Simulating the Effect of Voltage on Cation Diffusion Across a Selectively-Permeable Membrane.

This simulation contains extra quantitative information to help you track how the red (cation) particles diffuse. Numerals in the top left-hand and right-hand panels provide a running total in, respectively, the left (inside) and right (outside) compartment. The middle panel on top shows the voltage in the left compartment compared with the right.

Along the bottom of the simulation is a strip chart recorder that always shows the fraction of red particles in the outside or right-hand compartment. Note the strip chart scrolls from right to left and at first, only blank (gray) chart appears. Initially, the chart records no red ink because all the particles begin diffusing in the left-hand compartment. Watch how the strip chart changes with time, in conjunction with cation diffusion.

At the very bottom of the simulation are menu bars that allow the voltage to be changed and the simulation to be stopped (if you wish). Once the initial simulation has become "steady" (or technically, has reached a "steady state"), try changing the voltage and see what happens. Record your observations. Watch very carefully the behavior of the particles very close to the membrane.

Also, clicking your mouse inside the simulation restarts it at the cursor point. Use this feature to restart the simulation in the right-hand compartment, and watch what happens. Compare these observations with the results of a simulation begun at the mirror-image location in the left compartment. Again, record your observations for future reference.

We use "inside" to refer to the left compartment, and "outside" for the right-hand compartment, because many text books use a similar convention for static illustrations of the diffusion of potassium ions down their concentration gradient from high levels inside a cell to lower levels outside. For reasons that will become evident in the next section, the voltage illustrated is measured in the left-hand compartment (and represents the electromotive potential difference across the membrane, between a cell and its environment).