The movement of a substance across a biological membrane, such as living cells, against a concentration (diffusion) gradient with the help of metabolic energy usually provided by ATP (adenosine triphosphate). Active transport serves to maintain the normal balance of ions in cells and, in particular, those of sodium and potassium ions, which play a vital role in nerve and muscle cells. Since a molecule is “pumped” across the membrane against its gradient, requiring the use of metabolic energy, it is referred to as “active” transport. The sodium-potassium “pump,” which exchanges sodium (Na⁺) for potassium (K⁺) across the plasma membrane of animal cells, is an example of the active transport mechanism. The carriage of a solute across a biological membrane from low to high concentration requires the expenditure of metabolic energy.