Membrane Structure and Composition
Core concepts
Every membrane is a fluid mosaic: a phospholipid bilayer studded with proteins, stiffened by cholesterol in animal cells, and asymmetrically decorated with carbohydrates on the extracellular face. Know the amphipathic logic - hydrophilic heads face water, hydrophobic tails face each other - because nearly every question about membrane behavior traces back to it. Saturated tails pack tightly and lower fluidity; unsaturated tails kink and raise it. Cholesterol is the classic trap: at body temperature it decreases fluidity by restricting tail motion, but at low temperature it prevents tight packing and thus increases fluidity. It is a buffer, not a one-way modifier.
Concepts to name
- Phospholipid, glycolipid, glycoprotein, integral vs peripheral protein
- Fluid mosaic model; membrane asymmetry (leaflet composition differs)
- Saturated vs unsaturated fatty acids; cholesterol as fluidity buffer
- Selective permeability as an emergent property of the bilayer
Traps
- Assuming cholesterol always increases or always decreases fluidity - it does both depending on temperature.
- Confusing integral (embedded, often transmembrane) with peripheral (surface-associated) proteins.
- Forgetting asymmetry: carbohydrates are almost always on the outside; the inner leaflet differs in lipid composition.
- Treating the membrane as static - proteins and lipids diffuse laterally.
Worked example / rule of thumb
Rule: "Tails set the baseline, cholesterol tunes it." If a question asks how adding unsaturated fatty acids changes a membrane, answer more fluid (kinks prevent tight packing). If it asks about cholesterol at high temperature, answer less fluid; at low temperature, more fluid. Anchor every fluidity answer to tail packing and temperature.