Facilitated diffusion is also called carrier-mediated dif-fusion because a substance transported in this mannerdiffuses through the membrane using a specific carrier protein to help. That is, the carrier facilitates diffusion of the substance to the other side.
Facilitated diffusion differs from simple diffusion in the following important way: Although the rate of simple diffusion through an open channel increases proportionately with the concentration of the diffus-ing substance, in facilitated diffusion the rate of diffusion approaches a maximum, called Vmax, as the concentration of the diffusing substance increases.This difference between simple diffusion and facilitated dif-fusion is demonstrated in Figure 4–6. The figure shows that as the concentration of the diffusing substance increases, the rate of simple diffusion continues to increase proportionately, but in the case of facilitated diffusion, the rate of diffusion cannot rise greater than the Vmax level.
What is it that limits the rate of facilitated diffusion?
A probable answer is the mechanism illustrated in Figure 4–7. This figure shows a carrier protein with a pore large enough to transport a specific molecule partway through. It also shows a binding “receptor” on the inside of the protein carrier. The molecule to be transported enters the pore and becomes bound. Then, in a fraction of a second, a conformational or chemi-cal change occurs in the carrier protein, so that the pore now opens to the opposite side of the membrane. Because the binding force of the receptor is weak, the thermal motion of the attached molecule causes it to break away and to be released on the opposite side of the membrane. The rate at which molecules can be transported by this mechanism can never be greater than the rate at which the carrier protein molecule can undergo change back and forth between its two states. Note specifically, though, that this mechanism allows the transported molecule to move—that is, to “diffuse”—in either direction through the membrane.
Among the most important substances that cross cell membranes by facilitated diffusion are glucose and most of the amino acids. In the case of glucose, the carrier molecule has been discovered, and it has a molecular weight of about 45,000; it can also transport several other monosaccharides that have structures similar to that of glucose, including galactose. Also, insulin can increase the rate of facilitated diffusion of glucose as much as 10-fold to 20-fold. This is the prin-cipal mechanism by which insulin controls glucose use in the body.
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