The partially permeable membrane forming the boundary of a cell. It consists mostly of protein and lipid (see lipid bilayer; fluid mosaic model) and plays various crucial roles in the cell’s activities. A key task is to regulate the flow of materials into and out of the cell; this selectivity of traffic is accomplished, for example, by membrane proteins that act as ion channels or transport proteins. Other membrane proteins are receptors for signal molecules (e.g. hormones, growth factors, cytokines) arriving at the cell surface; they relay the signal to other components inside the cell. Supported by the cell’s internal cytoskeleton, the plasma membrane is the site of junctions with neighbouring cells (see cell junction) and forms attachments to the extracellular matrix, thus ensuring tissue integrity. In plants, fungi, bacteria, and many protists, it helps in assembling a cell wall or capsule on its outer surface. Compare nuclear envelope.