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Topic: Membrane transport equation?  (Read 11212 times)

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Offline brasarehot

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Membrane transport equation?
« on: April 24, 2012, 01:37:11 PM »
I have an equation...
deltaG = RTln[A in / A out] + ZF(delta psi)
I know R is 8.314, T is 298K, Z is charge of ion, F is 96485, and psi is the membrane potential given in problem "-60mV"
My question is that I've seen this equation sometimes with a + and sometimes with a -. How do you know which to put.
Example situations...
Positive ion(in to out). Positive ion(out to in). Negative ion(in to out). Negative ion(out to in).

Offline Babcock_Hall

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Re: Membrane transport equation?
« Reply #1 on: April 24, 2012, 05:08:49 PM »
I use the form of the equation with a positive sign between the two terms.  Z can be positive or negative, depending on the nature of the ion.  The other thing I would do for ions moving from inside to outside is to think of the numerator in the first term as always being the final concentration and the denominator always being the initial concentration.

However, one has to be careful with respect to the sign of the membrane potential. I like to think in terms of two states, initial and final.  When delta psi is given in textbooks, it usually refers to a positive ion moving from outside to inside; in other words, the cation is falling in potential, and -60 mV is in line with typical values I have seen.  However, the same ion moving inside (initial) to outside (final) would experience a delta psi of +60 mV.

If I were to compute delta G for an ion moving from inside to outside, I would use +60 mV as the change in membrane potential, and I would set z = +1 for cations and = -1 for anions.  I would also be careful with respect to the concentrations, as I noted in my first paragraph.

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