In facilitated diffusion a carrier protein or channel protein is used as a passageway for molecules entering or leaving the cell. The carrier protein only allows a certain type of molecule to bind to it (such as a specific sugar molecule). The molecule is bound to the carrier protein and moved down the channel to the other side of the membrane. The carrier proteins change shape so that can happen. In a channel protein only a certain molecule or ion may use a specific protein, just like the carrier protein. The difference between the carrier and channel, is in the channel protein the protein does not have to change shape to accomodate the ion, the ions are allowed to move right through. This activity does not require energy but the molecules must move down the concentration gradient.