Key Concepts of Section 11.4

• An inside-negative electric potential (voltage) of about 60 to 70 mV exists across the plasma membrane of all cells.

• The electric potential generated by the selective flow of ions across a membrane can be calculated using the Nernst equation (see Equation 11-2).

• The resting membrane potential in animal cells is the result of the combined action of the ATP-powered Na⁺/K⁺ pump, which establishes Na⁺ and K⁺ concentration gradients across the membrane, and resting K⁺ channels, which permit selective movement of K⁺ ions back down their concentration gradient to the external medium (see Figure 11-3).

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• Unlike the more common gated ion channels, which open only in response to various signals, the nongated resting 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 α helices and a nonhelical P segment that lines the ion pore and forms the selectivity filter (see Figure 11-20).

• The ion specificity of K⁺ channels is due mainly to binding of the K⁺ ion with eight carbonyl oxygen atoms of specific amino acids in the P segments, which lowers the activation energy for the passage of K⁺ compared with Na⁺ or other ions (see Figure 11-21).

• 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 reagents on its activity (see Figure 11-22).

• Recombinant DNA techniques and patch clamping allow the expression and functional characterization of channel proteins in frog oocytes (see Figure 11-24).