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Carbohydrate Chemistry : Monosaccharides, Disaccharides and Specific Oligosaccharides
Browse All Figures Return to Figure. Starch is a polysaccharide. It is abundant in cereals wheat, maize, rice , potatoes, and processed food based on cereal flour , such as bread, pizza or pasta. Sugars appear in human diet mainly as table sugar sucrose, extracted from sugarcane or sugar beets , lactose abundant in milk , glucose and fructose, both of which occur naturally in honey , many fruits, and some vegetables.
Table sugar, milk, or honey are often added to drinks and many prepared foods such as jam, biscuits and cakes. Cellulose, a polysaccharide found in the cell walls of all plants, is one of the main components of insoluble dietary fiber. Although it is not digestible, insoluble dietary fiber helps to maintain a healthy digestive system  by easing defecation.
Other polysaccharides contained in dietary fiber include resistant starch and inulin , which feed some bacteria in the microbiota of the large intestine , and are metabolized by these bacteria to yield short-chain fatty acids. In scientific literature, the term "carbohydrate" has many synonyms, like "sugar" in the broad sense , "saccharide", "ose",  "glucide",  "hydrate of carbon" or " polyhydroxy compounds with aldehyde or ketone ".
Some of these terms, specially "carbohydrate" and "sugar", are also used with other meanings. In food science and in many informal contexts, the term "carbohydrate" often means any food that is particularly rich in the complex carbohydrate starch such as cereals, bread and pasta or simple carbohydrates, such as sugar found in candy, jams , and desserts. Often in lists of nutritional information , such as the USDA National Nutrient Database , the term "carbohydrate" or "carbohydrate by difference" is used for everything other than water, protein, fat, ash, and ethanol.
It also includes dietary fiber which is a carbohydrate but which does not contribute much in the way of food energy kilocalories , even though it is often included in the calculation of total food energy just as though it were a sugar. In the strict sense, " sugar " is applied for sweet, soluble carbohydrates, many of which are used in food.
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Formerly the name "carbohydrate" was used in chemistry for any compound with the formula C m H 2 O n. Following this definition, some chemists considered formaldehyde CH 2 O to be the simplest carbohydrate,  while others claimed that title for glycolaldehyde. For example, while the above representative formulas would seem to capture the commonly known carbohydrates, ubiquitous and abundant carbohydrates often deviate from this. For example, carbohydrates often display chemical groups such as: N -acetyl e. Natural saccharides are generally built of simple carbohydrates called monosaccharides with general formula CH 2 O n where n is three or more.
Examples of monosaccharides are glucose , fructose , and glyceraldehydes. However, some biological substances commonly called "monosaccharides" do not conform to this formula e. Monosaccharides can be linked together into what are called polysaccharides or oligosaccharides in a large variety of ways. Many carbohydrates contain one or more modified monosaccharide units that have had one or more groups replaced or removed.
For example, deoxyribose , a component of DNA , is a modified version of ribose ; chitin is composed of repeating units of N-acetyl glucosamine , a nitrogen -containing form of glucose. Carbohydrates are polyhydroxy aldehydes, ketones, alcohols, acids, their simple derivatives and their polymers having linkages of the acetal type. They may be classified according to their degree of polymerization, and may be divided initially into three principal groups, namely sugars, oligosaccharides and polysaccharides .
Monosaccharides are the simplest carbohydrates in that they cannot be hydrolyzed to smaller carbohydrates. They are aldehydes or ketones with two or more hydroxyl groups. Monosaccharides are important fuel molecules as well as building blocks for nucleic acids. 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.
Monosaccharides with three carbon atoms are called trioses , those with four are called tetroses , five are called pentoses , six are hexoses , and so on. For example, glucose is an aldohexose a six-carbon aldehyde , ribose is an aldopentose a five-carbon aldehyde , and fructose is a ketohexose a six-carbon ketone.
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Each carbon atom bearing a hydroxyl group -OH , with the exception of the first and last carbons, are asymmetric , making them stereo centers with two possible configurations each R or S. Because of this asymmetry, a number of isomers may exist for any given monosaccharide formula.
In the case of glyceraldehydes , an aldotriose, there is one pair of possible stereoisomers, which are enantiomers and epimers. The assignment of D or L is made according to the orientation of the asymmetric carbon furthest from the carbonyl group: in a standard Fischer projection if the hydroxyl group is on the right the molecule is a D sugar, otherwise it is an L sugar. The "D-" and "L-" prefixes should not be confused with "d-" or "l-", which indicate the direction that the sugar rotates plane polarized light.
This usage of "d-" and "l-" is no longer followed in carbohydrate chemistry. 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.
Rings with five and six atoms are called furanose and pyranose forms, respectively, and exist in equilibrium with the straight-chain form. During the conversion from straight-chain form to the cyclic form, the carbon atom containing the carbonyl oxygen, called the anomeric carbon , becomes a stereogenic center with two possible configurations: The oxygen atom may take a position either above or below the plane of the ring.
The resulting possible pair of stereoisomers is called anomers. Monosaccharides are the major source of fuel for metabolism , being used both as an energy source glucose being the most important in nature and in biosynthesis. When monosaccharides are not immediately needed by many cells they are often converted to more space-efficient forms, often polysaccharides. In many animals, including humans, this storage form is glycogen , especially in liver and muscle cells.
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In plants, starch is used for the same purpose. The most abundant carbohydrate, cellulose , is a structural component of the cell wall of plants and many forms of algae. Ribose is a component of RNA. Deoxyribose is a component of DNA. Lyxose is a component of lyxoflavin found in the human heart. Galactose , a component of milk sugar lactose , is found in galactolipids in plant cell membranes and in glycoproteins in many tissues.
Mannose occurs in human metabolism, especially in the glycosylation of certain proteins. Fructose , or fruit sugar, is found in many plants and in humans, it is metabolized in the liver, absorbed directly into the intestines during digestion , and found in semen.
Trehalose , a major sugar of insects, is rapidly hydrolyzed into two glucose molecules to support continuous flight. Two joined monosaccharides are called a disaccharide and these are the simplest polysaccharides. Examples include sucrose and lactose. They are composed of two monosaccharide units bound together by a covalent bond known as a glycosidic linkage formed via a dehydration reaction , resulting in the loss of a hydrogen atom from one monosaccharide and a hydroxyl group from the other.
The formula of unmodified disaccharides is C 12 H 22 O Although there are numerous kinds of disaccharides, a handful of disaccharides are particularly notable. Sucrose , pictured to the right, is the most abundant disaccharide, and the main form in which carbohydrates are transported in plants.