Hello! This has been driving me crazy for aaaaaaaaaaaaaaaaaaaaaaaaaaaages. It's about equilibirum potential. Okay, say I have a cell with High K+ concentration inside and low K+ concentration outside, and the opposite for Na+. If for each cation there is an anion, then the voltage across the cell membrane will be zero. If K+ is then allowed to move, an accumulation of negative charge takes place in the cell. This opposes the move of K+ out of the cell; and an equilibirum is reached (i.e. there is no net movement, the movement of ions out of the cell due to random movement is opposed by the negative charge acting on the K+ ions to move them back into the cell, and these two opposing 'factors' are equal; although I guess there is a charge repulsion between K+ driving them out to some extent and random movement moving them into to some extent). The Na+ ions have no role, since they do no cross the membrane. I think all this is correct; I get confused when we increase the permeability of the membrane to Na+. If Na+ had the same permeability as K+, then the cell would not be able to distonguidh between the two cations, and so they can be treated as a single type of ion. However, if the membrane is less permeable to Na+, apparently they make unequal contributions to the voltage across the membrane at equilibirum. I don't undertsnd this. If Na+ moves more slowly, it just means that say, a K+ ion moves across, back and forth, more frequently; so it will approach the voltage that would be obtained for K+ since it moves muuch more quickly than Na+. However, if the Na+ concentration is unequal in the opposite set-up to K+, then the negatve charge in the cell will act to move the Na+ into the cell, as will its concentration gradient. Overtime, the Na+ will move in, this neutralises the negative charge and allows more K+ to move out. See what I mean? We would keep going until we get to a point where the net movement is zero and the rate of movement of Na+ in and out is equal, and the same is true for K+, except K+ has a much higher rate of movement. The euilibrium then, is when the proportion of total cations that are Na+ is the same for both inside and outside (also true for K+); the total cation conentrations do not have to be equal, but the net movement is zero, as determined by random movement and the voltage across the membrane. This is annoying me no end. Any help appreciated. Thaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaanks! Nobahar.