Prokaryotes: Structure, Function, Diversity, and Impact

Prokaryotes: Structure, Function, Diversity, and Impact

Introduction to Prokaryotes

Prokaryotes are single-celled organisms that comprise the domains Bacteria and Archaea. They are the most abundant and diverse organisms on Earth, thriving in a wide range of environments, including extreme conditions. Their structural and functional adaptations have enabled them to colonize virtually every habitat.

Structural and Functional Adaptations of Prokaryotes

Cell Size and Shape

• Most prokaryotic cells are 0.5–5 µm in diameter, much smaller than typical eukaryotic cells (10–100 µm).

• Common shapes include cocci (spheres), bacilli (rods), and spirilla (spirals).

• Some species form colonies, but most are unicellular.

Cell-Surface Structures

• The cell wall maintains cell shape, protects the cell, and prevents lysis in hypotonic environments.

• In hypertonic environments, prokaryotes lose water and may undergo plasmolysis; salt is used as a preservative to inhibit their growth.

• Bacterial cell walls contain peptidoglycan, a network of sugar polymers cross-linked by polypeptides.

• Archaeal cell walls lack peptidoglycan and instead contain various polysaccharides and proteins.

• Eukaryotic cell walls (if present) are made of cellulose (plants) or chitin (fungi).

Gram Staining

• The Gram stain differentiates bacteria based on cell wall composition:

  • Gram-positive bacteria: Thick peptidoglycan layer; stain purple; more susceptible to antibiotics targeting peptidoglycan.

  • Gram-negative bacteria: Thin peptidoglycan layer and an outer membrane with lipopolysaccharides; stain pink; more resistant to antibiotics.

Capsules and Slime Layers

• Many prokaryotes have a sticky outer layer:

  • Capsule: Dense and well-defined.

  • Slime layer: Loosely organized.

• Functions: Adherence, protection from dehydration, and defense against host immune systems.

Endospores

• Some bacteria form endospores—dormant, tough structures that can survive extreme conditions for centuries.

• Formed when essential resources are scarce; the cell's chromosome is encased in a multilayered structure.

Fimbriae and Pili

• Fimbriae: Hairlike appendages for attachment to surfaces or other cells.

• Pili (sex pili): Longer than fimbriae; facilitate DNA transfer between cells during conjugation.

Motility

• About half of prokaryotes are motile, often using flagella for movement.

• Taxis: Directed movement toward or away from stimuli (e.g., chemotaxis—movement in response to chemicals).

• Prokaryotic flagella differ structurally and functionally from eukaryotic flagella.

Evolution of Flagella

• Bacterial flagella are complex structures composed of a motor, hook, and filament (42 proteins).

• Flagella likely evolved through exaptation: modification of existing proteins for new functions.

Internal Organization and DNA

• Prokaryotes lack membrane-bound organelles.

• Some have infolded membranes for metabolic functions.

• Genetic material:

  • Single circular chromosome located in the nucleoid (no membrane).

  • May also have plasmids: small, independently replicating DNA rings.

• Differences in DNA processes (replication, transcription, translation) between prokaryotes and eukaryotes are exploited by antibiotics.

Reproduction

• Prokaryotes reproduce asexually by binary fission (simple cell division).

• They have short generation times and can rapidly increase population size under optimal conditions.

Genetic Diversity in Prokaryotes

Sources of Genetic Diversity

• Three main factors:

  1. Rapid reproduction

  2. Mutation

  3. Genetic recombination

• Mutations, though rare per division, accumulate quickly due to large populations and rapid reproduction, fueling adaptation by natural selection.

Genetic Recombination Mechanisms

• Genetic recombination: Combining DNA from different sources.

• Mechanisms:

  • Transformation: Uptake of foreign DNA from the environment.

  • Transduction: Transfer of DNA via bacteriophages (viruses that infect bacteria).

  • Conjugation: Direct transfer of DNA between cells via a pilus and mating bridge.

• Horizontal gene transfer: Movement of genes between different species.

Conjugation and the F Factor

• The F factor (fertility factor) is required for pilus formation and DNA transfer.

• F factor can be a plasmid (F plasmid) or integrated into the chromosome (Hfr cell).

• F+ cells (with F plasmid) are donors; F– cells are recipients.

• Hfr cells can transfer chromosomal genes to F– cells, creating recombinant cells.

R Plasmids and Antibiotic Resistance

• R plasmids carry genes for antibiotic resistance and can be transferred between cells, spreading resistance rapidly.

• Some R plasmids confer resistance to multiple antibiotics and encode pili for conjugation.

Nutritional and Metabolic Diversity

Major Nutritional Modes

• Prokaryotes are classified by energy and carbon sources:

  • Phototrophs: Use light as energy source.

  • Chemotrophs: Use chemicals as energy source.

  • Autotrophs: Use CO2 or related compounds as carbon source.

  • Heterotrophs: Require organic compounds as carbon source.

Mode	Energy Source	Carbon Source	Example Organisms
Photoautotroph	Light	CO2, HCO3–	Cyanobacteria, plants
Chemolithoautotroph	Inorganic chemicals	CO2, HCO3–	Nitrosomonas
Photoheterotroph	Light	Organic compounds	Rhodobacter
Chemoheterotroph	Organic compounds	Organic compounds	Many prokaryotes, animals, fungi

Additional info: Table inferred from context and standard biology sources.

Oxygen and Metabolism

• Obligate aerobes: Require O2 for cellular respiration.

• Obligate anaerobes: Poisoned by O2; use fermentation or anaerobic respiration.

• Facultative anaerobes: Can use O2 if present or switch to anaerobic metabolism if not.

Nitrogen Metabolism

• Nitrogen is essential for amino acids and nucleic acids.

• Some prokaryotes perform nitrogen fixation: conversion of atmospheric N2 to ammonia (NH3).

• Example: Anabaena cyanobacteria have specialized cells (heterocysts) for nitrogen fixation.

Metabolic Cooperation and Biofilms

• Prokaryotes may cooperate metabolically (e.g., exchange of nutrients between specialized cells).

• Biofilms: Surface-coating colonies of prokaryotes; cells communicate and share resources.

• Biofilms can cause industrial corrosion, medical device contamination, tooth decay, and chronic infections.

Diversity and Classification of Prokaryotes

Bacterial Diversity

• Estimated 700,000–1.4 million bacterial species (about 16,000 described).

• Major groups:

  • Proteobacteria: Gram-negative; includes photoautotrophs, chemoautotrophs, and heterotrophs. Examples: Thiomargarita namibiensis, Neisseria gonorrhoeae, Vibrio cholerae, Helicobacter pylori.

  • Chlamydias: Animal cell parasites; gram-negative; lack peptidoglycan. Example: Chlamydia trachomatis.

  • Spirochetes: Helical, gram-negative heterotrophs; some are pathogens. Examples: Treponema pallidum (syphilis), Borrelia burgdorferi (Lyme disease).

  • Cyanobacteria: Gram-negative photoautotrophs; ancestors of plant chloroplasts; important in aquatic ecosystems.

  • Gram-positive bacteria: Diverse; includes Streptomyces (antibiotic producers), Staphylococcus aureus, Bacillus anthracis, Clostridium botulinum.

Archaeal Diversity

• Archaea share features with both bacteria and eukaryotes, but also have unique traits.

• Major groups:

  • Extremophiles: Live in extreme environments.

    • Extreme halophiles: Thrive in high-salt environments.

    • Extreme thermophiles: Thrive at high temperatures (even above 100°C).

  • Methanogens: Obligate anaerobes that produce methane; found in swamps, marshes, animal guts, and under ice.

  • Euryarchaeota: Includes many halophiles, methanogens, and some thermophiles.

  • TACK supergroup: Includes Thaumarchaeota, Aigarchaeota, Crenarchaeota (most thermophiles), and Korarchaeota.

  • Lokiarchaeotes: Recently discovered; closely related to TACK; may be sister group to eukaryotes.

Ecological Roles of Prokaryotes

Chemical Recycling

• Prokaryotes decompose dead organisms and wastes, recycling elements like carbon and nitrogen.

• Autotrophic prokaryotes produce sugars and oxygen; nitrogen-fixing bacteria make nitrogen available to plants.

• Some prokaryotes immobilize nutrients, reducing their availability to other organisms.

Ecological Interactions

• Symbiosis: Close ecological relationship between two species (host and symbiont).

• Types:

  • Mutualism: Both benefit.

  • Commensalism: One benefits, the other is unaffected.

  • Parasitism: Parasite harms the host (pathogens cause disease).

Prokaryotes and Humans

Beneficial Prokaryotes

• Human intestines host 500–1,000 bacterial species, outnumbering human cells tenfold.

• Many are mutualists, aiding in digestion and nutrient synthesis (e.g., Bacteroides thetaiotaomicron).

Pathogenic Prokaryotes

• All known pathogenic prokaryotes are bacteria; cause about half of all human diseases (e.g., tuberculosis, Lyme disease).

• Pathogens may be transmitted by other species (e.g., ticks for Lyme disease).

• Pathogenicity often involves:

  • Exotoxins: Secreted proteins causing disease even after bacteria are gone (e.g., cholera toxin).

  • Endotoxins: Lipopolysaccharide components of gram-negative bacteria released upon cell death (e.g., Salmonella).

• Horizontal gene transfer can spread virulence genes to harmless bacteria.

Antibiotic Resistance

• Antibiotic resistance has evolved rapidly since the 1940s; new antibiotics are not keeping pace.

• Resistance genes spread quickly via horizontal gene transfer and rapid reproduction.

• Drug-resistant strains of Mycobacterium tuberculosis are a major global health concern.

• New antibiotics (e.g., malacidins) are being discovered, but resistance remains a challenge.

Prokaryotes in Research and Technology

• Prokaryotes are used in food production (cheese, yogurt, beer, wine, sauerkraut, soy sauce).

• The CRISPR-Cas9 system, derived from prokaryotes, is a powerful gene-editing tool.

• Some bacteria produce biodegradable plastics (e.g., PHA polymers).

Summary Table: Key Differences Between Bacteria and Archaea

Feature	Bacteria	Archaea
Cell Wall	Contains peptidoglycan	No peptidoglycan; various polysaccharides/proteins
Membrane Lipids	Unbranched hydrocarbons	Branched hydrocarbons
RNA Polymerase	One kind	Several kinds, similar to eukaryotes
Initiator Amino Acid for Protein Synthesis	Formyl-methionine	Methionine
Growth at >100°C	No	Some species

Additional info: Table inferred from standard biology comparisons.