The process of moving sodium and potassium ions across the cell membrance is an active transport process involving the hydrolysis of ATP to provide the necessary energy. It involves an enzyme referred to as Na+/K+-ATPase. This process is responsible for maintaining the large excessof Na+ outside the cell and the large excess of K+ ions on the inside. A cycle of the transport process is sketched below. It accomplishes the transport of three Na+ to the outside of the cell and the transport of two K+ ions to the inside. This unbalanced charge transfer contributes to the separation of charge across the membrane. The sodium-potassium pump is an important contributer to action potential produced by nerve cells. This pump is called a P-type ion pump because the ATP interactions phosphorylates the transport protein and causes a change in its conformation.