The simulation above is just like the one we saw on the previous page. 100 each of the red and the blue particles start diffusing from 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 has an added element, however: in the middle is a vertical black line that represents a selectively permeabile membrane between the two halves. In this case, the membrane is partially permeable to the red particles, but completely impermeable to the blue.
Watch what happens with time. The red particles cross until they gradually fill the right compartment to an equal extent as the left. The blues remain, and are in the end twice as concentrated as the reds on the left.
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 the permeant particle is water and the impermeant one, solute.