The role of solvent pressure in osmotic systems.

Abstract

The os-motic potential (solute pressure) of a limp leaf cell is in close balance with the negative hydrostatic pressure of the solute-free xylem sap. A somewhat different interpretation of the osmotic is needed. When a capillary matrix like the xylem contains water under reduced or negative pressure, there is necessarily a balance between the matrix compression on one hand and the surface tension on the other, and one observes that the vapor pressure is lowered correspondingly. If water is added, the matrix swells by the same pressure. The fact that water can be pressed out of a gel, and that the swelling pressure has been found to be equivalent to the lowering of its vapor pressure, suggests that the mechanism is the same as in the capillary system, i.e., that the hydrostatic pressure is equivalent but opposite (negative) to the swelling (matrix) pressure. When a solution is forced against a semipermeable membrane, only the solute molecules are held back by the barrier, and hence their osmotic pressure can be measured. However, when the barrier is simply the free surface of the solvent, the solute molecules must exert their force upon it, thereby causing the solvent pressure to drop 24 atm per mole, with an equivalent drop in the vapor pressure. Viewed in this manner, not only the activity but indeed the hydrostatic pressure of the solvent is the same in all compartments of an osmotic system at equilibrium. Seeding solute and matrix pressure as analogues gives, therefore, a uniform explanation for the water relations in capillary, colloidal, and solute systems. It explains quantitatively the osmotic pressure and the lowering of the vapor pressure, and does away with the contradictory hydrostatic solvent gradients in the membrane at equilibrium.