The uptake kinetics of sorbitol, sucrose, glucose and fructose across the plasma membrane using protoplasts isolated from apple fruit flesh (Malus pumila Mill. var. domestica Schneid.) was investigated. When sorbitol was taken up into the cell, PCMBS-sensitive saturable transport was distinguishable from the diffusive transport. At a low sorbitol concentration, the saturable transport system accounted for more than 50% of the total uptake, whereas at a high concentration the diffusive transport system was more dominant. The saturable transport was suggested be a carrier-mediated transport system coupled with ATP because the system was inhibited CCCP or orthovanadate. The Km value for sorbitol was computed to be 3.6mM. A carrier-mediated transport system coupled with ATP was also observed for glucose and fructose with corresponding Km values of 5.0 and 2.5 mM. However, no saturable transport for sucrose was observed over a range of 0.1 to 10 mM sucrose concentration. The relationship among these sugar transport systems across the plasma membrane, apoplastic unloading, and sugar accumulation vacuoles are discussed.
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Yamaki et al. (1988) studied this question.