Biology Forums - Study Force

Describe the process of facilitated diffusion and the proteins involved in that process.

Facilitated diffusion describes a process whereby molecules that cannot move through the membrane are allowed to cross the membrane through membrane proteins called carriers. Carrier proteins have binding sites for specific molecules that allow them to selectively move those molecules across the membrane. However, those molecules will always move down their electrochemical gradient. Thus, these proteins can move molecules in either direction (in or out of cell). Once the molecule has attached to its binding site, the molecule goes through a conformational change that allows that molecule to be released on the other side of the membrane. Due to the presence of these binding sites, the carriers can be saturated as the concentration gradient is increased. Thus, at a high concentration, the carriers move the molecules across the membrane at their maximal rate of flux. The saturation kinetics of individual carriers will alter the rate of flux as will the number of active carriers that are present on the membrane. Since it is the electrochemical gradient of a molecule that drives facilitated diffusion, anything that affects the electrochemical driving force will alter the rates of ion movement through these proteins.