Facilitative transporters (also known as uniporters) are integral membrane proteins that mediate the transport of membrane-impermeant solutes down a concentration gradient across biological membranes. This process is referred to as facilitated diffusion (also known as facilitated transport or uniport). Facilitated diffusion is a form of passive transport and, therefore, the direction of transport is always down a concentration gradient. A prominent example of a facilitative transporter is the glucose transporter (GLUT) found in the plasma membrane of all body cells, which mediate the entry of glucose into cells across the plasma membrane. The transport cycle can be described by the alternating access model. The cartoon (animation) shown here is a simplified conceptual representation of the workings of a typical facilitative transporter. According to the alternating access model, the substrate binding site of the transport protein is alternately exposed to one or the other side of the membrane such that at no time is there an open and unrestricted permeation pathway through the transporter connecting the two fluid compartments separated by the membrane. It is postulated that binding of the substrate (transported molecule) to its binding site induces a transporter conformation referred to as the occluded state. The occluded state refers to a transporter conformation in which the substrate binding site is exposed neither to one or the other side of the membrane. Following the occluded state, additional conformational changes expose the substrate to the other side of the membrane. Dissociation of the substrate from the binding site is followed by the reorientation of the binding site to the original side of the membrane such that the cycle can be repeated for as long as substrate molecules are present and that there is a substrate concentration gradient across the membrane.

See also:
Facilitated Diffusion (in Lecture Notes)
Facilitated Diffusion (in Glossay)