Glucose (CAS: 50-99-7) is a monosaccharide containing six carbon atoms and an aldehyde group and is therefore referred to as an aldohexose. The glucose molecule can exist in an open-chain (acyclic) and ring (cyclic) form, the latter being the result of an intramolecular reaction between the aldehyde C atom and the C-5 hydroxyl group to form an intramolecular hemiacetal. In aqueous solution, both forms are in equilibrium and at pH 7 the cyclic one is predominant. Glucose is a primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. In animals, glucose arises from the breakdown of glycogen in a process known as glycogenolysis. Glucose is synthesized in the liver and kidneys from non-carbohydrate intermediates, such as pyruvate and glycerol, by a process known as gluconeogenesis. D-Glucose is found to be associated with 3-methyl-crotonyl-glycinuria, growth hormone deficiency, and primary hypomagnesemia, which are inborn errors of metabolism. Glucose is used therapeutically in fluid and nutrient replacement. A glucoside is a glycoside that is derived from glucose. Glucosides are common in plants, but rare in animals. Glucose is produced when a glucoside is hydrolyzed by purely chemical means, or decomposed by fermentation or enzymes. Classification of the glucosides is quite difficult. One system based on the chemical constitution of the non-glucose part of the molecules has been proposed. This system outlines four groups: (1) ethylene derivatives, (2) benzene derivatives, (3) styrolene derivatives, and (4) anthracene derivatives. A group may also be made to include the cyanogenetic glucosides (i.e. those containing prussic acid). Other classifications follow a botanical classification, which has several advantages; in particular, plants of allied genera contain similar compounds.