Evidence for a mobile membrane carrier mediating the uphill monosaccharide transport in the yeastRhodotorula gracilis is based on two types of observations: (1) Countertransport was found with(14)C-labelledD-xylose,L-xylose,L-rhamnose and withL-rhamnose in a cell suspension preincubated with unlabelledD-xylose. This finding indicates, moreover, that both the hexoses and the pentose share the same membrane carrier. (2) The mobility of occupied carrier molecules is higher than that of free carrier molecules. This conclusion has been drawn from: (a) comparison of the initial rates of uptake of a labelled sugar into cells preincubated in the absence and in the presence of unlabelled sugar; (b) comparison on the half-saturation constant of transport with the dissociation constant of the sugar-carrier complex; and (c) comparison of the initial rates of efflux of a labelled sugar into sugar-free and sugar-containing medium.

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