There is a misconception among plant scientists that osmosis is driven by the tendency of solutes to dilute water. In this opinion article, we discuss the quantitative and qualitative failures of this view, and go on to review the correct kinetic picture of osmosis as it appears in physics textbooks.
…The quantitative failure of dilution-based arguments was first emphasized in studies of water transport across cell membranes during the 1950s. Solomon and coworkers9,10 measured the diffusive entry of tritiated water into red blood cells under isotonic conditions and compared it to the observed rate of water influx under a gradient of osmotically active solutes. This allowed them to make a direct comparison between the flux of water in response to a water concentration gradient and the flux of water in response to an equivalent osmotic gradient. They found that the osmotic flux was 2–6× larger than the flux driven by a water concentration gradient. This result has subsequently been confirmed repeatedly, as reviewed in.11
…The force driving osmosis: So, if the degree to which solutes dilute water does not play a role in understanding osmosis, what is the explanation? The correct molecular explanation of osmosis was published in English at least as early as 1951,14 and has subsequently become the standard in biophysics textbooks15,16. This explanation considers the forces exerted on solution molecules in the neighborhood of an aquaporin protein or other membrane-bound water channel (Box 1 & Figure 1). 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 (yellow semicircles in Figure 1). 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 (for dissolved ions, the time scale for momentum sharing is ~10−12s)17. 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. This is the counterintuitive idea at the center of osmotic theory: a pore that lets water molecules pass freely will effectively repel the water if solute is present. If there are different concentrations of solute at either end of the pore, this can produce an unbalanced force that drives water through the pore into the compartment with more solute.
