MECHANISM OF GLUCOSE TRANSPORT ACROSS THE YEAST CELL MEMBRANE

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Abstract
The kinetics of D-glucose and L-sorbose transport was studied in Saccharomyces cerevisiae inhibited with iodoacetic acid under nitrogen to prevent glucose metabolism. D-glucose was found to compete with L-sorbose for a common membrane transport system with an apparent affinity greater than 25 times that of sorbose. A comparison of the net rate of glucose and sorbose transport at 50 and 500 mM external concentration showed that glucose transport is greater than that of sorbose from the lower concentration, but sorbose transport is greater than glucose at the higher concentration. This reversal of transport rate of two sugars with markedly different affinities is predicted by the membrane carrier theory. A further prediction of carrier theory was confirmed by the demonstration that the rate of glucose transport into fructose-loaded cells is greater than into unloaded cells.