Cell membrane structure, composition, and function

Definition and roles of the cell membrane

The cell membrane (plasma membrane) is a semipermeable biological barrier that separates and protects the interior of a cell from the extracellular environment. It is primarily a lipid bilayer, typically composed of phospholipids and glycolipids, with sterols (such as cholesterol in animals) interspersed to help maintain appropriate membrane fluidity across different temperatures. The membrane also contains proteins—integral (transmembrane) proteins that span the membrane and act as transporters or receptors, and peripheral proteins that associate with the membrane surface and can function as enzymes or signaling components. Beyond acting as a selective barrier, the cell membrane controls the movement of substances into and out of the cell based on selective permeability to ions and organic molecules. It also supports multiple cellular roles including cell adhesion, ion conductivity, and cell signaling, and provides an attachment surface for extracellular structures such as the cell wall and the glycocalyx (carbohydrate coat). In addition, the membrane is connected to the intracellular cytoskeleton, helping maintain cell shape and organization. In synthetic biology, cell membranes can even be artificially reassembled, and in laboratory settings membrane-like lipid vesicles (liposomes) are used to study membrane permeability and protein function.

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Semipermeability and selective transport across membranes

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