Question

When chloride channels open, chloride ions will flow through the membrane down their electrochemical gradient. If you have channels in a cell membrane with a membrane potential of –30 mV inside, and you have chloride concentrations of 40 mM inside the cell and 120 mM outside. What would the spontaneous direction of the chloride flow be, and what would the free energy change of transport be in the spontaneous direction?

Answer 1

The concentration gradient for Cl- ions favors movement into the cell, but the electrical gradient opposes inward movement of Cl-.

Calculate the free energy change that results from moving chloride (Cl-) from the external solution to the cytoplasm of a cell at 37C. Assume that [Cl-]I = 40 mM and [Cl-]o = 120 mM. The external electropotential difference was measured to be 30/1000 V =0.03V. Remember that the ion does not change during transport, so there is no standard free energy change (G0 = 0). Use the Nernst-Planck Equation:

(delta)G = RT ln [Cl-]I/[Cl-]o + zF(delta)Em

T = (37 + 273)K

R = 8.314 X 10-3 KJ/(mole K)

Z = -1 equiv/mole (equivalent of ion is the amount that has 1 mole of charge)

(delta)Em = membrane potential

F = Faraday constant (96.5KJ/(volt equiv mole)

(delta)G = (8.314 X 10-3)(310) ln 40/120 + (-1)(96.5)(-0.03)

= 0.096369677 KJ/mole