Functional Anatomy of Prokaryotic and Eukaryotic Cells: Microbiology Study Guide

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Comparing Prokaryotic and Eukaryotic Cells

This section introduces the fundamental differences between prokaryotic and eukaryotic cells, which are central to understanding microbial life.

• Prokaryotes: Cells lacking a true nucleus; genetic material is not enclosed in a membrane.

• Eukaryotes: Cells with a true nucleus; genetic material is enclosed within a nuclear membrane.

• Key Differences:

  • Prokaryotes: Usually one circular chromosome, no histones, no membrane-bound organelles, cell walls (peptidoglycan in bacteria, pseudomurein in archaea), divide by binary fission.

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

• Shapes: Bacillus (rod-shaped), Coccus (spherical), Spiral (vibrio, spirillum, spirochete), Star-shaped, Rectangular.

• Arrangement: Pairs (diplococci, diplobacilli), Clusters (staphylococci), Chains (streptococci, streptobacilli), Groups of four (tetrads), Cubelike groups of eight (sarcinae).

• Monomorphic: Most bacteria have a single shape.

• Pleomorphic: Some bacteria can have multiple shapes.

Structure of a Prokaryotic Cell

Prokaryotic cells have distinct structural features that contribute to their function and survival.

• Cell wall: Provides shape and protection.

• Plasma membrane: Controls entry and exit of substances.

• Cytoplasm: Contains DNA, ribosomes, and inclusions.

• External structures: Capsule, flagella, fimbriae, pili.

Structures External to the Cell Wall

External structures play key roles in protection, motility, and adherence.

Glycocalyx

• Definition: Viscous, gelatinous layer external to cell wall; made of polysaccharide and/or polypeptide.

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

• Functions: Contributes to virulence, prevents phagocytosis, helps microbes adhere to surfaces, forms biofilms.

• Examples: Bacillus anthracis, Streptococcus pneumoniae, Klebsiella pneumoniae, Streptococcus mutans, Vibrio cholerae.

Flagella

• Definition: Filamentous appendages for motility; made of flagellin protein.

• Structure: Filament (outermost), Hook (attaches filament), Basal body (anchors flagellum).

• Function: Movement toward/away from stimuli (taxis); "run" and "tumble" motion.

• Flagella proteins: H antigens used for serovar identification (e.g., E. coli O157:H7).

Axial Filaments and Archaella

• Axial filaments: Endoflagella found in spirochetes; rotation causes corkscrew movement.

• Archaella: Motility structures in archaea; made of archaellins, rotate like flagella, use ATP for energy.

Fimbriae and Pili

• Fimbriae: Hairlike appendages for attachment; involved in biofilm formation and adherence to surfaces (e.g., Neisseria gonorrhoeae, E. coli O157).

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

The Cell Wall

The cell wall is essential for maintaining cell shape, protecting against osmotic lysis, and contributing to pathogenicity.

• Composition: Peptidoglycan in bacteria; pseudomurein in archaea.

• Function: Prevents osmotic lysis, protects membrane, site of antibiotic action.

• Peptidoglycan: Polymer of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by polypeptides, forming a lattice.

• Penicillin: Interferes with peptide cross-bridges, weakening cell wall.

Gram-Positive vs. Gram-Negative Cell Walls

• Gram-Positive: Thick peptidoglycan, teichoic acids, high susceptibility to penicillin, disrupted by lysozyme, two rings in basal body of flagella, produce exotoxins.

• Gram-Negative: Thin peptidoglycan, outer membrane (lipopolysaccharide, lipoproteins, phospholipids), four rings in basal body of flagella, produce endotoxins and exotoxins, low susceptibility to penicillin.

• Gram Stain Mechanism: Crystal violet-iodine crystals retained in gram-positive (alcohol dehydrates peptidoglycan), washed out in gram-negative (alcohol dissolves outer membrane).

Feature	Gram-Positive	Gram-Negative
Peptidoglycan	Thick	Thin
Teichoic acids	Present	Absent
Outer membrane	Absent	Present
Flagella basal body	2 rings	4 rings
Toxins	Exotoxins	Endotoxins & Exotoxins
Penicillin susceptibility	High	Low

Atypical Cell Walls

• Acid-fast cell walls: Similar to gram-positive, thick peptidoglycan, waxy mycolic acid, stain with carbolfuchsin, genera: Mycobacterium, Nocardia.

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

• Archaea: Wall-less or walls of pseudomurein (lack NAM and D-amino acids).

Damage to the Cell Wall

• Lysozyme: Hydrolyzes bonds in peptidoglycan, weakens cell wall.

• Penicillin: Inhibits peptide bridge formation.

• Protoplast: Wall-less gram-positive cell.

• Spheroplast: Wall-less gram-negative cell.

• L forms: Wall-less cells, irregular shapes, susceptible to osmotic lysis.

Structures Internal to the Cell Wall

Internal structures are vital for cell function, genetic information, and survival.

The Plasma (Cytoplasmic) Membrane

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

• Fluid mosaic model: Membrane is flexible, proteins and lipids move freely.

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

Movement of Materials Across Membranes

• Passive processes: No energy required; substances move from high to low concentration.

• Active processes: Energy required; substances move from low to high concentration.

Passive Processes

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

• Facilitated diffusion: Integral proteins act as channels/carriers; movement with concentration gradient.

• Osmosis: Net movement of water across a selectively permeable membrane from high to low water concentration.

• Isotonic solution: Equal solute concentrations; no net water movement.

• Hypotonic solution: Lower solute outside; water moves into cell.

• Hypertonic solution: Higher solute outside; water moves out of cell.

Active Processes

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

Cytoplasm

• Definition: Thick, aqueous, elastic substance inside plasma membrane; 80% water plus proteins, carbohydrates, lipids, ions.

• Includes: DNA (nucleoid), ribosomes, inclusions, cytoskeleton (cell division, shape, growth, DNA movement).

The Nucleoid

• Bacterial chromosome: Circular, double-stranded DNA; not enclosed in nuclear envelope.

• Plasmids: Small, extrachromosomal DNA circles; carry noncrucial genes (antibiotic resistance, toxin production), replicate independently, transferable.

Ribosomes

• Function: Sites of protein synthesis.

• Structure: Made of protein and ribosomal RNA; 70S type in prokaryotes.

• Antibiotics: Streptomycin, gentamicin, erythromycin, chloramphenicol interfere with prokaryotic ribosomal function.

Inclusions

• Function: Reserve deposits of nutrients.

• Types: Metachromatic granules (phosphate), polysaccharide granules (energy), lipid inclusions (energy), sulfur granules (energy), carboxysomes (RuBisCO enzyme for photosynthesis), gas vacuoles (buoyancy), magnetosomes (iron oxide).

Endospores

• Definition: Resting cells produced when nutrients are depleted; highly resistant to desiccation, heat, chemicals, radiation.

• Produced by: Bacillus and Clostridium genera.

• Sporulation: Endospore formation.

• Germination: Endospore returns to vegetative state.

• Importance: Survival mechanism, not reproductive; significant in food industry.

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