Animation showing the mechanism of action of the sodium-potassium pump, an essential membrane protein found in all animal cells. The pump is responsible for maintaining the correct level of ions in the cell. The cell interior is at bottom. Three sodium ions (Na+, yellow) enter the open pump, and a molecule of ATP (adenosine triphosphate) then phosphorylates the pump, losing a phosphate group (P) to it, and becoming ADP (adenosine diphosphate). The phosphorylation changes the structure of the pump, causing it to open outwards and expel the sodium ions. Two potassium ions (K+, green) from the outside then bind to the pump, which dephosphorylates it, converting back to its starting form and moving the potassium ions into the cell. The pump is then ready for three more sodium ions.