Voltage Also Moves Charged Particles

In earlier chapters (Simple Diffusion, Diffusion of Many Particles), random, thermal energy provided the main driving force for particle movement, which was best characterized as a "random walk". In addition to diffusion, charged particles such as inorganic ions also move in response to electrical differences in their environment.

In the simulation on the right, 100 (red) cations begin diffusing randomly from a point located in the left-hand compartment while, as in earlier simulations, solvent molecules are invisible. Gradually, by diffusion the cations fill the left-hand compartment, which is separated from the right-hand compartment by a membrane.

Over time red particles diffuse across the membrane into the right-hand compartment. From an earlier simulation (Diffusion Across a Selectively-Permeable Membrane) recall the uncharged red particles diffused until they had reached equal concentrations on either side of the membrane. In the present simulation, however, the red particles are cations and carry positive charge. A voltage difference (starting value of -60 mV), however,  has been set up between the left and the right compartments. Since the cations are charged, this voltage or membrane potential influences the way they move across the membrane.  (For the moment, put aside the question of how this voltage or potential difference arises!)

What happens to the behavior of cations when they diffuse to the right across a membrane with a -60 mV potential difference? Their diffusion does not continue until approximately equal numbers are present in both compartments. Why not? Vary the voltage of the left-hand compartment and observe the effects.  Then go on to the next page to explore the answer to this interesting question and to raise others.