The process of active transport differs from diffusion in that molecules are transported away from thermodynamic equilibrium; hence, energy is required. This energy can come from the hydrolysis of ATP, from electron movement, or from light. The maintenance of electrochemical gradients in biologic systems is so important that it consumes perhaps 30\u201340% of the total energy expenditure in a cell. In general, cells maintain a low intracellular Na+ concentration and a high intracellular K+ concentration, along with a net negative electrical potential inside. The pump that maintains these gradients is an ATPase that is activated by Na+ and K+ (Na+-K+ATPase).\r\nThe ATPase is an integral membrane protein and requires phospholipids for activity. The ATPase has catalytic centers for both ATP and Na+ on the cytoplasmic side of the membrane, but the K+ binding site is located on the extracellular side of the membrane. Ouabain or digitalis inhibits this ATPase by binding to the extracellular domain. Inhibition of the ATPase by ouabain can be antagonized by extracellular K+.