Using the Nernst equation and the ion concentrations given in Table 12–1, calculate the equilibrium membrane potential of K⁺ and Na⁺ that is, the membrane potential where there would be no net movement of the ion across the plasma membrane (assume that the concentration of intracellular Na⁺ is 10 mM). What membrane potential would you predict in a resting animal cell? Explain your answer. What would happen if a large number of Na⁺ channels suddenly opened, making the membrane much more permeable to Na+ than to K⁺?

What would you predict would happen next if the Na⁺ channels closed again?

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TABLE 12-1 A COMPARISON OF ION CONCENTRATIONS Intracellular Concentration (mm) lon Cations Na+ K+ Mg2+ Ca2+ H+ Anions** CH 5-15 140 0.5* 10-4* 7 x 10-5 (10-7.2 M or pH 7.2) 5-15 INSIDE AND OUTSIDE A TYPICAL MAMMALIAN CELL Extracellular Concentration (mM) 145 5 1-2 1-2 4 x 10-5 (10-7.4 M or pH 7.4) 110