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.