The system by which sodium ions (Na+) are actively transported out of the cell whilst potassium ions (K+) are actively transported into the cell, which maintains a chemical disequilibrium. The movement of ions is regulated by the transmembrane proteins found in the plasma membranes of virtually all animal cells. For every one molecule of ATP that is hydrolysed to release energy, two K+ and three Na+ molecules are actively transported across the membrane. This process ensures the resting membrane potential of the cell is maintained and requires energy in the form of ATP.
Initially, K+ moves out of the cell down a chemical gradient, through the K+ leak channel, resulting in a slight accumulation of positive charge outside the cell. This process is possible due to the high permeability of the lipid bilayer to K+. In conjunction with this process is the active transport of Na+ out of and K+ into the cell, through the sodium-potassium pump. Na+ would move into the cell in the absence of ATP because the concentration is 10-30 times higher in the extracelluar fluid than in the cell, therefore the ion wants to move down its chemical gradient. The presence of ATP, however, ensures it is actively transported into the cell. At the same time as Na+ is pulled out of the cell, K+ is pushed in against its concentration gradient.
This electrogenic process is essential for the regulation of cytosolic pH of the cell, and the cell devotes a third of its energy to the pump.