plasma membrane

Meaning: The plasma membrane separates a cell's cytoplasm from the extracellular fluid. In the fluid mosaic model it is a phospholipid bilayer: hydrophilic heads face the watery surroundings and hydrophobic tails face inward. Cholesterol helps regulate fluidity, and embedded proteins act as channels, carriers, pumps, receptors and anchors. Carbohydrates on the outer surface take part in cell recognition. The membrane is selectively permeable: small nonpolar molecules such as O₂ and CO₂ cross by simple diffusion, whereas ions and large polar molecules usually need membrane proteins.

On the MCAT: Passages ask you to decide whether a solute crosses by simple diffusion, facilitated diffusion or active transport, or to predict how temperature, unsaturated fatty acids or cholesterol change fluidity. In graphs, simple diffusion rises linearly with concentration gradient, while carrier-mediated transport levels off when the carriers saturate.

Common confusion: The plasma membrane is not the cell wall. Plants, fungi and bacteria have a wall outside the membrane, but the membrane is the layer that controls selective passage. Also, cholesterol does not simply make a membrane more fluid: it restrains phospholipid movement at warm temperatures and prevents tight packing at cool ones, so it buffers fluidity.