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As a teacher of high school and introductory college chemistry, I used textbooks that defined osmosis as a flow of water (only) through a membrane that prevents other substances such as dissolved salts from passing through. But in more advanced text, discussing osmosis through aquaporins in the plasma-membrane, bulk flow by osmosis implies that the water carries along dissolved substances. So which is it: pure water or a solution that moves by osmosis? (I'm somewhat naive about biology, as my degrees are in physical chemistry and metallurgy. Biology is way more complicated!)

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  • $\begingroup$ Can you provide a citation or quote? I'll answer by my guess about what's being talked about, but it would be nice to be able to confirm. $\endgroup$ – Bryan Krause Jan 9 at 17:54
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Bulk flow and osmosis are different; I'm guessing your text might be talking about using osmosis to drive bulk flow, but they are occurring at different stages.

For example, if you get your water from a water tower, your house's water pressure is driven by gravity. However, the water tower could be filled by an electric pump. You could say your water pressure is driven by an electric pump, but the pump isn't really pushing water into your house, it's just pushing water up a tall tower so that gravity can act, and if you didn't realize this fact you might be confused about how you can still have water pressure even when the power goes out.

Similarly, you can drive bulk flow by osmosis. If you have a closed space (e.g., a "tube") and pump salt (or anything else water soluble) into it, it can pull water from the surrounding tissues via osmosis. As the pressure of water increases in that closed space, it can now flow in bulk through that tube. Plants can use this mechanism to deliver nutrients up a stem: they pump in ions/molecules inside the stem, which pulls in water from the soil around. Because the space is rigid, instead of expanding the water goes up the stem in "bulk": anything else carried in the water will be pushed up with it. Then, when target tissues pull in solutes, osmosis again causes water to follow, and by only pumping solutes in one place and out another you produce a bulk flow of water.

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  • $\begingroup$ Bryan, thanks for your answer. But please clarify the example of plants delivering nutrients up a stem. Does this mean that the liquid that enters the roots from the soil by osmosis is essentially pure water, but once it has entered the tube-like stem it carries with it nutrients that were in a lower place in the stem? $\endgroup$ – S. Fishman Jan 11 at 20:13
  • $\begingroup$ @S.Fishman The general concept is that cells in the roots pump in any ions, etc, from the soil that the plant needs, and therefore water is pulled in via osmosis. Not totally pure water, but also anything else that can freely cross membranes. The root cells then pump those ions into other cells or cavities and water follows. The whole system is actually pretty complex and plants use a lot of combined forces to move solutes in a couple different systems, so you can read more broadly about xylem and phloem. $\endgroup$ – Bryan Krause Jan 11 at 20:22
  • $\begingroup$ I should also add that I simplified a bit by talking about a "tube", because in the context of nutrient transport in plants you can also have living cells that are connected to each other via openings between cells; they aren't really just a tube anymore in that context, but from the perspective of water and stuff dissolved in water the principle is the same. $\endgroup$ – Bryan Krause Jan 11 at 20:27

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