The sodium potassium pump (sodium potassium ATPase) is an enzyme found in the membrane of all animal cells. Its job is to move NA+ out of the cell and K+ in the cell against their concentration gradient. This allows the cell to keep a concentration gradient and generates a resting membrane potential. In order to do so, the pump needs energy and therefore it is ATP dependent. When the pump binds ATP, it can then bind 3 Na+ ions intracellularly. Hydrolysis of ATP causes the phosphorylation of the pump which now undergoes a conformational change and opens towards the exterior of the cell and releases the 3 Na+ ions. On the exterior, it binds 2 K+ ions. This causes a dephosphorylation and an opening of the enzyme towards the inside of the cell. The dephosphorylated form has now a greater affinity for Na+ so K+ is released. The pump binds ATP and the whole process starts again