Definition: Those carbohydrates (saccharides) having the general composition (CH2O)n, and simple derivatives thereof. Although the simple monomeric sugars (glycoses) are often written as polyhydroxy aldehydes or ketones, HOCH2–(CHOH)4–CHO for aldohexoses (glucose) or HOCH2–(CHOH)3–CO–CH2OH for 2-ketoses (fructose), cyclization can give rise to varied structures as described below. Sugars are generally identifiable by the ending -ose or, if in combination with a nonsugar (aglycon), -oside or -osyl. Sugars, especially d-glucose, are the chief source of energy from oxidation in nature, and they and their derivatives (d-glucosamine, d-glucuronic acid), in polymeric form, are major constituents of mucoproteins, bacterial cell walls, and plant structural material (cellulose). sugars are often found in combination with steroids (steroid glycosides) and other aglycons.