Carbohydrate
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ClockworkSoul
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[[WP:UNDO|Undid]] revision 225741338 by [[Special:Contributions/61.95.178.56|61.95.178.56]] ([[User talk:61.95.178.56|talk]])
[[Image:Lactose.svg|thumb|right|310px|[[Lactose]] is a [[disaccharide]] found in [[milk]]. It is composed of a molecule of [[galactose|D-galactose]] and a molecule of [[glucose|D-glucose]] bonded by a β-1-4 [[glycosidic linkage]].]]
'''Carbohydrates''' (from '[[hydrate]]s of [[carbon]]') or '''saccharides''' ([[Greek language|Greek]] ''σάκχαρον'' meaning "[[sugar]]") are the most abundant of the four major classes of [[biomolecule]]s, which also include [[protein]]s, [[lipid]]s and [[nucleic acid]]s. They fill numerous roles in living things, such as the storage and transport of [[energy]] ([[starch]], [[glycogen]]) and structural components ([[cellulose]] in plants, [[chitin]] in animals). Additionally, carbohydrates and their derivatives play major roles in the working process of the [[immune system]], [[fertilization]], [[pathogenesis]], [[blood clotting]], and [[developmental biology|development]].<ref>{{cite book
| last = Maton
| first = Anthea
| authorlink =
| coauthors = Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright
| title = Human Biology and Health
| publisher = Prentice Hall
| date = 1993
| location = Englewood Cliffs, New Jersey, USA
| pages = 52-59
| url =
| doi =
| id =
| isbn = 0-13-981176-1}}</ref>
Chemically, carbohydrates are simple [[organic compound]]s that are [[aldehyde]]s or [[ketone]]s with many [[hydroxyl]] groups added, usually one on each carbon atom that is not part of the aldehyde or ketone [[functional group]]. The basic carbohydrate units are called [[monosaccharide]]s, such as [[glucose]], [[galactose]], and [[fructose]]. The general [[stoichiometry|stoichiometric]] [[chemical formula|formula]] of an unmodified monosaccharide is (C·H<sub>2</sub>O)<sub>n</sub>, where n is any number of three or greater; however, the use of this word does not follow this exact definition and many molecules with formulae that differ slightly from this are still called carbohydrates, and others that possess formulae agreeing with this general rule are not called carbohydrates (eg formaldehyde).<ref>Matthews, C. E.; K. E. Van Holde; K. G. Ahern (1999) ''Biochemistry''. 3rd edition. Benjamin Cummings. ISBN 0-8053-3066-6</ref>
Monosaccharides can be linked together into [[polysaccharide]]s in almost limitless 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-acetylglucosamine]], a [[nitrogen]]-containing form of glucose. The names of carbohydrates often end in the suffix [[-ose]].
==Monosaccharides==
{{main|Monosaccharide}}
[[Image:D-glucose color coded.png|130px|right|thumb|[[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 hydrolyzed to smaller carbohydrates. The general [[chemical formula]] of an unmodified monosaccharide is (C•H<sub>2</sub>O)<sub>n</sub>, where n is any number of three or greater.
=== Classification of monosaccharides ===
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[[Image:Alpha-D-glucopyranose-2D-skeletal.png|185px]]
[[Image:Beta-D-glucopyranose-2D-skeletal.png|185px]]
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The [[α]] and [[Beta (letter)|β]] [[anomer]]s of glucose. Note the position of the anomeric carbon (red or green) relative to the CH<sub>2</sub>OH group bound to carbon 5: they are either on the opposite sides (α), or the same side (β).
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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. 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|stereocenter]]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. The aldohexose D-glucose, for example, has the formula (C·H<sub>2</sub>O)<sub>6</sub>, of which all but two 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]], 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 glyceraldehyde, is a symmetric molecule with no stereocenters). 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. Because D sugars are biologically far more common, the D is often omitted.
===Conformation===
[[Image:Glucose Fisher to Haworth.gif|thumb|200px|right||[[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.
During the conversion from straight-chain form to cyclic form, the carbon atom containing the carbonyl oxygen, called the [[anomeric carbon]], becomes a chiral 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 are 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''. Because the ring and straight-chain forms readily interconvert, both anomers exist in [[chemical equilibrium|equilibrium]].
===Use in living organisms===
Monosaccharides are the major source of fuel for [[metabolism]], being used both as an energy source (glucose being the most important in nature) and in [[biosynthesis]]. When monosaccharides are not needed by cells they are quickly converted into another form, such as [[polysaccharide]]s.
==Disaccharides==
[[Image:Saccharose.svg|thumb|right|250px|[[Sucrose]], also known as table [[sugar]], is a common disaccharide. It is composed of two monosaccharides: [[glucose|D-glucose]] (left) and [[fructose|D-fructose]] (right).]]
{{main|Disaccharide}}
Two joined monosaccharides are called [[disaccharide]]s and represent the simplest polysaccharides. Examples include [[sucrose]] and [[lactose]]. They are composed of two monosaccharide units bound together by a [[covalent]] bond known as a [[glycosidic linkage]] formed via a [[dehydration reaction]], resulting in the loss of a [[hydrogen]] atom from one monosaccharide and a [[hydroxyl group]] from the other. The [[chemical formula|formula]] of unmodified disaccharides is C<sub>12</sub>H<sub>22</sub>O<sub>11</sub>. Although there are numerous kinds of disaccharides, a handful of disaccharides are particularly notable.
[[Sucrose]], pictured to the right, is the most abundant disaccharide and the main form in which carbohydrates are transported in [[plant]]s. It is composed of one [[glucose|D-glucose]] molecule and one [[fructose|D-fructose]] molecule. The [[systematic name]] for sucrose, ''O''-α-D-glucopyranosyl-(1→2)-D-fructofuranoside, indicates four things:
* Its monosaccharides: glucose and fructose
* Their ring types: glucose is a [[pyranose]], and fructose is a [[furanose]]
* How they are linked together: the oxygen on carbon number 1 (C1) of α-D-glucose is linked to the C2 of D-fructose.
* The ''-oside'' suffix indicates that the [[anomeric carbon]] of both monosaccharides participates in the glycosidic bond.
[[Lactose]], a disaccharide composed of one [[galactose|D-galactose]] molecule and one [[glucose|D-glucose]] molecule, occurs naturally in [[milk]]. The [[systematic name]] for lactose is ''O''-β-D-galactopyranosyl-(1→4)-D-glucopyranose. Other notable disaccharides include [[maltose]] (two D-glucoses linked α-1,4) and cellulobiose (two D-glucoses linked β-1,4).
==Oligosaccharides and polysaccharides==
[[Image:Amylose.svg|thumb|right|350px|[[Amylose]] is a linear [[polymer]] of [[glucose]] mainly linked with α(1→4) bonds. It can be made of several thousands of glucose units. It is one of the two components of [[starch]], the other being [[amylopectin]].]]
{{main|Oligosaccharide|Polysaccharide}}
Oligosaccharides and polysaccharides are composed of longer chains of monosaccharide units bound together by glycosidic bonds. The distinction between the two is based upon the number of monosaccharide units present in the chain. Oligosaccharides typically contain between two and nine monosaccharide units, and polysaccharides contain greater than ten monosaccharide units. Definitions of how large a carbohydrate must be to fall into each category vary according to personal opinion. Examples of oligosaccharides include the disaccharides mentioned above, the trisaccharide [[raffinose]] and the tetrasaccharide stachyose.
Oligosaccharides are found as a common form of [[protein]] [[posttranslational modification]]. Such posttranslational modifications include the Lewis and ABO oligosaccharides responsible for [[blood group]] incompatibilities, the alpha-Gal epitope responsible for hyperacute rejection in xenotransplanation, and O-GlcNAc modifications.
Polysaccharides represent an important class of biological [[polymer]]s. Their [[function (biology)|function]] in living organisms is usually either structure or storage related. [[Starch]] is used as a storage polysaccharide in plants, being found in the form of both [[amylose]] and the branched [[amylopectin]]. In animals, the structurally similar but more densely branched [[glycogen]] is used instead. Glycogen's properties allow it to be metabolized more quickly, which suits the active lives of locomotive animals.
[[Cellulose]] and [[chitin]] are examples of structural polysaccharides. Cellulose is used in the [[cell wall]]s of plants and other organisms, and is claimed to be the most abundant organic molecule on earth.<ref>N.A.Campbell (1996) ''Biology'' (4th edition). Benjamin Cummings NY. p.23 ISBN 0-8053-1957-3</ref> It has a variety of uses including in the paper and textile industry and as a feedstock for the production of rayon (in the viscose process), cellulose acetate, celluloid and nitrocellulose. Chitin has a similar structure to cellulose but has [[nitrogen]] containing side branches, increasing its strength. It is found in [[arthropod]] [[exoskeleton]]s and in the cell walls of some [[fungi]]. It has a variety of uses, for example in [[surgical thread]]s.
Other polysaccharides include [[callose]] or [[laminarin]], [[xylan]], [[mannan]], fucoidan, and [[galactomannan]].
==Nutrition==
[[Image:starchy-foods..jpg|thumb|[[cereal|Grain]] products: rich sources of complex and simple carbohydrates]]
Carbohydrates require less [[water]] to digest than [[protein]]s or [[fat]]s and are the most common source of energy. Proteins and fat are vital building components for body [[biological tissue|tissue]] and [[cell (biology)|cells]] and are also a source of energy for the body.
Carbohydrates are not [[essential nutrient]]s: the body can obtain all its energy from protein and fats<ref>[http://www.ajcn.org/cgi/content/full/75/5/951-a Is dietary carbohydrate essential for human nutrition? - Westman 75 (5): 951 - American Journal of Clinical Nutrition<!-- Bot generated title -->]</ref><ref>[http://jn.nutrition.org/cgi/reprint/136/5/1256?ijkey=ebf0450b5cf21e8d83dd43f62b5559254694f65f A High-Protein, High-Fat, Carbohydrate-Free Diet Reduces Energy Intake, Hepatic Lipogenesis, and Adiposity in Rats - Pichon et al. 136 (5): 1256 - Journal of Nutrition<!-- Bot generated title -->]</ref>. The brain cannot burn fat and needs glucose for energy, but the body can make this glucose from protein. Carbohydrates contain 3.75 and proteins 4 [[kilocalories]] per gram, respectively, while fats contain 9 kilocalories and alcohol contains 7 kilocalories per gram.{{Fact|date=August 2007}}
Foods that are high in carbohydrates include [[bread]]s, [[pasta]]s, [[bean]]s, [[potato]]es, [[bran]], [[rice]] and [[cereal]]s.
Based on evidence for risk of heart disease and obesity, the [[Institute of Medicine]] recommends that American and Canadian adults get between 40-65% of [[food energy|dietary energy]] from carbohydrates.<ref>Food and Nutrition Board (2002/2005). ''[http://newton.nap.edu/books/0309085373/html Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids]''. Washington, DC: The [[National Academies Press]]. Page [http://newton.nap.edu/books/0309085373/html/769.html 769]. ISBN 0-309-08537-3</ref>
The [[Food and Agriculture Organization]] and [[World Health Organization]] jointly recommend that national dietary guidelines set a goal of 55-75% of total energy from carbohydrates, but only 10% should be from Free sugars (their definition of simple carbohydrates).<ref>Joint WHO/FAO expert consultation (2003). ''[http://www.who.int/hpr/NPH/docs/who_fao_expert_report.pdf Diet, Nutrition and the Prevention of Chronic Diseases]'' ([[Portable Document Format|PDF]]). Geneva: [[World Health Organization]]. Pages 55-56. ISBN 92-4-120916-X</ref>
===Classification===
[[Dietitian]]s and [[nutritionist]]s commonly classify carbohydrates as simple ([[monosaccharide]]s and [[disaccharide]]s) or complex ([[oligosaccharide]]s and [[polysaccharide]]s). The term ''complex carbohydrate'' was first used in the Senate Select Committee publication ''Dietary Goals for the United States'' (1977), where it denoted "fruit, vegetables and whole-grains".<ref>Joint WHO/FAO expert consultation (1998), ''Carbohydrates in human nutrition'', [http://www.fao.org/docrep/W8079E/w8079e07.htm chapter 1]. ISBN 92-5-104114-8.</ref> Dietary guidelines generally recommend that complex carbohydrates and nutrient-rich simple carbohydrates such as [[fruit]] and [[dairy product]]s make up the bulk of carbohydrate consumption. The [[United States Department of Agriculture|USDA's]] ''Dietary Guidelines for Americans 2005'' dispenses with the simple/complex distinction, instead recommending fiber-rich foods and whole grains.<ref>[[United States Department of Health and Human Services|DHHS]] and [[United States Department of Agriculture|USDA]], ''Dietary Guidelines for Americans 2005'', [http://www.health.gov/dietaryguidelines/dga2005/document/html/chapter7.htm Chapter 7 Carbohydrates]</ref>
The [[glycemic index]] and [[glycemic load]] systems are popular alternative classification methods which rank carbohydrate-rich foods based on their effect on [[blood glucose]] levels. The [[insulin index]] is a similar, more recent classification method which ranks foods based on their effects on [[insulin|blood insulin]] levels. This system assumes that high glycemic index foods and low glycemic index foods can be mixed to make the intake of high glycemic foods more acceptable. {{Fact|date=February 2007}}
==Metabolism==
{{Main|Carbohydrate metabolism}}
{{Expand-section|date=June 2008}}
===Catabolism===
Catabolism is the metabolic reaction cells undergo in order to extract energy. There are two major [[metabolic pathway]]s of monosaccharide [[catabolism]]:
# [[Glycolysis]]
# [[Citric acid cycle]]
Oligo/polysaccharides are cleaved first to smaller monosaccharides by enzymes called [[Glycoside hydrolase]]s. The monosaccharide units can then enter into monosaccharide catabolism.
==Carbohydrate chemistry==
Carbohydrates are reactants in many [[organic reaction]]s. For example:
* [[Carbohydrate acetalisation]]
* [[Cyanohydrin reaction]]
* [[Lobry-de Bruyn-van Ekenstein transformation]]
* [[Amadori rearrangement]]
* [[Nef reaction]]
* [[Wohl degradation]]
* [[Koenigs-Knorr reaction]]
==See also==
{{commonscat|Carbohydrates}}
<!-- Please keep alphabetical -->
* [[Biochemistry]]
* [[Gluconeogenesis]]
* [[Glycolipid]]
* [[Glycoprotein]]
* [[Macromolecules]]
* [[Nutrition]]
* [[Pentose phosphate pathway]]
* [[Photosynthesis]]
* [[Sugar]]
{{metabolism}}
{{Food chemistry}}
{{Carbohydrates}}
==References==
<references/>
==External links==
* [http://www2.ufp.pt/~pedros/bq/carb_en.htm Carbohydrates, including interactive models and animations] (Requires [http://www.mdl.com/products/framework/chime/ MDL Chime])
* [http://www.chem.qmw.ac.uk/iupac/2carb/ IUPAC-IUBMB Joint Commission on Biochemical Nomenclature (JCBN): Carbohydrate Nomenclature]
* [http://www.cem.msu.edu/~reusch/VirtualText/carbhyd.htm Carbohydrates detailed]
* [http://www.biochemweb.org/carbohydrates.shtml Carbohydrates and Glycosylation - The Virtual Library of Biochemistry and Cell Biology]
* [http://www.functionalglycomics.org/static/consortium/ Consortium for Functional Glycomics]
{{Carbohydrates}}
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[[Category:Nutrition]]
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