A Comparison of the Relative Rates of Transport of Ascorbate and Glucose Across the Thylakoid, Chloroplast and Plasmalemma Membranes of Pea Leaf Mesophyll Cells

Mesophyll protoplasts, chloroplasts and thylakoids were prepared from the leaves of 10-12 day old pea seedlings. They contained 1618 ± 351, 197 ± 30 and 25 ± 7nmol ascorbate per milligram of chlorophyll respectively equivalent to average concentrations of 8.4, 9.8 and 3.8 mM ascorbate in the intact protoplasts, intact chloroplasts and thylakoid membranes respectively. Transport of [ 14C]-ascorbate and [ 14C]-glucose across the plasmalemma, chloroplast envelope and thylakoid membranes was measured in these fractions. Saturation kinetics for [ 14C]-ascorbate and [ l4C]-glucose uptake suggest that the plasmalemma and chloroplast envelope membranes contain carriers for both ascorbate and glucose. In contrast, the thylakoid membranes appear to have no transport system for ascorbate, permeability resulting from diffusion alone. Dehydroascorbate inhibited ascorbate transport but not glucose transport across the chloroplast envelope membrane. Ascorbate stimulated [ 14C]-glucose transport across the plasmalemma and chloroplast envelope membranes. Similarly, glucose had a positive effect on [ l4C]-ascorbate transport across these membranes. These observations suggest the presence of independent carriers for ascorbate and glucose into both protoplasts and chloroplasts. Unlike animal membranes where the hexose transporters have been found to transport dehydroascorbate the plant counterparts does not appear to do so, either in the chloroplast envelope or the plasmalemma membranes. The ascorbate transporters on these membranes are hence discrete entities not involved with general hexose transport. However, the observed synergistic effects of either glucose or ascorbate on the transport of the other metabolite suggest coordination of the two processes.