Why bother with equilibrium potentials?
The equilibrium potential was developed to describe a very special circumstance: an ionic concentration gradient across a membrane permeable to only one ion. Since biological membranes are generally permeable to large numbers of ions, at first glance this special situation can appear to be an irrelevancy. Not so! There are two important reasons the ability to quantify equilibrium potentials is very valuable: • There are some circumstances when a biological membrane becomes very permeable to one ion, so permeable that the permeabilities to other ions become relatively unimportant, and the membrane behaves as if it was a single ion-selective membrane. The equilibrium potential for the ion involved then becomes useful in predicting membrane potential. • Even under ordinary circumstances when the membrane is permeable to many ions, comparison of membrane potential and a particular ion equilibrium potential can allow us to assess whether or not an ion is at equilibrium across the membrane.
