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Biology · Glossary

What is The plasma membrane?

Also known as: plasma membrane · lipid bilayer · membrane protein · fluid mosaic model

Definition 7.1 University Biology — Year 1 · Chapter 7 — Membranes and Membrane Transport

The plasma membrane — and every membrane of the cell — is a lipid bilayer about 5 nm5\,\mathrm{nm} thick: two sheets of phospholipids (Chapter 9) with their hydrophobic tails facing each other and their polar heads facing the two aqueous sides, with cholesterol between the tails, and membrane proteins embedded in it or attached to it. In the fluid mosaic model (Singer and Nicolson, 1972) the bilayer is a two-dimensional fluid in which lipids and proteins diffuse laterally, and the proteins are a mosaic of independent units, not a continuous coat. The two faces differ: sugars attached to lipids and proteins face outward only (the glycocalyx), and some phospholipids are confined to one leaflet.

The fluid mosaic. Two leaflets of phospholipids, tails inward, with cholesterol among them; integral proteins spanning the bilayer (a channel with its pore, a carrier), a peripheral protein on one face, and sugar chains on the outer face only.
The fluid mosaic. Two leaflets of phospholipids, tails inward, with cholesterol among them; integral proteins spanning the bilayer (a channel with its pore, a carrier), a peripheral protein on one face, and sugar chains on the outer face only.
A red blood cell, a platelet and a lymphocyte under the scanning electron microscope (colourised). The red cell’s biconcave shape is held by a protein mesh under its membrane; it survives four months of squeezing through capillaries narrower than itself. Image: National Cancer Institute, public domain.
A red blood cell, a platelet and a lymphocyte under the scanning electron microscope (colourised). The red cell’s biconcave shape is held by a protein mesh under its membrane; it survives four months of squeezing through capillaries narrower than itself. Image: National Cancer Institute, public domain.

Examples

Example 7.17 (Cholesterol delivery)

Cholesterol travels in the blood inside protein-coated particles too large for any carrier. A cell needing cholesterol displays receptors for the particle; particle and receptor gather in a coated pit, which pinches off, and the vesicle delivers the particle to a lysosome where the cholesterol is freed. A person whose receptors are defective cannot clear the particles: blood cholesterol doubles and arteries clog in early adulthood. One membrane protein, one disease.

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