Trends in Plant Science
Opinion
Osmosis is not driven by water dilution
Section snippets
The challenge of osmosis
Osmosis, the flow of water across a semipermeable membrane from a region of lower to higher solute concentration, is of central importance to plant physiology, in particular for cell turgor, tissue growth, and phloem transport. The thermodynamic explanation of osmosis in terms of the chemical potential of solvent and solute was first published by the physicist J.W. Gibbs in 1897. This explanation appears in modern plant biology textbooks via the equivalent concept of water potential, usually
Osmosis and dilution
The kinetic explanation of osmosis familiar to most plant biologists appears in most introductory college-level textbooks on chemistry 7, 8, and is substantially incorrect. This explanation focuses on the rate at which water molecules arrive at the aperture of a membrane pore (in biological systems, typically the hydrophilic channel of an aquaporin protein). The model assumes that the volume occupied by the solute displaces some water molecules and thereby decreases the number of water
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 textbooks 15, 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 and Figure 1). The key interactions take place
Acknowledgments
The authors are grateful to Tobias Baskin, Peter Hepler, and Larry Winship for helpful discussions. This work was supported in part by a grant from the National Science Foundation (Grant No. IOS 0815453).
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