<prompt> Equilibrium potential for an ion is calculated using-Gibbs-Donnan equationNernst equationGoldman equationFicks law  <response>: Ans. is b i.e., Nernst equation Nernst equation o When two ionic solutions ions A and B of different concentration CA and CB of an ion separated by a permeable membrane, the ions tend to diffuse along their concentration gradient. Since ions are charged particles their diffusion can be stopped by an appropriate electrical potential E applied across the membrane o The magnitude and polarity of the potential equilibrium potential that must be applied to side A of the membrane for stopping the diffusion of ions EA is given by Nernst equation, i.e., the equilibrium potential for an ion is calculated by Nernst equation. o For example, if solution A contains an ion with a negative charge in more concentration than the concentration of the same ion in solution B, the ion will diffuse from A to B along the concentration gradient. If an electric gradient is created in opposite direction, the magnitude of the potential that completely counteracts this diffusion from A to B is called equilibrium potential. EMF - 61 CACB Where EMF is electromotive force or equilibrium potential in millivolts. o For example, the application of Nernst equation to the concentration of K in ICF 150 mEqL and in ECF 5 mEqL gives a value of - 90 mV. This is called the Nernst potential for K EK. EK -61 --- 1 log 150 --- 5 -90 mV Ion Concentration meqL Nemst potential or Equilibrium potential mV Inside cell ICF Outside cell ECF Na K C1-- 15?0 150?0 9?0 150?0 5?5 125?0 60 -90 -70 o Resting membrane potential is -70 mV. So, resting membrane potential has a value similar to equilibrium potential or isoelectric potential of Cl . 