On simultaneous transport of water and solute through plant cell membranes: Evidence for the absence of solvent drag effect and insensitivity of the reflection coefficient

The simultaneous efflux of tritiated water and ¹⁴C labelled ethanol from inner epidermal cells of the bulb scale of Allium cepa was measured with a specially designed efflux chamber. It was found that water and ethanol moved essentially independently. Rates of efflux of tritiated water and ¹⁴C ethanol were essentially the same in the presence or absence of a simultaneous influx of water. Using the same technique the efflux of tritiated water from the epidermal cells was measured during a simultaneous flow of nonlabelled ethanol. When tritiated water and ethanol moved in opposite directions, the water permeability values became slightly reduced depending upon the concentration of ethanol. When ethanol and tritiated water moved in the same direction, however, no effect on water permeability values could be detected. These results are best explained by the molecular theory of diffusion across lipid bilayer membranes, and are consistent with the above findings of lack of interaction between water and ethanol as they are transported across the cell membrane. In another study, the solute permeability coefficients (K_s) for non-electrolytes such as urea and methyl urea were measured by plasmolyzing the epidermal cells and transferring them to equimolal solutions of urea and methyl urea. This method was also used to measure the reflection coefficient (σ) for these nonelectrolytes. The K_s values for methyl urea were 16 times greater than the ones for urea. The values of σ for both of these solutes, however, were very close to 1. Using the K_s data available in the literature for the subepidermal cells of the Pisum sativum stem basis, the σ values were calculated for malonamide, glycerol, methyl urea, ethyl urea, dimethyl urea, and formamide. Again the K_s values for these nonelectrolytes varied by several orders of magnitude, whereas all σ values were found to be close to 1. These findings point out that σ is an insensitive parameter and that K_s, the solute permeability constant, has to be used for characterizing solute transport through the membrane. The present study shows that fast (e.g. ethanol, formamide) as well as slowly permeating molecules do not interact with water as they are transported across the cell membrane. Aqueous pores for the simultaneous transport of water and solutes, therefore, are absent in the plant cell membranes investigated here.