## B Sample calculation with the Nernst equation

- If the intracellular [Na+] is 15 mM and the extracellular [Na+] is 150 mM, what is the equilibrium potential for Na+ ?

where:

z LOeJ

Note: You need not remember which concentration goes in the numerator. Because it is a log function, perform the calculation either way to get 60 mV, and then use an "intuitive approach" to determine the correct sign. (Intuitive approach: The [Na+] is higher in extracellular fluid than in intracellular fluid, so Na" ions will diffuse from extracellular to intracellular, making the inside of the cell positive [i.e., +60 mV at equilibrium].)

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