The sodium-potassium pump (also called the sodium-potassium ATPase) is an ion pump in the membranes of cells that pumps sodium out as it pumps potassium into the cell (Figure 10.25). The energy source for the pumping action is the hydrolysis of ATP. The sodium-potassium pump is important in maintaining the osmotic balance of the cell. It can be inhibited by the glycoside called ouabain.