Active transport is a cellular process of moving molecules across a cellular membrane (into or outside a cell) with the use of cellular energy. Active transport is further divided into primary active transport and secondary active transport. Primary active transport is carried out by the enzyme transmembrane ATPase with the use of ATP as the source of energy. An example of primary active transport in human physiology is the sodium/potassium pump that maintains membrane potential by pumping three ions of sodium out and two ions of potassium into the cell. Secondary active transport uses electrochemical potential as the source of energy to drive the movement of a molecule across a cellular membrane. In this form of active transport, both molecules being transported may move either in the same direction or opposite directions. When they move in the same direction, the process is known as symport, and when they move in opposite directions, it is known as antiport.