Selective Movement of Ions Creates a Transmembrane Electric Potential Difference

To help explain how an electric potential across the plasma membrane can arise, we first consider a set of simplified experimental systems in which a membrane separates a 150 mM NaCl/15 mM KCl solution on the right from a 15 mM NaCl/150 mM KCl solution on the left. A potentiometer (voltmeter) is connected to both solutions to measure any difference in electric potential across the membrane. If the membrane is impermeable to all ions, no ions will flow across it and no electric potential will be generated, as shown in Figure 7-13a.

Now suppose that the membrane contains Na+-channel proteins that accommodate Na+ ions but exclude K+ and Cl_ ions (Figure 7-13b). Na+ ions then tend to move down their concentration gradient from the right side to the left, leaving an excess of negative Cl_ ions compared with Na+ ions on the right side and generating an excess of positive Na+ ions compared with Cl_ ions on the left side. The excess Na+ on the left and CP on the right remain near the respective surfaces of the membrane because the excess positive charges on one side of the membrane are attracted to the excess negative charges on the other side. The resulting separation of charge across the membrane constitutes an electric

► EXPERIMENTAL FIGURE 7-13 Generation of a transmembrane electric potential (voltage) depends on the selective movement of ions across a semipermeable membrane. In this experimental system, a membrane separates a 15 mM NaCl/150 mM KCl solution (left) from a 150 mM NaCl/15 mM KCl solution (right); these ion concentrations are similar to those in cytosol and blood, respectively. If the membrane separating the two solutions is impermeable to all ions (a), no ions can move across the membrane and no difference in electric potential is registered on the potentiometer connecting the two solutions. If the membrane is selectively permeable only to Na+ (b) or to K+ (c), then diffusion of ions through their respective channels leads to a separation of charge across the membrane. At equilibrium, the membrane potential caused by the charge separation becomes equal to the Nernst potential ENa or EK registered on the potentiometer. See the text for further explanation.

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