also called SIMPLE SUGAR, any of the basic compounds that serve as the building blocks of carbohydrates. are polyhydroxy aldehydes or ketones; that is, they are molecules with more than one hydroxyl group (-OH), and a carbonyl group (C=O) either at the terminal carbon atom (aldose) or at the second carbon atom (ketose). The carbonyl group combines in aqueous solution with one hydroxyl group to form a cyclic compound (hemi-acetal or hemi-ketal). are classified by the number of carbon atoms in the molecule; trioses have three, tetroses four, p ....