Cell Membrane

The cell membrane (or plasma membrane) is the thin outer layer of the cell that differentiates the cell from its environment. In animals, the cell membrane establishes this separation alone, whereas in yeast, bacteria and plants, an additional cell wall forms the outermost boundary, providing primarily mechanical support.

As a semi-permeable barrier, the cell membrane maintains an essential balance between individual distinctness and communal interaction: it functions to retain key components of the cell and to keep out toxic or unwanted substances, while selectively controlling the flow of nutrients and biochemical signals into the cell.

The cell membrane is composed mainly of phospholipid and protein molecules arranged in organized but flexible sheets. The phospholipid components form a bilayer that contributes structural stability and creates the semi-permeable environment, while the proteins are responsible for most of the dynamic processes carried out by cell membranes, such as the transport of molecules into and out of the cell.

Transport across the cell membrane underlies a variety of physiological processes, from the beating of an animal’s heart to the opening of tiny pores in leaves that enables gas exchange with the environment. A major cellular manifestation of motor neuron disease is the inability of nerve cells to stimulate the opening of channels through the membranes of musclecells, which would result in normal muscle function.

The regulation of transport, though a crucial function of the cell membrane, is not its only role. Cell membranes assist in the organization of individual cells to form tissues. They are also involved in biological communication: the binding of a specific substance to the exterior of the membrane can initiate, modify, or turn off a cell function.