Pumps, also called transporters, are transmembrane proteins that actively move ions and/or solutes against a concentration or electrochemical gradient across biological membranes. Pumps generate a membrane potential by creating an electrochemical gradient across the membrane.

Ion pumps can be distinguished from ion channels on the basis that ion pumps actively transport ions against a concentration gradient, while ion channels allow ions to passively flow down a concentration gradient.

Pumps can be classified as either primary or secondary active transporters based on the method they use to move ions across the gradient.

Primary active transporters are usually transmembrane ATPases, that hydrolyse ATP to produce energy in order to transport ions up a concentration gradient.

Secondary active transporters, also known as co-transporters, pump ions against the concentration gradient by using the electrochemical gradient created across the membrane by pumping ions in or out of the cell. There are two types of secondary active transporters, which are classified based on the direction that they move ions.

Antiporters pump two different ions or solutes in opposite directions across the membrane. One moves with the concentration gradient (high to low) which powers the movement of the other against the gradient (low to high). Examples: CLCN3, NHE3.

Symporters pump two different ions or solutes in the same direction, moving one with the concentration gradient (high to low), and the other against the concentration gradient (low to high). Examples: KCC2, NCC, NIS, NKCC2.

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