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Osmosis Confusion: 60 Years and Counting

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The phenomenon of osmosis is familiar to most readers from their junior high school science class. A dialysis bag containing sugar is placed in beaker containing pure water. It is explained that the bag is semi-permeable: water molecules can pass through microscopic pores in the membrane, but sugar molecules are too large to pass. The student watches as water gradually flows into the bag, inflating it until it seems ready to burst. The mystery of osmosis is that water continues to flow in, even after the pressure within the bag exceeds that outside.

This seemingly simple phenomenon is vitally important for plants and animals. Osmosis plays a role in the blood circulation, keeping just the right balance between the water content of the blood and the surrounding tissues. Osmosis drives fluid flow in the kidneys, preventing waste products from accumulating to dangerous levels. Osmosis is also the driving force behind plant cell expansion, playing a role in flower and fruit growth.

It’s likely that you learned the following explanation for the flow of water: the sugar molecules inside the bag displace some of the water molecules, so the number of water molecules per unit volume is lower inside that out. Thus, water molecules undergo diffusion, through the membrane pores, from the region of higher water concentration to the region of lower concentration. In short, osmosis is a special case of diffusion.

Easy as pie, right?

Except that it’s wrong.

When a salt dissolves in water, it tends to disrupt the hydrogen bonds that keep water molecules evenly spaced. Most salts tend to increase the spacing, but there are a few – sodium fluoride, for example – that decrease the spacing. The number of water molecules per unit volume actually goes up. In such cases, the diffusion theory would predict an osmotic water flow in the wrong direction!

The correct atomic theory of osmosis emerged gradually during the first half of the twentieth century. But it wasn’t until 1951 that an English language textbook presented a clear and fully realized alternative to the diffusion model (Theoretical Physics, Georg Joos). Shortly after this, Harvard biophysicist Arthur Solomon decided to tackle the issue from the experimental side. He led a research group that spent several years measuring the rate at which water flows in and out of red blood cells during osmosis. They showed that the flow was much faster than the diffusion theory could explain.

Thus, the 1950s saw the end of the diffusion explanation for osmosis, at least as far as physicists were concerned. Modern biophysics textbooks present an explanation of osmosis that does not differ from that first published in 1951. Here’s my own description of the force that drives water across the membrane, as it appears in the April 2013 issue of Trends in Plant Science, (“solute” means any sugars, salts, etc., dissolved in the water):

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.

Eric M. Kramer About the Author: Eric M. Kramer received his Ph.D. in physics from the University of Chicago in 1996, and is professor of physics at Bard College at Simon's Rock. His research focus is transport in plants, with occasional forays into cat whiskers and crumpled candy wrappers.

The views expressed are those of the author and are not necessarily those of Scientific American.

Comments 11 Comments

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  1. 1. Matthew235 9:27 am 05/1/2013

    Very interesting argument. With a physiology background, I find that I’ve been laboring under the wrong idea for many years. Also, in the UK, many examination boards for A levels (secondary/high school) will not be expecting the true answer. I wonder how long it will take them to catch up?

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  2. 2. Benton44 2:00 pm 05/1/2013

    I fear I do not understand the explanation above. Why should the pore repel solute molecules and ions but not the water molecules?
    My preferred explanation is based on vapour pressure – solutes reduce the vapour pressure of water (they may add their own if they are volatile) so that in the pores, which are surely larger than some solute ions/molecules, the water passes through essentially as vapour, just as if you had two pots one with water one with sugar solution completely enclosed together, the water would gradually all end up in the sugar solution.

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  3. 3. narmstro 2:37 pm 05/1/2013

    I can imagine many students would have difficulty visualizing the process you describe. Perhaps this is something a good animation could help illustrate more effectively?

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  4. 4. fboondoggle 2:52 pm 05/1/2013

    This is very wrong. There is no requirement that solute molecules be “repelled” by the pore. They merely have to be blocked by it, without entailing any local momentum transfer to solvent molecules. A correct explanation in terms of entropy or – equivalently – free energy is given on the Wikipedia page. Both ironic and sad that SA provides an unedited platform for this sort of bad science – in the name of correcting a probably mostly non-existent “myth”.

    See here:

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  5. 5. Shmick 8:08 pm 05/1/2013

    It would be nice if a more than one paragraph explanation of your model were available without a pay-wall…

    I’m willing to accept I may have been taught the wrong mechanism, but I’d quite like to read your full explanation so I can reconcile it all in my head.

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  6. 6. emkramer 12:43 am 05/3/2013

    Replies from the author:

    To Shmick: sorry about the pay-wall. It may help to know that the details of our argument can also be found in the textbooks “Biological Physics”, by Nelson, and “Physics with Illustrative Examples from Medicine and Biology: Statistical Physics” by Benedek. Perhaps your library can arrange a loan.

    To fboondoggle: the widely discussed explanation in terms of free energy is also correct (first published by the American physicist JW Gibbs in 1897). In this blog, and the technical papers it cites, we are concerned with the atomic kinetic theory as distinct from Gibbs’ thermodynamic arguments.

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  7. 7. elderlybloke 5:39 pm 05/6/2013

    Yes narmstro, I too have difficulty visulising this process.
    I have only ever known the Diffusion Model,as has my Grandson-who lectured me about it when he learned it a couple of years ago.

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  8. 8. billlee42 12:31 am 05/7/2013

    This is fascinating! I was taught the simple diffusion model 55 years ago and had no idea it wasn’t correct!

    @fboondoggle: The Wikipedia entry begins with a prominent warning that it is in dispute, and the Talk tab discusses at length how & why it is incorrect!! So it’s probably *not* a good resource. What a muddle!!

    One of the comments on the Talk tab references this article:

    Caveat: I’m very much a lay person, so I’m not qualified to pass judgment on this topic!

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  9. 9. Cleon Ross 2:19 am 05/7/2013

    Any of the four editions of Plant Physiology by Salisbury and Ross give a physical chemical explanation of osmosis developed by physical chemists and soil physicists that uses the second law of thermodynamics to correctly explain osmosis. One cannot use red blood cells to explain the phenomenon, because they don’t build up a pressure; plant cells do of course, which drives their great increase in size. The idea that membranes repel solutes and that repulsion causes osmosis is naïve. Of course, that idea would require that membranes of all organisms and even artificial membranes repel all osmotically active solutes, a far-fetched and irrelevant idea I think.

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  10. 10. emkramer 1:53 pm 05/7/2013

    A reply from the author:

    To Cleon Ross: it may be helpful if I clarify that “repulsion” between membrane and solute doesn’t necessarily mean the long-range repulsive force you may be imagining. In physics, the term “repulsion” includes the contact forces between atoms that prevent objects from passing through each other. Thus, the simple fact that a solute can’t pass through the membrane is adequate evidence for a repulsive force between them, even if that force extends only over a very short distance (e.g. the diameter of one or two atoms).

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  11. 11. emkramer 2:20 pm 05/7/2013

    A further comment from the author:

    The textbook by Salisbury and Ross (cited by Cleon Ross – perhaps one of the authors?) is excellent, and was a key reference for me as I learned about these topics. Their thermodynamic treatment of osmosis is correct, as far as I know. But the thermodynamic theory does not provide a mental picture of how the solute, solvent, and membrane interact to drive water flow. This is what my coauthor and I are hoping to address.

    Link to this

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