Glucose. Computer-generated model of glucose (C6H12O6). Glucose is composed of carbon (green), hydrogen (grey-white) and oxygen (red). Glucose is a monosaccharide (sugar). It consists of a ring of five carbon atoms and one oxygen atom. Attached around the ring are hydroxyl (OH) groups. Glucose is present in many plants and is the sugar carried by the blood. It is a constituent of starch, cellulose, glycogen and sucrose.