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), 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
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:
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
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Protein | |
Protein metabolism (Protein synthesis/Amino acid synthesis/Catabolism) |
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Carbohydrate | |
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Lipid | |
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