Functional Anatomy of Prokaryotic Cells: Structure, Function, and Diversity

Functional Anatomy of Prokaryotic Cells

Comparing Prokaryotic and Eukaryotic Cells: An Overview

Understanding the differences between prokaryotic and eukaryotic cells is fundamental in microbiology. These differences influence cellular structure, function, and the mechanisms by which organisms grow and reproduce.

• Prokaryotes: Derived from Greek for 'prenucleus.' Characterized by a single circular chromosome, lack of membrane-bound organelles, absence of a defined nucleus, and division by binary fission.

• Eukaryotes: Derived from Greek for 'true nucleus.' Possess a nucleus with a nuclear membrane, histones, and membrane-bound organelles. Divide by mitosis.

The Size, Shape, and Arrangement of Bacterial Cells

Bacteria exhibit a variety of shapes and arrangements, which are important for identification and classification.

• Size: Typically 0.2–2.0 µm in diameter and 2–8 µm in length.

• Monomorphic: Most bacteria maintain a single shape.

• Pleomorphic: Some bacteria can have multiple shapes.

Common Shapes

• Bacillus: Rod-shaped

• Coccus: Spherical-shaped

• Spiral: Curved forms, including:

  • Vibrio: Comma-shaped, less than one turn

  • Spirillum: Helical, rigid, moves by flagella

  • Spirochete: Helical, flexible, moves by axial filament

• Less Common Shapes: Star-shaped and rectangular (often found in halophilic archaea)

Arrangements

• Pairs: Diplococci, diplobacilli

• Clusters: Staphylococci

• Chains: Streptococci, streptobacilli

• Groups of Four: Tetrads

• Cubelike Groups of Eight: Sarcinae

The Structure of a Prokaryotic Cell

Prokaryotic cells possess unique external and internal structures that contribute to their survival and pathogenicity.

Glycocalyx

• Definition: A viscous, gelatinous sugar coat external to the cell wall, composed of polysaccharide and/or polypeptide.

• Types:

  • Capsule: Well-organized and firmly attached to the cell wall.

  • Slime Layer: Loosely attached and unorganized.

• Functions:

  • Contributes to virulence by preventing phagocytosis and aiding in adherence to surfaces.

  • Helps form biofilms (e.g., Streptococcus mutans in tooth decay).

  • Protects from dehydration.

Flagella

• Structure: Composed of filament (flagellin), hook, and basal body.

• Function: Provides motility, allowing bacteria to move toward or away from stimuli (taxis).

• Arrangements:

  • Peritrichous: Flagella all around

  • Monotrichous: Single flagellum

  • Lophotrichous: Multiple flagella at one end

  • Amphitrichous: Flagella at both ends

  • Atrichous: No flagella

• Movement:

  • Run: Counterclockwise rotation, straight movement

  • Tumble: Clockwise rotation, random change in direction

  • Stimuli: Chemotaxis (chemicals), phototaxis (light)

Axial Filaments

• Also called endoflagella; found in spirochetes (e.g., Treponema pallidum, Borrelia burgdorferi).

• Enable corkscrew movement.

Fimbriae and Pili

• Fimbriae: Short, hairlike appendages for attachment; important in biofilm formation and pathogenicity (e.g., Neisseria gonorrhoeae).

• Pili: Longer; involved in motility and DNA transfer (conjugation pili).

The Cell Wall

The cell wall is a semi-rigid structure that provides protection, maintains shape, and prevents osmotic lysis. Its composition is crucial for bacterial classification and pathogenicity.

• Peptidoglycan: Main component, consisting of repeating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by polypeptides.

• Penicillin: Inhibits peptide cross-bridge formation, weakening the cell wall.

Gram-Positive Cell Walls

• Thick peptidoglycan layer

• Contains teichoic acids (lipoteichoic and wall teichoic acids)

• Retains crystal violet dye (appears purple)

• Provides antigenic specificity

Gram-Negative Cell Walls

• Thin peptidoglycan layer

• Three layers: outer membrane, cell wall, plasma membrane

• Outer membrane contains lipopolysaccharide (LPS), lipoproteins, and phospholipids

• LPS components: O polysaccharide (antigen), Lipid A (endotoxin)

• Porins allow selective passage of molecules

• More resistant to antibiotics and phagocytosis

Gram Stain Mechanism

1. Crystal violet-iodine complexes form inside cells

2. Alcohol dehydrates peptidoglycan in gram-positive cells (retains dye)

3. Alcohol dissolves outer membrane in gram-negative cells (dye washes out)

4. Safranin counterstain colors gram-negative cells pink/red

Atypical Cell Walls

• Acid-fast cell walls: Thick peptidoglycan with mycolic acid (waxy lipid); stains red with carbolfuchsin (e.g., Mycobacterium, Nocardia).

Damage to the Cell Wall

• Lysozyme: Hydrolyzes peptidoglycan bonds, weakening the wall.

• Penicillin: Inhibits peptide bridge formation.

• Protoplast: Wall-less gram-positive cell

• Spheroplast: Wall-less gram-negative cell (retains outer membrane)

• Both are susceptible to osmotic lysis.

The Plasma (Cytoplasmic) Membrane

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer and proteins. It is essential for cellular metabolism and transport.

• Fluid Mosaic Model: Describes the dynamic nature of the membrane, with proteins and phospholipids moving laterally.

• Functions: Selective permeability, ATP production, enzymatic activity.

Movement of Materials Across Membranes

• Passive Processes:

  • Simple diffusion: Movement from high to low concentration (e.g., O2, CO2).

  • Facilitated diffusion: Uses transport proteins for ions/large molecules.

  • Osmosis: Water movement across a selectively permeable membrane.

  • Osmotic pressure: Pressure needed to stop water movement.

  • Isotonic, hypotonic, hypertonic solutions affect water movement and cell integrity.

• Active Processes:

  • Active transport: Requires ATP to move substances against the gradient.

  • Group translocation (prokaryotes only): Substance is chemically modified during transport (e.g., glucose to glucose-6-phosphate).

Cytoplasm

The cytoplasm is the internal matrix of the cell, containing water, proteins, carbohydrates, lipids, ions, and essential cellular structures.

• Includes the nucleoid (DNA), ribosomes, and inclusions.

The Nucleoid

• Bacterial chromosome: Circular, double-stranded DNA, not membrane-bound.

• Plasmids: Small, circular DNA molecules; carry genes for antibiotic resistance and toxins; replicate independently.

Ribosomes

• Sites of protein synthesis

• Composed of protein and rRNA

• 70S ribosome: 50S (large) + 30S (small) subunits

Inclusions

• Reserve deposits for nutrients

• Types:

  • Metachromatic granules (phosphate reserves)

  • Polysaccharide granules (energy reserves)

  • Lipid inclusions (energy reserves)

  • Sulfur granules (energy reserves)

  • Carboxysomes (CO2 fixation)

  • Gas vacuoles (buoyancy)

  • Magnetosomes (iron oxide, destroy H2O2)

Endospores

• Highly durable, dormant structures formed in response to adverse conditions

• Resistant to desiccation, heat, chemicals, and radiation

• Produced by Bacillus and Clostridium species

• Sporulation: Formation of endospores

• Germination: Return to vegetative state

Additional info: The above notes integrate foundational microbiology concepts with expanded academic context for clarity and exam preparation.