Osmosis Revisited

Statkraft's osmotic energy plant

A few people have told me that they do not understand what is full happening with the egg and the osmosis experiment so I thought I would try to explain what is going on a little differently. So first of all diffusion, again, is the transport of some substance in solution (aka the solute) from a point of high concentration to a point of low concentration. It will continue to happen until the entire solution is the exact same (watch the glass with red dye for an example).In the end the dye (the solute) is found at equal concentrations all over the glass.
Osmosis is a special type of diffusion where the water moves to equal out the concentrations instead of the solute. Osmosis occurs because there is a barrier that does not allow the solute to pass but does allow water (aka semi permeable membrane). So imagine a fish tank filled with water. If you were to put a semi permeable membrane that split the fish tank in to two compartments water levels would not change. The water has no driving force left or right. If you now dump a lot of salt in to the left side of the tank water would begin to flow from across the membrane from the right to the left. Since the concentration of salt on the left is higher and salt can not pass through the membrane water will move to equalize the concentrations by flowing to the left side and diluting the concentration of salt. So in the case of the egg, which has lots of proteins, ions and other solutes particles inside, water was flowing in to the egg to try to equalize the concentration of solute inside the egg to match that of outside the egg. Osmosis can only pull water in with so much force and eventually it will reach equilibrium with the force of the pressure created by moving the water into the egg. At that point water will no flow.
The cup where I added salt and soy sauce to the water, the water did not flow into the egg because there was as much osmosis pulling water out of the egg as in to it. In the cup with red dye water still flowed into the egg because the dye did not act like a typical solute. It was able to flow through the membrane with the water therefore it maintained equal concentrations and did not create any osmotic forces.
I hope this has helped clear up the topic of osmosis. It really is an important phenomena that is being use in engineering projects like reverse osmosis water filtration systems and in Norway Statkraft is actually building a power plant based off of osmotic energy. The idea is rather old but we are just beginning to be able to build the membranes needed for large scale processing of osmotic energy. This is a very exciting new form of renewable energy with firm grounds in biotechnology.