Copyright © 1996 by Paul O. Lewis
This is a demonstration of the principle of osmosis (diffusion through a semipermeable membrane). Normally osmosis is used to describe the diffusion of water across a biological membrane, which allows water molecules to easily pass but which blocks by default most charged molecules (e.g. sodium ions) and larger molecules (e.g. sucrose). The small black particles and the larger blue ones represent molecules of two different substances. You can imagine if you like that the small black balls are water molecules and the larger blue balls are sucrose molecules, and the membrane is a cell membrane. Ordinarily, these molecules would be moving quite fast (and also colliding with one another), but things have been simplified in this demonstration. Note that the black particles are able to pass through the membrane, and thus this substance increases in concentration on the right side of the membrane despite the fact that the overall solute concentration is initially the same on both sides. The black particles will increase in number on the right side until the concentration of the black molecules is about the same on both sides of the membrane, at which point there is no net change in concentration since as many black particles cross from left to right as from right to left (on average). At this point, the equilibrium concentration has been reached.
The cell membranes of living organisms provide examples of such semipermeable membranes. So much water enters a red blood cell by osmosis when it is placed in fresh water that it bursts! In this case, think of the solid black circles as water molecules - the inside of the cell would be to the right of the membrane. Osmosis is also responsible for the fact that celery (for example) becomes more crisp and rigid when placed in fresh water. There is a net movement of water into the cells of the celery in this case. The opposite effect can be observed (i.e., water moving out of the celery cells) by placing the celery stalk in a bowl of salty water.
Like the diffusion applet, this is of course quite simplistic. Probably water molecules do not bounce around a few times while actually inside the membrane (but maybe they do!). It is designed to be an entertaining way to think about the concept of osmosis, not an attempt to present a realistic simulation of the process