Chapter 27: Bacteria and Archaea – Structure, Function, and Diversity

Prokaryotic Ubiquity and Adaptations

Characteristics Enabling Prokaryotic Ubiquity

Prokaryotes are found in nearly every environment on Earth due to several key characteristics that enhance their survival and reproduction.

• Small Size & Rapid Reproduction: Their small cell size allows for efficient nutrient uptake and rapid cell division, often by binary fission.

• Mutations & Rapid Evolution: High reproduction rates increase the chance of mutations, fueling rapid evolutionary adaptation.

• Endospores: Some prokaryotes form endospores, dormant structures that withstand extreme conditions such as heat, desiccation, and chemicals.

• Ubiquity: The ability to live in diverse environments, from deep-sea vents to the human gut.

Structural Adaptations of Prokaryotes

Cellular Structure and Morphology

• Unicellular: Most prokaryotes exist as single cells, though some form colonies or simple multicellular structures.

• Very Small: Typical size ranges from 0.5 to 5 μm in diameter.

• Shapes: Common shapes include cocci (spherical), bacilli (rod-shaped), and spirilla (spiral).

Cell-Surface Structures

• Bacterial Cell Wall: Composed primarily of peptidoglycan, a polymer of sugars and amino acids.

• Archaeal Cell Wall: Lacks peptidoglycan; instead, contains polysaccharides and proteins (e.g., pseudomurein).

Gram Staining and Cell Wall Classification

• Gram-Positive Bacteria: Thick peptidoglycan layer; stains purple in Gram stain.

• Gram-Negative Bacteria: Thin peptidoglycan layer and an outer membrane; stains pink/red.

Example: Escherichia coli is Gram-negative; Bacillus subtilis is Gram-positive.

Protective Structures

• Capsule: A sticky layer of polysaccharide or protein outside the cell wall; protects against desiccation and immune attack.

• Endospores: Metabolically inactive, highly resistant structures formed under stress.

Surface Appendages

• Fimbriae: Hair-like projections for attachment to surfaces or other cells.

• Pili: Longer than fimbriae; involved in DNA transfer (conjugation).

• Flagella: Used for motility; structurally distinct from eukaryotic flagella.

Specialized Membrane Structures

• Some prokaryotes have infolded plasma membranes or internal membranes for metabolic processes (e.g., photosynthesis, respiration).

Genetic Material and Reproduction

Genetic Structures

• Circular Chromosome: Main DNA molecule, located in the nucleoid region.

• Plasmids: Small, circular DNA molecules carrying accessory genes (e.g., antibiotic resistance).

• Ribosomes: Sites of protein synthesis; structurally different from eukaryotic ribosomes and targeted by some antibiotics.

Reproduction and Mutation

• Binary Fission: Asexual reproduction; one cell divides into two identical cells.

• Mutation Rates: Generally low per division, but high overall due to rapid reproduction.

Genetic Diversity and Recombination

• Transformation: Uptake of naked DNA from the environment.

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

• Conjugation: Direct transfer of DNA between cells via a pilus (sex pilus).

F+ Cell: Contains the F plasmid (fertility factor); can donate DNA. F- Cell: Lacks the F plasmid; can receive DNA.

Metabolic and Nutritional Diversity

Nutritional Modes

Prokaryotes are classified by how they obtain energy and carbon:

• Photo-: Energy from light.

• Chemo-: Energy from chemicals.

• Auto-: Carbon from CO2 or inorganic sources.

• Hetero-: Carbon from organic compounds.

Example: Photoautotrophs (e.g., cyanobacteria) use light and CO2; chemoheterotrophs (e.g., most bacteria) use organic molecules for both energy and carbon.

Oxygen Relationships

• Obligate Aerobes: Require oxygen for cellular respiration.

• Facultative Anaerobes: Can use oxygen but also grow without it.

• Obligate Anaerobes: Poisoned by oxygen; use fermentation or anaerobic respiration.

Nitrogen Fixation

• Conversion of atmospheric nitrogen (N2) to ammonia (NH3), making nitrogen available for biosynthesis.

Equation: $\mathrm{N_2 + 8H^+ + 8e^- \rightarrow 2NH_3 + H_2}$

Genetic Diversity Transmission

• Horizontal Gene Transfer (HGT): Movement of genes between organisms other than by descent (vertical gene transfer).

• HGT complicates evolutionary relationships; traits like shape or metabolism do not always reflect ancestry.

Major Prokaryotic Groups and Features

Domain Relationships

• Archaea are more closely related to Eukaryotes than to Bacteria.

Proteobacteria

• Mostly Gram-negative; highly diverse in metabolism (photoautotrophs, chemoautotrophs, heterotrophs).

• Mitochondria are believed to have evolved from an ancestral proteobacterium.

Chlamydia

• Intracellular parasites; Gram-negative.

Spirochetes

• Spiral-shaped; Gram-negative; some are parasites (e.g., Treponema pallidum, syphilis agent).

Cyanobacteria

• Gram-negative; perform oxygenic photosynthesis; likely ancestors of chloroplasts.

Gram-Positive Bacteria

• Thick peptidoglycan cell wall; includes Streptomyces (antibiotic producers).

Archaea as Extremophiles

• Halophiles: Thrive in high-salt environments.

• Thermophiles: Thrive in high-temperature environments.

• Acidophiles: Thrive in acidic environments.

• Alkalinophiles: Thrive in basic (alkaline) environments.

Comparison of the Three Domains of Life

The following table summarizes key differences among Bacteria, Archaea, and Eukarya:

Feature	Bacteria	Archaea	Eukarya
Cell Type	Prokaryotic	Prokaryotic	Eukaryotic
Cell Wall Composition	Peptidoglycan	Polysaccharides, proteins, pseudomurein	Cellulose (plants), chitin (fungi), or none
Membrane Lipids	Ester-linked, unbranched hydrocarbons	Ether-linked, branched hydrocarbons	Ester-linked, unbranched hydrocarbons
Genetic Material	Circular chromosome, plasmids	Circular chromosome, plasmids	Linear chromosomes in nucleus
Introns in Genes	Rare	Present in some genes	Common
Membrane-bound Organelles	No	No	Yes
Ribosome Size	70S	70S	80S
Examples	Escherichia coli, Streptomyces	Halobacterium, Thermoproteus	Plants, animals, fungi, protists

Additional info: Table entries inferred and expanded for completeness based on standard biology textbooks.