The Nernst equation is an equation that relates the the total voltage, i.e. the electromotive force, of the full cell at any point in time to the standard electrode potential, temperature, activity, and reaction quotient of the underlying reactions and species used. I have been wondering why Universal gas constant (R) is included in Nernst and Goldmann equations while describing the steady state of membrane potential?

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    $\begingroup$ I'm voting to close this question as off-topic because this should go to physics $\endgroup$
    – AliceD
    Jul 11 '16 at 8:45
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    $\begingroup$ Are questions about neuron membrane properties off-topic here? I see a lot of them around, so I am not clear why this is any different. If such questions are off-topic, then this would be on-topic for the proposed Neuroscience stack exchange site on Area 51. $\endgroup$ Jul 11 '16 at 14:29
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    $\begingroup$ @TheBlackCat, if the question is relevant for understanding membrane potentials, we ask the OP (or anyone else) to add that part in the post. Not everybody is knowledgeable about neuroscience at such a detailed level. Providing more context to the question would prevent ambiguity and confusion of the topic. $\endgroup$ Jul 11 '16 at 15:59
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    $\begingroup$ @RobinKramer: It is at the end of the post, "...while describing the steady state of membrane potential?" $\endgroup$ Jul 11 '16 at 16:23
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    $\begingroup$ You are absolutely right. I completely missed that part of the question. Then, since two people apparently missed it, perhaps some more emphasis on the neuro part may be handy, also to avoid further confusion :) $\endgroup$ Jul 11 '16 at 16:31

As in the ideal gas law, the universal gas constant allows for calculation of amount of energy associated with a certain group of molecules (see https://en.wikipedia.org/wiki/Gas_constant). As the Nernst equation compares the "osmotic pressure" to "electrical pressure", the universal gas constant is needed to convert amount of an ion on the two sides of a membrane ("osmotic concentrations") to the amount of energy associated with that concentration differential ("osmotic pressure").

Carefully looking at the units of all of the constituents of the equation can help clear things up. R is in units of J / (mol * degree), and V is J/coulomb. zF in the denominator gets you your coulombs.
In short, both the left and right sides of the equation are more or less measures of energy. R converts amounts of ions to the energy associated with their osmotic pressure so that it can be compared with the electrical energy associated with their charge.

  • $\begingroup$ Thank you so much! That adds some sense to my knowledge about R now ! $\endgroup$
    – sarannns
    Jul 11 '16 at 21:19

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