Chapter Summary

3.1

  • Bacterial cells are protected by a thick cell envelope.
  • Compact genomes maximize reproductive potential with minimal resources.
  • The model bacterial cell contains a highly ordered cytoplasm in which DNA replication, RNA transcription, and protein synthesis occur coordinately.
  • The biochemical composition of bacteria includes relatively high nucleic acid content, as well as proteins, phospholipids, and other organic and inorganic constituents.
  • Proteins in the cell vary, depending on the species and environmental conditions.

3.2

  • Subcellular fractionation isolates cell parts for structural, biochemical, and genetic analysis.
  • X-ray crystallography shows the three-dimensional form of cell components at the atomic level.
  • Genetic analysis shows which genes express the proteins of subcellular complexes such as the ribosome. Mutation of a gene leads to altered function of the cell.

3.3

  • The cell membrane consists of a phospholipid bilayer containing hydrophobic membrane proteins. Membrane proteins serve diverse functions, including transport, cell defense, and cell communication.
  • Small uncharged molecules, such as oxygen, can penetrate the cell membrane by diffusion.
  • Weak acids and weak bases exist partly in an uncharged form that can diffuse across the membrane and increase or decrease, respectively, the H+ concentration within the cell.
  • Polar molecules and charged molecules require membrane proteins to mediate transport. Such facilitated transport can be active or passive.
  • Active transport requires input of energy from a chemical reaction or from an ion gradient across the membrane.
  • Ion gradients generated by membrane pumps store energy for cell functions.
  • Diverse fatty acids are found in different microbial species and in microbes grown under different environmental conditions.
  • Archaeal membranes have ether-linked terpenoids, which confer increased stability at high temperature and acidity. Some species have diglycerol tetraethers, which generate a lipid monolayer.

3.4

  • The cell wall maintains turgor pressure. The cell wall is porous, but its rigid network of covalent bonds protects the cell from osmotic shock.
  • The gram-positive cell envelope has multiple layers of peptidoglycan, interpenetrated by teichoic acids.
  • The S-layer, composed of proteins, is highly porous but can prevent phagocytosis and phage infection. In archaea, the S-layer serves the structural function of a cell wall.
  • The capsule, composed of polysaccharide and glycoprotein filaments, protects cells from phagocytosis. Either gram-positive or gram-negative cells may possess a capsule.
  • The gram-negative outer membrane regulates nutrient uptake and excludes toxins. The envelope layers include protein pores and transporters of varying selectivity.
  • Eukaryotic microbes are protected from osmotic shock by polysaccharide cell walls or by a contractile vacuole.

3.5

  • The nucleoid region contains loops of DNA, supercoiled and bound to DNA-binding proteins.
  • DNA is transcribed in the cytoplasm, often at the same time that it is being replicated.
  • The ribosome translates RNA to make proteins, which are folded by chaperones and in some cases secreted at the cell membrane.
  • DNA is replicated bidirectionally by the replisome.
  • Cell expansion and septation are coordinated with DNA replication.
  • Cell shape is determined by the FtsZ “Z ring” and other cytoskeletal proteins.

3.6

  • Phototrophs possess thylakoid membrane organelles packed with photosynthetic apparatus and carboxysomes for carbon dioxide fixation. Other subcellular structures may include sulfur granules from H2S photolysis and gas vesicles for buoyancy in the water column.
  • Storage granules store polymers for energy.
  • Magnetosomes orient the swimming of magnetotactic anaerobic bacteria.
  • Adherence structures enable prokaryotes to remain in an environment with favorable environmental factors. Major adherence structures include pili, or fimbriae (protein filaments), and the holdfast (a cell extension).
  • Flagellar motility occurs by rotary motion of helical flagella.
  • Chemotaxis involves a biased random walk up a gradient of an attractant substance or down a gradient of repellents.