Key Concepts of Section 11.4

Key Concepts of Section 11.4

Nongated Ion Channels and the Resting Membrane Potential

  • 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).