AJP - Cell Physiology, Vol 256, Issue 3 C694-C698, Copyright © 1989 by American Physiological Society
Water movement: does thermodynamic interpretation distort reality?
A. Essig and S. R. Caplan
Department of Physiology, Boston University School of Medicine, Massachusetts 02118.
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.
