- An Overview of Procaryotic
Cell Structure
- Size, shape, and
arrangement
- Procaryotes come in a
variety of shapes including spheres (cocci), rods (bacilli), ovals
(coccobacilli), curved rods (vibrios), rigid helices (spirilla), and
flexible helices (spirochetes)
- During the
reproductive process, some cells remain attached to each other to form
chains, clusters, square planar configurations (tetrads), or cubic
configurations (sarcinae)
- A few bacteria are
flat and some lack a single, characteristic form and are called
pleomorphic
- Procaryotic cells
vary in size although they are generally smaller than most eucaryotic
cells; recently, however, several large prokaryotes have been
discovered, which grow as large as 750mm in diameter and can be seen
without the aid of a microscope
Procaryotic cells
contain a variety of internal structures, although not all structures are
found in every genus; procaryotes are morphologically distinct from
eucaryotic cells and have fewer internal structures.
Procaryotic Cell Membranes
The plasma membrane
The plasma membrane
of bacteria consists of a phospholipid bilayer with hydrophilic surfaces
(interact with water) and a hydrophobic interior (insoluble in water);
such asymmetric molecules are said to be amphipathic; most bacterial
membranes lack sterols
Many archaeal
membranes have a monolayer instead of a bilayer; archaeal membranes are
describe in more detail in chapter 20
The fluid mosaic
model is the most widely accepted model of membrane structure. It
distinguishes two types of proteins associated with the membrane:
peripheral (loosely associated and easily removed) and integral
(embedded within the membrane and not easily removed)
The membrane is
highly organized, asymmetric, flexible, and dynamic
The plasma membrane
serves several functions
It retains the
cytoplasm and separates the cell from its environment
It serves as a
selectively permeable barrier, allowing some molecules to pass into or
out of the cell while preventing passage of other molecules
It is the location
of a variety of crucial metabolic processes including respiration,
photosynthesis, lipid synthesis, and cell wall synthesis
It may contain
special receptor molecules that enable detection of and response to
chemicals in the surroundings
Internal membrane
systems
Mesosomes are
structures formed by invaginations of the plasma membrane that may play
a role in cell wall formation during cell division and in chromosome
replication and distribution; however, mesosomes may be artifacts
generated during chemical fixation for electron microscopy
Photosynthetic
bacteria may have complex infoldings of the plasma membrane that
increase the surface area available for photosynthesis
Bacteria with high
respiratory activity may also have extensive infoldings that provide a
large surface area for greater metabolic activity
These internal
membranes may be aggregates of spherical vesicles, flattened vesicles,
or tubular membranes
The Cytoplasmic Matrix
A.
The cytoplasmic matrix is the substance between the
membrane and the nucleoid; it is featureless in electron micrographs but is
often packed with ribosomes and inclusion bodies; although lacking a true
cytoskeleton, the cytoplasmic matrix of bacteria does have a cytoskeleton-like
system of proteins
B.
Inclusion Bodies
1.
Many inclusion bodies are granules of organic or
inorganic material that are stockpiled by the cell for future use; some are not
bounded by a membrane, but others are enclosed by a single-layered membrane
2.
Gas vacuoles are a type of inclusion body found in
cyanobacteria and some other aquatic forms; they provide buoyancy for these
organisms and keep them at or near the surface of their aqueous habitat
3.
Magnetosomes are inclusion bodies that contain iron in
the form of magnetite; they are used by some bacteria to orient in the Earthís
magnetic field
C.
Ribosomes
0.
Ribosomes are complex structures consisting of protein
and RNA
1.
They are responsible for the synthesis of cellular
proteins
2.
Procaryotic ribosomes are similar in structure to, but
smaller than, eucaryotic ribosomes
1.
The Nucleoid
.
The nucleoid is an irregularly shaped region in which
the chromosome of the procaryote is found
0.
In most procaryotes, the nucleoid contains a single
circular chromosome, though some have more than one chromosome or have one or
more linear chromosomes
1.
The nucleoid is not bounded by a membrane, but it is
sometimes found to be associated with the plasma membrane or with mesosomes
A.
The bacterial chromosome is an efficiently packed DNA
molecule that is looped and coiled extensively
B.
In addition to the chromosome, many bacteria contain
plasmids; plasmids are usually small, closed circular DNA molecules
0.
They can exist and replicate independently of the
bacterial chromosome
1.
They are not required for bacterial growth and
reproduction, but they may carry genes that give the bacterium a selective
advantage (e.g., drug resistance, enhanced metabolic activities, etc.)
- The Procaryotic Cell Wall
.
The cell wall is a rigid structure that lies just
outside the plasma membrane; it provides the characteristic shapes of the
various procaryotes and protects them from osmotic lysis
0.
The cell walls of most bacteria contain peptidoglycan;
the cell walls of archaea lack peptidoglycan and instead are composed of
proteins, glycoptoteins, or polysaccharides
1.
The cell walls of gram-positive bacteria and
gram-negative bacteria differ greatly, but both have a periplasmic space, which
usually contains a variety of proteins; these proteins can be involved in
nutrient acquisition, electron transport, peptidoglycan synthesis or in
modification of toxic compounds
A.
Peptidoglycan (murein) is a polysaccharide polymer
found in bacterial cell walls; it consists of polysaccharide chains
cross-linked by peptide bridges
B.
Gram-positive cell walls-consist of a thick layer of
peptidoglycan and large amounts of teichoic acids
C.
Gram-negative cell walls
0.
They consist of a thin layer of peptidoglycan
surrounded by an outer membrane composed of lipids, lipoproteins, and a large
molecule known as lipopolysaccharide (LPS). LPS can play a protective role and
can also act as an endotoxin, causing some of the symptoms characteristic of
gram-negative bacterial infections; there are no teichoic acids in
gram-negative cell walls.
1.
The outer membrane is more permeable than the plasma
membrane because of porin proteins that form channels through which small
molecules (600-700 daltons) can pass
D.
The mechanism of Gram staining-involves constricting
the thick peptidoglycan layer of gram-positive cells, thereby preventing the
loss of the crystal violet stain during the brief decolorization step; the
thinner, less cross-linked peptidoglycan layer of gram-negative bacteria cannot
retain the stain as well, and these bacteria are thus more readily decolorized
when treated with alcohol
E.
The cell wall and osmotic protection-the cell wall
prevents swelling and lysis of bacteria in hypotonic solutions. However, in
hypertonic habitats, the plasma membrane shrinks away from the cell wall in a
process known as plasmolysis
- Components External to the
Cell Wall
.
Capsules, slime layers and S layers
0.
Capsules and slime layers (also known as glycocalyx)
are layers of polysaccharides lying outside the cell wall; they protect the
bacteria from phagocytosis, desiccation, viral infection, and hydrophobic toxic
materials such as detergents; they also aid bacterial attachment to surfaces
and gliding motility a. Capsules are well organized b. Slime layers are diffuse
and unorganized
1.
S layers are regularly structured layers of protein or
glycoprotein observed in both bacteria and archaea, where it may be the only
structure outside the plasma membrane; they protect against ion and pH
fluctuations, osmotic stress, hydrolytic enzymes, or the predacious bacterium
Bdellovibrio
A.
Pili and fimbriae are short, thin, hairlike appendages
that mediate bacterial attachment to surfaces (fimbriae) or to other bacteria
during sexual mating (pili)
B.
Flagella and motility 1. Flagella are threadlike
locomotor appendages extending outward from the plasma membrane and cell wall;
they may be arranged in various patterns:
.
Monotrichous-a single flagellum
a.
Amphitrichous-a single flagellum at each pole
b.
Lophotrichous-a cluster (tuft) of flagella at one or
both ends
c.
Peritrichous-a relatively even distribution of flagella
over the entire surface of the bacterium
C.
Flagellar ultrastructure: The flagellum consists of a
hollow filament composed of a single protein known as flagellin. The hook is a
short curved segment that links the filament to the basal body, a series of
rings that drives flagellar rotation.
D.
Flagellar synthesis involves many genes for the hook
and basal body, as well as the gene for flagellin. New molecules of flagellin
are transported through the hollow filament so that the growth of the flagellum
is from the tip, not from the base.
E.
The mechanism of flagellar movement appears to be
rotation; the hook and helical structure of the flagellum causes the flagellum
to act as a propeller, thus driving the bacterium through its watery
environment
.
Counterclockwise rotation causes forward motion (called
a run)
a.
Clockwise rotation disrupts forward motion (resulting
in a tumble)
F.
Procaryotes can move by other mechanisms; in
spirochetes, axial filaments cause movement by flexing and spinning; other
procaryotes exhibit gliding motility-a mechanism by which they coast along
solid surfaces; no visible structure is associated with gliding motility
- Chemotaxis
.
Chemotaxis is directed movement of bacteria either
towards a chemical attractant or away from a chemical repellent
A.
The concentrations of these attractants and repellents
are detected by chemoreceptors in the surfaces of the bacteria
B.
Directional travel toward a chemoattractant (biased
random walk toward attractant) is caused by lowering the frequency of tumbles
(twiddles), thereby lengthening the runs when traveling up the gradient, but
allowing tumbling to occur at normal frequency when traveling down the gradient
C.
Directional travel away from a chemorepellent (biased
random walk away from repellent) involves similar but opposite responses
D.
The mechanism of control of tumbles and runs is
complex, involving numerous proteins and several mechanisms (conformation
changes, methylation, and phosphorylation) to modulate their activity; despite
this complexity chemotaxis is fast, with responses occurring in as little as
200 meters/second
- The Bacterial Endospore
.
The bacterial endospore is a special, resistant,
dormant structure formed by some bacteria, which enables them to resist harsh
environmental conditions
A.
Endospore formation (sporulation) normally commences
when growth ceases because of lack of nutrients; it is a complex, multistage
process
B.
Transformation of dormant endospores into active
vegetative cells is also a complex, multistage process that includes activation
(preparation) of the endospore, germination (breaking of the endosporeís
dormant state), and outgrowth (emergence of the new vegetative cell)