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: Possess a true nucleus surrounded by a nuclear membrane. Contain membrane-bound organelles (mitochondria, ER, Golgi, etc.). 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 to cell wall. Increases virulence by protecting against phagocytosis.

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

• Virulence: Capsules enable bacteria to evade immune responses, e.g., Streptococcus pneumoniae.

Bacterial 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 whether cells remain attached after division.

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

Types and Functions

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

• Flagella: Used for motility. Arrangements include:

  • Monotrichous: Single flagellum at one end

  • Lophotrichous: Tuft of flagella at one end

  • Amphitrichous: Flagella at both ends

  • Peritrichous: Flagella all over the surface

  • Polar: Flagella at one or both ends

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

• Fimbriae: Short, numerous; used for attachment.

• Pili: Longer, fewer; used for attachment and conjugation (genetic exchange).

• Conjugation Pilus: Facilitates transfer of DNA between bacteria.

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

Cell Wall Types and Comparison

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 layer, teichoic acids. Example: Staphylococcus.

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

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

• Archaea: Variable cell wall; may lack peptidoglycan.

• Mycoplasmas: Lack cell wall; have sterols in membrane. Example: Mycoplasma pneumoniae.

Peptidoglycan and Antibiotic Action

Importance and Penicillin Mechanism

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.

Prokaryotic Plasma Membrane

Structure, Chemistry, and Functions

The plasma membrane is a phospholipid bilayer with embedded proteins, controlling transport and metabolic processes.

• Structure: Phospholipid bilayer, proteins, no sterols (except mycoplasmas).

• Functions: Selective permeability, energy generation, transport.

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

Transport Mechanisms

Diffusion, Osmosis, Active Transport

Cells use various mechanisms to move substances across membranes.

• Simple Diffusion: Movement of molecules from high to low concentration.

• Facilitated Diffusion: Uses transport proteins; still passive.

• Osmosis: Diffusion of water across membrane.

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

• Isotonic: Equal solute concentration inside and outside cell.

• Hypotonic: Lower solute outside; cell may swell.

• Hypertonic: Higher solute outside; cell may shrink.

Equation:

Nucleoid Function

Genetic Material Storage

The nucleoid is the region in prokaryotes where DNA is concentrated.

• Function: Contains the bacterial chromosome; site of DNA replication and transcription.

Ribosomes

Protein Synthesis and Differences

Ribosomes are essential for protein synthesis; prokaryotic and eukaryotic ribosomes differ in size and structure.

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

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

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

Prokaryotic DNA Structure

Chromosome Organization

Prokaryotic DNA is typically a single, circular chromosome located in the nucleoid.

• Structure: Circular, double-stranded DNA.

Plasmids

Definition and Function

Plasmids are small, circular DNA molecules separate from the chromosome, often carrying genes for antibiotic resistance.

• Function: Can be transferred between bacteria; confer advantageous traits.

Inclusions

Storage Granules

Inclusions are reserve deposits found in prokaryotic cells.

• Example: Polyphosphate granules, glycogen granules, gas vesicles.

Endospores

Structure and Function

Endospores are highly resistant structures formed by some bacteria for survival under adverse conditions.

• Function: Allow bacteria to withstand heat, desiccation, chemicals.

• Species: Bacillus and Clostridium produce endospores.

• Trigger: Nutrient depletion or environmental stress.

Example: Bacillus anthracis forms endospores in soil.