A non-nitrogen-containing compound based on carbon, hydrogen, and oxygen, generally with two hydrogen atoms per atom of oxygen. The molecules may be small (glucose) or large (cellulose, starch). Most carbohydrates comprise one or more 6-carbon units, of which glucose is by far the most abundant. Starch is a polymer of glucose which is digestible by humans. Cellulose, another glucose polymer, is not digestible by higher animals, but is fermented by bacteria in the gut. Carbohydrates are not nutritionally essential, but prolonged intake of carbohydrate-free diets can cause a type of abnormal body metabolism (ketosis).
Carbohydrates or saccharides (Greek sakcharon, "sugar") are simple molecules that are straight-chain aldehydes or ketones 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 monosaccharides, such as glucose and fructose. The general chemical formula of an unmodified monosaccharide is (C·H2O)n, where n is any number of three or greater. Two joined monosaccharides are called disaccharides, such as sucrose and lactose. Carbohydrates containing between about three to six monosaccharide units are termed oligosaccharides;
Many carbohydrates contain one or more modified monosaccharide units that have had one or more groups replaced or removed.
Monosaccharides
Monosaccharides, also called simple sugars, are the basic units of carbohydrates. They are straight-chain aldehydes or ketones with many hydroxyl groups added, usually one on each carbon atom that is not part of the aldehyde or ketone functional group. The general chemical formula of an unmodified monosaccharide is (C·H2O)n, where n is any number of three or greater.
Classification of monosaccharides
Monosaccharides are classified according to three different characteristics: the placement of its carbonyl group, the number of carbon atoms it contains, and its 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. The smallest possible monosaccharide, those with three carbon atoms, are called trioses.
Each carbon atom bearing a hydroxyl group (-OH), with the exception of the first and last carbons, are asymmetric, making them stereocenters with two possible configurations each (the -H and -OH may be on either side). Because of this asymmetry, a number of isomers may exist for any given monosaccharide formula. The assignment of D or L is made according to the orientation of the asymmetric carbon furthest from the carbonyl group: if the hydroxyl group is on the right the molecule is a D sugar, otherwise it is an L sugar.
Conformation
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.
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.
Disaccharides
Disaccharides are the simplest polysaccharides. They are composed of two monosaccharide units bound together by a covalent glycosidic bond formed via a dehydration reaction, resulting in the loss of a hydrogen atom from one monosaccharide and a hydroxyl group from the other, so the formula of unmodified disaccharides is C12H22O11.
Sucrose, pictured to the right, is the most abundant disaccharide and the main form in which carbohydrates are transported in plants. 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're linked together: the oxygen on the number 1 carbon (C1) of α-glucose is linked to the C2 of fructose.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.
Oligosaccharides are found as a common form of protein posttranslational modification.
Nutrition
Carbohydrates require less water to digest than proteins or fats and are the most common source of energy.
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.
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). 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 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 cycleOligo/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.
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