N

Carrier protein and

.conformational change mmm A.rnmm

MMWM PxtoWM

Release of binding

Figure 4-7

Postulated mechanism for facilitated diffusion.

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 chemical 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 principal mechanism by which insulin controls glucose use in the body, as discussed in Chapter 78.

Factors That Affect Net Rate of Diffusion

By now it is evident that many substances can diffuse through the cell membrane. What is usually important is the net rate of diffusion of a substance in the desired direction. This net rate is determined by several factors.

Effect of Concentration Difference on Net Diffusion Through a Membrane. Figure 4-8A shows a cell membrane with a substance in high concentration on the outside and low concentration on the inside. The rate at which the substance diffuses inward is proportional to the concentration of molecules on the outside, because this concentration determines how many molecules strike the outside of the membrane each second. Conversely, the rate at which molecules diffuse outward is proportional to their concentration inside the membrane. Therefore, the rate of net diffusion into the cell is proportional to the concentration on the outside minus the concentration on the inside, or:

in which Co is concentration outside and Ci is concentration inside.

Effect of Membrane Electrical Potential on Diffusion of Ions— The "Nernst Potential." If an electrical potential is applied across the membrane, as shown in Figure 4-85, the electrical charges of the ions cause them to move through the membrane even though no concentration difference exists to cause movement. Thus, in the left panel of Figure 4-85, the concentration of negative ions is the same on both sides of the membrane, but a positive charge has been applied to the right side of the membrane and a negative charge to the left, creating an electrical gradient across the membrane. The positive charge attracts the negative ions, whereas the negative charge repels them. Therefore, net diffusion occurs from left to right.After much time, large quantities of negative ions have moved to the

Inside

Membrane

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Essentials of Human Physiology

Essentials of Human Physiology

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