plasmamembrans

The plasma membrane, also called the cell membrane, is the flexible boundary that encloses the cytoplasm of a cell. It separates the interior from the extracellular environment and regulates the exchange of substances. The structure is commonly described by the fluid mosaic model: a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrate moieties, allowing lateral movement of components and self-sealing properties.

Lipids form the basic matrix: amphipathic phospholipids arrange into a bilayer with hydrophilic heads facing outward and hydrophobic tails inward. Cholesterol modulates fluidity and membrane thickness; glycolipids and glycoproteins occur on the extracellular surface, contributing to cell recognition and protection. The membrane is asymmetric, with different lipid compositions on each side.

Proteins play diverse roles: integral or transmembrane proteins function as channels, transporters, receptors, and enzymes; peripheral proteins associate with the membrane surface. Carbohydrates attached to proteins or lipids form the glycocalyx, aiding cell–cell recognition and adhesion. Some proteins anchor the cytoskeleton, helping shape and stabilize the membrane.

Functions include forming a selective barrier that controls diffusion and transport, generating and maintaining the cell’s membrane potential, and enabling communication with the external environment. Transport occurs via passive diffusion, facilitated diffusion through channels and carriers, and active transport. The membrane also participates in endocytosis and exocytosis for material uptake and secretion.