IB Biology Topic B2.1.12: Cholesterol and Membrane Fluidity in Animal Cells Notes

Cholesterol Regulates Membrane Fluidity in Animal Cells

1. Cholesterol helps keep the cell membrane of eukaryotes stable and working.
2. It makes up 20 to 40% of the lipids in the plasma membrane and is a steroid, not a glyceride.
3. Its structure and position in the membrane let it regulate membrane fluidity as conditions change.

The Structure of Cholesterol Anchors It in the Bilayer

1. Hydrophobic region: most of the molecule is hydrophobic and sits among the hydrocarbon tails of the phospholipids in the membrane core.
2. Hydrophilic region: a single hydroxyl group (-OH) at one end lines up with the phosphate heads at the membrane surface.
3. Position: cholesterol sits between the phospholipids, with the hydroxyl group facing outward.

How Cholesterol Buffers Membrane Fluidity

1. Membranes sit in a liquid-ordered phase, where lipids are packed closely but can still move sideways.
2. Cholesterol holds the balance between fluidity and stability as temperature changes.
3. At high temperatures
   1. Cholesterol stabilises the membrane and stops it becoming too fluid.
   2. By limiting the sideways movement of phospholipids, it keeps the membrane from becoming too leaky.
4. At low temperatures
   1. Cholesterol keeps the fatty acid tails from packing solid, so the membrane stays flexible.
   2. This stops stiffening that could restrict cell movement and damage the cell.