Cell membrane

📌 Definition
Cell membrane = membrane separating cytoplasm from surroundings.
Also called Plasmalemma / Plasma membrane.
📌 Thickness
≈ 7.5 nm
📌 Light Microscopy (L/M)
❌ Not visible with H&E.
✅ Stains: PAS (red), Silver stain (brown).
📌 Electron Microscopy (E/M)
Shows detailed membrane structure between adjacent cells.
📌 Molecular Structure
Mainly lipids + proteins + carbohydrates.
🟠 Glycocalyx
Carbohydrate coating from glycoproteins + glycolipids.
📌 Functions:
1️⃣ Cell Adhesion – adhesion molecules for binding.
2️⃣ Antigens – includes MHC for immune recognition.
3️⃣ Cell Repulsion – negative charge → repel adjacent cells.
🟣 Functions of Cell Membrane
🖇 1. Passive Transport (No energy)
Simple diffusion: small nonpolar (O₂, N₂) & small polar (H₂O, CO₂, glycerol).
Facilitated diffusion: uses proteins.
Ion channels: specific ion passage.
Carrier proteins: change shape to transport.
🖇 2. Active Transport (Requires ATP)
Moves substances against gradient.
Examples:
Na⁺/K⁺ pump
Glucose transport (some types)
ABC transporters
🖇 3. Endocytosis (Intake of large molecules)
Phagocytosis: "cell eating" – engulfs solids.
Pinocytosis: "cell drinking" – engulfs fluids.
🖇 4. Exocytosis
Expulsion of materials outside the cell.
🖇 5. Receptor-Mediated Endocytosis
Specific receptors → selective uptake.
Explains selective action of drugs, hormones, bacteria, viruses.