Carbohydrate

Carbohydrates are chemical compoundsthat contain oxygen, hydrogen, and carbonatoms. They consist of monosaccharidesugarsof varying chain lengths and that have the general chemical formulaC_n(H₂O)_n or are derivativesof such.

Certain carbohydrates are an important storage and transport form of energyin most organisms, including plantsand animals. Carbohydrates are classified by their number of sugar units: monosaccharides (such as glucose), disaccharides(such as saccharose), oligosaccharides, and polysaccharides(such as starch, glycogen, and cellulose). About 55% of your calories come from carbohydrates.

Structure

Image:D-glucose.png Image:D-fructose.png Pure carbohydrates contain carbon, hydrogen, and oxygenatoms, in a 1:2:1 molarratio, giving the general formulaC_n(H₂O)_n. (This applies only to monosaccharides, see below, although all carbohydrates have the more general formula C_n(H₂O)_m.) However, many important "carbohydrates" deviate from this, such as deoxyriboseand glycerol, although they are not, in the strict sense, carbohydrates. Sometimes compounds containing other elements are also counted as carbohydrates (e.g. chitin, which contains nitrogen).

The simplest carbohydrates are monosaccharides, which are small straight-chain aldehydesand ketoneswith many hydroxylgroups added, usually one on each carbon except the functional group. Other carbohydrates are composed of monosaccharide units and break down under hydrolysis. These may be classified as disaccharides, oligosaccharides, or polysaccharides, depending on whether they have two, several, or many monosaccharide units.

Monosaccharides

Monosaccharidesmay be divided into aldoses, which have an aldehydegroup on the first carbon atom, and ketoses, which typically have a ketone group on the second. They may also be divided into trioses, tetroses, pentoses, hexoses, and so forth, depending on how many carbon atoms they contain. For instance, glucoseis an aldohexose, fructosea ketohexose, and ribosean aldopentose.

Further, each carbon atom that supports a hydroxyl group (except for the first and last) is optically active, allowing a number of different carbohydrates with the same basic structure. For instance, galactoseis an aldohexose but has different properties from glucose because the atoms are arranged differently.

Image:Ribose.png

The straight-chain structure described here is only one of the forms a monosaccharide may take. The aldehyde or ketone group may react with a hydroxyl group on a different carbon atom to form a hemiacetalor hemiketal, in which case there is an oxygen bridge between the two carbon atoms, forming a heterocyclic ring. Rings with five and six atoms are called furanose and pyranose forms and exist in equilibrium with the straight-chain form.

It should be noted that the ring form has one more optically active carbon than the straight-chain form, and so has both an alpha and a beta form, which interconvert in equilibrium. However, the carbohydrate may further react with an alcohol to form an acetalor ketal, in which case the two forms become distinct. This is the basic type of link between the monosaccharide units of larger carbohydrates.

Disaccharides

Disaccharidesare composed of two monosaccharide units bound together by a covalentglycosidicbond. The binding between the two sugars results in the loss of a hydrogen atom (H) from one molecule and a hydroxyl group(OH) from the other.

The most common disaccharides are sucrose(cane or beet sugar - made from one glucose and one fructose), lactose(milk sugar - made from one glucose and one galactose) and maltose(made of two glucoses). The formulaof these disaccharides is C₁₂H₂₂O₁₁.

Oligosaccharides and polysaccharides

Oligosaccharidesand polysaccharidesare 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 three 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 however.

Oligosaccharides are found as a common form of proteinposttranslational modification. Polysaccharides represent an important class of biological polymer. Examples include starch, celluloseand chitin. Table and powdered sugar are some of the foods you find disaccharides in.

Nutrition

Image:Starchy-foods..jpg

Strictly speaking, carbohydrates are not necessary for human nutritionbecause proteinscan be converted to carbohydrates. The traditional diet of some culturesconsists of very little carbohydrate, and these people remain relatively healthy. However, carbohydrates require less water to digest than proteinsor fatsand are the most abundant source of energy. Proteins and fat are vital building components for body tissueand cells, and thus it could be considered advisable not to deplete such resources.

Very low carbohydrate diets can slow down brainand neuralfunction because the nervous systemespecially relies on glucose.

Some problems have been cited for the long term effects of a no-carbohydrate diet for some individuals. Athletes, for instance, or those that participate in high intensity activities, will have a considerable reduction in performance, due to having little to no glycogen supplies stored in muscle tissue. Additionally, nephrotoxicitymay occur, particularly in persons that are not very well hydrated.

Some examples of different carbohydrate rich foods are beans, breadand pasta.

Catabolism

There are two major metabolic pathwaysof carbohydrate catabolism:

1. Glycolysis
2. Citric acid cycle

See also

• Biochemistry
• Macromolecules
• Glycolysis
• Gluconeogenesis
• Pentose phosphate pathway

External links

• IUPAC-IUBMB Joint Commission on Biochemical Nomenclature (JCBN): Carbohydrate Nomenclature
• Carbohydrates Information
• Carbohydrates detailed
• Carbohydrates Overview
• Carbohydrates and Glycosylation - The Virtual Library of Biochemistry and Cell Biology
• Consortium for Functional Glycomicsca:Hidrat de carboni

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This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Carbohydrate Wikipedia article Carbohydrate.