Prokaryotes and Protists: Structure, Diversity, and Function

Prokaryotes and Protists

Introduction

This study guide covers the biology of prokaryotes and protists, focusing on their structure, diversity, metabolic adaptations, and ecological roles. These topics are central to understanding microbial life and the evolutionary origins of eukaryotes.

Prokaryotes

Differences Between Prokaryotes and Eukaryotes

• Cellular Organization: Prokaryotes lack a membrane-bound nucleus and organelles, while eukaryotes possess both.

• Genetic Material: Prokaryotic DNA is typically a single circular chromosome located in the nucleoid region; eukaryotic DNA is linear and contained within a nucleus.

• Cell Size: Prokaryotes are generally smaller (0.5–5 μm) than eukaryotic cells (10–100 μm).

Common Shapes of Prokaryotes

• Spherical (Cocci): Round-shaped cells.

• Rod-shaped (Bacilli): Cylindrical cells.

• Spiral (Spirilla/Spirochetes): Helically coiled cells.

Cell-Surface Structures

• Cell Wall: Maintains shape, protects the cell, and prevents lysis in hypotonic environments.

• Bacterial Cell Walls: Contain peptidoglycan, a polymer of sugars and amino acids.

• Archaeal Cell Walls: Composed of polysaccharides and proteins, lacking peptidoglycan.

• Eukaryotic Cell Walls: Made of cellulose (plants) or chitin (fungi).

Gram Staining and Cell Wall Types

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

• Gram-Negative Bacteria: Thin peptidoglycan layer and outer membrane with lipopolysaccharides; stain pink/red. Lipid portions can be toxic and confer antibiotic resistance.

Additional Cell-Surface Features

• Capsule: Sticky, protective outer layer aiding in adherence and evasion of host defenses.

• Endospores: Dormant, highly resistant cells formed under stress, capable of surviving extreme conditions.

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

• Pili (Sex Pili): Longer appendages used in DNA transfer (conjugation).

Motility

• Flagella: Most common structure for movement; prokaryotic and eukaryotic flagella differ in structure and function.

• Flagella Evolution: Bacterial and archaeal flagella are structurally similar but composed of different proteins (convergent evolution).

Internal Organization and DNA

• Lack of Compartmentalization: Prokaryotes do not have membrane-bound organelles.

• Specialized Membranes: Some have infoldings for metabolic functions (e.g., respiratory or thylakoid membranes).

• Genome: Single circular chromosome in the nucleoid; may also have plasmids (small, circular DNA molecules).

Nutritional and Metabolic Adaptations

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

• Heterotrophs: Require organic compounds for carbon.

Mode	Energy Source	Carbon Source	Types of Organisms
Photoautotroph	Light	CO2, HCO3-	Photosynthetic prokaryotes (e.g., cyanobacteria), plants, certain protists
Chemoautotroph	Inorganic chemicals (e.g., NH3, H2S)	CO2, HCO3-	Certain prokaryotes (e.g., Sulfolobus)
Photoheterotroph	Light	Organic compounds	Certain aquatic and salt-loving prokaryotes
Chemoheterotroph	Organic compounds	Organic compounds	Many prokaryotes, protists, fungi, animals, some plants

The Role of Oxygen in 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.

Nitrogen Metabolism

• Nitrogen Fixation: Some prokaryotes convert atmospheric N2 to ammonia (NH3), making nitrogen available to other organisms.

Metabolic Cooperation

• Biofilms: Surface-coating colonies of prokaryotes that cooperate metabolically; can cause medical and industrial problems.

Reproduction and Genetic Diversity

• Binary Fission: Asexual reproduction by cell division; rapid due to small size and short generation times.

• Genetic Variation: Promoted by rapid reproduction, mutation, and genetic recombination.

• Genetic Recombination: Includes transformation (uptake of foreign DNA), transduction (gene transfer by phages), and conjugation (direct transfer via pili).

• Horizontal Gene Transfer: Movement of genes between different species, increasing diversity.

Prokaryotic Diversity

• Bacteria: Includes most familiar prokaryotes; diverse in metabolism and nutrition.

• Proteobacteria: Large clade of gram-negative bacteria with subgroups (alpha, beta, gamma, delta, epsilon) exhibiting diverse lifestyles.

• Examples: Rhizobium (nitrogen-fixing), Agrobacterium (genetic engineering), Nitrosomonas (nitrification), Salmonella (pathogen), Escherichia coli (model organism).

*Additional info: Further details on Archaea, ecological roles, and protist diversity are covered in subsequent sections of the original material.*