MD

AGRC1021 - Biological Concepts and Plant Science

Defining Prokaryotes

  • Prokaryotes appeared approximately 3.5 billion years ago and were the first cellular life forms on Earth.
  • They are single-celled organisms that lack internal membrane-bound organelles, distinguishing them from eukaryotes.
  • Typical cell size ranges from 0.5 to 5.0 μm.
  • Prokaryotes have a remarkable ability to rapidly adapt to changing and extreme environments due to their genetic flexibility and quick reproduction rates.
  • Possess cell walls which provide structure and protection.

Gram Staining

  • Gram staining is a differential staining technique used to classify bacteria based on their cell wall structure.
  • Gram-positive bacteria:
    • Have thick cell walls.
    • Retain the crystal violet stain, appearing purple under a microscope.
    • This can be a sign of antibiotic susceptibility, but it's not always the case.
  • Gram-negative bacteria:
    • Have thinner cell walls.
    • Possess a second, outer membrane.
    • The outer membrane can impede the entry of drugs into the cell, contributing to antibiotic resistance.

Capsules and Slime Layers

  • Capsules: Well-organized layers of polysaccharides or proteins on the outer surface of prokaryotes.
  • Slime layers: Disorganized layers of polysaccharides or proteins on the outer surface of prokaryotes.
  • Function:
    • Reduce dehydration.
    • Protect against attack by the host organism's immune system (important for pathogenic prokaryotes).

Endospore Formation

  • Endospore formation is a survival strategy employed by some bacteria.
  • Triggered by limited resources or environmental stressors like drying out.
  • The cell replicates its DNA and encapsulates one copy within a multilayered protective structure (the endospore).
  • The endospore is dehydrated, making it highly resistant to harsh conditions.
  • Endospores can survive in the soil for extended periods, potentially for centuries.

Fimbriae

  • Fimbriae are hairlike appendages found on the surface of bacteria.
  • Function: Attach bacteria to surfaces, including other bacteria or host cells (e.g., cells of mucous membranes).

Motility

  • Many prokaryotes exhibit taxis, which is directed movement towards or away from stimuli.
  • Stimuli can include nutrients, oxygen, or harmful compounds.
  • Flagella are a key mechanism for movement in prokaryotes.
  • Prokaryotic flagella rotate using a complex protein motor embedded in the cell wall.
  • The motor is powered by a proton gradient.
    Protons are pumped out of the cell by the ETC

Internal Organization and DNA

  • Prokaryotes lack complex compartmentalization found in eukaryotes.
  • Some prokaryotes may possess specialized internal membranes formed by infolding of the plasma membrane.
  • Prokaryotes have a single, circular chromosome.
  • The structure and complexity of prokaryotic DNA is less than that of eukaryotes.
  • DNA is not membrane-bound (i.e., there is no nucleus).
  • Prokaryotes may also have smaller rings of DNA called plasmids, which carry a few genes.

Reproduction and Evolution

  • Bacteria can reproduce rapidly, with reproductive cycles as short as 20 minutes.
  • Population sizes are typically very high.
  • Due to the large population sizes, genetic mutations occur rapidly.
  • This leads to increased genetic diversity and rapid evolution.
  • The small size and adaptability of bacteria make them highly adaptable, even more so than eukaryotes.

Genetic Recombination

  • Three processes allow prokaryotes to undergo genetic changes:
    • Transformation:
      • Uptake of foreign DNA from the environment.
      • The foreign DNA is absorbed and incorporated into the prokaryote's genome, producing a new recombinant.
      • The DNA often comes from similar species.
    • Transduction:
      • Transfer of DNA from one prokaryote to another via a viral bacteriophage (a virus that infects bacteria).
    • Conjugation:
      • Bacteria temporarily join, and one cell donates DNA to another through a hollow pilus (a bridge-like structure).

Nutritional and Metabolic Adaptations

  • Prokaryotes exhibit diverse nutritional adaptations, reflecting their genetic variation.
  • The range of adaptations in prokaryotes is broader than that found in eukaryotes.

Bacteria Groups

  • There are 5 key groups of bacteria:
    • spirochetes
    • proteobacteria
    • cyanobacteria
    • chlamydia
    • grampositive bacteria
  • Most are familiar as environmental, food-associated, or pathogenic species.

The Archaea

  • Archaea have some chemical and biochemical differences compared to Bacteria.
  • Some archaea live in extreme environments (extremophiles):
    • Extreme halophiles: Thrive in high-salt concentrations.
    • Extreme thermophiles: Thrive in high-temperature environments.
    • Anaerobes (e.g., methanogens): Use CO2 to oxidize H2, releasing methane.
  • The lifestyles of some Archaea suggest the possibility of life forms on other planets.

Pathogenic Bacteria

  • All known pathogenic prokaryotes are bacteria.
  • Bacteria cause approximately half of human diseases.
  • Some pathogenic bacteria can be transmitted by vectors such as fleas and ticks.

Antibiotic Resistance

  • The rapid reproduction rate of bacteria contributes to the development of antibiotic resistance.
  • Broadscale use and misuse of antibiotics can accelerate this process.
  • Resistance genes can be transferred via horizontal gene transfer, spreading resistance among bacteria.

Positive Uses of Prokaryotes

  • The metabolic capabilities of prokaryotes can be harnessed in many useful ways:
    • Bacteria convert milk to cheese and yogurt.
    • Fermentation in beer, wine, meats, cabbage, soybeans, and other foods.
    • Tools in genetic research and technologies.
    • Genetically engineered to produce vitamins, antibiotics, hormones, and other products.
    • Reduce dependence on petroleum products by creating bioplastics and bioethanol.
    • Used in bioremediation of polluted ecosystems.