one of a group of carbohydrates known as simple sugars (monosaccharides). Glucose (from Greek glykys; “sweet”) has the molecular formula C₆H₁₂O₆. It is found in fruits and honey and is the major free sugar circulating in the blood of higher animals. It is the source of energy in cell function, and the regulation of its metabolism is of great importance (see fermentation; gluconeogenesis). Molecules of starch, the major energy-reserve carbohydrate of plants, consist of thousands of glucose units, as do those of cellulose (q.v.). Also composed of glucose is glycogen (q.v.), the reserve carbohydrate in most vertebrate and invertebrate animal cells, as well as those of numerous fungi and protozoans. See also polysaccharide.