Editing Carbohydrate (section)

== Structure ==
Formerly the name "carbohydrate" was used in [[chemistry]] for any compound with the formula C<sub>''m''</sub> (H<sub>2</sub>O)<sub>''n''</sub>. Following this definition, some chemists considered [[formaldehyde]] (CH<sub>2</sub>O) to be the simplest carbohydrate,<ref name="coulter">{{cite book | vauthors = Coulter JM, Barnes CR, Cowles HC | year = 1930 | url = https://books.google.com/books?id=WyZnVpCiTHIC&q=simplest+carbohydrate&pg=PA375 | title = A Textbook of Botany for Colleges and Universities | publisher = BiblioBazaar | isbn = 978-1113909954 | access-date = April 24, 2022 | archive-date = April 17, 2022 | archive-url = https://web.archive.org/web/20220417005854/https://books.google.com/books?id=WyZnVpCiTHIC&q=simplest+carbohydrate&pg=PA375 | url-status = live }}</ref> while others claimed that title for [[glycolaldehyde]].<ref name="tietz">{{cite book | vauthors = Burtis CA, Ashwood ER, Tietz NW | year = 2000 | url = https://books.google.com/books?id=l5hqAAAAMAAJ&q=simplest+carbohydrate | title = Tietz fundamentals of clinical chemistry | publisher = W.B. Saunders | isbn = 9780721686349 | access-date = January 8, 2016 | archive-date = June 24, 2016 | archive-url = https://web.archive.org/web/20160624073749/https://books.google.com/books?id=l5hqAAAAMAAJ&q=simplest+carbohydrate | url-status = live }}</ref> Today, the term is generally understood in the biochemistry sense, which excludes compounds with only one or two carbons and includes many biological carbohydrates which deviate from this formula. For example, while the above representative formulas would seem to capture the commonly known carbohydrates, ubiquitous and abundant carbohydrates often deviate from this. For example, carbohydrates often display chemical groups such as: ''N''-acetyl (e.g., [[chitin]]), [[sulfate]] (e.g., [[glycosaminoglycan]]s), [[carboxylic acid]]  and deoxy modifications (e.g., [[fucose]] and [[sialic acid]]).

Natural saccharides are generally built of simple carbohydrates called [[monosaccharide]]s with general formula (CH<sub>2</sub>O)<sub>''n''</sub> where ''n'' is three or more. A typical monosaccharide has the structure H–(CHOH)<sub>''x''</sub>(C=O)–(CHOH)<sub>''y''</sub>–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]]. Examples of monosaccharides are [[glucose]], [[fructose]], and [[glyceraldehyde]]s. However, some biological substances commonly called "monosaccharides" do not conform to this formula (e.g., [[uronic acid]]s 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<sub>2</sub>O and [[inositol]] (CH<sub>2</sub>O)<sub>6</sub>).<ref>{{cite book | vauthors = Matthews CE, Van Holde KE, Ahern KG | year = 1999 | title = Biochemistry | edition = 3rd | publisher = Benjamin Cummings | isbn = 978-0-8053-3066-3 }}{{page needed|date=January 2018}}</ref>

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.

Monosaccharides can be linked together into what are called [[polysaccharide]]s (or [[oligosaccharide]]s) in a large variety of ways. Many carbohydrates contain one or more modified monosaccharide units that have had one or more groups replaced or removed. For example, [[deoxyribose]], a component of [[DNA]], is a modified version of [[ribose]]; [[chitin]] is composed of repeating units of [[N-acetyl glucosamine]], a [[nitrogen]]-containing form of glucose.