Functional Anatomy of Prokaryotic and Eukaryotic Cells

Comparing Prokaryotic and Eukaryotic Cells

Prokaryotic and eukaryotic cells differ fundamentally in their structure and organization. Understanding these differences is essential for microbiology, as it informs classification, physiology, and pathogenicity.

• Prokaryote: "Prenucleus"; lacks a membrane-bound nucleus.

• Eukaryote: "True nucleus"; possesses a membrane-bound nucleus.

Size, Shape, and Arrangement of Bacterial Cells

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

• Size: 0.2–2.0 μm diameter, 2–8 μm length

• Shape: Bacillus (rod), Coccus (sphere), Spiral (Vibrio, Spirillum, Spirochete), Star-shaped, Rectangular

• Arrangement: Diplococci/diplobacilli (pairs), Staphylococci (clusters), Streptococci/streptobacilli (chains), Tetrads (groups of 4), Sarcinae (cubelike groups of 8)

Example: Bacillus genus; rod-shaped bacteria.

Prokaryotic Cell Structures & Functions

Glycocalyx

The glycocalyx is an external layer that enhances bacterial survival and pathogenicity.

• Structure: Viscous, gelatinous; made of polysaccharide and/or polypeptide.

• Types: Capsule (organized, attached), Slime layer (unorganized, loose)

• Function: Virulence factor; prevents phagocytosis, aids in adherence, forms biofilms.

• Examples: Bacillus anthracis, Streptococcus pneumoniae, Klebsiella pneumoniae (capsule); Streptococcus mutans, Vibrio cholerae (biofilm formation)

Flagella, Archaella, and Axial Filaments

These structures provide motility, which is crucial for colonization and infection.

• Flagella: Filamentous appendages; composed of flagellin; three parts: filament, hook, basal body.

• Function: Movement (taxis); rotation causes "run" or "tumble"; H antigens distinguish serovars (e.g., E. coli).

• Archaella: Motility structure in Archaea; made of archaellins; uses ATP.

• Axial filaments: Endoflagella in spirochetes; corkscrew motion.

Example: Spirochetes use axial filaments for motility.

Fimbriae and Pili

Fimbriae and pili are surface appendages involved in attachment and genetic exchange.

• Fimbriae: Hairlike; enable attachment and biofilm formation; e.g., Neisseria gonorrhoeae, E. coli O157.

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

Cell Wall

The cell wall provides structural support, protection, and is a key target for antibiotics.

• Composition: Peptidoglycan (bacteria); pseudomurein (archaea).

• Function: Prevents osmotic lysis; contributes to pathogenicity; differentiates bacterial groups.

Peptidoglycan Structure

• Polymer of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

• Rows linked by polypeptides; forms a lattice.

• Penicillin inhibits peptide cross-bridges, weakening the wall.

Gram-Positive vs. Gram-Negative Cell Walls

• Gram stain mechanism: Crystal violet-iodine complex retained in Gram-positive (purple/blue); washed out in Gram-negative (pink).

Atypical Cell Walls

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

• Mycoplasmas: Lack cell walls; sterols in membrane.

• Archaea: Wall-less or pseudomurein (lacks NAM, D-amino acids).

Damage to Cell Walls

• Lysozyme: Hydrolyzes glycan bonds; weakens Gram-positive walls.

• Penicillin: Inhibits peptide bridge formation.

• Protoplast: Wall-less Gram-positive cell.

• Spheroplast: Wall-less Gram-negative cell.

• L forms: Irregular, wall-less cells; susceptible to osmotic lysis.

Example: Gram-negative bacteria less susceptible to penicillin due to outer membrane.

The Plasma (Cytoplasmic) Membrane

The plasma membrane is a selectively permeable barrier, crucial for cellular function and integrity.

• Structure: Phospholipid bilayer; peripheral, integral, and transmembrane proteins; glycoproteins and glycolipids.

• Fluid mosaic model: Membrane is dynamic; proteins and lipids move freely.

• Function: Selective permeability, ATP production, photosynthetic pigments (chromatophores).

Destruction by Antimicrobial Agents

• Disinfectants: Alcohols, quaternary ammonium compounds.

• Antibiotics: Polymyxin damages membrane.

• Result: Leakage of cell contents.

Movement of Materials Across Membranes

Cells transport substances across membranes via passive and active processes.

• Passive: No energy required; moves from high to low concentration.

• Active: Requires energy; moves from low to high concentration.

• Osmotic pressure: Pressure to stop water movement.

• Isotonic: Equal solute; no net water movement.

• Hypotonic: Lower solute outside; water enters cell.

• Hypertonic: Higher solute outside; water leaves cell.

Cytoplasm

The cytoplasm is the internal matrix of the cell, containing essential components for metabolism and growth.

• Composition: 80% water, proteins, carbohydrates, lipids, ions.

• Includes: DNA (nucleoid), ribosomes, inclusions.

• Cytoskeleton: Fibers for cell division, shape, growth, DNA movement.

Nucleoid and Plasmids

The nucleoid contains the bacterial chromosome, while plasmids carry additional genetic information.

• Bacterial chromosome: Circular, double-stranded DNA; not membrane-bound; no histones.

• Plasmids: Small, extrachromosomal DNA; 5–100 genes; may encode antibiotic resistance, toxins; replicate independently; transferable.

Ribosomes

Ribosomes are the site of protein synthesis and are targeted by several antibiotics.

• Prokaryotic ribosome: 70S (30S + 50S subunits).

• Antibiotics: Streptomycin, gentamicin, erythromycin, chloramphenicol inhibit prokaryotic ribosomes.

• Svedberg unit (S): Measures sedimentation rate.

Inclusions

Inclusions are storage sites for nutrients and other substances.

• Metachromatic granules: Phosphate reserves.

• Polysaccharide granules: Energy reserves.

• Lipid inclusions: Energy reserves.

• Sulfur granules: Energy reserves.

• Carboxysomes: RuBisCO enzyme for CO2 fixation.

• Gas vacuoles: Buoyancy.

• Magnetosomes: Iron oxide; destroy H2O2.

Endospores

Endospores are highly resistant, dormant structures formed by certain bacteria for survival.

• Produced by: Bacillus, Clostridium.

• Resistant to: Desiccation, heat, chemicals, radiation.

• Process: Sporulation (formation), germination (return to vegetative state).

Eukaryotic Cell Structures & Functions

Flagella and Cilia

Eukaryotic flagella and cilia are projections used for movement and are structurally distinct from prokaryotic flagella.

• Flagella: Long, few; cilia: short, numerous.

• Structure: Microtubules (tubulin); 9+2 array.

• Movement: Wavelike.

Cell Wall and Glycocalyx

Eukaryotic cell walls and glycocalyx provide structural support and facilitate cell interactions.

• Cell wall: Plants (cellulose), fungi (chitin), yeasts (glucan, mannan).

• Glycocalyx: Carbohydrates bonded to proteins/lipids; strengthens surface, aids attachment, cell recognition.

Plasma (Cytoplasmic) Membrane

The eukaryotic plasma membrane is similar to prokaryotes but contains sterols and carbohydrates for additional functions.

• Structure: Phospholipid bilayer; integral/peripheral proteins; sterols; carbohydrates.

• Function: Simple diffusion, selective permeability, endocytosis (phagocytosis, pinocytosis, receptor-mediated).

Cytoplasm and Cytoskeleton

The cytoplasm contains organelles and a cytoskeleton for structural support and intracellular movement.

• Cytosol: Fluid portion.

• Cytoskeleton: Microfilaments, intermediate filaments, microtubules.

• Cytoplasmic streaming: Movement of cytoplasm.

Ribosomes

• 80S: Large (60S) and small (40S) subunits; membrane-bound (ER) or free (cytoplasm).

• 70S: In chloroplasts and mitochondria.

Nucleus

The nucleus is the control center of the cell, containing genetic material.

• Structure: Double membrane (nuclear envelope).

• DNA: Complexed with histones (chromatin); condenses to chromosomes during division.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; protein synthesis.

• Smooth ER: No ribosomes; synthesizes membranes, fats, hormones.

Golgi Complex

• Function: Modifies, sorts, and transports proteins from ER via vesicles.

Organelles

• Lysosomes: Digestive enzymes; formed in Golgi.

• Vacuoles: Storage, shape, formed by Golgi or endocytosis.

• Mitochondria: Double membrane; cristae and matrix; ATP production; 70S ribosomes; circular DNA; self-replicating.

• Chloroplasts: Photosynthesis; thylakoids with chlorophyll; 70S ribosomes; circular DNA.

• Peroxisomes: Oxidize fatty acids; destroy H2O2.

• Centrosomes: Pericentriolar matrix and centrioles; organize mitotic spindle; cell division.

The Evolution of Eukaryotes

Endosymbiotic Theory

The endosymbiotic theory explains the origin of eukaryotic cells from prokaryotic ancestors.

• Process: Larger bacterial cells engulfed smaller ones, forming eukaryotes.

• Nucleus: Formed from plasma membrane infolding.

• Chloroplasts: From ingested photosynthetic bacteria.

• Mitochondria: From ingested aerobic bacteria.

Additional info: The endosymbiotic theory is widely accepted and explains the presence of prokaryotic features in mitochondria and chloroplasts.