Two concentrations of calcium ions inside and outside the membrane

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msoric
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This isn't homework - just working through questions in my text. The question gives two concentrations of calcium ions inside and outside the membrane and asks to find the Nernst potential - ok no problem there.

Then it asks if the calcium ions are in equilibrium if the resting potential is 70mV. If they were in equilibrium, then

E = (RT/zF)(ln([outisde]/[inside])

But then, at calcium equilibrium, the ratio would be 1... ln(1) = 0 and there would be no potential... am I correct?

Thanks,

Mark
 
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I can't exactly figure out what you mean, but I wonder if you are mixing up a couple of ideas. At equilibrium, there is not equal concentrations; there is no current flow. The difference in concentration is balanced by the resting potential (which is the E in your equation). Don't forget calcium has 2 charges.
 


I still can't see how to figure out if the calcium is in equilibrium, I would only be able to get the ratio of the two concentrations... no?

mark
 


I suppose that's true, the concentration *gradient* is what drives motion, not the absolute value of concentration. But again, I think I am not really understanding your original question. It almost seems as if you are treating the Nernst and resting potential to be the same (and I don't think they are).