Facilitated Diffusion Animation (Passive Transport)
Shown to the right is an animation which illustrates protein mediated, facilitated diffusion out of a cell. The protein illustrated is clearly a uniporter, transporting one substrate across the membrane. Other mechanisms of facilitated diffusion are also possible. When we are dealing with facilitated diffusion, the protein allows molecules or ions to enter or leave the cell moving DOWN their concentration gradient. In this particular example, the concentration of molecules or ions (illustrated by the red dots) is greater inside the cell than outside the cell. Thus, the protein carrier allows the red dots to leave, down their concentration gradient. If the concentration of red dots was higher outside of the cell than inside of the cell the protein would allow the dots to pass into the cell just as easily. Note that no ATP or ion gradient is involved with facilitated diffusion.
You might ask, "When does a cell let materials diffuse out using a facilitated diffusion system? A good example of this situation is found in the liver cells which control the concentration of glucose in the blood. Liver cells store excess glucose as glycogen when blood sugar levels are high (just after a high carbohydrate meal) and then breakdown the glycogen as glucose-1-phosphate which is converted to glucose-6-phosphate which is finally converted to free glucose. The breakdown of glycogen is closely controlled by hormones. The glucose-1-P and the glucose-6-P produced by glycogen breakdown are impermeable to the membrane and there is no transport protein to allow them to leave the cell. However, there is a transport protein which functions by facilitated diffusion which allows glucose to freely pass back and forth across the membrane.