lecturer 4 cell morphology

• Plasma membrane

  • Separates the cell from its environment
  • Phospholipid molecules oriented so that water-loving heads directed outward and water-hating tails directed inward
  • Proteins embedded in two layers of lipids (lipid bilayer)

  
• Membranecell

  • Highly selective permeability barrier constructed of lipids and proteins that forms a bilayer with hydrophilic exteriors and a hydrophobic interior
  • The attraction of the nonpolar fatty acid portions of one phospholipid layer for the other layer helps to account for the selective permeability of the cell membrane
  • Other molecules, such as sterols and hopanoids, may strengthen the membrane as a result of their rigid planar structure
  • Integral proteins involved in transport and other functions traverse the membrane

  
• Bacteria and Eukarya, have ester linkages bond fatty acids to glycerol (Note: In Archae, it is ether bond instead.)

  
• The three classes of transporters are uniporters, symporters, and antiporters

  
• Peptidoglycan(cell wall)

  • is A huge polymer of interlocking chains of identical peptidoglycan monomers
  • Provides rigid support while freely permeable to solutes
  • Backbone of peptidoglycan molecule composed of two derivatives of glucose: N-acetylglucosamine (NAG) and N-acetlymuramic acid (NAM)
  • NAG / NAM strands are connected by interpeptide bridges

  
• Cell Wall

  • Major function is to prevent bacterial cells from rupturing due to water pressure
  • Maintain shape
  • Point of anchorage for flagella
  • Ability to cause disease
  • The site for antibiotic action
• Composition of cell wall


  • Peptidoglycan (murein): consists of repeating disaccharide attached by polypeptides
  • Disscharide is made up of N-acetylglucosamin (NAG) and N-acetylmuramic acid (NAM)
  • Alternating NAM and NAG molecules are linked in row of 10-65 → glycan
  • Adjacent row are linked by polypeptides (peptido-)
  • Amino acids occur in an alternating pattern of D and L-forms
  • Many sheets of peptidoglycan can be present, depending on the organism
  • Archaea lack peptidoglycan but contain walls made of other polysaccharides or protein
  • The enzyme lysozyme destroys peptidoglycan, leading to cell lysis

  
• Peptidoglycan


  • Composed of two sugar derivatives: N-acetylglucosamine and N-acetylmuramic acid
  • Amino acids: L-alanine, D-alanine, D-glutamic acid, Lysine or diaminopimelic acid (DAP)
• Gram-Positive Cell Walls


  • Consists of thick, rigid and many layers of peptidoglycan
  • Contain teichoic acids→lipoteichoic acid which spans the peptidoglycan layer and linked to plasma membrane

  →wall teichoic acid which link to the peptidoglycan layer

  • Negatively charge→bind and regulate movement of cations in and out of cell

  
• Gram-Negative Cell Walls

  • Consists of one or few layers of peptidoglycan
  • Outer membrane where peptidoglycan is bonded to lipoprotein
  • Outer membrane consists of lipopolysaccharides (LPS), lipoptoteins and phospholipids
  • Strong negative charge→evading phagocytosis

  → barrier to certain antibiotics, digestive enzymes

  • LPS has 2 important characteristics: i) O-polysccharides (as antigen) ii) Lipid A (endotoxin)
  • Periplasm→fluid filled space between the outer membranve and the plasma membrane

  → contains degradative enzymes and transport proteins

  • Do not contain teichoic acids

  
• Antibiotic such as penicillin inhibit the linking of the peptidoglycan rows by peptide cross-bridges
• Glycocalyx

  DEFINITION: An additional layer outside of the cell wall.

  Can come in one of two forms: Glycoproteins loosely associated with the cell wall or Slime layer causes bacteria to adhere to solid surfaces and helps prevent the cell from drying out
• Streptococcus


  • The slime layer allows it to accumulate on tooth enamel (one of the causes of cavities)

  Other bacteria in the mouth become trapped in the slime and form a biofilm & eventually a buildup of plaque
• Prokaryotic cell surface structures

  • Fimbriae and pili ,S-layers, Capsules and Slime layers

  
• Function of prokaryotic cell surface structures


  Attaching cells to a solid surface
• GlycocalyX (SUGARCOAT)


  • Contains substances that surround the cells
  • Can be polysaccharide or polypeptides
  • Functions in virulence, attachment, protection

  
• Internal granules of prokaryotic cells


  • Function as storage materials or in magnetotaxis
  • Poly-β-hydroxyalkanoates (PHAs) and glycogen are produced as storage polymers when carbon is in excess
  • Poly-β-hydroxybutyrate (PHB) is a common storage material

  
• Types of bacterial flagella arrangement


  • Monotrichous (single polar)
  • Amphitrichous (both ends)
  • Lophotrichous (2 or more at one pole)
  • Peritrichous (entire cell)

  
• Flagella


  3 basic parts: Filament, Hook, Basal Body (motor)

  • Filament is made of the protein flagellin.
• Bacteria movement

  • Flagellated bacteria are motile
  • Flagella move by rotation, clockwise or anti-clockwise
  • "Run" = movement in one direction for a length of time
  • "Tumble"= random changes in direction (reversal of flagellar rotation)
  • Movement toward or away from a stimulus is called taxis (e.g. chemotaxis, phototaxis)
• Atypical Cell Walls (No cell walls)


  • Members of the genus Mycoplasma have no walls or very little wall material
  • Smallest known bacteria with smallest genome
  • Require cholesterol for growth

  
• Endospores


  • Dormant, tough, non-reproductive structure produced by small number of bacteria
  • Resistant to radiation, desiccation, lysozyme, temperature, starvation, and chemical disinfectants
  • Commonly found in soil and water, where they may survive for very long periods of time
  • fuction- allow to survive in in harsh condition
• The members of the Clostridium genus have a couple of bacterial "superpowers" that make them particularly tough pathogens.

  superpower- (spores formation, anaerobic growth)
• Clostridium sp.


  • Known to produce a variety of toxins, some of which are fatal
  • Examples: Clostridium tetani (agent of tetanus), C. botulinum (agent of botulism), C. perfringens (one of the agents of gas gangrene), C. difficile (can proliferate and cause pseudomembranous colitis)
• Types of Microscopes


  • Bright field
  • Dark Field
  • Fluorescence
  • Phase contrast
  • TEM
  • SEM
• Function Cytoplasmic Membrane

  • Permeability Barrier - Prevents leakage and functions as a gateway for transport of nutrients into and out of the cell
  • Protein Anchor - Site of many proteins involved in transport, bioenergetics, and chemotaxis
  • Energy Conservation - Site of generation and use of the proton motive force
• Simple transport


  1. Driven by the energy in the proton motive force

  2. Transported substance moves out
• Group translocation


  Chemical modification of the transported substance driven by phosphoenolpyruvate
• ABC system


  Periplasmic binding proteins are involved and energy comes from ATP
• Preparation of Bacterial Smear


  1. Spread a small sample of the bacterial culture onto a glass slide

  2. Allow to air dry

  3. Heat fixation may be used to adhere the bacteria to the slide and kill the bacteria
• Primary Stain (Crystal Violet)

  1. Flood the smear with a primary stain, typically crystal violet

  2. Crystal violet binds to the peptidoglycan layer of the bacterial cell wall in both Gram-positive and Gram-negative bacteria, staining all cells purple
• Iodine Treatment (Mordant)


  1. Rinse off the excess crystal violet

  2. Treat the smear with iodine solution (Gram's iodine)

  3. Iodine serves as a mordant, forming a complex with the crystal violet and trapping it within the cell wall of the bacteria
• Decolorization


  1. Rinse the smear with a decolorizing agent, such as ethanol or acetone

  2. This step differentiates between Gram-positive and Gram-negative bacteria

  3. Gram-positive bacteria retain the crystal violet-iodine complex and remain purple

  4. Gram-negative bacteria lose the stain and become colorless
• Counterstain (Safranin)


  1. Stain the smear with a contrasting counterstain, such as safranin or fuchsin

  2. Gram-negative bacteria, having lost the crystal violet stain, take up the counterstain and appear pink or red

  3. Gram-positive bacteria retain the purple color of the crystal violet-iodine complex
• Gram staining


  A mechanism used to differentiate between Gram-positive and Gram-negative bacteria