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Simulating Diffusion Across a Selectively-Permeable Membrane

The simulation above resembles the one we saw on the previous page. Identical numbers of the red and the blue particles (about 100 each) start diffusion in the upper left corner. They move only by random small jumps, and like the previous simulation, they quickly fill the left-hand side of the compartment.

This simulation above, however, has an added element: a selectively permeable membrane that divides the chamber vertically into two equal compartments. In this case, the membrane is permeable to the red particles, but completely impermeable to the blue ones.

Watch what happens with time. The red particles diffuse gradually across the membrane until they reach equilibrium, until as many particles occupy the right compartment as the left. At this point, there is no net movement of red particles between compartments. The membrane, however, restricts the diffusion of the blue particles and at equilibrium they are twice as concentrated as the reds on the left compartment.

Click the mouse in either compartment to reset all the particles to a new location. Click near the membrane to see the direct result of the "filtering" action of the membrane.

Lets next examine what happens when one of the diffusing particles are water molecules. Alternatively, if the red particles are monovalent cations (such as potassium) and the blue particles are anions (such as chloride), selective diffusion can create an actual voltage across the membrane or a diffusion potential.