The movement of substances through membranes in living cells, often against a concentration gradient: a process requiring metabolic energy. Organic molecules and inorganic ions are transported into and out of both cells and their organelles. The substance binds to a transport protein embedded in the membrane, which carries it through the membrane and releases it on the opposite side. Primary active transport uses energy from the hydrolysis of ATP directly, as in the sodium/potassium ATPase sodium pump. Secondary active transport is driven by the movement of ions down a pre-existing concentration gradient established by a separate energy-consuming mechanism, as in antiporters and symporters. Active transport serves chiefly to maintain the normal balance of ions in cells, especially the concentration gradients of sodium and potassium ions crucial to the activity of nerve and muscle cells. Compare facilitated diffusion.