Active Transport

Active transport is the transport of substances through the cell membrane by means of an energy-consuming transport system (transport ATPase). Here, again, ATP serves as universal fuel. Such a transport process can move a substance through the membrane against a concentration gradient. Thus cells have the ability to maintain in their interior stable ion concentrations, for example, that are clearly different from their concentrations in the extracellular fluid. These active transport processes are served by specialized proteins in the cell membrane that can move several ions simultaneously. In this process, the coupled transport of substances can occur in the same direction (cotransport) or in opposite directions (countertransport). For instance, in the kidney the transport of amino acids is coupled with an active Na+ transport. Additionally, active ion transport through cell membranes is necessary for the formation of  membrane or resting potentials.