Overview
This lecture covers the structure and function of prokaryotic and eukaryotic cells, focusing on bacterial morphology, cell wall differences, and key cellular processes important in microbiology.
Bacterial Cell Morphology
- Prokaryotic cells are simple in structure and have defined shapes: coccus (spherical), rod (cylindrical), vibrio (curved rod), spirillum (spiral), pleomorphic (variable shape).
- Bacterial cells may group in characteristic ways: pairs (e.g., Neisseria), chains (e.g., Streptococcus), and clusters (e.g., Staphylococcus).
- Some bacteria form multicellular structures like fruiting bodies (Myxobacteria) under stress or biofilms for surface adherence.
Prokaryotic Cell Structures
- Filamentous appendages: flagella (for motility) built from flagellin; movement called "taxis" (e.g., chemotaxis, phototaxis).
- Pili: hair-like projections for attachment (fimbriae) or DNA transfer (sex pili).
- Capsules and slime layers: polysaccharide coatings aiding in adherence and immune evasion.
- The cell wall made of peptidoglycan provides structural integrity and prevents osmotic lysis; mycoplasma lack a cell wall and are pleomorphic.
Gram-Positive vs. Gram-Negative Bacteria
- Gram-positive: thick peptidoglycan layer, no outer membrane (e.g., Bacillus, Clostridium, Staphylococcus, Streptococcus).
- Gram-negative: thin peptidoglycan layer, outer membrane with lipopolysaccharides; most other bacteria.
- Gram staining: crystal violet binds peptidoglycan, resulting in purple (positive) or pink (negative) coloring.
Plasma Membrane and Transport
- Plasma membrane is a phospholipid bilayer, selectively permeable to water, gases, and small hydrophobic molecules.
- Types of transport: simple diffusion, facilitated diffusion (via channels), osmosis (water), active transport (requires ATP).
- Bacterial plasma membrane is the site of the electron transport chain and secretion.
Effects of Solutions on Bacterial Cells
- Hypotonic: water enters, but cell wall prevents bursting; with lysozyme (no wall), cell lyses.
- Hypertonic: water exits, cell undergoes plasmolysis (shrinking inside the wall); without wall, cell visibly shrinks.
- Isotonic: no net water movement; cell unchanged.
Internal Structures of Prokaryotes
- Circular single chromosome in nucleoid (no nuclear membrane).
- Plasmids: small, extra DNA carrying non-essential genes (e.g., antibiotic resistance).
- Ribosomes (70S) for protein synthesis; different from eukaryotic (80S) ribosomes.
- Cytoskeleton for internal organization; storage granules without membranes.
- Endospores (Bacillus, Clostridium): dormant, resistant cells with dipicolinic acid for DNA protection.
Eukaryotic Cell Structure (Review)
- DNA enclosed in a nuclear envelope; membrane-bound organelles (mitochondria, ER, Golgi, lysosomes).
- Plasma membrane enables endocytosis (uptake) and exocytosis (release).
- Eukaryotic flagella are structurally distinct from prokaryotic flagella.
- Mitochondria and chloroplasts have circular DNA and 70S ribosomes, supporting the endosymbiotic theory.
- Lysosomes and peroxisomes digest and process cellular material.
Key Terms & Definitions
- Coccus — spherical bacterial shape.
- Rod (Bacillus) — cylindrical bacterial shape.
- Pleomorphic — variable shape, lacking a rigid wall.
- Flagella — whip-like structures for bacterial motility.
- Pili (Fimbriae) — hairlike appendages for attachment or DNA transfer.
- Capsule — polysaccharide outer layer for protection and adherence.
- Peptidoglycan — bacterial cell wall component.
- Gram-positive — thick peptidoglycan, stains purple.
- Gram-negative — thin peptidoglycan, outer membrane, stains pink.
- Osmosis — water movement across a membrane.
- Plasmolysis — cell membrane shrinking inside the wall.
- Endospore — dormant, resistant bacterial cell.
- Endosymbiotic theory — origin of mitochondria/chloroplasts from engulfed bacteria.
Action Items / Next Steps
- Memorize the four Gram-positive genera: Bacillus, Clostridium, Staphylococcus, Streptococcus.
- Review eukaryotic organelle functions if needed.
- Prepare for matching bacterial species with correct morphology and Gram reaction for the first exam.