In a two-compartment model of a cell with a K - and Cl --permeable membrane and a 10-fold excess of KCl in the inside compartment, how would the membrane potential change if all the beginning KCl solutions (including inside and outside) were replaced by NaCl

When the solutions (including inside and outside) were replaced by NaCl, the membrane potential changes from a negative value of -60 mV to a positive value of + 60 mV.

Explanation:

According to the Nernst equation the potential of the membrane for a two-compartment model of a cell for positive gradient of K⁺ ions is V = - 60 mVlogK'/K where K' = inside concentration of K⁺ ions and K = outside concentration of K⁺ ions. For a 10 fold excess of KCl in the inside compartment, K'/K = 10. So,

V = - 60 mVlogK'/K = - 60 mVlog10 = -60 mV.

For a negative gradient of Na ions is V = + 60 mVlogK'/K where K' = inside concentration of Na⁺ ions and K = outside concentration of Na⁺ ions. (Since the cell is selectively permeable to Na⁺ ions. So, Na ions to not flow out but in.)For a 10 fold excess of NaCl in the inside compartment, K'/K = 10. So,

V = + 60 mVlogK'/K = + 60 mVlog10 = +60 mV.

So, when the solutions (including inside and outside) were replaced by NaCl, the membrane potential changes from a negative value of -60 mV to a positive value of + 60 mV.

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