a membrane-bound transporter found in nearly all mammalian cells that transports potassium ions into the cytoplasm from the extracellular fluid while simultaneously transporting sodium ions out of the cytoplasm to the extracellular fluid. The pump transports both types of ions against large electrochemical potential gradients and maintains a much higher concentration of potassium (and a much lower concentration of sodium) in the cytoplasm than exists outside the cell. The pump is an enzyme that transports two potassium ions in exchange for three sodium ions in a reaction driven by hydrolysis of one molecule of ATP to form ATP plus one inorganic phosphate ion. SYN: sodium-potassium ATPase.