Why is glucose a structural isomer?

A structural isomer is when molecules have the same formula, but a different structure. Glucose is a structural isomer because Pyruvate and Ribose have the same formula (CH2O), but have a different number of carbons, hydrogens and oxygens:

C3H6O3 - Pyruvate

C5H10O5 - Ribose

C6H12O6 - Glucose

Hope this helps! :)