in cellular physiology, a protein that has been identified in many cells that maintains the internal concentration of potassium ions 【K⁺】 higher than that in the surrounding medium (blood, body fluid, water) and maintains the internal concentration of sodium ions 【Na⁺】 lower than that of the surrounding medium. The pump, which has adenosine-triphosphatase (ATPase) activity, traverses the cell membrane and is activated by external 【K⁺】 and internal 【Na⁺】. This enzyme uses metabolic energy to transport (pump) Na⁺ outward and K⁺ inward. The resting potential of cells and related bioelectric phenomena such as the action potential depend on the steady state difference in concentrations of Na⁺ and K⁺ maintained by the pump. Other ion pumps, transporting different ions, have also been identified.