Key Points: Plasma Membrane and Cell Wall
Fluid Mosaic Model (Singer & Nicolson, 1972): The plasma membrane is a phospholipid bilayer in which the hydrophilic heads face outward (toward aqueous environments) and the hydrophobic tails face inward. Proteins are embedded in the bilayer or attached to its surface, forming a mosaic pattern. The bilayer is fluid — phospholipids and many proteins move laterally.
Membrane Components:
- Phospholipids: amphipathic molecules forming the bilayer
- Integral (intrinsic) proteins: span the full bilayer; serve as channels, carriers, receptors
- Peripheral (extrinsic) proteins: attached to the surface; serve as enzymes, structural support
- Cholesterol: found only in animal cell membranes; regulates fluidity (stabilizes at high temp, fluidizes at low temp)
- Glycoproteins + Glycolipids: form glycocalyx on outer surface; cell recognition, adhesion, immune response
Cell Wall:
- Bacteria: Peptidoglycan (murein) — rigid, protects against osmotic lysis; target of penicillin
- Plants: Cellulose (primary wall) + Calcium pectate (middle lamella) + Lignin (secondary wall, if present)
- Fungi: Chitin (N-acetylglucosamine polymer; also in arthropod exoskeletons)
- Animals: No cell wall; plasma membrane is the outermost boundary
Middle Lamella: Composed of calcium pectate. Shared between adjacent plant cells. Formed first during cell division (from the cell plate). Acts as cement between cells. Distinct from cellulose cell wall.
Key NEET Points:
- Fluid Mosaic Model = Singer + Nicolson + 1972 (memorize all three)
- Middle lamella = calcium pectate (NOT cellulose — frequent error)
- Cholesterol = animal cells only (plants use phytosterols)
- Membrane is selectively permeable — not freely permeable