Monosaccharides are the simplest form of carbohydrates. They may be classified based on the number of constituent carbon atoms. For instance, hexose is a monosaccharide with six carbon atoms. They may also be classified based on the functional group present: aldose and ketose. Glucose, galactose, and fructose with the same chemical formula: C6H12O6 are hexose monosaccharides. Glucose and galactose are aldoses whereas fructose is a ketose.
Glucose occurs naturally and ubiquitous. It can join with other monosaccharide units to form disaccharides: maltose (i.e. two glucose molecules), lactose (i.e. glucose and galactose molecules), and sucrose (i.e. glucose and fructose molecules).
Glucose is one of the products of photosynthesis in plants and other photosynthetic organisms. In plants, glucose molecules are stored as repeating units of sugar (e.g. starch). It is also an important component of amylopectin and cellulose. Thus, it occurs abundantly in fruits, plant juices, and many other plant organs.
Glucose also serves as an important metabolic intermediate of cellular respiration and a major source of energy (via aerobic respiration, anaerobic respiration, or fermentation). In animals, it circulates in the blood and as such referred to as blood sugar. An excess of glucose is stored as glycogen.
Glucose may either be dextro- (D-) or l(a)evo- (L-) in form. This nomenclature based on Fischer projection designates D- when the glucose stereoisomer rotates the plane polarized light in the clockwise direction. L- is when it rotates the plane polarized light in a counterclockwise direction.
Word origin: Greek gleukos ("sweetness"), from glykys ("sweet")
- blood sugar