Many carbohydrates contain one or more modified monosaccharide units that have had one or more groups replaced or removed. Monosaccharides can be linked together into what are called polysaccharides (or oligosaccharides) in a large variety of ways. The open-chain form of a monosaccharide often coexists with a heterocyclic compound|closed ring form where the aldehyde/ ketone carbonyl group carbon (C=O) and hydroxyl group (-OH) react forming a hemiacetal with a new C-O-C bridge. However, some biological substances commonly called "monosaccharides" do not conform to this formula (e.g., uronic acids and deoxy-sugars such as fucose), and there are many chemicals that do conform to this formula but are not considered to be monosaccharides (e.g., formaldehyde CH 2O and inositol (CH 2O) 6). Examples of monosaccharides are glucose, fructose, and glyceraldehyde. A typical monosaccharide has the structure H-(CHOH) x(C=O)-(CHOH) y-H, that is, an aldehyde or ketone with many hydroxyl groups added, usually one on each carbon atom that is not part of the aldehyde or ketone functional group. Natural saccharides are generally built of simple carbohydrates called monosaccharides with general formula (CH 2O) n where n is three or more. Today the term is generally understood in the biochemistry sense, which excludes compounds with only one or two carbons atoms. While others claimed that title for glycolaldehyde. Following this definition, some chemists considered formaldehyde CH 2O to be the simplest carbohydrate, Earlier the name "carbohydrate" was used in chemistry for any compound with the formula C m(H 2O) n.
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