Facilitative hexose transporter that mediates the transport of glucose, fructose and galactose. Likely mediates the bidirectional transfer of glucose across the plasma membrane of hepatocytes and is responsible for uptake of glucose by the beta cells; may comprise part of the glucose-sensing mechanism of the beta cell. May also participate with the Na(+)/glucose cotransporter in the transcellular transport of glucose in the small intestine and kidney. Also able to mediate the transport of dehydroascorbate. D-glucose(out) = D-glucose(in) D-fructose(out) = D-fructose(in) L-dehydroascorbate(out) = L-dehydroascorbate(in) D-galactose(in) = D-galactose(out) D-glucose and maltose competitively inhibit fructose transport. D-glucose, D-fructose and maltose inhibit deoxyglucose transport. N-glycosylated; required for stability and retention at the cell surface of pancreatic beta cells. Belongs to the major facilitator superfamily. Sugar transporter (TC 2.A.1.1) family. Glucose transporter subfamily.