Fluid mosaic — what is fluid and what is not
The bilayer is a two-dimensional fluid: phospholipids diffuse laterally millions of times a second, while flipping across the bilayer is rare and requires flippase enzymes, because dragging a polar head group through the hydrophobic core is energetically costly. Fluidity falls as temperature drops, and two features resist that. Unsaturated fatty acid tails have cis double bonds that kink the chain and stop tails packing tightly, so membranes rich in them stay fluid when cold. Cholesterol is the one students misstate: it is a fluidity buffer, not a fluidiser — at warm temperatures it restrains movement, and at cool temperatures it wedges between tails and prevents solidification.