Prokaryotes: Structure, Function, and Interactions

Prokaryotes: Structure, Function, and Interactions

Limits to Prokaryotic Growth

Prokaryotic growth is influenced by several environmental and biological factors that limit their population expansion.

• Competition: Prokaryotes compete with each other and with other organisms for resources.

• Environmental Conditions: Accumulation of waste products and depletion of nutrients can inhibit growth.

• Lack of Nutrients: Essential nutrients must be available for prokaryotic growth.

The Microbiome

The microbiome refers to the community of prokaryotes (mainly bacteria) that live on and in the human body, often forming mutualistic relationships.

• Functions: Digestion, vitamin production, mood regulation, and immune system development.

• Inheritance: Some microbiome components are inherited from the mother (e.g., during vaginal delivery).

• Health Impact: The composition of the microbiome can influence susceptibility to autoimmune disorders and other health conditions.

Prokaryotes: Ancient and Ubiquitous

Origins and Habitats

Prokaryotes are among the oldest life forms on Earth and are found in a wide range of environments.

• Fossil Record: Earliest fossils are 3.5 to 3.8 billion years old.

• Habitats: Found in extreme environments (acidic, salty, hot, cold) and in more moderate conditions.

• Abundance: The number of prokaryotes in a handful of soil exceeds the number of people who have ever lived.

• Genetic Diversity: Two strains of E. coli can be more genetically different than a human and a duck-billed platypus.

Characteristics of Prokaryotes

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles.

• Genetic Material: DNA is circular and not enclosed in a nuclear envelope.

• Reproduction: Binary fission is the primary mode of reproduction.

• Domains: Bacteria and Archaea (not monophyletic).

Prokaryote vs. Eukaryote

Prokaryotes and eukaryotes differ in several fundamental ways:

Prokaryote	Eukaryote
Bacteria, Archaea	Plants, Animals, Fungi, Protists
No nucleus No membrane-bound organelles No DNA in nuclear envelope Circular chromosomes Unicellular	Nucleus Membrane-bound organelles DNA in nuclear envelope Linear chromosomes Multicellular (most)

Bacteria vs. Archaea

• Cell Membranes: Differ in the types of molecules that make up their membranes and cell walls.

• Transcription Machinery: Archaea use machinery more similar to eukaryotes.

• Extremophiles: Archaea are often found in extreme environments.

• Diversity: Bacteria are more diverse and better studied than archaea.

• Peptidoglycan: Found in bacterial cell walls, not in archaea or eukaryotes.

Prokaryotic Metabolism

Energy and Carbon Sources

Prokaryotes display diverse metabolic strategies, classified by their energy and carbon sources.

• Energy Sources:

  • Sunlight: Used by plants and some prokaryotes (phototrophs).

  • Organic Molecules: Used by humans and other heterotrophs.

  • Inorganic Molecules: Used by some prokaryotes (lithotrophs).

• Carbon Sources:

  • CO2: Used by autotrophs to make sugars.

  • Organic Molecules: Used by heterotrophs.

Types of Prokaryotic Metabolism:

• Photoautotroph: Energy from light, carbon from CO2 (e.g., plants, cyanobacteria).

• Chemoautotroph: Energy from inorganic chemicals, carbon from CO2.

• Photoheterotroph: Energy from light, carbon from organic compounds.

• Chemoheterotroph: Energy and carbon from organic compounds (e.g., humans, most bacteria).

• Organotroph: Uses organic molecules for electrons.

• Lithotroph: Uses inorganic molecules for electrons.

Note: Bolded terms are protist types.

Oxygen Requirements

• Obligate Aerobes: Require O2 for respiration.

• Obligate Anaerobes: Poisoned by O2.

• Facultative Anaerobes: Can use O2 but do not require it.

• Aerotolerant Anaerobes: Do not use O2 but are not poisoned by it.

Genomic Organization and Reproduction

Genomic Features

• Small Genomes: Prokaryotes have less DNA than eukaryotes.

• Nucleoid: Region where the chromosome is found (not membrane-bound).

• Plasmids: Small, circular DNA molecules that may carry antibiotic resistance genes.

Reproduction and Adaptation

• Binary Fission: Asexual reproduction; can occur rapidly (as little as 20 minutes).

• Rapid Adaptation: High mutation rates and short generation times allow quick adaptation to environmental changes.

• Genetic Recombination: Increases diversity and can occur via three main mechanisms:

1. Transformation: Uptake of naked DNA from the environment.

2. Transduction: DNA transfer via bacteriophages (viruses that infect bacteria).

3. Conjugation: Direct transfer of DNA between two cells via a pilus.

Prokaryotic Morphology and Motility

Cell Structure

• Size: Generally smaller than eukaryotic cells.

• Shape: Simple internal structure; most are unicellular.

• Cell Wall: Provides shape and protection; contains peptidoglycan in bacteria.

• Endospores: Some prokaryotes can produce resistant endospores to survive harsh conditions.

Motility

• About 50% of prokaryotes are motile, using structures such as:

  • Flagella: Whip-like structures for movement.

  • Cilia: Short, hair-like structures (rare in prokaryotes).

Prokaryotic Cell Wall

• Function: Maintains shape and protects from the environment.

• Composition: Bacterial cell walls contain peptidoglycan; archaea have different compounds.

• Gram Staining: Differentiates bacteria based on cell wall structure:

Gram-Positive	Gram-Negative
Thick peptidoglycan layer Retains violet stain More susceptible to penicillin	Thin peptidoglycan layer Outer envelope of lipopolysaccharides Less susceptible to penicillin Requires other antibiotics

Prokaryote Interactions

Types of Interactions

• Mutualism (+/+): Both partners benefit.

• Parasitism (+/-): One benefits at the expense of the other.

• Pathogenicity: Some prokaryotes cause disease.

Beneficial Interactions

• Medicine: Probiotics, antibiotics, and microbiome health.

• Food: Production of yogurt, cheese, and fermented foods.

• Agriculture: Nitrogen fixation for plants.

Pathogenic Bacteria and Disease

Virulence and Pathogenicity

• Virulence: The strength of infection varies among strains and individuals.

• Pathogenic Genes: Some bacteria have acquired genes for toxins or other virulence factors.

• Opportunistic Pathogens: Cause disease when the immune system is compromised.

Antibiotics and Resistance

• Antibiotics: Molecules that kill bacteria or stop their growth (e.g., penicillin).

• Mechanism: Target specific bacterial traits, such as cell wall synthesis or metabolic pathways.

• Antibiotic Resistance: Bacteria can acquire resistance via mutation or gene transfer, leading to treatment challenges.

Hygiene and Antibiotic Use

• Use antibiotics only when prescribed.

• Never share antibiotics or use leftovers.

• Antibiotic soaps are being phased out; regular soap is sufficient.

• Overuse of antibiotics can harm beneficial microbiota and contribute to autoimmune disorders.

Germ Theory and Koch's Postulates

• Germ Theory: Diseases are caused by microorganisms, not by 'bad air' or miasma.

• Koch's Postulates: Criteria to establish a causal relationship between a microbe and a disease.

Additional info:

• Some details about the diversity and ecological roles of prokaryotes were expanded for clarity.

• Definitions and examples were added to ensure the notes are self-contained and suitable for exam preparation.