Facilitated diffusion occurs through membrane-spanning protein molecules. It is a passive process, which goes down a concentration gradient. Usually, compounds that are not lipid-soluble tend to cross the membrane through this mechanism. Examples include charged ions, and compounds with relatively large molecular size, like glucose and amino acids.
One type of facilitated diffusion occurs on carrier protein. The substrate binds to the carrier protein on one side of the membrane. Through a change in the shape of the carrier protein, the substrate will be released onto the other side of the membrane.
For ions, they tend to pass through ion channels by facilitated diffusion. Travelling through a central pore between the component proteins, it can be opened or closed to regulate the flow of charged ions.
It is important to note that the kinetic consequences of facilitated diffusion and simple diffusion are different. Facilitated diffusion shows saturated kinetics, similar to that for many enzymes. In facilitated diffusion, the rate will eventually slow down when the protein carrier molecules become saturated with the substrates and the rate of entry will not increase beyond that point. It is different from a simple diffusion, because if the external concentration keeps on increasing, the rate of entry will keep on increasing too – there will not be a limit for this scenario.
The following graph shows the rate of uptake of glucose into erythrocytes by simple diffusion and facilitated diffusion, which highlights the different kinetic phenomena.
A final note is that one should distinguish between active transport and facilitated diffusion. While both processes involve some sort of membrane-spanning transporter proteins, the former requires an investment of energy while the latter does not.
by Ed Law
