ABSTRACT
The cell membrane is the essential barrier between the cell and its surroundings, keeping unwanted molecules away and preventing proteins, metabolites, and ions from leaking out. Yet, a completely impenetrable membrane would quickly strangle the cell. The membrane must be permeable to all kinds of molecules, large and small, but the permeability must be precisely regulated. Equally important, the cell must be able to transport molecules against their concentration gradient in order to extract nutrients from the environment or pump out toxic compounds. Transport processes of these kinds are carried out by specic classes of membrane proteins that act as channels (mediating passive diffusion across the membrane) or transporters (mediating active transport against a concentration gradient).
