Electrochemical Equilibrium

Diffusion does not continue until an equal number of cations are present on both sides of the membrane. Rather, approximately 8 or 9 cations end up in the right compartment, because we fixed (or more technically, "clamped") the membrane potential in this simulation at a negative voltage and allowed each cation to carry a current equivalent to about 7 mV at unitary resistance.

How does diffusion proceed when the membrane potential is 0?  If no voltage is applied to the membrane separating the two compartments, the cations are completely free to diffuse from one compartment into the other. An equilibrium is soon  reached, in which equal concentrations of cations are found in both compartments. Under these conditions, cations have an equal chance of diffusing across the membrane in either direction because they have equal electrochemical activities in both compartments.

Voltage affects the movement of charged particles across the membrane. A voltage difference applied across the membrane makes positively charged cations more likely to cross the membrane when they are moving towards the negatively charged side (remember that opposite charges attract each other). Conversely, cations are less likely to leave the negative side and cross to the positive side.

Thus, when a membrane potential of exists in the left compartment, cations more likely stay on that side. If, occasionally, cations do diffuse outward across the membrane, they are also much more likely to return. In this simulation, and in real cells, a membrane potential of -60 mV causes just about a 10-fold concentration difference between the two sides! Voltage dramatically affects ion distributions and their diffusion!

Alter the simulation by changing either the voltage or location of the particles. Based on these changes, estimate how the voltage and the differences in cation concentrations are related. Now consider how the membrane potential might be created across cellular membranes (rather than being set artificially). We'll see next this how the voltage/concentration relationship and selective membrane permeability play a major role in the generation of  diffusion (or Nernst) potentials.