prokaryotes and protoists

• Prokaryotes

  • Have a cellular organization fundamentally different from eukaryotes
  • Lack membrane-enclosed nucleus and organelles
  • Typically much smaller than eukaryotes
• The collective biological mass (biomass) of prokaryotes is at least 10 times that of all eukaryotes
• Metagenomics


  Researchers collect samples from an environment, isolate and sequence the DNA, then assemble the collection of genomes of individual species present, known as a microbiome
• Prokaryotes return vital chemical elements to the environment by decomposing dead organisms and other organic waste material
• Prokaryotes are indispensable components of the chemical cycle that makes nitrogen available to plants and other organisms
• If prokaryotes were to disappear, the chemical cycles that sustain life would halt, and all forms of eukaryotic life would be doomed
• If eukaryotes ceased to exist, prokaryotic life would undoubtedly continue alone on Earth, as it once did for more than a billion years
• Cocci


  Spherical prokaryotic cells
• Streptococci


  Cocci that occur in chains
• Staphylococci


  Cocci that occur in clusters
• Bacilli


  Rod-shaped prokaryotes
• Spirilla


  Relatively short and rigid spiral prokaryotes
• Spirochetes


  Longer, more flexible spiral prokaryotes
• Gram-positive bacteria


  Have simpler cell walls with a relatively thick layer of peptidoglycan
• Gram-negative bacteria


  Have more complex cell walls with an outer membrane containing lipids bonded to carbohydrates
• Capsule


  A sticky layer of polysaccharide or protein that covers the cell wall of many prokaryotes
• Flagella


  Adaptations that enable prokaryotes to move in response to chemical or physical signals
• Fimbriae


  Hairlike projections that enable some prokaryotes to stick to a surface or to one another
• Rapid reproduction generates a great deal of genetic variation in a prokaryote population
• Exposure to antibiotics may select for antibiotic resistance in a bacterial population
• Plasmids


  Small, circular DNA molecules that replicate independently of the chromosome and can provide genes that enhance survival under certain conditions
• Endospores


  Specialized resistant cells produced by some prokaryotes when environmental conditions become harsh
• Endospores can survive all sorts of trauma and remain dormant for decades, even centuries
• Rapid reproduction produces high genetic variation in populations of prokaryotes because each time DNA replicates, spontaneous mutations may occur
• Phototrophs


  Prokaryotes that capture energy from sunlight
• Chemotrophs


  Prokaryotes that harness the energy stored in chemicals, either organic molecules or inorganic chemicals
• Autotrophs


  Organisms that make their own organic compounds from inorganic sources
• Heterotrophs


  Organisms that obtain their carbon atoms from the organic compounds of other organisms
• Prokaryotes have greater nutritional diversity than eukaryotes, allowing them to inhabit almost every nook and cranny on Earth
• Sources of energy for prokaryotes


  • Phototrophs (capture energy from sunlight)
  • Chemotrophs (harness energy from chemicals)
• Prokaryotic phototrophs do not have chloroplasts, but some have thylakoid membranes where photosynthesis takes place
• Prokaryotic chemotrophs harness energy from organic molecules or inorganic chemicals like hydrogen sulfide, elemental sulfur, iron-containing compounds, or ammonia
• Sources of carbon for organisms


  • Autotrophs (make their own organic compounds from inorganic sources)
  • Heterotrophs (obtain carbon from organic compounds of other organisms)
• Modes of nutrition for prokaryotes


  • Photoautotrophs (harness sunlight for energy, use CO2 for carbon)
  • Photoheterotrophs (obtain energy from sunlight, get carbon from organic sources)
  • Chemoautotrophs (harvest energy from inorganic chemicals, use CO2 for carbon)
  • Chemoheterotrophs (acquire energy and carbon from organic molecules)
• Chemoheterotrophs are the largest and most diverse group of prokaryotes, able to use almost any organic molecule as food
• Biofilm


  Complex association of microbes attached to a surface in highly organized colonies
• Biofilm formation


  1. Prokaryotes secrete signaling molecules to attract nearby cells

  2. Cells produce a gooey coating that glues them to the support and each other

  3. Biofilm becomes a "city" of microbes that coordinate division of labor, defense, and other activities

  4. Channels allow nutrients and wastes to move through the biofilm
• Biofilms are common among bacteria that cause disease in humans, making infections difficult to treat
• Biofilms in the environment can be difficult to eradicate, clogging and corroding pipes, gumming up filters and drains, and coating ship hulls
• Biofilms are difficult to eradicate because the cells stick to the surface and each other, and the outer layer may prevent antimicrobial substances from penetrating