Editing Carbohydrate (section)

==Monosaccharides==
{{Main|Monosaccharide}}

[[Image:D-glucose color coded.png|upright=0.5|thumb|class=skin-invert|[[Glucose|D-glucose]] is an aldohexose with the formula (C·H<sub>2</sub>O)<sub>6</sub>. The red atoms highlight the [[aldehyde]] group and the blue atoms highlight the [[chirality (chemistry)|asymmetric center]] furthest from the aldehyde; because this -OH is on the right of the [[Fischer projection]], this is a D sugar.]]
Monosaccharides are the simplest carbohydrates in that they cannot be [[hydrolysis|hydrolyzed]] to smaller carbohydrates. They are aldehydes or ketones with two or more hydroxyl groups. The general [[chemical formula]] of an unmodified monosaccharide is (C•H<sub>2</sub>O)<sub>n</sub>, literally a "carbon hydrate". Monosaccharides are important fuel molecules as well as building blocks for nucleic acids. The smallest monosaccharides, for which n=3, are dihydroxyacetone and D- and L-glyceraldehydes.

===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>

===Ring-straight chain isomerism===
[[Image:Glucose Fisher to Haworth.gif|thumb|class=skin-invert|[[Glucose]] can exist in both a straight-chain and ring form.]]
The aldehyde or ketone group of a straight-chain monosaccharide will react reversibly with a hydroxyl group on a different carbon atom to form a [[hemiacetal]] or [[hemiketal]], forming a [[heterocyclic]] ring with an oxygen bridge between two carbon atoms. Rings with five and six atoms are called [[furanose]] and [[pyranose]] forms, respectively, and exist in equilibrium with the straight-chain form.<ref name=pigman>{{cite book | vauthors = Pigman W, Anet EF |title=The Carbohydrates: Chemistry and Biochemistry Vol 1A| veditors = Pigman W, Horton D  |edition=2nd|year=1972|publisher=Academic Press|location=San Diego|pages=165–194|chapter=Chapter 4: Mutarotations and Actions of Acids and Bases|isbn=978-0323138338}}</ref>

During the conversion from straight-chain form to the cyclic form, the carbon atom containing the carbonyl oxygen, called the [[anomeric carbon]], becomes a stereogenic center with two possible configurations: The oxygen atom may take a position either above or below the plane of the ring. The resulting possible pair of stereoisomers is called [[anomer]]s. In the ''α anomer'', the -OH substituent on the anomeric carbon rests on the opposite side ([[Cis-trans isomerism|trans]]) of the ring from the CH<sub>2</sub>OH side branch. The alternative form, in which the CH<sub>2</sub>OH substituent and the anomeric hydroxyl are on the same side (cis) of the plane of the ring, is called the ''β anomer''.{{citation needed|date=April 2025}}

===Use in living organisms===
Monosaccharides are the major fuel source for [[metabolism]], and glucose is an energy-rich molecule utilized to generate ATP in almost all living organisms.  Glucose is a high-energy substrate produced in plants through photosynthesis by combining energy-poor water and carbon dioxide in an endothermic reaction fueled by solar energy.  When monosaccharides are not immediately needed, they are often converted to more space-efficient (i.e., less water-soluble) forms, often [[polysaccharide]]s.  In animals, glucose circulating the blood is a major metabolic substrate and is oxidized in the mitochondria to produce ATP for performing useful cellular work.  In humans and other animals, serum glucose levels must be regulated carefully to maintain glucose within acceptable limits and prevent the deleterious effects of hypo- or hyperglycemia.  Hormones such as insulin and glucagon serve to keep glucose levels in balance: insulin stimulates glucose uptake into the muscle and fat cells when glucose levels are high, whereas glucagon helps to raise glucose levels if they dip too low by stimulating hepatic glucose synthesis.  In many animals, including humans, this storage form is [[glycogen]], especially in liver and muscle cells. In plants, [[starch]] is used for the same purpose. The most abundant carbohydrate, [[cellulose]], is a structural component of the [[cell wall#plant cell walls|cell wall]] of plants and many forms of algae. [[Ribose]] is a component of [[RNA]]. [[Deoxyribose]] is a component of [[DNA]]. [[Lyxose]] is a component of lyxoflavin found in the human [[heart]].<ref>{{cite encyclopedia |title=lyxoflavin |url=http://www.merriam-webster.com/medical/lyxoflavin |dictionary=Merriam-Webster |access-date=February 26, 2014 |archive-date=October 31, 2014 |archive-url=https://web.archive.org/web/20141031135041/http://www.merriam-webster.com/medical/lyxoflavin |url-status=live }}</ref> [[Ribulose]] and [[xylulose]] occur in the [[pentose phosphate pathway]]. [[Galactose]], a component of milk sugar [[lactose]], is found in [[galactolipid]]s in [[cell membrane#lipids|plant cell membranes]] and in [[glycoprotein]]s in many [[biological tissue|tissues]]. [[Mannose]] occurs in human metabolism, especially in the [[glycosylation]] of certain proteins. [[Fructose]], or fruit sugar, is found in many plants and humans, it is metabolized in the liver, absorbed directly into the intestines during [[digestion]], and found in [[semen]]. [[Trehalose]], a major sugar of insects, is rapidly hydrolyzed into two glucose molecules to support continuous flight.