Molecular transport at cell membranes

It has become increasingly evident during the last decade that cell membranes are equipped with factors that play specific roles in conveying both inorganic and organic substances from the cell exterior to the cell interior or from one compartment in the cell to another. The normal diffusion processes (often called passive diffusion) occur but these may be slow, the rates depending on the concentrations of the substances involved. Passive diffusion leads only to a concentration within the cell no higher than that outside except under circumstances where binding of the substance, to a cell constituent, takes place in the cell or in a cell compartment. The components in the cell membrane that control the rates of ionic and molecular transport, and which may lead to the attainment of concentrations of ions and molecules in the cell substantially higher than those outside, may play roles as important as those of the enzymes in the control of cell growth, metabolism and function. The process whereby a substance is transferred across the cell membrane in such a manner that it is specifically controlled, that it can lead to accumulation of the substance in the cell against a concentration gradient, and that it is energy assisted, i. e. coupled with metabolic processes, is often referred to as active transport. The specific components located at the cell membrane and involved in the transport processes are usually termed transport carriers.