Some Transporters Facilitate Diffusion of a Solute down an Electrochemical Potential Gradient

Some Transporters Facilitate Diffusion of a Solute down an Electrochemical Potential Gradient 

If the solute is uncharged, the free energy change associated with moving 1 mol of the material from a solution with concentration [Cj] to a solution with concentration 

This expression, which reflects the entropy increase resulting from distributing the molecules more randomly between the two solutions, holds for both free and facilitated diffusion. If [Cj] [C2], AG, 2 is negative and molecules diffuse spontaneously from solution 1 to solution 2. Net flux across the membrane ceases when [Cj] = [C2] and AG, 2 is zero. 

If the solute carries a net charge, there is an additional thermodynamic effect of moving the charge across any difference in electric potential that exists between the solutions on the two sides of the membrane. The free energy change then is 

We used this expression in chapter 14 to evaluate Ai// across the mitochondrial inner membrane. 

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