Fluxes of a nonelectrolyte and compartmentation in cells of carrot root tissue.

Permeation of thiourea into cells of carrot (Daucus carota L.) storage root discs was found to be governed by its concentration gradient only. The analysis of the course of thiourea efflux from preloaded discs shows two distinct exponential curves, apart from an initial very fast component. Reasons are given for relating the first exponential part of the curve to efflux from the cytoplasm and the second exponential part to efflux from the vacuole.The rate constant for thiourea efflux through the tonoplast is markedly temperature-dependent (activation energy 13.2 kcal mole(-)) and is lowered by 2,4-dinitrophenol and by phenyl mercuric acetate treatments. The rate constant for its efflux through the plasmalemma, on the other hand, has a low activation energy (4.2 kcal mole(-)), which is equal to that of free diffusion of thiourea in aqueous solution, and is not affected by the metabolic inhibitors. Possible reasons for the difference in the properties of the two membranes are discussed.