Re: What is considered a semi-permeable membrane?

Mary:

Thanks for the question, I hope this answer can settle the debate. Unfortunately, I fear your friend is correct. The short answer is that what you are observing is passive diffusion or just diffusion and not osmosis

A Gummi bear swelling in water is a great example of diffusion, which is the process by which solutes move from an area of high concnetration to low concentration. In this case, the gummi bear being made of almost entirely sugar and gelatin has a relatively low concentration of water, and a high concentration of sugar. Over time, water diffueses into the Gummi Bear and sugar and possibly some gelatin diffuses out. The bear swells because gelatin is not readily soluble in water at room tempaerature once it has polymerized. Since there is no membrane, semipermeable or otherwise, it cannot by definition be osmosis.

Osmosis is a type of diffusion in which solutes/analytes.solvent are moved across a membrane with or against a concentration gradient in order to achieve a concentration difference in the end. The net result of diffusion is that over time, the concentration gradient will achieve equilibrium with respect to the solute. In osmosis, the solvent does not necessarily across the membrane, only the chemical. Although water can be subject to osmosis itself, particularly in cells where water can pass freely in and out, but not other materials. This is why eggs can be made to swell or shrink in sugar water or water after you have dissolved the shell. This is diffusion of water, but not diffusion of the egg proteins fats, cholesterol etc. etc. (This is why it is osmosis and not diffusion)

If your Gummi bear selectively allowed sugar out/in, but not water, or vs.versa it would be a semi-permeable membrane, but these all flow freely in and out of the gummi bear.

You can also look at Wikipedia for further information Thanks for the question.

-Matt- http://en.wikipedia.org/wiki/Osmosis

http://en.wikipedia.org/wiki/Diffusion