Na-K membrane ion pump. Illustration of the Na-K-ATPase pump (red) transporting ions across a cell membrane (light blue). At left, the pump binds to three sodium ions (dark blue) in its E1 shape. Hydrolysis of ATP (adenosine triphosphate) produces ADP (adenosine diphosphate) and Pi (inorganic phosphate, pink). The phosphate attaches to the pump (centre left) triggering its E2 shape to release the sodium outside the cell. Two potassium ions (yellow) bind to the pump (centre right). When the phosphate is released, the pump reverts to its E1 shape to release potassium inside the cell (right). For this artwork with labels, see image C023/8779.