Trends in Plant Science
Volume 18, Issue 4, April 2013, Pages 195-197
Journal home page for Trends in Plant Science

Opinion
Osmosis is not driven by water dilution

https://doi.org/10.1016/j.tplants.2012.12.001 Get rights and content

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.

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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