The plasma membrane is the edge of life, the boundary that separates the living cell from its surroundings. The plasma membrane not only defines the borders of the cell, but also allows the cell to interact with its environment in a controlled way. It controls traffic into and out of the cell it surrounds. Like all biological membranes, the plasma membrane exhibits selective permeability; that is, it allows some substances to cross it more easily than others. In addition, they must able to communicate with other cells, identifying themselves and sharing information. One of the earliest episodes in the evolution of life may have been the formation of a membrane that enclosed a solution different from the surrounding solution while still permitting the uptake of nutrients and elimination of waste products. The ability of the cell to discriminate in its chemical exchanges with its environment is fundamental to life, and it is the plasma membrane and its component molecules that make this selectivity possible. The molecular makeup of each membrane allows compartmentalized specialization in cells.
In addition to an external cell membrane (called the plasma membrane), eukaryotic cells also contain internal membranes that form the boundaries of organelles such as mitochondria, chloroplasts, peroxisomes, and lysosomes. Functional specialization in the course of evolution has been closely linked to the formation of such compartments. Specific systems have evolved to allow targeting of selected proteins into or through particular internal membranes and, hence, into specific organelles.
Membranes are dynamic structures in which proteins float in a sea of lipids. The lipid components of the membrane form the permeability barrier, and protein components act as a transport system of pumps and channels that endow the membrane with selective permeability.