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 evolution and 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.

Prokaryotic Structure and Cell Surface Features

Structural Adaptations

• Unicellular: Most prokaryotes exist as single cells.

• Very Small: Typically 0.5–5 μm in diameter.

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

Cell-Surface Structures

• Bacteria: Cell wall composed of peptidoglycan, a polymer of sugars and amino acids.

• Archaea: Cell wall composed of pseudomurein (polysaccharides and proteins), lacking peptidoglycan.

Gram Staining and Cell Wall Classification

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

• 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, important for protection and adherence.

Other Surface Structures

• Fimbriae: Hair-like appendages for attachment to surfaces.

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

• Flagella: Used for motility; rotate to propel the cell.

Internal Structures and Genetic Material

Specialized Membranes

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

Genetic Material

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

• Plasmids: Small, circular DNA molecules carrying accessory genes.

• Ribosomes: Sites of protein synthesis; can be targeted by antibiotics.

Prokaryotic Reproduction and Genetic Diversity

Reproduction

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

• Mutation rates are generally low per division but high overall due to rapid reproduction.

Genetic Recombination Mechanisms

• Transformation: Uptake of foreign 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+ cells possess the F plasmid (fertility factor) and can donate DNA; F- cells lack the F plasmid and receive DNA.

Horizontal Gene Transfer (HGT)

• Genetic material is exchanged between organisms, not just from parent to offspring (vertical gene transfer).

• HGT contributes to genetic diversity and complicates evolutionary relationships.

Nutritional and Metabolic Diversity

Nutritional Modes

• 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 Requirements

• 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) into ammonia (NH3), a form usable by living organisms.

Equation:

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

Prokaryotic Diversity and Major Groups

Domain Relationships

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

Major Bacterial Groups

• Proteobacteria: Mostly Gram-negative; diverse nutritional modes; mitochondria likely evolved from this group.

• Chlamydia: Intracellular parasites.

• Spirochetes: Spiral-shaped, Gram-negative; some are parasites.

• Cyanobacteria: Gram-negative, photosynthetic; ancestors of chloroplasts.

• Gram-Positive Bacteria: Includes Streptomyces (antibiotic producers).

Archaea and 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

Table: Key Differences Among Bacteria, Archaea, and Eukarya

Feature	Bacteria	Archaea	Eukarya
Cell Type	Prokaryotic	Prokaryotic	Eukaryotic
Cell Wall Composition	Peptidoglycan	Pseudomurein (polysaccharides/proteins)	Cellulose (plants), chitin (fungi), or none
Membrane Lipids	Ester-linked, unbranched	Ether-linked, branched	Ester-linked, unbranched
Genetic Material	Circular chromosome, plasmids	Circular chromosome, plasmids	Linear chromosomes in nucleus
Introns in Genes	Rare	Some genes	Common
Organelles	Absent	Absent	Present (membrane-bound)
Ribosome Size	70S	70S	80S
Examples	Escherichia coli, Streptomyces	Halophiles, thermophiles	Plants, animals, fungi

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