Y

Free-energy change during transport of Na+ from outside to inside

Inside

Outside

▲ FIGURE 7-20 Transmembrane forces acting on Na+ ions. As with all Ions, the movement of Na+ Ions across the plasma membrane Is governed by the sum of two separate forces—the Ion concentration gradient and the membrane electric potential. At the Internal and external Na+ concentrations typical of mammalian cells, these forces usually act In the same direction, making the Inward movement of Na+ Ions energetically favorable.

K+ channels to the external medium. Unlike the more common gated ion channels, which open only in response to various signals, these nongated K+ channels are usually open.

■ In plants and fungi, the membrane potential is maintained by the ATP-driven pumping of protons from the cytosol to the exterior of the cell.

■ K+ channels are assembled from four identical subunits, each of which has at least two conserved membrane-spanning a helices and a nonhelical P segment that lines the ion pore (see Figure 7-15).

■ The ion specificity of K+ channel proteins is due mainly to coordination of the selected ion with the carbonyl oxygen atoms of specific amino acids in the P segments, thus lowering the activation energy for passage of the selected K+ compared with other ions (see Figure 7-16).

■ Patch-clamping techniques, which permit measurement of ion movements through single channels, are used to determine the ion conductivity of a channel and the effect of various signals on its activity (see Figure 7-17).

■ Recombinant DNA techniques and patch clamping allow the expression and functional characterization of channel proteins in frog oocytes (see Figure 7-19).

■ The electrochemical gradient across a semipermeable membrane determines the direction of ion movement through channel proteins. The two forces constituting the electrochemical gradient, the membrane electric potential and the ion concentration gradient, may act in the same or opposite directions (see Figure 7-20).

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