Water movement: does thermodynamic interpretation distort reality?
In a recent theoretical analysis of water flow, Finkelstein (Water Movement Through Lipid Bilayers, Pores, and Plasma Membranes: Theory and Reality, 1987) has attacked the contributions of irreversible thermodynamics, stating that "the thermodynamic treatment of uphill water flow completely distorts reality." Instead he presents a mechanistic formulation. For a porous membrane, water flow is attributed to convection generated by a favorable hydrostatic pressure gradient within pores, even when in the presence of permeant solutes water moves against its chemical potential gradient; water flow may "drag", solute, to an extent determined by the solute partition coefficient, but the possibility that solute flow may drag water is excluded. We argue that this formulation violates the second law of thermodynamics. Water cannot move against its chemical potential gradient because of the influence of only part of the chemical potential gradient. Furthermore, the proposed mechanism requires that at one of the membrane-solution interfaces water must move against both its concentration gradient and the hydrostatic pressure gradient. Also considered by Finkelstein is the nature of the reflection coefficient sigma, a kinetic variable, which he concludes can be evaluated (in a porous membrane) by measurement of the (equilibrium) solute partition coefficient. We claim that in general it is not possible to evaluate a kinetic variable from measurements of equilibrium parameters alone. A valid kinetic analysis must incorporate the contribution of all coupled flows.
Essig
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Department of Physiology, Boston University School of Medicine, Massachusetts 02118.
Caplan
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D001834:Body Water / Q000378:metabolism*; D002462:Cell Membrane / Q000502:physiology*; D008433:Mathematics; D008566:Membranes / Q000502:physiology*; D008962:Models, Theoretical; D013816:Thermodynamics; D014867:Water
D014867:Water