Final answer:
Glucose and fructose are called isomers due to having the same molecular formula, C6H12O6, but different structural arrangements, with glucose being an aldose and fructose a ketose.
Step-by-step explanation:
Glucose and fructose are two monosaccharides with the same molecular formula but vary in terms of their structure, thus they are called isomers to each other. Both glucose and fructose have the chemical formula C6H12O6, yet they have different arrangements of atoms within their structures. Glucose is an aldose with an aldehyde group, whereas fructose is a ketose with a ketone group, reflecting the classification based on their carbonyl group's location within the molecule.
Moreover, the differing arrangements of functional groups around the asymmetric carbons lead to distinct properties for each sugar. This variation is why glucose and fructose, although sharing a molecular formula, differ in their physical and chemical behaviors, an important notion for understanding carbohydrates in biology and chemistry.