This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General Physiology, Subject name: Physiology
Equilibrium potential for an ion is calculated by: 
Select and explain. 
A. Gibbs Donnan Equilibrium, B. Nernst Equation, C. Goldman Equation, D. None of the above
B: Nernst Equation
Answer: B: Nernst Equation 
Explanation: Gibbs Donna Equilibrium explains unequal distribution of ions in ICF and ECF due to non- diffusible anions (proteins) present intracellularly Nernst Equation: Nernst equilibrium potential (at which there is no net transpo of the molecule across the membrane) Where, R = the gas constant T = temperature in degrees Kelvin ln = natural logarithm i = the concentration of X inside the cell o = the concentration of X outside the cell zx = the valence of charged molecules F = the Faraday constant Vm = the membrane potential The Nernst equation, at 37degC, can be written for univalent ion as follows: If movement of ion is producing negativity inside the cell membrane, Nernst potential will be with negative sign and vice-versa Goldman-Hodgkin-Katz (GHK) equation state that when a membrane is permeable to several different ions, the diffusion potential that develops depends on three factors: polarity of the electrical charge of each ion permeability of the membrane (P) to each ion concentrations (C) of the respective ions on the inside (i) and outside (o) of the membrane