Functional Anatomy of Prokaryotic and Eukaryotic Cells: Microbiology Study Notes

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Overview: Prokaryotic vs. Eukaryotic Cells

This section introduces the fundamental differences between prokaryotic and eukaryotic cells, which are the two major cell types found in the microbial world. Understanding these differences is essential for studying microbial structure, function, and classification.

• Prokaryotes: Organisms whose cells lack a true nucleus and membrane-bound organelles. Includes Bacteria and Archaea.

• Eukaryotes: Organisms whose cells have a true nucleus enclosed by a nuclear membrane and possess membrane-bound organelles. Includes Fungi, Protozoa, Algae, and Helminths.

• Key Differences:

  • Prokaryotes: One circular chromosome (not in a membrane), no histones, no organelles, cell walls (peptidoglycan in bacteria, pseudomurein in archaea), divide by binary fission.

  • Eukaryotes: Paired chromosomes in a nuclear membrane, histones present, organelles present, polysaccharide cell walls (when present), divide by mitosis.

Size, Shape, and Arrangement of Bacterial Cells

General Characteristics

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

• Average Size: 0.2 to 2.0 μm in diameter, 2 to 8 μm in length.

• Monomorphic: Most bacteria maintain a single shape.

• Pleomorphic: Some bacteria can vary in shape.

Common Shapes

• Bacillus: Rod-shaped

• Coccus: Spherical-shaped

• Spiral: Includes vibrio (comma-shaped), spirillum (rigid spiral), and spirochete (flexible spiral)

• Other Shapes: Star-shaped, rectangular

Arrangements

• Pairs: Diplococci, diplobacilli

• Chains: Streptococci, streptobacilli

• Clusters: Staphylococci

• Groups of Four: Tetrads

• Cubelike Groups of Eight: Sarcinae

Example: Bacillus anthracis

Bacillus anthracis is a rod-shaped, Gram-positive bacterium that causes anthrax.

Structure of a Prokaryotic Cell

Prokaryotic cells have a simple structure but possess specialized features that contribute to their survival and pathogenicity.

Glycocalyx

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

• Types:

  • Capsule: Neatly organized and firmly attached.

  • Slime Layer: Unorganized and loosely attached.

• Functions: Contributes to virulence by preventing phagocytosis and aiding in biofilm formation.

Flagella

• Definition: Long, filamentous appendages that propel bacteria.

• Structure: Composed of three parts: filament (outermost), hook (connects filament to cell), and basal body (anchors flagellum).

• Function: Motility (movement toward/away from stimuli, called taxis); flagella rotate to produce "runs" and "tumbles".

• Antigenic Properties: Flagella proteins (H antigens) are used to distinguish bacterial serovars.

Archaella and Axial Filaments

• Archaella: Motility structures in archaea, composed of archaellins (glycoproteins), rotate like flagella.

• Axial Filaments (Endoflagella): Found in spirochetes, anchored at one end, rotation causes corkscrew movement.

Fimbriae and Pili

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

• Pili: Involved in motility (gliding, twitching) and DNA transfer (conjugation pili).

The Cell Wall

The bacterial cell wall is a rigid structure that maintains cell shape, prevents osmotic lysis, and contributes to pathogenicity. Its composition varies between Gram-positive and Gram-negative bacteria.

Peptidoglycan Structure

• Peptidoglycan: A polymer of repeating disaccharides (N-acetylglucosamine, NAG, and N-acetylmuramic acid, NAM) cross-linked by polypeptides.

Gram-Positive Cell Walls

• Thick peptidoglycan layer

• Teichoic acids: Lipoteichoic acid links cell wall to plasma membrane; wall teichoic acid links peptidoglycan.

• 2 rings in basal body of flagella

• Produce exotoxins

• High susceptibility to penicillin

• Disrupted by lysozyme

Gram-Negative Cell Walls

• Thin peptidoglycan layer

• Outer membrane: Contains lipopolysaccharide (LPS), lipoproteins, and phospholipids.

• Periplasmic space: Contains peptidoglycan.

• LPS: O polysaccharide (antigenic), Lipid A (endotoxin).

• 4 rings in basal body of flagella

• Produce endotoxins and exotoxins

• Low susceptibility to penicillin

Gram Stain Mechanism

• Gram-positive: Alcohol dehydrates peptidoglycan, crystal violet-iodine (CV-I) complexes are retained, cells appear purple.

• Gram-negative: Alcohol dissolves outer membrane, CV-I washes out, cells are colorless until counterstained with safranin (appear red/pink).

Atypical Cell Walls

• Acid-fast cell walls: Like Gram-positive but with waxy mycolic acid (e.g., Mycobacterium, Nocardia).

• Mycoplasmas: Lack cell walls, have sterols in plasma membrane.

• Archaea: May lack cell walls or have walls of pseudomurein (lack NAM and D-amino acids).

Damage to the Cell Wall

• Lysozyme: Hydrolyzes bonds in peptidoglycan.

• Penicillin: Inhibits peptide bridges in peptidoglycan.

• Protoplast: Wall-less Gram-positive cell.

• Spheroplast: Wall-less Gram-negative cell.

• L forms: Wall-less cells that swell into irregular shapes.

The Plasma (Cytoplasmic) Membrane

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins. It plays a critical role in transport, energy production, and cell signaling.

Structure: Fluid Mosaic Model

• Membrane is as viscous as olive oil.

• Proteins move freely for various functions.

• Phospholipids rotate and move laterally.

• Self-sealing property.

Functions

• Selective permeability: Allows passage of some molecules but not others.

• ATP production: Contains enzymes for energy generation.

• Photosynthetic pigments: Found on foldings called chromatophores in some bacteria.

Damage to the Membrane

• Alcohols, detergents, and antibiotics (e.g., polymyxin) can disrupt the membrane, causing leakage of cell contents.

Movement of Materials Across Membranes

Cells transport substances across their membranes using passive and active processes.

Passive Processes

• Simple diffusion: Movement of solute from high to low concentration until equilibrium is reached.

• Facilitated diffusion: Solute combines with a transporter protein; used for ions and larger molecules.

• Osmosis: Movement of water across a selectively permeable membrane from high to low water concentration, via lipid layer or aquaporins.

• Osmotic pressure: Pressure needed to stop water movement across the membrane.

• Isotonic solution: Equal solute concentrations inside and outside cell; no net water movement.

• Hypotonic solution: Lower solute outside; water enters cell, may cause lysis.

• Hypertonic solution: Higher solute outside; water leaves cell, causing plasmolysis.

Active Processes

• Active transport: Requires transporter protein and ATP; moves substances against concentration gradient.

• Group translocation: Requires transporter protein and phosphoenolpyruvic acid (PEP); substance is chemically altered during transport.

Cytoplasm and Internal Structures

The cytoplasm is the substance inside the plasma membrane, containing water, proteins, carbohydrates, lipids, ions, and the cytoskeleton.

The Nucleoid

• Bacterial chromosome: Circular DNA containing genetic information.

• Plasmids: Extrachromosomal DNA elements carrying non-essential genes (e.g., antibiotic resistance).

Ribosomes

• Function: Sites of protein synthesis.

• Structure: Composed of protein and rRNA; 70S (50S + 30S subunits) in prokaryotes.

Inclusions

• Metachromatic granules (volutin): Phosphate reserves.

• Polysaccharide granules, lipid inclusions, sulfur granules: Energy reserves.

• Carboxysomes: Contain RuBisCO for CO2 fixation in photosynthesis.

• Gas vacuoles: Protein-covered cylinders for buoyancy.

• Magnetosomes: Iron oxide inclusions for orientation in magnetic fields.

Endospores

• Definition: Resting, highly resistant cells formed by certain bacteria (e.g., Bacillus, Clostridium) when nutrients are depleted.

• Resistance: Endospores withstand desiccation, heat, chemicals, and radiation.

• Sporulation: Process of endospore formation.

• Germination: Return of endospore to vegetative state.