D-glucose is classified as an _______ and D-fructose as a _______.

D-glucose is classified as an aldose because it contains an aldehyde functional group. Aldoses are sugars with the carbonyl group (C=O) located at the end of the carbon chain, which are common in many monosaccharides. In contrast, D-fructose is classified as a ketose, meaning it possesses a ketone functional group. Ketoses have the carbonyl group situated within the carbon chain, specifically at the second carbon in the case of fructose.

This classification reflects the structural differences between glucose and fructose, despite both being six-carbon sugars, or hexoses, since they each contain six carbon atoms. Understanding these differences is crucial in biochemistry, particularly when studying metabolic processes and carbohydrate chemistry.