Originally Posted by Sana ri
What do we mean by concentration equilibrium potential ?
Does the permiability increase when we become near it ??
I am not entirely sure by what exactly mean by your question but I will give it a try anyways.
Equilibrium potential is the membrane potential at which diffusion for a particular ion stops. It is calculated using the Nerst equation. It is a balance between chemical and electrical forces. For example, let's take potassium. Potassium is a positively charged ion which is in a very high concentration intracellularly. So, "chemically" K+ wants to go outside. But, the more K+ goes outside, the more positive outside becomes, repelling it, and the more negative the inside becomes, attracting it. If there were no charges involved, diffusion would go on until concentrations of potassium are equal intra and extracellularly, but since there are charges involved, diffusion stops once we hit the equilibrium potential, which is the potential difference across the plasma membrane at which outward pulling chemical forces and inward pulling electrical forces balance each other out.
And for the second question the answer would be no. Permeability can be changed primarily by altering the membrane channels. Opening the channels increases the permeability, while closing the channels decreases it. When you get close to the equilibrium potential the "speed" of the diffusion decreases but permeability does not change. If that makes sense. You can have all the doors open and not have any net diffusion just because the forces are already equilibrated. So, good permeability does not always mean good diffusion and concentration gradient does not directly alter permeability.