The key interactions take place in the small region of space adjacent to a pore aperture that allows water molecules to pass but repels solute ... Each time a solute molecule enters this region, it is repelled. That is, the aperture gives to the solute molecule a small amount of momentum directed away from the membrane. Due to viscous interactions between solute and water, this momentum is rapidly shared among all nearby molecules, including both solute and water ... Thus, although the pore aperture repels only the solute, the net effect is a force directed away from the membrane acting on the solution as a whole.Unfortunately, communication on this topic between physics, chemistry and biology has not been good. In the 1960s, most introductory college-level textbooks in chemistry and biology continued to repeat the discredited, diffusion-based view.In the 1970s the situation grew even worse. A research group led by physiologist Harold Hammel at the University of California in San Diego began promoting a third theory of osmosis, which they called the solvent tension theory. In brief, they suggested that the sugar and the water in a solution could permanently co-exist at two different pressures. Water flowed into the dialysis bag, they suggested, because the water (i.e. the solvent) was only affected by the difference in water pressures across the membrane. A series of papers published in the September 1979 issue of the American Journal of Physiology refuted this theory point-by-point, after which it was regarded as fringe science. (Hammel's conviction was not shaken. When he passed away in 2005, the preferred theory was carved onto his headstone.)As far as I know, 1979 was the last time that the osmosis of dilute solutions was discussed as an active research topic. Physicists regard the matter as long-settled. However, chemistry and biology textbooks continue to repeat the incorrect, diffusion explanation. And it's not just textbooks. If you try an internet search, you're sure to find plenty of authoritative-sounding discussions that explain osmosis as a special case of diffusion. There are also links to educational movies and video games designed to amplify the point.In collaboration with my colleague, chemist David Myers, I have recently written two papers [2012, 2013] that try to bridge the gap between physics and biology. We are also working with the author of a popular physiology textbook to help him improve the next edition.After so many years of confusion, will we finally succeed in fixing the problem? Here's a quote from the biologist Howard J. Stein, who also recognized the problem and tried to address it back in 1966: "The time has passed when authors of textbooks should continue with the older, and deceptively simple, idea of osmosis as a special case of diffusion."Hope springs eternal.Image: Internet archive. Follow Scientific American on Twitter @SciAm and @SciamBlogs. Visit ScientificAmerican.com for the latest in science, health and technology news.
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- water molecules