From the standpoint of physiological processes, _glucuronic acid_ is the most interesting and important oxidation product of glucose. It is often found in the urine of animals, as the result of the partial oxidation of glucose in the animal tissues. Normally, glucose is oxidized in the body to its final oxidation products, carbon dioxide and water. But when many difficultly oxidizable substances, such as chloral, camphor, turpentine oil, aniline, etc., are introduced into the body, the organism has the power of combining these with glucose to form glucosides. These so-called "paired" compounds are then oxidized to the corresponding glucuronic acid derivatives and eliminated from the body in the urine. No phenomenon similar to this occurs in plants, however, and glucuronic acid has never been found in plant tissues. =Synthesis and Degradation of Hexoses.=--Monosaccharides of any desired number of carbon atoms can be produced from aldoses having one less carbon atoms, by way of the familiar "nitrile" reaction. Aldoses, like all other aldehydes, combine directly with hydrocyanic acid, forming compounds known as nitriles, which contain one more carbon atom than was present in the original aldehyde; the cyanogen group can easily be converted into a COOH group; and this, in turn, reduced to an aldehyde, thus producing an aldose with one more carbon atom than was present in the initial sugar. These changes may be illustrated by the following equations: (1) CHO + HCN CHOH·CN CN | | | (CHOH)_{3} = (CHOH)_{3} or (CHOH)_{4} | | | CH{2}OH CH_{2}OH CH_{2}OH Aldopentose Nitrile (2) CN + H_{2}O COOH + NH_{3} | | (CHOH)_{4} = (CHOH)_{4} | | CH_{2}OH CH_{2}OH Nitrile Acid (3) COOH - O CHO | | (CHOH)_{4} = (CHOH)_{4} | | CH_{2}OH CH_{2}OH Acid Aldohexose It is possible, by this process, to advance step by step from formaldehyde to higher sugars, Emil Fischer and his students having carried the process as far as the production of glucodecose (C_{10}H_{20}O_{10}). It usually happens, however, that two stereo-isomers result from the "step-up" by way of the nitrile reaction; thus, arabinose yields a mixture of glucose and mannose, glucose yields glucoheptose and mannoheptose, etc.