Equilibrium Potentials II Answer 3

Here are the answers for Figure 8, when sodium is permeable:

• In the equilibrium condition, we computed that the internal sodium ion concentration was $\left[\text{Na}^+\right]_\text{in} = 12\ \text{mM}$, and we were given that its external concentration is $\left[\text{Na}^+\right]_\text{out} = 120\ \text{mM}$.
• Using the Nernst equation, the predicted equilibrium potential for sodium across the membrane is $$E_{\text{Na}^+} = 58\ \text{mV} \times \log_{10} \frac{120\ \text{mM}}{12\ \text{mM}} = +58\ \text{mV},$$ which is very different from the $-81\ \text{mV}$ equilibrium potential that we calculated for the chloride and potassium ions.