🧫 The cell membrane (plasma membrane) is a dynamic, selectively permeable barrier that surrounds every cell. It regulates movement of substances in/out, preserves cellular integrity, and enables communication and recognition between cells. Think of it as the "security system + interface" of the cell. 🔑

🧬 Structure

Phospholipid Bilayer: Foundation of the membrane.
- 💧 Hydrophilic heads face outward (towards aqueous environment).
- 💨 Hydrophobic tails face inward, forming a nonpolar barrier that blocks most polar molecules.
Proteins: Essential for function and transport.
- 🔗 Integral proteins: Span the bilayer, acting as channels, pumps, and receptors.
- ⚙️ Peripheral proteins: Loosely attached, providing support, anchorage, and signalling roles.
Cholesterol: 🧴 Inserts between phospholipids.
- Stabilises membrane fluidity — prevents rigidity in cold and excess fluidity in heat.
Carbohydrates (Glycocalyx): 🍬 Found as glycoproteins & glycolipids on outer surface.
- Enable cell recognition (immune self vs non-self).
- Protects membrane, aids adhesion, and mediates immune response.

⚡ Functions

Selective Permeability: 🚪 Controls entry/exit of molecules, blocking harmful agents while allowing nutrients and ions.
Transport Mechanisms:
- 🌬️ Passive: Diffusion & facilitated diffusion → down gradient, no ATP needed.
- ⚡ Active: Requires ATP → pumps ions/molecules against gradient (e.g. Na⁺/K⁺ ATPase).
Cell Communication: 📡 Surface receptors bind hormones, neurotransmitters → trigger signalling cascades.
Cell Recognition: 🧾 Glycocalyx tags identify cell type → critical for immunity, development, and tissue organisation.
Structural Support: 🏗️ Anchors to cytoskeleton → maintains shape, stabilises cell–cell junctions.

🩺 Clinical Relevance

🫁 Cystic Fibrosis: Defective CFTR channel → abnormal Cl⁻ transport → thick mucus, lung infections.
🩸 Diabetes: Faulty insulin receptor signalling → impaired glucose uptake.
🎯 Cancer: Mutations in membrane receptors (e.g. EGFR) → uncontrolled growth signalling.
🛡️ Immune defence: Glycocalyx enables immune system to distinguish self vs pathogen.

📌 Summary

🔑 The cell membrane is more than a barrier — it’s an active, intelligent system. It preserves cell integrity, regulates exchange, and mediates communication, adhesion, and recognition. Disruption of its structure or signalling underlies major diseases, from cystic fibrosis to cancer.

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The Cell membrane

🧬 Structure

⚡ Functions

🩺 Clinical Relevance

📌 Summary

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