The movement of molecules against a concentration gradient is called active transport. A concentration gradient occurs when the concentration of molecules or ions on one side of a permeable membrane is higher than on the other side. Molecules naturally move with the gradient; therefore, active transport requires enzymes and energy.
Animal cells maintain an ion concentration gradient by use of active transport. During active transport, the cell pumps two potassium ions into the cell, while pushing three sodium ions out. When the cell receives a signal, the active transport stops and the ions begin to move with the gradient. In nerve cells, active transport occurs until the nerve must send an impulse. When the active transport pump stops, rapid diffusion allows the impulse to occur almost instantly.