Na-K membrane ion pump. Illustration of how sodium (Na+) and potassium (K+) ions are pumped through a channel (red) in a cell membrane (blue). The cell exterior (top) has high sodium-low potassium concentrations. The cell interior (cytosol, bottom) has high potassium-low sodium concentrations. The Na-K-ATPase pump (red) transports 3 sodium ions out and 2 potassium ions in for every molecule of ATP (adenosine triphosphate) hydrolysed. This produces an electrochemical gradient. The ATP hydrolysis produces ADP (adenosine diphosphate) and Pi (inorganic phosphate, yellow). For this artwork without labels, see image C023/8778.