverse environmental
conditions by forming endospores,dormant
structures resistant to heat,desiccation and many
chemicals
2.1 Overview of the structure of microbial cells
2.2 Procaryotic cell wall
2.3 Cytoplasmic membrane
2.4 Cellular genetic information
2.5 Cytoplasmic matrix – Ribosome and Inclusions
2.6 Components external to the cell wall
2.7 Bacterial endospores
2.8 Comparison of the prokaryotic and eukaryotic cell
Chapter outline
A procaryotic cell
A eucaryotic cell
Overview of cell structure
3,Their cell wall almost always contain the complex
polysaccharide peptidoglycan
The prokaryotic cell
1,Their genetic material (DNA) is not enclosed
within a membrane and they lack other membrane
– bounded organelles
2,Their DNA is not associated with histidine
4,They are very small!!
Size,Most bacteria fall within a range from 0.2 to 2.0
um in diameter and from 2 to 8 um in length
A rod-shaped prokaryote
is typically about 1-5
micrometers (μm) long
and about 1 μm wide
Microorganisms in
general are very small
and are completely
invisible to the naked
eye.
A cyanobacterium 8
x 50 um
size comparison of microorganisms
Visibility scaleMeters Relative size of Microbes
Prokaryotes
Eukaryotes
Viruses
Naked eye
Light microscope
Electron
microscope
a cell increases in size,
its surface area – to –
volume ratio decreases
Surface area and volume relationships in cells
spirallum
Shape,Bacteria have a few basic shapes
spherical coccus
Rod-shaped bacillus
? The cell wall of the bacterial cell is a complex,
semi-rigid structure that is responsible for the
characteristic shape of the cell,The cell wall
surrounds the underlying,fragile plasma
(cytoplasmic) membrane and protects it and
internal parts of the cell from adverse changes in
the surrounding environment.
? Almost all prokaryotes have cell walls.
Prokaryotic cell wall
Gram+ Gram-
Schematic diagram of bacterial cell walls
Bacteria can be divided into two major groups,called
gram-positive and gram-negative,The original distinction
between gram-positive and gram-negative was based on a
special staining procedure,the Gram stain
The Gram-positive cell wall has a peptidoglycan layer
that is relatively thick (ca,40 nm) and comprises
approximately 90% of the cell wall,The cell walls of
most Gram-positive eubacteria also have teichoic acids.
Gram-positive cell wall
Structure of the Repeating Unit in Peptidoglycan
These constituents are
connected to form a
repeating structure,the
glycan tetrapeptide.
Peptidoglycan is composed of two sugar derivatives,N-
acetylglucosamine (NAG) and N-acetylmuramic acid
(NAM),and a small group of amino acids consisting of
L-alanine,D-alanine,D-glutamic acid,and either lysine
or diaminopimelic acid (DAP),
Peptide and glycan units are connected
in formation of the peptidoglycan sheet
Gram-positive Bacteria frequently have acidic
polysaccharides called teichoic acids attached to their
cell wall,The term teichoic acids includes all wall,
membrane,or capsular polymers containing
glycerophosphate or ribitol phosphate residues,These
polyalcohols are connected by phosphate esters and
usually have other sugars and D-alanine attached.
Teichoic acids
Teichoic acids and lipoteichoic acids are arranged in
the overall wall structure of gram-positive Bacteria,
Teichoic acidLipoteichoic acid
The Gram-negative cell wall is a thin layer attached to an outer
membrane via lipoproteins,The outer membrane contains
phospholipid on its inner surface and lipopolysaccharide (LPS) on
its outer surface,The space between the outer membrane and the
cytoplasmic membrane is called the periplasmic space.Teichoic
acids do not occur in Gram-negative bacterial cell walls..
O side chain
Core
polysaccharide
Lipid A
Chemical structure of Lipopolysaccharide
Molecular model of E.coli lipopolysaccharide
? The bonds between the carbohydrates in
pseudopeptidoglycan are β 1-3 instead of'β1-4 as
in peptidoglycan.
CELL WALLS OF ARCHAEBACTERIA
? The archaebacteria do not contain peptidoglycan in
their cell walls as occurs in eubacteria.
? N-acetylmuramic acid and D-amino acids are not
found in the cell walls of archaebacteria.
( Differences from eubacteria )
? Some archaebacteria have walls composed of
pseudopeptidoglycan,which resembles the
peptidoglycan of eubacteria but contains N-
acetyltalosaminuronic acid instead of N-
acetylmuramic acid and L.-amino acids instead of
the D-amino acids in eubacterial cell walls.
Protoplast Formation
Peptidoglycancan be destroyed by certain agents for instance
lysozyme,that breaks the 1,4-glycosidic bonds between N-
acetylglucosamine and N-acetylmuramic acid in the molecule,
The difference between gram-positive and gram-negative
bacteria is due to the physical nature of their cell walls,
If the cell wall is removed from gram-positive bacteria,
they become gram negative,
The peptidoglycan seems to act as a permeability
barrier preventing loss of crystal violet,Gram-
negative peptidoglycan is very thin,not as highly
cross-linked,and has larger pores,Alcohol treatment
also may extract enough lipid from the gram negative
wall to further increase its porosity,For these
reasons,alcohol more readily removes the purple
crystal violet-iodine complex from gram-negative
bacteria.
The Mechanism of Gram Staining
Procedures of Gram Staining
Gram positive or Gram negative?
Morphology of a gram-positive bacterial cell
? Structure of cytoplasmic membrane
? Function of cytoplasmic membrane
2.3 Cytoplasmic membrane
A,The typical cytoplasmic membrane of
prokaryotic and eukaryotic cells is a lipid
bilayer,as illustrated here showing the
orientations of the hydrophilic (tan spheres)
and hydrophobic (black) ends of
phospholipids that make up this structure.
B,Colorized electron micrograph of'the
cytoplasmic membrane (CM) of the
bacterium Bacillus subtilis reveals the
characteristic railroad track appearance of
this lipid bilayer.
Structure of cytoplasmic membrane
It is a typical UNIT MEMBRANE !
The cytoplasmic membrane,a highly selective
barrier,is constructed principally of lipid,within
which certain proteins are embedded.
Membranes contain
both lipids and
proteins,although
the exact proportions
of lipid and protein
vary widely.
1,Permeability barrier -
prevents leakage and function
as gate way for transport of
nutrients into and out of the
cell.
2,Protein anchor-
site of many proteins involved
in transport,bioenergetics,
and chemotaxis.
3,Energy conservation- site
of generation and use of the
proton motive force.
Function of membrane
Co m p a r a t i v e p e r m e a b i l i t y o f
m e m b r a n e s t o v a r i o u s m o l e c u l e s
S u b s t a n c e Ra t e o f p e r m e a b i l i t y *
W a t e r
G l y c e r o l
T r y p t o p h a n
G l u c o s e
Ch l o r i d e i o n ( Cl
-
)
P a t a s s i u m i o n ( K
+
)
S o d i u m i o n ( Na
+
)
100
0, 1
0, 0 0 1
0, 0 0 1
0, 0 0 0 0 0 1
0, 0 0 0 0 0 0 1
0, 0 0 0 0 0 0 0 1
* R e l a t iv e r a te – P e r m e a b il i ty w it h r e s p e c t to p e r m e a b il it y o f w a t e r g iv e n a s 1 0 0
Intracellular membrane system
? Bacteria cells don’t contain membrane-
enclosed organelles,However,bacteria may
have specialized invaginations of the
cytoplasmic membrane,
? Their function may be to provide a larger
membrane surface for greater metabolic
activity.
Structure of Mesosome
Mesosome may be involved in wall formation
during division or play a role in chromosome
replication and distribution to daughter cells,It
may also be involved in secretory processes
2.4 Cellular genetic information
? Bacterial Chromosome
1,Supercoiling and chromosome structure
2,Chromosomal copy number
? Plasmids
Micrograph of a bacterium showing the
nucleoid region (green) within the cytoplasm
where the bacterial chromosome occurs
The bacterial chromosome is a circular DNA
macromolecule except in Streptomyces where it is
linear and Rhodobacter sphaffoides,which has two
separate chromosomes.
Bacterial chromosome
The bacterial chromosome is usually a single
covalently closed circular molecule.
The term nucleoid is used to describe aggregated
DNA in the prokaryotic cell.
Range of genome sizes in virous groups of
organisms and the organellesof eukarya
The bacterial chromosome
and supercoiling:
Example of E,coli cell
? There are over 50 supercoiled domain in the E.coli
chromosome.
? The total amount of DNA is about 4600 kb.
? If the total DNA is opened and linearized,it would be
1 mm in length.
? The the cell is only about 2-3 um long,
So to package this much DNA into the cell requires
that the DNA be highly supercoiled.
Electron micrograph of an isolated nucleoid
released from E.coli.
Chromosome copy number
? Bacteria that reproduce asexually are typically
haploid in genetic complement.
? Rapidly growing cells contain more than 1
copy of the chromosome,and only when cell
growth has ceased does the chromosome
number approach one per cell.
Reproduction of
a bacterial cell requires the replication of the
bacterial chromosome,The micrograph shows the
sequence of synthesis of new circular loops of
double helical DNA.
Bacteria normally reproduce
by binary fission,The
inward growth of the septum
divides the parent cell to
produce two equal-sized
progeny cells.
Plasmids don’t contain the
genetic information for the
essential metabolic
activities of the cell,but
they generally do contain
genetic information for
special features.
Plasmid
Prokaryotic cells have small extra-chromosomal
genetic elements called plasmids.
Resistant plasmids
Col plasmids
Conjugative plasmids
Metabolic plasmids
Major types of plasmids
2.5 Cytoplasmic matrix – Ribosome
and Inclusions
All eucaryotic and procaryotic cells
contain ribosomes,which function
as the sites of protein synthesis,
Ribosomes are composed of two
subunits
Procaryotic ribosomes are called
70S ribosomes,and those of
eucaryotic cells are known as 80S
ribosomes
Ribosomes
The letter S refers to Svedberg units,which
indicate the relative rate of sedimentation
during ultra-high-speed centrifugation
Within the cytoplasm of procaryotic (and eucaryotic)
cells are several kinds 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 a
basis for identification,Among the more prominent
bacterial inclusions are the following:
Carbon storage polymers – PHB and glycogen
Phosphate polymers
Sulfur Granules
Gas Vacuoles
INCLUSIONS
Polyhydroxybutyric acid
(PHB)
PHB is a lipidlike compound - one of the most common
inclusion bodies in prokaryotic organisms,
PHB is commonly found as a storage material and unique
to bacteria
Glycogen is a starchlike polymer of glucose subunits,
Glycogen granules are usually smaller than PHB granules.
A Vibrio species
Many microorganisms accumulate granules of
polyphosphate,which are large reserves of
inorganic phosphates that can be used in the
synthesis of ATP
Polyphosphate granule
in a bacterial cell
A Pseudomonas species
The sulfur globules
inside the cells of
purple sulfur bacterium
Chromatium buderi
Some bacteria,including many photosynthetic
bacteria,accumulate elemental sulfur granules
as a result of their metabolism.
Gas vacuoles (blue)
and storage granules (red)
in the cyanobacterium
Microcystis
The formation of gas vacuoles by aquatic bacteria
provides a mechanism for adjusting the buoyancy
of the cell.Many aquatic cyanobacteria use their
gas vacuoles to move up and down in the water
column.
2.6 Components external to the cell wall
? Flagella
? Fimbriae and pili
? Capsules and slime layers
Motility allows the cell to reach different regions of its
environment,In the struggle for survival,movement to a
new location may mean the difference between survival
and death of the cell,But,as in any physical process,cell
movement is closely tied to an energy expenditure,and
the movement of flagella is no exception.
Many prokaryotes are motile,and this
ability to move independently is usually
due to a special structure,the flagellum
(plural,flagella),
Four basic types of flagellar arrangements
a,monotrichous b,amphitrichous
c,lophotrichous
d,peritrichous
Flagella are arranged differently on different bacteria,
In polar flagellation the flagella are attached at one or
both ends of the cell,Occasionally a tuft (group) of
flagella may arise at one end of the cell,an
arrangement called lophotrichous,In peritrichous
flagellation the flagella are inserted at many places
around the cell surface (peri means "around"),The
type of flagellation,polar or peritrichous,is often used
as a characteristic in the classification of bacteria.
The flagellum of a Gram-negative bacterium
The filament of bacterial flagella is composed of
subunits of a protein called flagellin.
The base of the flagellum is different in structure from
that of the filament,There is a wider region at the base
of the flagellum called the hook,The hook consists of a
single type of protein and functions to connect the
filament to the motor portion of the flagellum.
The basal body is anchored in the cytoplasmic
membrane and cell wall,The basal body consists of a
small central rod that passes through a system of rings.
In gram-negative Bacteria,an outer ring is anchored in
the lipopolysaccharide layer and another in the
peptidoglycan layer of the cell wall,and an inner ring is
located within the cytoplasmic membrane.
In gram-positive Bacteria,which lack the outer
lipopolysaccharide layer,only the inner pair of rings is
present,Surrounding the inner ring and anchored in the
cytoplasmic membrane are a pair of proteins called
Mot, These proteins actually drive the flagellar motor
causing a torque that rotates the filament,A final set of
proteins,called the Fli proteins function as the motor
switch,reversing rotation of the flagella in response to
intracellular signals.
The movement of a procaryotic flagellum results from
rotation of its basal body and is similar to the movement
of the shaft of an electric motor,Bacterial cells can alter
the speed and direction of rotation of flagella and thus
are capable of various patterns of motility.
Fimbriae can be evenly distributed over the entire
surface of the cell,They can number anywhere from a few
to several hundred per cell,Fimbriae allow a cell to adhere
to surfaces including the surfaces of other cells,
Pili are usually longer than fimbriae
and number only one or two per cell,
Pili function to join bacterial cells prior
to the transfer of DNA from one cell to
another.
F pilus
Many prokaryotic organisms secrete on their surfaces
slimy or gummy materials,A variety of these structures
consist of polysaccharide,and a few consist of protein.
The terms capsule and slime layer are frequently used to
describe polysaccharide layers.
Demonstration of the
presence of a capsule
is usually by means
of negative staining
Capsules and Slime Layers
Many prokaryotes contain a cell surface layer composed
of a two-dimensional array of protein,These layers are
called S-layers,S-layers have been detected in
representatives of virtually every phylogenetic grouping
of Bacteria and are nearly universal among Archaea,In
some species of Archaea the S-layer is also the cell wall,
The major function of S-layers is unknown,However,as
the interface between the cell and its environment it is
likely that in cells that produce them the S-layer at least
functions as an external permeability barrier,allowing
the passage of low-molecular-weight substances while
excluding large molecules,
Paracrystalline Surface Layers (S-Layers)
2.7 Bacterial endospores
? Certain species of bacteria produce special
structure called endospores.
? They are very resistant to heat and can not
be destroyed easily,even by harsh
chemicals,Endospores are also resistant to
other harmful agents such as drying,
radiation,acids and chemical disinfectants.
Sporulating cell
Central core
Cortex
Spore coat /
membrane
exosporium
Micrograph of a endospore
Vegetative cell
Containing abundant DPA
(dipicolinic acid) which is combined with
calcium ions.
? Lower water content – only 10-30% of the
water content of the vegetative cell.
? Low pH value,and contains small acid-soluble
spore proteins (SASPs)
Structure of DPA
Properties of endospore and its resistance
Sporulation involves a very complex series of events
in cellular differentiation,
Bacterial sporulation does not occur when cells are
dividing exponentially but only when growth ceases
owing to the exhaustion of an essential nutrient,Thus,
cells of Bacillus cease vegetative growth and begin
sporulation when a key nutrient such as the carbon or
nitrogen source becomes limiting.
Endospore formatiom
1,Axial filament formation
2,Septum formatiom
3,Engulfment of forespore
4,Cortex formation
5,Coat synthesis
6,Completion of coat synthesis,
Increase in refractility and heat
resistance
7,Lysis of sporangium,spore
liberation
Stages in endospore formation
Spore germination
1,Activation – Usually results from treatments like heating.
2,Germination – Breaks spore’s dormant state,This process
is characterized by spore swelling,loss of resistance to heat and
other stresses,loss of refractility and increase in metabolic activity.
3,Outgrowth – The spore protoplast makes new components
and develops once more into an active bacterium.
Parasporal Crystal
(Spore-companioned crystal)
Several Bacillus species,most notably
B.popilliae and B,thuringiensis,produce
intracellular crystals of toxic glycoproteins
when they sporulate.
Toxic crystals
Please carefully read and be familiar with
the contents of table 3.3 on the page 100.
Comparison of the prokaryotic and eucaryotic cell
Their DNA is associated
with histone proteins.
membrane-bounded
organelles
9+2 type flagella
Eucaryotic cell
The chief distinguishing characteristics of
procaryotic cells are:
1,Their genetic material (DNA) is not enclosed within a
membrane.
2,They lack other membrane-bounded organelles.
3,Their DNA is not associated with histone proteins.
4,Their cell walls almost always contain the complex
polysaccharide peptidoglycan.
5,They usually divide by binary fission,During this
process,the DNA is copied and the cell splits into two
cells.
Summary
1,Prokaryotic genetic material is located in
an area called the nucleoid and is not
enclosed by a membrane.
2,Most bacteria have a cell wall outside the
plasma membrane to give them shape and
protect them from osmotic lysis.
Bacterial walls are chemically complex and
usually contain peptidoglycan or murein.
3,Bacteria often are classfied as either gram
positive or gram negative based on differences in
cell wall structure and their response to Gram
staining,
Gram-positive walls have thick,homogeneous
layers of peptidoglycan and teichoid acid,Gram-
negative bacteria have a thin peptidoglycan layer
surrounded by a complex outer membrane
containing lipopolysaccharides and other
components
4,Some bacteria are motile,usually by means of
flagella,and bacterial species differ in the number
and distribution of their flagella.
5,Structures such as capsules,fimbriae,and sex
pili are found outside of some bacterial cell wall.
6,Some bacteria survive adverse environmental
conditions by forming endospores,dormant
structures resistant to heat,desiccation and many
chemicals
