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Microbiology, Fifth Edition
Microbiology, 5/e
Lansing M Prescott, Augustana College
Donald A Klein, Colorado State University
John P Harley, Eastern Kentucky University
Procaryotic Cell Structure and Function

Study Outline

  1. An Overview of Procaryotic Cell Structure
    1. Size, shape, and arrangement
      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 pleomorphic
      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 systems
      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
      1. 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
      2. 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
      3. Ribosomes
        1. Ribosomes are complex structures consisting of protein and RNA
        2. They are responsible for the synthesis of cellular proteins
        3. Procaryotic ribosomes are similar in structure to, but smaller than, eucaryotic ribosomes
    1. The Nucleoid
      1. The nucleoid is an irregularly shaped region in which the chromosome of the procaryote is found
        1. In most procaryotes, the nucleoid contains a single circular chromosome, though some have more than one chromosome or have one or more linear chromosomes
        2. The nucleoid is not bounded by a membrane, but it is sometimes found to be associated with the plasma membrane or with mesosomes
      2. The bacterial chromosome is an efficiently packed DNA molecule that is looped and coiled extensively
      3. In addition to the chromosome, many bacteria contain plasmids; plasmids are usually small, closed circular DNA molecules
        1. They can exist and replicate independently of the bacterial chromosome
        2. 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.)
  4. The Procaryotic Cell Wall
    1. 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
      1. The cell walls of most bacteria contain peptidoglycan; the cell walls of archaea lack peptidoglycan and instead are composed of proteins, glycoptoteins, or polysaccharides
      2. 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
    2. Peptidoglycan (murein) is a polysaccharide polymer found in bacterial cell walls; it consists of polysaccharide chains cross-linked by peptide bridges
    3. Gram-positive cell walls-consist of a thick layer of peptidoglycan and large amounts of teichoic acids
    4. Gram-negative cell walls
      1. 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.
      2. 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
    5. 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
    6. 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
  5. Components External to the Cell Wall
    1. Capsules, slime layers and S layers
      1. 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
      2. 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
    2. Pili and fimbriae are short, thin, hairlike appendages that mediate bacterial attachment to surfaces (fimbriae) or to other bacteria during sexual mating (pili)
    3. 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:
      1. Monotrichous-a single flagellum
      2. Amphitrichous-a single flagellum at each pole
      3. Lophotrichous-a cluster (tuft) of flagella at one or both ends
      4. Peritrichous-a relatively even distribution of flagella over the entire surface of the bacterium
    4. 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.
    5. 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.
    6. 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
      1. Counterclockwise rotation causes forward motion (called a run)
      2. Clockwise rotation disrupts forward motion (resulting in a tumble)
    7. 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
  6. Chemotaxis
    1. Chemotaxis is directed movement of bacteria either towards a chemical attractant or away from a chemical repellent
    2. The concentrations of these attractants and repellents are detected by chemoreceptors in the surfaces of the bacteria
    3. 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
    4. Directional travel away from a chemorepellent (biased random walk away from repellent) involves similar but opposite responses
    5. 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
  7. The Bacterial Endospore
    1. The bacterial endospore is a special, resistant, dormant structure formed by some bacteria, which enables them to resist harsh environmental conditions
    2. Endospore formation (sporulation) normally commences when growth ceases because of lack of nutrients; it is a complex, multistage process
    3. 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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