lipid rafts

Meaning: Lipid rafts are small, dynamic patches of the plasma membrane enriched in cholesterol and sphingolipids such as sphingomyelin. Their long, mostly saturated tails pack tightly against rigid cholesterol, so a raft is thicker and more ordered than the surrounding glycerophospholipid bilayer, which has more kinked unsaturated tails. Rafts are thought to concentrate certain proteins, such as GPI-anchored proteins and some receptors, so they serve as platforms for signaling and sorting.

On the MCAT: A passage may describe a detergent-resistant membrane fraction or a receptor that clusters in cholesterol-rich regions. Typical questions: what happens to the protein's clustering or signaling if cholesterol is removed (rafts break up, the protein spreads out), or whether lateral diffusion is faster or slower in a raft (slower). Rafts show why the fluid mosaic model is not uniform across the membrane.

Common confusion: Rafts are not vesicles or organelles. They are regions within one continuous bilayer, and lipids and proteins can move in and out. "Ordered" does not mean solid; a raft is still fluid-like, just less fluid than its surroundings. Their size and lifetime are debated, so textbooks present them as a model.