HEXOSE UPTAKE BY STORAGE PARENCHYMA OF POTATO AND SUGAR-BEET AT DIFFERENT CONCENTRATIONS AND DIFFERENT THICKNESSES OF TISSUE-SLICES

At low substrate concentrations, the rate of monosaccharide uptake by plant parenchymal is limited by the diffusion of sugar in the free space. This was demonstrated in experiments with slices of sugar-beet parenchyma and potato parenchyma of various thicknesses. A diffusion equilibrium between the free space even of thin tissue slices (approximately 1 mm in thickness) and the external solution can only be assumed for sugar uptake at high substrate concentrations. The concentration required for an apparent diffusion equilibrium is above 0.1 mol/l in the case of potato slices but is already reached at 0.001 mol/l for parenchymal slices of sugar-beet roots. The limiting role of diffusion among the cells of a multicellular tissue means that kinetic constants of membrane transport cannot be simply determined from the concentration dependence of uptake.