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In the previous simulation you saw that a voltage difference between two compartments has a strong influence on the movement of ions crossing a membrane between them. This is the first step in understanding the generation of cellular membrane potentials. The simulation was highly unrealistic, however, because only cations were illustrated, with no anions to balance their charge. Such a solution, if it could exist, would draw bolts of lightning to restore the balance!

Another reason the previous simulation was unrealistic is that normally nothing holds the voltage between the two compartments constant.  Moreover, real membranes surrounding most cells are far more permeable to certain cations than they are to anions.

The generation of membrane potentials is depicted in the simulation on the right, which starts with equal number of anions and cations in the left compartment, and none in the right. As these ions diffuse and fill the compartment, they approach a selectively permeable membrane. The cations cross as before, but the membrane is impermeable to anions.  Thus, selective cation permeability leads to an imbalance in charge distribution and an additional voltage difference.

As we saw in the previous simulation, voltage both hinders the crossing of more cations and promotes the reentry of those already crossed. The two tendencies - diffusion rightwards down the gradient and attraction leftwards by the voltage - oppose one another. Watch the simulation to see if they ever come into balance.

The voltage difference established between the two compartments is thus a direct result of the selective diffusion of ions across the membrane. The membrane potential becomes more negative as more cations diffuse from left to right, and the voltage in turn limits their outward diffusion. At some point the tendency of a cation to diffuse down its concentration gradient is balanced by its equal tendency to move in the opposite direction "down" the electrical gradient.  We call such voltage differences diffusion potentials because they are generated by selective ion diffusion.

This balance of electrical and diffusion tendencies creates an electrochemical equilibrium between the opposing chemical or concentration and electrical forces.   (Consequently, diffusion potentials are also often called Nernst potentials after the scientist who formally characterized the electrochemical equilibrium.) Over time, about the same number of cations diffuse into the outside compartment as diffuse in the opposite direction, and no additional membrane potential develops. (There may, however, be short-term fluctuations in the cation gradient between the two compartments which in turn leads to temporary fluctuations in membrane potentials, for the same reasons solvent fluctuations occurred in the Osmosis simulation. See no net movement.)