Permeability of valonia to water and solutes: Apparent absence of aqueous membrane pores

Abstract

The permeability of the marine alga Valonia ventricosa to water and small solutes was studied in cells perfused internally by two micropipets inserted into the vacuole. The protoplast of Valonia was highly permeable to alcohols but relatively impermeable to water and small hydrophilic solutes. For methanol the permeability coefficient of the protoplast was 3.1 · 10 −4 cm · sec −1, and the reflection coefficient was highly negative, about −1.3. The permeability coefficient for urea, on the other hand, was about 10 −6 cm · sec −1, and the reflection coefficient was unity. No apparent solvent-solute interactions occurred for methanol, urea and water crossing the protoplast, as indicated either by a simple test for “solvent drag” or by a test based on irreversible thermodynamics. The osmotic and diffusional permeability coefficients for water were similar, about 2.4 · 10 −4 cm · sec −1. These results suggest that movements of water, urea and methanol across the protoplast of Valonia are rate limited by a non-porous plasma membrane.