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Carbohydrates are chemical compounds that contain oxygen, hydrogen, and carbon atoms. They consist of monosaccharide sugars of varying chain lengths and that have the general chemical formula Cn(H2O)n or are derivatives of such.

Certain carbohydrates are an important storage and transport form of energy in most organisms, including plants and animals. Carbohydrates are classified by their number of sugar units: monosaccharides (such as glucose and fructose), disaccharides (such as sucrose and lactose), oligosaccharides, and polysaccharides (such as starch, glycogen, and cellulose).

Structure

D-glucose

Glucose as a straight-chain carbohydrate (Fischer projection)

D-fructose

Fructose (Fischer projection)

Pure carbohydrates contain carbon, hydrogen, and oxygen atoms, in a 1:2:1 molar ratio, giving the general formula Cn(H2O)n. (This applies only to monosaccharides, see below, although all carbohydrates have the more general formula Cn(H2O)m.) However, many important "carbohydrates" deviate from this, such as deoxyribose and 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 aldehydes and ketones with many hydroxyl groups 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

Monosaccharides may be divided into aldoses, which have an aldehyde group 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, glucose is an aldohexose, fructose a ketohexose, and ribose an 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, galactose is an aldohexose but has different properties from glucose because the atoms are arranged differently.

Ribose

A heterocyclic form of ribose (Haworth projection)

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 hemiacetal or 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 acetal or ketal, in which case the two forms become distinct. This is the basic type of link between the monosaccharide units of larger carbohydrates.

Disaccharides

Disaccharides are composed of two monosaccharide units bound together by a covalent glycosidic bond. 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 formula of these disaccharides is C12H22O11.

Oligosaccharides and polysaccharides

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 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 protein posttranslational modification. Polysaccharides represent an important class of biological polymer. Examples include starch, cellulose and chitin. Table and powdered sugar are some of the foods you find disaccharides in.

Nutrition

Starchy-foods.

Unrefined grain products are rich sources of complex carbohydrates

Strictly speaking, carbohydrates are not necessary for human nutrition because proteins can be converted to carbohydrates. The traditional diet of some cultures consists of very little carbohydrate, and these people remain relatively healthy. However, carbohydrates require less water to digest than proteins or fats and are the most abundant source of energy. Proteins and fat are vital building components for body tissue and cells, and thus it could be considered advisable not to deplete such resources.

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 dietary energy from carbohydrates.[1] 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.[2]

Very low carbohydrate diets can slow down brain and neural function because the nervous system especially 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, nephrotoxicity may occur, particularly in persons that are not very well hydrated.

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

Catabolism

There are two major metabolic pathways of carbohydrate catabolism:

  1. Glycolysis
  2. Citric acid cycle

See also

References

  1. ^ Food and Nutrition Board (2002/2005). Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, DC: The National Academies Press. Page 769. ISBN 0-309-08537-3
  2. ^ Joint WHO/FAO expert consultation (2003). Diet, Nutrition and the Prevention of Chronic Diseases (PDF). Geneva: World Health Organization. Pages 55-56. ISBN 92-4-120916-X

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