Membranes of Valonia ventricosa: apparent absence of water-filled pores.

Osmotic and diffusional permeabilities to water have been measured in internally perfused cells of Valonia ventricosa. The osmotic and diffusional permeability coefficients for the protoplast are identical, 2.4 x 10(-4) centimeter per second. Thus, both osmotic and diffusional flows can occur by the same mechanism, that is, by diffusion; and there is no need to postulate the existence of water-filled pores in the membranes of this cell. Supporting evidence for this conclusion is the absence of solvent-solute interactions, that is, "solvent drag," for water, urea, and methanol crossing the protoplast of Valonia.