Editing Carbohydrate (section)

===Classification of monosaccharides===
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[[Image:Alpha-D-glucopyranose-2D-skeletal.svg|185px|class=skin-invert]]
[[Image:Beta-D-glucopyranose-2D-skeletal.svg|185px|class=skin-invert]]
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The [[α]] and [[Beta (letter)|β]] [[anomer]]s of glucose. Note the position of the hydroxyl group (red or green) on the anomeric carbon relative to the CH<sub>2</sub>OH group bound to carbon 5: they either have identical absolute configurations (R,R or S,S) (α), or opposite absolute configurations (R,S or S,R) (β).<ref>{{cite book | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK1955/#_ch2_s4_ | chapter = Structural Basis of Glycan Diversity | title = Essentials of Glycobiology | edition = 3rd | publisher = Cold Spring Harbor Laboratory Press | location = Cold Spring Harbor (NY) | vauthors = Bertozzi CR, Rabuka D | isbn = 978-1-621821-32-8 | year = 2017 | pmid = 20301274 | access-date = August 30, 2017 | archive-date = May 19, 2020 | archive-url = https://web.archive.org/web/20200519081218/https://www.ncbi.nlm.nih.gov/books/NBK1955/#_ch2_s4_ | url-status = live }}</ref>
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Monosaccharides are classified according to three different characteristics: the placement of its [[carbonyl]] group, the number of [[carbon]] atoms it contains, and its [[chirality (chemistry)|chiral]] handedness. If the carbonyl group is an [[aldehyde]], the monosaccharide is an [[aldose]]; if the carbonyl group is a [[ketone]], the monosaccharide is a [[ketose]]. Monosaccharides with three carbon atoms are called [[triose]]s, those with four are called [[tetrose]]s, five are called [[pentose]]s, six are [[hexose]]s, and so on.<ref>{{cite book | vauthors = Campbell NA, Williamson B, Heyden RJ | title = Biology: Exploring Life | publisher = Pearson Prentice Hall | year = 2006 | location = Boston, Massachusetts | url = http://www.phschool.com/el_marketing.html | isbn = 978-0-13-250882-7 | access-date = December 2, 2008 | archive-date = November 2, 2014 | archive-url = https://web.archive.org/web/20141102041816/http://www.phschool.com/el_marketing.html | url-status = live }}</ref> These two systems of classification are often combined. For example, [[glucose]] is an [[aldohexose]] (a six-carbon aldehyde), [[ribose]] is an [[aldopentose]] (a five-carbon aldehyde), and [[fructose]] is a [[ketohexose]] (a six-carbon ketone).

Each carbon atom bearing a [[hydroxyl group]] (-OH), with the exception of the first and last carbons, are [[Chirality (chemistry)|asymmetric]], making them [[Stereogenic|stereo center]]s with two possible configurations each (R or S). Because of this asymmetry, a number of [[isomer]]s may exist for any given monosaccharide formula. Using [[Le Bel-van't Hoff rule]], the aldohexose D-glucose, for example, has the formula (C·H<sub>2</sub>O)<sub>6</sub>, of which four of its six carbons atoms are stereogenic, making D-glucose one of 2<sup>4</sup>=16 possible [[stereoisomer]]s. In the case of [[glyceraldehyde]]s, an aldotriose, there is one pair of possible stereoisomers, which are [[enantiomers]] and [[epimer]]s. [[Dihydroxyacetone|1, 3-dihydroxyacetone]], the ketose corresponding to the aldose glyceraldehydes, is a symmetric molecule with no stereo centers. The assignment of D or L is made according to the orientation of the asymmetric carbon furthest from the carbonyl group: in a standard Fischer projection if the hydroxyl group is on the right the molecule is a D sugar, otherwise it is an L sugar. The "D-" and "L-" prefixes should not be confused with "d-" or "l-", which indicate the direction that the sugar  [[Levorotation and dextrorotation|rotates]] plane [[Polarization (waves)|polarized light]]. This usage of "d-" and "l-" is no longer followed in carbohydrate chemistry.<ref>{{cite book | vauthors = Pigman W, Horton D | title=The Carbohydrates: Chemistry and Biochemistry Vol 1A| veditors = Pigman W, Horton D |edition=2nd|year=1972|publisher=Academic Press|location=San Diego|pages=1–67|chapter=Chapter 1: Stereochemistry of the Monosaccharides|isbn=978-0323138338}}</ref>