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, histones present, membrane-bound organelles, 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-shaped)

  • Spiral (includes vibrio, spirillum, spirochete)

  • Star-shaped and rectangular (rare)

• Arrangements:

  • 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 a simple structure but contain specialized components for survival and function.

• 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

Glycocalyx

The glycocalyx is a viscous, gelatinous layer external to the cell wall, composed of polysaccharides and/or polypeptides.

• Capsule: Neatly organized and firmly attached; increases virulence by preventing phagocytosis and aiding adherence.

• Slime layer: Unorganized and loose; helps in biofilm formation.

• Function: Protects cells, aids in attachment, and contributes to pathogenicity.

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

Flagella

Flagella are filamentous appendages that provide motility to bacteria.

• Structure: Composed of filament, hook, and basal body.

• Function: Movement toward or away from stimuli (taxis); rotation causes "run" or "tumble".

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

Axial Filaments and Archaella

• Axial filaments: Also called endoflagella; found in spirochetes; rotation causes corkscrew movement.

• Archaella: Motility structures in archaea; composed of archaellins; rotate and use ATP for energy.

Fimbriae and Pili

• Fimbriae: Hairlike appendages for attachment; important 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 a critical structure for bacterial survival, providing shape, protection, and contributing to pathogenicity.

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

• Composition: Peptidoglycan (bacteria), pseudomurein (archaea), no cell wall (mycoplasmas).

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

• Penicillin: Inhibits peptide cross-bridge formation, 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 complex retained in gram-positive cells; alcohol decolorizes gram-negative cells, which are then stained with safranin.

Atypical Cell Walls

• Acid-fast cell walls: Thick peptidoglycan, mycolic acid (waxy lipid), stain with carbolfuchsin; genera include Mycobacterium and 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, weakening cell wall.

• Penicillin: Inhibits peptide bridge formation.

• Protoplast: Wall-less gram-positive cell.

• Spheroplast: Wall-less gram-negative cell.

• L forms: Wall-less cells that swell into irregular shapes; susceptible to osmotic lysis.

Structures Internal to the Cell Wall

The Plasma (Cytoplasmic) Membrane

The plasma membrane is a phospholipid bilayer that encloses the cytoplasm and regulates transport.

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

  • Simple diffusion: Movement of solute until equilibrium is reached.

  • Facilitated diffusion: Transporter proteins enable movement of ions or larger molecules.

  • Osmosis: Net movement of water across a selectively permeable membrane.

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

  • Active transport: Requires transporter protein and ATP.

Osmosis and Solutions

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

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

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

Cytoplasm

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

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

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

The Nucleoid

• Bacterial chromosome: Circular, double-stranded DNA; not membrane-bound.

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

Ribosomes

• Function: Protein synthesis.

• Composition: Protein and ribosomal RNA.

• 70S ribosomes: Targeted by antibiotics (streptomycin, gentamicin, erythromycin, chloramphenicol).

Inclusions

• Function: Reserve deposits of nutrients.

• Types:

  • Metachromatic granules (phosphate reserves)

  • Polysaccharide granules (energy reserves)

  • Lipid inclusions (energy reserves)

  • Sulfur granules (energy reserves)

  • Carboxysomes (contain RuBisCO for photosynthesis)

  • Gas vacuoles (buoyancy)

  • Magnetosomes (iron oxide inclusions)

Endospores

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

• Produced by: Bacillus and Clostridium genera.

• Sporulation: Formation of endospores.

• Germination: Return to vegetative state.

• Importance: Survival mechanism; significant in food industry.

Table: Comparative Characteristics of Gram-Positive and Gram-Negative Bacteria

This table summarizes the main differences between gram-positive and gram-negative bacteria.

Additional info: Table entries inferred from textbook context and standard microbiology knowledge.