Cell membrane

• Cell morphology:

  • Cell shape distinguishes different microbial cells and has ecological significance
• Major morphologies of prokaryotic cells:

  • Coccus: Spherical or ovoid
  • Bacillus: cylindrical
  • Spirilla: curved or loose spiral shapes
  • Spirochetes: tightly coiled bacteria (long tubes/stalks)
  • Filamentous: long, thin cells or chains of cells
• Bacteria and archaea cells vary in size:

  • Small cells have a larger surface area relative to cell volume
  • Larger cells have a higher surface-to-volume ratio
  • As cell size increases, surface-to-volume ratio decreases
  • Surface-to-volume ratio of cells controls properties like growth rate and evolution
  • Higher surface-to-volume ratio of small cells leads to a faster rate of nutrient and waste exchange per unit of cell volume
• DNA replication and mutations:

  • Mutation rates are roughly the same in all cells
  • More chromosomal replications lead to a greater total number of mutations in the cell population
  • Mutation is the raw material of evolution, providing larger pools and greater evolutionary possibilities
  • Prokaryotic cells are small and genetically haploid, allowing mutations to be expressed immediately
• Cytoplasmic membrane:

  • Surrounds the cytoplasm
  • Physically weak but ideal for selective permeability
• Bacterial cytoplasmic membrane:

  • Phospholipid bilayer containing embedded proteins
  • Fatty acids and glycerol molecules with phosphate and other functional groups
  • Lipid bilayer/unit membrane: outer surface faces the environment while the inner surface faces the cytoplasm
  • Proteins attached/integrated to the membrane with hydrophobic and hydrophilic domains
  • Integral membrane proteins are significantly embedded in the membrane
  • Peripheral membrane proteins are more loosely attached, including lipoproteins
• Archaeal membranes:

  • Lipids of Archaea contain ether bonds between glycerol and a hydrophobic side chain
  • Constructed from phosphoglycerol diethers or diphosphoglycerol tetraethers
  • Tetraether lipid structure forms a lipid monolayer membrane
• Cell wall:

  • Peptidoglycan is a rigid polysaccharide found in bacterial cell walls, conferring structural strength
  • Peptidoglycan is composed of alternating repeats of modified glucose residues and amino acids connected to form a glycan tetrapeptide
  • Gram-negative cell walls have at least two layers, while gram-positive cell walls are thicker and primarily composed of a single type of molecule
• Lipopolysaccharide (LPS) in gram-negative bacteria:

  • LPS is a second lipid bilayer in the outer membrane
  • Composed of core polysaccharide and O-specific polysaccharide
  • LPS confers structural strength and acts as a barrier against harmful agents
  • LPS is toxic to animals and impermeable to other proteins
• Periplasm and porins:

  • Periplasm is the space between the cytoplasmic membrane and the outer membrane
  • Periplasm contains hydrolytic enzymes, binding proteins, chemoreceptors, and proteins for constructing extracellular structures
  • Porins in the outer membrane function as channels for the entrance and exit of solutes
• Archaeal cell walls:

  • Pseudomurein is a polysaccharide similar to peptidoglycan found in methane-producing Archaea
  • S-layers are paracrystalline surface layers consisting of interlocking molecules of protein or glycoprotein
• Cell morphology:

  • Cell shape distinguishes different microbial cells and has ecological significance
• Major morphologies of prokaryotic cells:

  • Coccus: Spherical or ovoid
  • Bacillus: cylindrical
  • Spirilla: curved or loose spiral shapes
  • Spirochetes: tightly coiled bacteria (long tubes/stalks)
  • Filamentous: long, thin cells or chains of cells
• Bacteria and archaea cells vary in size:

  • Small cells have a larger surface area relative to cell volume
  • Larger cells have a higher surface-to-volume ratio
  • As cell size increases, surface-to-volume ratio decreases
  • Surface-to-volume ratio of cells controls properties like growth rate and evolution
  • Higher surface-to-volume ratio of small cells leads to a faster rate of nutrient and waste exchange per unit of cell volume
• DNA replication and mutations:

  • Mutation rates are roughly the same in all cells
  • More chromosomal replications lead to a greater total number of mutations in the cell population
  • Mutation is the raw material of evolution, providing larger pools and greater evolutionary possibilities
  • Prokaryotic cells are small and genetically haploid, allowing mutations to be expressed immediately
• Cytoplasmic membrane:

  • Surrounds the cytoplasm
  • Physically weak but ideal for selective permeability
• Bacterial cytoplasmic membrane:

  • Phospholipid bilayer containing embedded proteins
  • Fatty acids and glycerol molecules with phosphate and other functional groups
  • Lipid bilayer/unit membrane: outer surface faces the environment while inner surface faces the cytoplasm
  • Proteins attached/integrated to the membrane with hydrophobic and hydrophilic domains
• Archaeal membranes:

  • Lipids contain ether bonds between glycerol and a hydrophobic side chain that is not a fatty acid
  • Constructed from phosphoglycerol diethers or diphosphoglycerol tetraethers
  • Tetraether lipid structure forms a lipid monolayer membrane
• Cell wall:

  • Peptidoglycan is a rigid polysaccharide found in bacterial cell walls, conferring structural strength
  • Peptidoglycan is composed of alternating repeats of N-acetylglucosamine, N-acetylmuramic acid, and amino acids connected in an ordered way to form the glycan tetrapeptide
• Gram-negative cell walls:

  • Consist of at least two layers
  • Cross-link often contains a short peptide "interbridge"
  • Peptidoglycans can be destroyed by lysozyme, weakening and causing cell lysis
• Lipopolysaccharide (LPS) in outer membrane:

  • Acts as a barrier against many substances
  • Composed of core polysaccharide and O-specific polysaccharide
  • Lipid portion (lipid A) contains fatty acids bonded through amine groups
  • LPS is toxic to animals and impermeable to other proteins
• Periplasm and porins:

  • Periplasm is the space between the cytoplasmic membrane and inner surface of the outer membrane
  • Periplasmic proteins include hydrolytic enzymes, binding proteins, chemoreceptors, and proteins for extracellular structures
  • Porins function as channels for the entrance and exit of solutes, composed of three identical polypeptides
• Archaeal cell walls:

  • Pseudomurein is a polysaccharide similar to peptidoglycan found in methane-producing Archaea
  • S-layers are paracrystalline surface layers found in Archaea and some Bacteria, serving as a selective sieve between the cell and its environment