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 An Overview of Procaryotic Cell Structure

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PostSubject: An Overview of Procaryotic Cell Structure   Wed Feb 18, 2009 7:15 pm

  1. An Overview of Procaryotic
    Cell Structure

    1. Size, shape, and

      1. Procaryotes come in a
        variety of shapes including spheres (cocci), rods (bacilli), ovals
        (coccobacilli), curved rods (vibrios), rigid helices (spirilla), and
        flexible helices (spirochetes)
      2. During the
        reproductive process, some cells remain attached to each other to form
        chains, clusters, square planar configurations (tetrads), or cubic
        configurations (sarcinae)
      3. A few bacteria are
        flat and some lack a single, characteristic form and are called
      4. 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

    2. 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.

  2. Procaryotic Cell Membranes

    1. The plasma membrane

      1. 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
      2. Many archaeal
        membranes have a monolayer instead of a bilayer; archaeal membranes are
        describe in more detail in chapter 20
      3. 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)
      4. The membrane is
        highly organized, asymmetric, flexible, and dynamic
      5. The plasma membrane
        serves several functions

        1. It retains the
          cytoplasm and separates the cell from its environment
        2. It serves as a
          selectively permeable barrier, allowing some molecules to pass into or
          out of the cell while preventing passage of other molecules
        3. It is the location
          of a variety of crucial metabolic processes including respiration,
          photosynthesis, lipid synthesis, and cell wall synthesis
        4. It may contain
          special receptor molecules that enable detection of and response to
          chemicals in the surroundings

    2. Internal membrane

      1. 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
      2. Photosynthetic
        bacteria may have complex infoldings of the plasma membrane that
        increase the surface area available for photosynthesis
      3. Bacteria with high
        respiratory activity may also have extensive infoldings that provide a
        large surface area for greater metabolic activity
      4. These internal
        membranes may be aggregates of spherical vesicles, flattened vesicles,
        or tubular membranes

  3. 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 are complex structures consisting of protein
and RNA

They are responsible for the synthesis of cellular

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.)

  1. 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

  1. 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

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

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

  1. 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

  1. 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

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)
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