Prokaryotic and Eukaryotic Cell Structure

Comparison of Cell Types

Prokaryotes and eukaryotes differ fundamentally in their cellular organization, structure, and complexity. Understanding these differences is essential for microbiology.

• Prokaryotes: Lack a membrane-bound nucleus; DNA is located in the nucleoid. No membrane-bound organelles. Cell wall usually present.

• Eukaryotes: Have a membrane-bound nucleus and organelles (e.g., mitochondria, endoplasmic reticulum). Cell wall present in plants and fungi, absent in animals.

• Example: Escherichia coli (prokaryote), human cell (eukaryote).

Glycocalyx Structure and Function

Capsule vs. Slime Layer

The glycocalyx is an external layer found in many prokaryotes, providing protection and aiding in adherence.

• Capsule: Organized, firmly attached layer; protects against phagocytosis and desiccation.

• Slime Layer: Unorganized, loosely attached; aids in adherence to surfaces.

• Virulence: Capsules increase bacterial virulence by preventing immune detection and phagocytosis.

• Example: Streptococcus pneumoniae capsule increases pathogenicity.

Bacterial Cell Morphology and Arrangements

Shapes and Arrangements

Bacteria exhibit diverse shapes and arrangements, which are important for identification and classification.

• Shapes:

  • Coccus: Spherical

  • Bacillus: Rod-shaped

  • Spiral: Includes vibrio (curved), spirillum (rigid spiral), spirochete (flexible spiral)

• Arrangements:

  • Random: No specific grouping

  • Staphylo: Clusters

  • Strepto: Chains

  • Diplo: Pairs

  • Tetrad: Groups of four

  • Sarcinae: Cubical packets

• Reason for Arrangements: Determined by patterns of cell division and separation.

Surface Structures: Flagella, Axial Filaments, Fimbriae, and Pili

Types and Functions

Prokaryotes possess various surface appendages for movement, attachment, and genetic exchange.

• Flagella: Used for motility; arrangements include monotrichous (single), lophotrichous (tuft), amphitrichous (both ends), peritrichous (all over), polar (at one or both ends).

• Axial Filaments: Found in spirochetes; enable corkscrew movement.

• Fimbriae: Short, numerous; used for attachment.

• Pili: Longer, fewer; involved in conjugation (genetic exchange).

• Conjugation Pilus: Transfers DNA between cells.

• Movement: Bacteria move via flagella; chemotaxis is movement toward/away from chemical stimuli.

Cell Wall Types in Prokaryotes

Gram-Positive, Gram-Negative, Acid-Fast, Archaea, and Mycoplasmas

Bacterial cell walls vary in structure and composition, affecting staining, susceptibility to antibiotics, and pathogenicity.

• Gram-Positive: Thick peptidoglycan, teichoic acids; stains purple. Example: Staphylococcus

• Gram-Negative: Thin peptidoglycan, outer membrane with lipopolysaccharide (LPS); stains pink. Example: E. coli

• Acid-Fast: Waxy mycolic acid layer; resists decolorization. Example: Mycobacterium

• Archaea: Diverse cell wall types; lack peptidoglycan.

• Mycoplasmas: No cell wall; sterols in membrane. Example: Mycoplasma pneumoniae

Peptidoglycan and Antibiotics

Importance and Mechanism of Penicillin

Peptidoglycan is a critical component of most bacterial cell walls, providing structural integrity.

• Peptidoglycan: Polymer of sugars and amino acids; forms mesh-like layer.

• Penicillin: Inhibits transpeptidase enzyme, preventing cross-linking of peptidoglycan.

• Equation:

Lipopolysaccharide (LPS)

Structure and Importance

LPS is a major component of the outer membrane of gram-negative bacteria, contributing to pathogenicity.

• Structure: Lipid A (toxic), core polysaccharide, O antigen.

• Importance: Acts as endotoxin; triggers immune response and septic shock.

Prokaryotic Plasma Membrane

Structure, Chemistry, and Functions

The plasma membrane controls transport, energy production, and cell integrity.

• Structure: Phospholipid bilayer with embedded proteins.

• Functions: Selective permeability, ATP synthesis, transport.

• Disruption: Detergents, alcohols, antibiotics can damage membrane.

Transport Mechanisms

Diffusion, Osmosis, Active Transport

Cells use various mechanisms to move substances across membranes.

• Simple Diffusion: Movement from high to low concentration.

• Facilitated Diffusion: Uses transport proteins.

• Osmosis: Movement of water across membrane.

• Active Transport: Requires energy (ATP) to move substances against gradient.

• Isotonic: Equal solute concentration.

• Hypotonic: Lower solute outside; cell swells.

• Hypertonic: Higher solute outside; cell shrinks.

Nucleoid Function

Genetic Material in Prokaryotes

The nucleoid is the region containing the bacterial chromosome.

• Function: Stores genetic information; controls cell activities.

Ribosomes

Structure and Function

Ribosomes are the site of protein synthesis.

• Prokaryotic Ribosomes: 70S (50S + 30S subunits)

• Eukaryotic Ribosomes: 80S (60S + 40S subunits)

• Importance: Antibiotics can target 70S ribosomes without affecting eukaryotic cells.

Prokaryotic DNA Structure

Organization of Genetic Material

Bacterial DNA is typically a single, circular chromosome.

• Structure: Circular, double-stranded DNA.

Plasmids

Definition and Function

Plasmids are small, circular DNA molecules separate from the chromosome.

• Function: Carry genes for antibiotic resistance, virulence, and metabolism.

Inclusions

Storage Granules

Inclusions are reserve deposits found in prokaryotic cells.

• Example: Polyphosphate granules, glycogen, sulfur granules.

Endospores

Structure and Function

Endospores are highly resistant, dormant structures formed by some bacteria.

• Function: Survival under harsh conditions (heat, desiccation, chemicals).

• Species: Bacillus and Clostridium produce endospores.

• Trigger: Nutrient depletion or environmental stress.