which changes can occur during the opening and closing of the protein molecular gates. At one voltage potential, the channel may remain closed all the time or almost all the time, whereas at another voltage level, it may remain open either all or most of the time. At in-between voltages, as shown in the figure, the gates tend to snap open and closed intermittently, giving an average current flow somewhere between the minimum and the maximum.

Patch-Clamp Method for Recording Ion Current Flow Through Single Channels. One might wonder how it is technically possible to record ion current flow through single protein channels as shown in Figure 4-5A. This has been achieved by using the "patch-clamp" method illustrated in Figure 4-55. Very simply, a micropipette, having a tip diameter of only 1 or 2 micrometers, is abutted against the outside of a cell membrane. Then suction is applied inside the pipette to pull the membrane against the tip of the pipette. This creates a seal where the edges of the pipette touch the cell membrane. The result is a minute membrane "patch" at the tip of the pipette through which electrical current flow can be recorded.

Alternatively, as shown to the right in Figure 4-5B, the small cell membrane patch at the end of the pipette can be torn away from the cell. The pipette with its sealed patch is then inserted into a free solution. This allows the concentrations of ions both inside the micropipette and in the outside solution to be altered as desired. Also, the voltage between the two sides of the membrane can be set at will—that is, "clamped" to a given voltage.

It has been possible to make such patches small enough so that only a single channel protein is found in the membrane patch being studied. By varying the concentrations of different ions, as well as the voltage across the membrane, one can determine the transport characteristics of the single channel and also its gating properties.

Facilitated Diffusion

Facilitated diffusion is also called carrier-mediated diffusion because a substance transported in this manner diffuses 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 diffusing 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 diffusion 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

Simple diffusion Facilitated diffusion

Simple diffusion Facilitated diffusion

Concentration of substance

Figure 4-6

Effect of concentration of a substance on rate of diffusion through a membrane by simple diffusion and facilitated diffusion. This shows that facilitated diffusion approaches a maximum rate called the Vmax.

Transported molecule

.Binding point

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