Prokaryotes: Structure, Diversity, and Metabolism – General Biology Study Notes

Prokaryotes: Diversity and Characteristics

Overview of Prokaryotic Diversity

Prokaryotes, which include bacteria and archaea, represent the most diverse group of organisms on Earth. They are adapted to a wide range of habitats and play essential roles in ecological and biological processes.

• Unicellular: Most prokaryotes are single-celled organisms.

• Habitat Adaptation: Found in nearly every environment, from deep oceans to extreme environments.

• Cell Structure: Lack membrane-bound organelles; DNA is not enclosed in a nucleus.

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

Prokaryotic Cell Structure

Common Features

Prokaryotic cells possess several unique structural features that distinguish them from eukaryotic cells.

• Cell Wall: Maintains cell shape and protects the cell. Most bacterial cell walls contain peptidoglycan.

• Plasmolysis: In hypertonic environments, prokaryotes lose water and may experience plasmolysis, leading to cell shrinkage.

• Capsules: Some bacteria have a sticky, polysaccharide or protein layer outside the cell wall, aiding in protection and adherence.

• Fimbriae: Hairlike appendages that help cells stick to surfaces or each other.

• Pili (sex pili): Longer than fimbriae, used in DNA exchange during conjugation.

• Flagella: Structures for movement; prokaryotic flagella differ from eukaryotic flagella in structure and composition.

Peptidoglycan and Gram Staining

Peptidoglycan is a network of sugar polymers cross-linked with polypeptides, forming the main component of bacterial cell walls. Gram staining is a technique used to classify bacteria based on cell wall structure.

Type	Peptidoglycan Layer	Gram Stain Result
Gram-positive bacteria	Thick	Purple
Gram-negative bacteria	Thin, with outer membrane	Pink

Gram-positive bacteria are more susceptible to antibiotics targeting peptidoglycan (e.g., penicillin), while gram-negative bacteria are often more resistant due to their outer membrane.

Unique Features of Bacterial Cells

• Capsules: Protect against dehydration and help adhere to surfaces.

• Endospores: Dormant, tough, non-reproductive structures formed under stress; highly resistant and can survive for centuries.

• Fimbriae and Pili: Aid in attachment and genetic exchange.

• Flagella: Enable motility; structure differs from eukaryotic flagella.

Prokaryotic Reproduction and Genetic Diversity

Binary Fission

Prokaryotes reproduce asexually by binary fission, a process in which the cell divides into two genetically identical daughter cells.

• Can occur every 1-3 hours under optimal conditions.

• High rates of reproduction lead to rapid population growth and genetic diversity.

Equation:

$\text{Binary fission:} \quad \text{Parent cell} \rightarrow 2 \times \text{Daughter cells}$

Genetic Recombination

Genetic diversity in prokaryotes is enhanced by several mechanisms:

• Transformation: Uptake of foreign DNA from the environment.

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

• Conjugation: Direct transfer of DNA between cells via sex pili.

• Horizontal gene flow: Movement of genes between individuals of different species.

Prokaryotic Metabolism

Classification by Energy and Carbon Source

Prokaryotes are classified based on how they obtain energy and carbon:

• Autotrophs: Use CO2 or inorganic compounds as a carbon source.

• Heterotrophs: Require organic nutrients (e.g., glucose).

• Phototrophs: Obtain energy from light.

• Chemotrophs: Obtain energy from chemicals.

Autotrophs

• Photoautotrophs: Use light as energy source; e.g., cyanobacteria.

• Chemoautotrophs: Use inorganic chemicals; e.g., Sulfolobus.

Prokaryotic Metabolism: Oxygen

Prokaryotes can be classified by their response to oxygen:

• Obligate aerobes: Require O2 for cellular respiration.

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

• Facultative anaerobes: Can use O2 or not, depending on conditions.

Prokaryotic Metabolism: Nitrogen

Nitrogen is essential for amino acids and nucleic acids. Prokaryotes metabolize nitrogen in various forms, including nitrogen fixation:

$\text{N}_2 \rightarrow \text{NH}_3$

• Nitrogen fixation: Conversion of atmospheric nitrogen (N2) to ammonia (NH3).

• Some cyanobacteria specialize in nitrogen fixation.

Prokaryotic Diversity

Bacteria

• Most diverse domain; estimated 700,000 to 1.4 million species.

• Major groups include Proteobacteria, Gram-positive bacteria, Spirochetes, and Cyanobacteria.

Major Bacterial Groups

Group	Key Features	Examples
Proteobacteria	Gram-negative; includes photoautotrophs, chemoautotrophs, heterotrophs	Helicobacter pylori (ulcers), Neisseria gonorrhoeae (gonorrhea)
Spirochetes	Helical, gram-negative, move with flagella	Syphilis, Lyme disease
Cyanobacteria	Oxygen-generating photoautotrophs	Freshwater and marine phytoplankton
Gram-positive bacteria	Diverse, soil-dwelling, some pathogenic	Streptomyces (antibiotics), Bacillus anthracis (anthrax)

Archaea

• Lack peptidoglycan in cell walls.

• Can grow in extreme environments (extremophiles).

• Major types: Halophiles (salt-loving), Thermophiles (heat-loving), Methanogens (produce methane).

Prokaryotic Ecological Interactions

Symbiosis

Symbiosis is an ecological relationship where two species live in close contact.

• Mutualism: Both species benefit (e.g., bacteria in human intestines).

• Commensalism: One benefits, the other is unaffected.

• Parasitism: One benefits, the other is harmed; disease-causing bacteria are called pathogens.

Antibiotic Resistance

Mechanisms and Implications

Antibiotic resistance has evolved rapidly in bacteria due to natural selection and horizontal gene transfer. This poses significant challenges for treating bacterial infections.

• Rapid reproduction and genetic diversity facilitate resistance.

• Horizontal gene transfer spreads resistance genes between species.

• Every major antibiotic now faces at least one species of resistant bacteria.

Applications of Prokaryotes

Food Production

Prokaryotes are used in the production of many foods, such as cheese, yogurt, beer, wine, sauerkraut, and soy sauce.

Human Health

• Many human diseases are caused by pathogenic bacteria (e.g., Mycobacterium tuberculosis).

• Mutualistic bacteria aid in digestion and other bodily functions.

Summary Table: Prokaryotic Features

Feature	Bacteria	Archaea
Cell Wall	Peptidoglycan	No peptidoglycan
Membrane Lipids	Unbranched hydrocarbons	Branched hydrocarbons
Habitat	Wide range	Often extreme environments
Pathogenicity	Many pathogens	Few known pathogens

Additional info: These notes expand on the original bullet points with academic context, definitions, and examples to provide a comprehensive overview suitable for General Biology students.