Found in the membranes of all animal cells, the sodium-potassium pump is a protein complex used to transport potassium ions (K⁺) into the cell and sodium ions (Na⁺) out of the cell. For every three Na⁺ the pump sends out of the cell, two K⁺ are pushed in. This activity results in the net loss of positive charges within the cell and sets up a concentration gradient of both ions.

Because the ions must be transported against their natural tendencies to equalize the concentration inside and outside the cell, the pump is an active process, requiring the hydrolysis of one molecule of adenosine triphosphate (ATP) for every cycle. This hydrolysis releases free energy, which is then used to transport the ions.

The pump is composed of two proteins: large and small subunits. The large subunit spans the cell membrane and is the catalytic subunit. This protein...