Chapter 20, Problem 30CQ

Note that in Figure 20.28, both the concentration gradient and the Coulomb force tend to move Na+ ions into the cell. What prevents this?

Figure 20.28 The semipermeable membrane of a cell has different concentrations of ions inside and out. Diffusion moves the K+ and Cl ions in the direction shown, until the Coulomb force halts further transfer. This results in a layer of positive charge on the outside, a layer of negative charge on the inside, and thus a voltage across the cell membrane. The membrane is normally impermeable to Na+.