Membrane Potential Ionic Steady State

In Chapter 4, we learned that in a Donnan equilibrium, two permeant ions can be at equilibrium provided the membrane potential is simultaneously equal to the Nernst potentials for both ions. However, real animal cells are permeable to sodium, and thus there are three major ions potassium, chloride, and sodium that can cross the plasma membrane. This chapter will be concerned with the effect of sodium permeability on membrane potential and with the quantitative relation between ion permeabilities and ion concentrations on the one hand and electrical membrane potential on the other.

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