Carbohydrates are molecules that contain oxygen, hydrogen, and carbon atoms. They may also contain other elements such as sulfur or nitrogen, but these are usually minor components. They consist of monosaccharide sugars, of varying chain lengths, that have the general chemical formula Cn(H2O)n or are derivatives of such.The smallest value for n is 3. A 3-carbon sugar is referred to as a triose, whereas a 6-carbon sugar is called a hexose (see monosaccharides below).
Certain carbohydrates are important for storing and transporting energy in most organisms, including plants and animals, and are major structural elements in many organisms (eg cellulose in plants). In addition they play major roles in cell to cell communication, the immune system, fertilization, pathogenesis, blood clotting, and development. Carbohydrates can be classified by the number of constituent sugar units: monosaccharides (such as glucose and fructose), disaccharides (such as sucrose and lactose), oligosaccharides, and polysaccharides (such as starch, glycogen, and cellulose).
Glucose as a straight-chain carbohydrate (Fischer projection)
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. Sometimes compounds containing other elements are also counted as carbohydrates (e.g. glucosamine and chitin, which contain 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.
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Contents
- 1 Monosaccharides
- 2 Disaccharides
- 3 Oligosaccharides and polysaccharides
- 4 Nutrition
- 4.1 Foods that are high in carbohydrates
- 4.2 Classification
- 5 Catabolism
- 6 Anabolism
- 7 See also
- 8 References
- 9 External links
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Monosaccharides
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Main article: Monosaccharide
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 a stereogenic centre, allowing a number of different enantiomers and stereoisomers for carbohydrates with the same basic structure. For instance, galactose is an aldohexose but has different properties from glucose because the atoms are arranged differently.
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 stereogenic centre 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
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Main article: Disaccharide
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), maltose (made of two glucoses linked alpha-1,4) and cellobiose (made of two glucoses linked beta-1,4). The formula of these disaccharides is C12H22O11.
Other examples of disaccharides include trehalose, chitobiose, laminaribiose, kojibiose and xylobiose.
Oligosaccharides and polysaccharides
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Main articles: Oligosaccharide and 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 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. Examples include starch, cellulose, chitin, glycogen, callose, laminarin, xylan, and galactomannan.
Nutrition
Unrefined grain products are rich sources of complex carbohydrates
Carbohydrates require less water to digest than proteins or fats and are the most common 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 by necessitating their use in energy production. Carbohydrates, like proteins, contain 4 kilocalories per gram while fats contain 9 kilocalories and alcohol contains 7 kilocalories per gram.
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]
Foods that are high in carbohydrates
Breads, pastas, beans, potatoes, bran, rice and cereals are all high in carbohydrates.
Classification
Dietitians and nutritionists commonly classify carbohydrates as simple (monosaccharides and disaccharides) or complex (oligosaccharides and polysaccharides), depending on their chemical structure. 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".[3] Dietary guidelines generally recommend that complex carbohydrates and nutrient-rich simple carbohydrates such as fruit and dairy products should make up the bulk of carbohydrate consumption. The USDA's Dietary Guidelines for Americans 2005 dispenses with the simple/complex distinction, instead recommending fiber-rich foods and whole grains.[4]
The glycemic index and glycemic load systems are popular alternative classification methods which rank carbohydrates based on their effect on blood glucose levels.
Catabolism
There are two major metabolic pathways of monosaccharide catabolism:
- Glycolysis
- Citric acid cycle
Oligo/polysaccharides are cleaved first to smaller monosaccharides by enzymes called Glycoside hydrolases. The monosaccharide units can then enter into monosaccharide catabolism.
Anabolism
Complex carbohydrates are assembled from sugar nucleotides by the action of glycosyltransferases.
See also
- Biochemistry
- Macromolecules
- Glycolysis
- Gluconeogenesis
- Pentose phosphate pathway
- Glycoside hydrolase
- Glycosyltransferase
References
- ^ 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
- ^ 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
- ^ Joint WHO/FAO expert consultation (1998), Carbohydrates in human nutrition, chapter 1. ISBN 92-5-104114-8.
- ^ DHHS and USDA, Dietary Guidelines for Americans 2005, Chapter 7 Carbohydrates
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 Glycomics
- Carbohydrate content in Food
Categories: Carbohydrates | Nutrition
