fluidity

Meaning: Membrane fluidity describes how freely phospholipids and membrane proteins diffuse laterally in the bilayer, as in the fluid mosaic model. Tight packing of the hydrophobic tails lowers it. Four factors matter: 1. Temperature: warmer membranes are more fluid, and cooling can push a membrane toward a gel-like state. 2. Saturation: cis double bonds put kinks in the tails, which disrupt packing and raise fluidity. Saturated tails are straight and pack tightly. 3. Chain length: longer tails have more van der Waals contact, so fluidity is lower. 4. Cholesterol: at higher temperature it restrains the lipids and lowers fluidity. At low temperature it keeps tails from packing tightly and helps preserve fluidity.

On the MCAT: Passages often compare membranes from organisms in cold and warm environments, or from cells grown on different fatty acids. Predict that membranes with more unsaturated, shorter-chain lipids are more fluid, and that cells in the cold tend to make more of them. You may also read a fluorescence recovery after photobleaching (FRAP) experiment, where faster recovery of fluorescence means faster lateral diffusion and higher fluidity.

Common confusion: Students say cholesterol simply makes membranes more rigid or more fluid. It does either, depending on temperature. Also, lateral movement is fast, but flipping a lipid between leaflets is slow without a flippase.