Makindo Medical Notes

🧱 Structure of the Cell Membrane

• The cell membrane (plasma membrane) is a thin, flexible barrier that surrounds the cell. It maintains cell integrity and regulates the movement of substances in and out. Its design is best explained by the fluid mosaic model 🧩, which highlights its dynamic and flexible nature.
• Phospholipid Bilayer: The foundation is a double layer of phospholipids. 🟡 Hydrophilic heads face outward (towards extracellular fluid and cytoplasm). ⚫ Hydrophobic tails face inward, away from water.
• Membrane Proteins: Essential for transport, signalling, and anchoring.
  • Integral proteins: Span the entire bilayer; act as channels, carriers, or receptors.
  • Peripheral proteins: Loosely attached on the surface; support structure and signalling.
• Cholesterol: 🧴 Scattered throughout the bilayer, it adds rigidity while maintaining fluidity — preventing the membrane from becoming too rigid (at low temps) or too fluid (at high temps).
• Carbohydrate Chains: 🍬 Attached to proteins (glycoproteins) or lipids (glycolipids) on the outer surface. They function in cell recognition, immune defence, and intercellular communication.
• Fluid Mosaic Model: Emphasises that components (lipids, proteins, carbohydrates) move fluidly within the bilayer — giving flexibility, adaptability, and resilience to the membrane.

📌 Teaching Pearl: The cell membrane is not just a “barrier” — it’s an active communication hub, essential for signalling, nutrient uptake, and maintaining ion gradients ⚡.

🌊 Diffusion

Diffusion = random movement of molecules from an area of high concentration ➝ low concentration, driven by kinetic energy. In cells, diffusion allows gases, nutrients, and ions to move across membranes without energy input.

• Simple Diffusion: Passive movement through the lipid bilayer (if lipid-soluble, e.g. O₂, CO₂) or via water-filled channels in proteins. Rate depends on substance concentration, molecular motion, and available openings.
• Facilitated Diffusion: Requires carrier proteins 🚪. The molecule binds to a transporter, which changes shape to shuttle it across (e.g. glucose transport via GLUT proteins).
• Key Point: Both simple and facilitated diffusion are passive processes — no ATP is needed ⚡.

⚡ Intracellular vs Extracellular Electrolyte Concentrations

💡 Clinical Insight: The steep Na⁺ and K⁺ gradients are maintained by the sodium–potassium pump (Na⁺/K⁺-ATPase). This pump is vital for nerve conduction, muscle contraction, and maintaining cell volume. Disturbances in these gradients (e.g. hypokalaemia, hyponatraemia) ➝ severe clinical consequences.