Transporters and channels allow small molecules to cross the cell membrane. Molecules will spontaneously flow "downhill" from a region of high concentration to a region of low concentration, provided a pathway exists. Such movements are called passive, because they need no other driving force. If, for example, a solute is present at a higher concentration outside the cell than inside and an appropriate channel or transporter is present in the plasma membrane, the solute will move spontaneously across the membrane down its concentration gradient into the cell by passive transport (sometimes called facilitated diffusion), without expenditure of energy by its membrane transport protein. All channels and many transporters act as conduits for such passive transport.To move a solute against its concentration gradient, however, a membrane transport protein must do work: it has to drive the flow "uphill" by coupling it to some other process that provides energy. Transmembrane solute movement driven in this way is termed active transport, and it is carried out only by special types of transporters that can harness some energy source to the transport process. Because they drive the transport of solutes against their concentration gradient, many of these transporters are called pumps.