Membrane fluidity, cholesterol and vesiclesSpec B2.1.11–B2.1.13
In short
Membrane fluidity depends on the fatty acids in the bilayer: unsaturated fatty acids have lower melting points and keep membranes fluid and flexible, while saturated fatty acids make membranes stronger at high temperatures. Cholesterol modulates fluidity in animal cells. Fluidity lets membranes form vesicles in endocytosis and fuse with vesicles in exocytosis.
Fatty acids and fluidity
Unsaturated fatty acids have one or more cis double bonds, which put kinks in the hydrocarbon chain. The chains cannot pack closely, so they have lower melting points and the membrane stays fluid, and therefore flexible, at the temperatures the cell experiences. Saturated fatty acids have straight chains that pack tightly. They have higher melting points and make membranes stronger at higher temperatures.
Organisms adapt membrane composition to their habitat. Fish adapted to the near-freezing waters around Antarctica have a higher proportion of unsaturated fatty acids in their membrane phospholipids than related fish from warmer waters, so their membranes do not become too rigid in the cold. Bacteria such as Escherichia coli increase the proportion of unsaturated fatty acids in their membranes when they grow at lower temperatures, and the proportion of saturated fatty acids when they grow at higher temperatures.
Cholesterol
In animal cell membranes, cholesterol sits within the bilayer. Its hydroxyl (–OH) group lies among the phosphate heads, and its rigid steroid rings and hydrocarbon tail lie among the fatty acid tails. Cholesterol acts as a modulator (adjustor) of membrane fluidity.
- At higher temperatures it restricts the movement of the phospholipid tails, stabilising the membrane.
- At lower temperatures it stops the tails packing closely together, preventing the membrane from stiffening.
Vesicles: endocytosis and exocytosis
Because the bilayer is fluid, membranes can bend, pinch off and fuse with each other, then reseal without leaking. Endocytosis removes a small area of plasma membrane as a vesicle; exocytosis adds the vesicle membrane back to the plasma membrane.
- Endocytosis
- The plasma membrane folds inwards around material outside the cell and pinches off to form a vesicle inside the cell. Example: phagocytosis of bacteria by white blood cells.
- Exocytosis
- A vesicle moves to the plasma membrane, fuses with it and releases its contents outside the cell. Examples: secretion of digestive enzymes by pancreatic cells; release of neurotransmitter at a synapse.
Do not just write that cholesterol makes membranes less fluid. It reduces fluidity at high temperatures but prevents stiffening at low temperatures.
Written and checked against the IB Biology HL specification · Updated October 2026