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
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
Inclusion Bodies
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
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
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
Ribosomes
Ribosomes are complex structures consisting of protein and RNA
They are responsible for the synthesis of cellular proteins
Procaryotic ribosomes are similar in structure to, but smaller than,
eucaryotic ribosomes
The Nucleoid
The nucleoid is an irregularly shaped region in which the chromosome
of the procaryote is found
In most procaryotes, the nucleoid contains a single circular chromosome,
though some have more than one chromosome or have one or more linear
chromosomes
The nucleoid is not bounded by a membrane, but it is sometimes found
to be associated with the plasma membrane or with mesosomes
The bacterial chromosome is an efficiently packed DNA molecule that
is looped and coiled extensively
In addition to the chromosome, many bacteria contain plasmids; plasmids
are usually small, closed circular DNA molecules
They can exist and replicate independently of the bacterial chromosome
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
The cell walls of most bacteria contain peptidoglycan; the cell walls
of archaea lack peptidoglycan and instead are composed of proteins, glycoptoteins,
or polysaccharides
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
Peptidoglycan (murein) is a polysaccharide polymer found in bacterial
cell walls; it consists of polysaccharide chains cross-linked by peptide
bridges
Gram-positive cell walls-consist of a thick layer of peptidoglycan and
large amounts of teichoic acids
Gram-negative cell walls
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.
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
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
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
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
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
Pili and fimbriae are short, thin, hairlike appendages that mediate bacterial
attachment to surfaces (fimbriae) or to other bacteria during sexual mating
(pili)
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
Amphitrichous-a single flagellum at each pole
Lophotrichous-a cluster (tuft) of flagella at one or both ends
Peritrichous-a relatively even distribution of flagella over the entire
surface of the bacterium
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.
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.
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)
Clockwise rotation disrupts forward motion (resulting in a tumble)
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
The concentrations of these attractants and repellents are detected by
chemoreceptors in the surfaces of the bacteria
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
Directional travel away from a chemorepellent (biased random walk away
from repellent) involves similar but opposite responses
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
Endospore formation (sporulation) normally commences when growth ceases
because of lack of nutrients; it is a complex, multistage process
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)
