Active transport requires energy because it is pumping particles, such as proteins, ions and sugar molecules, against a concentration gradient: from areas of lower to higher solute concentration. The main energy source for active transport is adenosine triphosphate, or ATP, although electrochemical and light energy are also used.
The process of transporting biochemicals along a concentration gradient is classified into two: active and passive transport. Passive transport moves with the concentration gradient, while active transport works against it. The two types of active transport are primary and secondary active transport. Primary active transport utilizes the energy released from ATP hydrolysis, as well as photon energy and energy harvested from electrochemical gradients. Secondary transport uses the energy produced from forcing ions out of the cell.