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Prokaryotic Cell Structure and Function

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					Prokaryotic Structure and Function
I. Morphological
  features of
  Prokaryotes
Prokaryotic versus Eukaryotic Cells

 Prokaryotic cells are much simpler than
 eukaryotic cells
Size, shape, and
  arrangement
          Shape and Arrangement
 Coccus
   Spherical cells
   Pairs of cocci = diplococcus (Neisseria
      gonorrhoeae, Streptococcus pneumoniae) (remain
      associated after cell division)
     Chains of cocci (Streptococcus spp., Enterococcus
      spp., Lactococcus spp.) (remain associated after
      repeated rounds of cell division)
     Clumps (Staphylococcus spp., Micrococcus spp.)
      (divide in several planes, can be symmetrical)
     Tetrads = square (Pediococcus spp.)
     Cubical = eight cell cluster (Sarcina spp.)
 Rods (bacilli)
   Divide only in one plane
   Not all rods belong to the genus Bacillus
   Cigar-shaped
   Variation in length to width ratio (can be short and
    wide, e.g. ovals (coccobacilli)
   Variations in end of rod
      Flat
      Round
      Cigar-shapted
      Bifurcated
   Can also associate to form pairs of chains
    (streptobacilli)
   Some rods are curved (vibrios),
 Filamentous bacteria
   Multinucleate – hyphae
   Mycelium formed if
    branching occurs (e.g.
    Actinomyces)
   Figure = Anabaena
    flos-aquae
 Buds and Stalks
  (Hyphomicrobium)
 Spirals
   Rigid helices (spirilla)
      Corkscrews, mobile usually by polar
      flagella
      Rhodospirillum rubrum, Spirillum volutans
   Flexible helices (spirochetes)
      Axial filaments between cell wall and
      plasma membrane
     Treponema pallidum
 Some bacteria are flat (Walsby’s square
  bacterium)
 Pleiomorphic
   Variable, no single shape (e.g.
    Corynebacterium – Chinese letters
    arrangement)
                    Size

 Overall size
   Prokaryotic cells vary in size
   They are generally smaller than most
    eukaryotic cells
   Exception: a large prokaryote,
    Epulopiscium fisheloni was discovered that
    grows as large as 600m X 80 m, a littler
    smaller than a printed hyphen
II. Prokaryotic Cellular
       Structures
             Cell Membranes
 The plasma membrane
  Fluid Mosaic Model (Singer and Nicholson)
     Proteins and lipids are embedded
     assymetrically
    Phospholipid bilayer with hydrophilic surfaces
     (interact with water) and a hydrophobic interior
     (insoluble in water)
      Such asymmetric molecules are said to be amphipathic
  5-10nm thick
  Amphipathic lipids are often phospholipids
   (e.g. phosphatidylethanolamine)
 Hopanoids are often present in bacterial
 membranes (similar to the sterol
 cholesterol)
   Stabilization and strength
Two types of proteins in the plasma
           membrane
 Peripheral (extrinsic) proteins
    Associated with surface of PM (usually the underside)
    Polar, water-soluble
    20-30% of PM protein
 Integral (intrinsic) proteins
    Amphipathic
    Span the PM and interact with internal and/or external
     environments of the cell
    Hydrophobic, non-polar regions is embedded in the
     lipid portion
    Hydrophilic, polar domains stick out of membrane
    Lateral movement in PM
    If carbohydrates are attached, then called
     glycoproteins
     Various Functions of PM

 Boundary
  Retains the cytoplasm and separates
   the cell from its environment
 Selectively permeable barrier
  Determines what goes in and out of
   the cell
 Transport systems
  Uptake of nutrients
  Secretion of proteins
  Elimination of waste products
 Metabolism
  Respiration, photosynthesis, lipid
   synthesis, and cell wall synthesis
 Receptor molecules
  For detection and response to signals from
   the external environment (attractants and
   repellants)
       Intracellular membranes
 Mesosomes
  Plasma membrane pinches in on itself to
   form vesicle, tubules or lamellae
  Function: (??)
     Cell wall formation during division
     Secretory roles
     Respiratory activity/metabolism
     Metabolism
     Provide larger surface area for photosynthesis
     (chromatophores)
The Cytoplasmic Matrix
(material between the membrane
       and the nucleoid)
 Inclusion bodies (for storage)
   Granules make up of organic or inorganic
    substances
   Often visible using light microscopy
   Can be bound or unbound by membrane
 Examples
    Glycogen (glucose polymers) – C source
    Poly beta hydroxybutyrate (PHB) – C source
    Cyanophycin granules (Arg + Asp in 1:1 ratio) – N
     source
    Carboxysomes (contain RUBISCO)            - CO2
     fixation
    Gas vacuole – buoyancy
    Polyphosphate granules (volutin granules, aka
     metachromatic granules) – phosphate source
    Sulfur granules (e.g. Thiobacillus spp.) – S source
 Magnetosomes
   Inclusion body used for
    purpose other than
    storage
   Bacteria use
    magnetosomes to
    orient in the earth’s
    magnetic field
   Magnetite – iron
    (greigite and pyrite)
 Bacteria containing
  magnetosomes
  migrate in waves
  following exposure to
  a magnetic field
 Ribosomes
   Comprised of RNA + protein
   Prokaryotic ribosomes = 70S  50S + 30S
    subunits (eukaryotic ribosomes = 80 S  60S
    + 40S subunits)
      S = Svedberg unit (sedimentation coefficient) 
       affected by MW, volume and shape
   Sites of translation (protein synthesis)
      Matrix ribosomes produce proteins for intracellular
       usage
      PM-associated ribosomes proteins destined for
       export out of the cell
 Molecular chaperones
   Aid in the folding of nascent polypeptides as they come
    off of ribosomes
   Include SecB, DNnaK, DnaJ, GroEL, GroES and GrpE
     all involved in folding a protein to its final native
    conformation
   Concentration increases dramatically when cells are
    subjected to stress (e.g. 42 degrees Celsius)
       May aid in the refolding of denatured polypeptides
   AKA heat shock proteins
 Molecular chaperones also function
 to keep secretory proteins in an
 export-competent state until they
 are translocated across the plasma
 membrane
         The
       Nucleoid
  (aka nuclear body,
     chromatin body,
     nuclear region)

   a. = Bacillus nucleoids
      b. = TEM – E. coli
c. = model of metabolically
     active nucleoids – with
            extensions
 The DNA of bacteria is USUALLY a
  single circular ds molecule located in the
  nucleoid
 Following DNA replication, 2 nucleoids
  are present prior to cell division
 Sometimes found to be associated with the
  plasma membrane or with mesosomes
 Projections are associated with sites of
  transcription by RNA polymerases
 Plasmid may also contribute to genetic
 material
   Not essential for host growth
   Usually confer selective advantage to bacteria
    (e.g. Amp resistance, Colicin resistance,
    toluene utilization)
   Replicate independently (autonomously) of
    the host chromosome
   Also circular, dsDNA, but MUCH SMALLER
    than the genome
The Prokaryotic Cell
        Wall
 a rigid structure that results in the
  characteristic shapes of the
  various prokaryotes and protects
  them from osmotic lysis
 This topic will be revisited in LAB.
Components External to
      the Cell Wall
                    Capsule
 Layer of highly organized material external to
  the cell wall
 Composed of polysaccharide (usuallly) or
  polypeptide (sometimes, e.g Bacillus has D-
  glutamic acid)
 Functional importance:
   Protection from phagocytosis
   Protection from hostile environments (dessication)
   Contributes to virulence of certain pathogens
    (Streptococcus pneumoniae, Neisseria meningitidis,
    Klebsiella pneumoniae, Alcaligenes faecalis)
 Part of glycocalyx
             Slime Layer
 Part of glycocalyx
 Less well organized than capsules
 Aid in adherence to surfaces (biofilms)
 Not as tight bound to cell as capsules
 Protects against dehydration and loss of
 nutrients
                S Layer
 Part of glycocalyx
 Highly structured layer made up of protein
  or glycoprotein (crystalline)
 Adheres to OM in Gram-negative bacteria
 Adheres to PTG in Gram-positive bacteria
 Provides protection from
     Ion and pH fluctuations
     Osmotic stress
     Digestion by enzymes
     Ingestion by bacteria (Bdellovibrio)
     Phagocytosis by WBC (virulence factor)
 Aids in adherence to surfaces (virulence factor)
 Protects from complement attack (virulence
  factor)
         Fimbriae (aka pili)
 Very thin appendages (3-10nm diameter)
 Involved in attachment, not motility
 Made of pilin (phosphate-carbohydrate-
  protein complex)
 Often called adhesins
               Sex Pilus
 Similar to fimbriae
 9-10 nm diameter
 Reguired for bacterial conjugation (F
  pilus, fertility)
 Often have receptors for bacteriophages
  (filamentous phage)
                  Flagellum
 Confer motility
 20 nm diameter
 Helical
 Present on almost all spirals, ~1/2 of bacilli,
  almost no cocci
 Requires a mordant to visualize by light
  microscopy (builds up diameter, then stained)
 Structure – three main parts
 1. Filament
   From cell surface to top
   Hollow, made of flagellin protein
 2. Basal body
   Embedded in cell
   Anchors flagellum
 3. Hook
   Hooks filament to the basal body
 Basal body is made up of a series of rings
   Gram-negative bacteria have four rings
    connected to a central rod (L,M, P and S)
   Gram-positive bacteria have two rings
      One attaches to PM
      Other attaches to PTG
 Location:
   Monotrichous – polar, one flagellum
   Lophotrichous – tufts of flagella at one or both
    ends
   Amphitricous – one flagellum at each end
   Peritrichous – flagella evenly spread on
    surface
 Synthesis
   Involves expression and regulation of > 30
    genes coding for various components
   Flagellin subunits thought to be transported
    through center of hollow filament, associated
    at tip – SELF ASSEMBLY at the growing end
 Mechanism of flagellar movement
   Direction of movement determined by
    direction of rotation
      Counterclockwise  forward
      Clockwise  tumbles (bundles are disrupted,
       bacterium travels randomly)
   Driving force
      Passage of protons from the exterior of cell to
       cytoplasm through the basal body past the rings
 Gliding motility is a mechanism
  used by some prokaryotes by
  which they coast along solid
  surfaces;
 no visible structure is associated
  with this form of motility
                 Chemotaxis
 Definition: Movement towards a gradient of
  attractants (nutrients) or away from repellents
   CCW  straight line
   CW  tumble aimlessly
 Attractants and repellents are detected by
  chemoreceptors
   ~20 for attractants
   ~10 for repellents
   Binds chemical, transmits signal to chemosensing
    system which influences direction of rotation
Positive Bacterial Chemotaxis: Left = motile colony; outer ring =
bacteria
consuming serine, second ring = bacteria consuming aspartate
(less powerful attractant)
Upper right = motile, nonchemotatic
Lower right = nonmotile bacteria
Negative bacterial chemotaxis: Disk plugs containing acetate – increasing
in concentration from 0 to 3 M acetate. (Bacteria free zones increase as
concentration increases.
A = Random movement in absence of concentration gradient.

B = Movement in an attractant gradient. Tumbling frequencing is
reduced, therefore runs are longer.
 Negative chemotaxis
   Tumbles frequently and moves down the
    gradient away from the undesirable substance
 Positive chemotaxis
   Tumbles less frequently (longer runs) and
    travels up a chemical gradient towards a
    desirable substance
 Responses are triggered by methylation
 and/or phosphorylation of target proteins
 called methyl-accepting chemotaxis proteins
 (MCPs) to cycle them between active and
 inactive forms
The Bacterial
  Endospore
 A dormant structure which enables
  bacteria to resist harsh environmental
  conditions
 Formed by Gram-positive bacteria (e.g.
  Bacillus, Clostridium, Sporosarcina,
  Sporolactobacillus, Desulfotomaculum)
   Medically significant: anthrax, botulism,
    gas gangrene, tetanus
 Resistant to harsh environments
   Heat (owing to high concentration of calcium
    dipicolinic acid)
   UV radiation (increased cysteine amino acids)
   Dessication
   Disinfectants (impermeable cell coat)
   Mechanical stress
 Survival strategy when nutrients are
 limiting
 Sporogenesis (sporulation) induced when
 nutrient concentration is low
   A septum separates the DNA to be
    encapsulated
   The sporangium = the mother cell housing the
    spore
   Location and size of endospore varies
    (Genus-specific)
   DNA stabilized by calcium dipicolinate
 Generation of active vegetative cell:
   Activation
      Prepares for germination
      Often heat-induced process
   Germination
      Breaks out of spore coat
   Outgrowth
      Active growth – vegetative state
 Dormancy for thousands of years is
 possible
CR = Core     N = Nucleoid           CW = Cell Wall
CX = Cortex   SC = Spore Cell Wall   EX =

				
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posted:10/17/2011
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