Facilitated diffusion

Many membrane transporters (all channel proteins and many carrier proteins) only allow molecules to cross the membrane from the side with the higher concentration to the side with the lower concentration, a process called facilitated diffusion. Such transport down a concentration gradient, or downhill, does not require energy and is therefore also known as passive transport. For molecules that carry a charge, the membrane potential also influences the rate and direction of transport. The difference in electrical potential and the difference in concentration across the membrane can be combined in a single driving force known as the electrochemical gradient. Typically, the inside of the cell is more negative compared to the outside. The transport of molecules against their electrochemical gradient requires energy and is known as active transport.