Editing Biochemistry (section)

===Carbohydrates===
{{Main|Carbohydrate|Monosaccharide|Disaccharide|Polysaccharide}}
<div class='skin-invert-image'>{{multiple image
| align     = right
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| header    = [[Carbohydrates]]
| image1    = Beta-D-Glucose.svg
| width1    = 220
| caption1 = Glucose, a [[monosaccharide]]
| image2   = Sucrose-inkscape.svg
| width2   = 220
| caption2 = A molecule of [[sucrose]] ([[glucose]] + [[fructose]]), a [[disaccharide]]
| image3   = Amylose 3Dprojection.svg
| width3   = 220
| caption3 =[[Amylose]],  a [[polysaccharide]] made up of several thousand [[glucose]] units
}}</div>
Two of the main functions of carbohydrates are energy storage and providing structure. One of the common [[sugar]]s known as [[glucose]] is a carbohydrate, but not all carbohydrates are sugars. There are more carbohydrates on Earth than any other known type of biomolecule; they are used to store energy and [[Deoxyribose|genetic information]], as well as play important roles in cell to [[Cell–cell interaction|cell interactions]] and [[Cell signaling|communications]].{{cn|date=April 2023}}

The simplest type of carbohydrate is a [[monosaccharide]], which among other properties contains [[carbon]], [[hydrogen]], and [[oxygen]], mostly in a ratio of 1:2:1 (generalized formula C<sub>''n''</sub>H<sub>2''n''</sub>O<sub>''n''</sub>, where ''n'' is at least 3). [[Glucose]] (C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>) is one of the most important carbohydrates; others include [[fructose]] (C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>), the sugar commonly associated with the [[sweet taste]] of [[fruit]]s,<ref name="Whiting1970">[[#Whiting|Whiting]] (1970), pp. 1–31.</ref>{{Efn|Fructose is not the only sugar found in fruits. Glucose and sucrose are also found in varying quantities in various fruits, and sometimes exceed the fructose present. For example, 32% of the edible portion of a [[date (fruit)|date]] is glucose, compared with 24% fructose and 8% sucrose. However, [[peach]]es contain more sucrose (6.66%) than they do fructose (0.93%) or glucose (1.47%).<ref name=Whiting1970p5>[[#Whiting|Whiting]], G.C. (1970), p. 5.</ref>}} and [[deoxyribose]] (C<sub>5</sub>H<sub>10</sub>O<sub>4</sub>), a component of [[DNA]].  A monosaccharide can switch between [[Open-chain compound|acyclic (open-chain) form]] and a [[cyclic compound|cyclic]] form. The open-chain form can be turned into a ring of carbon atoms bridged by an [[oxygen]] atom created from the [[carbonyl group]] of one end and the [[hydroxyl]] group of another. The cyclic molecule has a [[hemiacetal]] or [[hemiketal]] group, depending on whether the linear form was an [[aldose]] or a [[ketose]].<ref>[[#Voet|Voet]] (2005), pp. 358–359.</ref>

In these cyclic forms, the ring usually has '''5''' or '''6''' atoms. These forms are called [[furanose]]s and [[pyranose]]s, respectively—by analogy with [[furan]] and [[pyran]], the simplest compounds with the same carbon-oxygen ring (although they lack the carbon-carbon [[double bond]]s of these two molecules).  For example, the aldohexose [[glucose]] may form a hemiacetal linkage between the hydroxyl on carbon 1 and the oxygen on carbon 4, yielding a molecule with a 5-membered ring, called [[glucofuranose]].  The same reaction can take place between carbons 1 and 5 to form a molecule with a 6-membered ring, called [[glucopyranose]]. Cyclic forms with a 7-atom ring called [[heptoses]] are rare.{{cn|date=April 2023}}

Two monosaccharides can be joined by a [[Glycosidic bond|glycosidic]] or [[ester bond]] into a ''[[disaccharide]]'' through a [[dehydration reaction]] during which a molecule of water is released. The reverse reaction in which the glycosidic bond of a disaccharide is broken into two monosaccharides is termed ''[[hydrolysis]]''.  The best-known disaccharide is [[sucrose]] or ordinary [[sugar]], which consists of a [[glucose]] molecule and a [[fructose]] molecule joined. Another important disaccharide is [[lactose]] found in milk, consisting of a glucose molecule and a [[galactose]] molecule. Lactose may be hydrolysed by [[lactase]], and deficiency in this enzyme results in [[lactose intolerance]].

When a few (around three to six) monosaccharides are joined, it is called an ''[[oligosaccharide]]'' (''oligo-'' meaning "few"). These molecules tend to be used as markers and [[Cell signaling|signals]], as well as having some other uses.<ref name="Varki_1999">[[#Varki|Varki]] (1999), p. 17.</ref> Many monosaccharides joined form a [[polysaccharide]]. They can be joined in one long linear chain, or they may be [[Branching (polymer chemistry)|branched]]. Two of the most common polysaccharides are [[cellulose]] and [[glycogen]], both consisting of repeating glucose [[monomer]]s.  ''Cellulose'' is an important structural component of plant's [[cell wall]]s and ''[[glycogen]]'' is used as a form of energy storage in animals.

[[Sugar]] can be characterized by having [[Reducing sugar|reducing]] or non-reducing ends. A [[reducing end]] of a carbohydrate is a carbon atom that can be in equilibrium with the open-chain [[aldehyde]] ([[aldose]]) or keto form ([[ketose]]). If the joining of monomers takes place at such a carbon atom, the free [[hydroxy group]] of the [[pyranose]] or [[furanose]] form is exchanged with an OH-side-chain of another sugar, yielding a full [[acetal]]. This prevents opening of the chain to the aldehyde or keto form and renders the modified residue non-reducing. Lactose contains a reducing end at its glucose moiety, whereas the galactose moiety forms a full acetal with the C4-OH group of glucose. [[Saccharose]] does not have a reducing end because of full acetal formation between the aldehyde carbon of glucose (C1) and the keto carbon of fructose (C2).