In membrane potential determination of a cell, the Goldman Hodgkin Katz equation says the contribution of an ion to diffusion potential is dependent on its membrane permeability. So in case an ion, one that has a zero permeability, is present outside the cell, using the equation , one will get contribution to diffusion potential as zero but thinking rationally, the charge on the ion must contribute to the electrical potential across the membrane. Though I see that the equation is meant for diffusion potential, I see that the same equation is being used to find the resting membrane potential. Here my questions are 1. Are membrane potential and net diffusion potential the same? 2. Isn't there any contribution to membrane Potential by an ion with zero permeability? If so how? Is there a different equation for it? Or will its presence influence the membrane permeability for other ions and thus have an effect?