Definition of carbohydrates:
Carbohydrates can be stated as polyhydroxy aldehydes or ketones, or as substance which yield one of these compounds on hydrolysis. Several but not all carbohydrates contain empirical formula (CH2O)n , where n is three (3) or greater than 3. Though, this formula doesn't fit in for all carbohydrates as some carbohydrates have been found to have nitrogen, phosphorus or sulfur where as some are deoxysugars e.g. deoxyribose. Occurrence of ratio of one molecule of water to one atom of carbon led to name hydrates of carbon or carbohydrates. Though, this name isn't appropriate to all carbohydrates due to aforementioned reason of presence of nitrogen, phosphorus or sulfur.
Categorization of carbohydrates:
Carbohydrates are categorized in three major classes. These are: Monosaccharides, Oligosaccharides and Polysaccharides. Word saccharide is derived from Greek word sackaron which means sugar. Monosaccharides are simple sugars comprising of single polyhydroxy aldelyde or ketone unit. Most plentiful monosaccharide in nature is six-carbon sugar D-glucose. Other monosaccharides comprise: Mannose, Galactose and Fructose. Oligosaccharides comprises of short chains of monosaccharide units or residues attached by characteristic linkage known as glycosidic bonds. Most abundant ones are diasaccharides that have two monosaccharide units joined together by glycosidic bond. Typical example is sucrose that comprises of two six carbon sugars, D-glucose and D-fructose.
Polysaccharides are sugar polymers which happen in continuous range of sizes, they generally have more than 20 monosaccharide units. Polysaccharides may contain hundreds and thousands of monosaccharides units attached together by glycosidic bonds.
Functions of carbohydrates:
Physical property of carbohydrates:
Stereochemistry of carbohydrates:
Carbohydrates, as organic compounds show stereoisomerisms, various molecule in which order of bonding is same but spatial relationship among atoms is different. Enantiomers are stereoisomers which are non superimposable mirror images of each other. Concept of enatiomerism needs the presence of chiral carbon atom. The chiral carbon (also known as asymmetric atom) is one which is attached to four different groups:
Enatiomers will be differentiated from each other by designations D for dextrorotatory and L for leavorotatory. Maximum numbers of stereoisomers possible is 2n, where n is number of chiral carbon atoms. In sugars, what finds whether it is a D or L is the position of -OH group on carbon atom adjacent or next to carbon atom which is most isolated from aldehyde or ketone functional group in sugar.
Diastereoisomers are stereoisomers which are not mirror image of each other and need not have chiral atoms. Epimers are diastereoisomers which have more than one chiral carbon and vary in configuration about only one asymmetric carbon. Example of epimers comprise: galactose, glucose and mannose. Epimers thus show concept of epimerism (varying around only one chiral carbon atom) Anomers are special form of carbohydrate sterioisomers in which difference is especially about anomeric carbon. Carbohydrates which show difference around anomeric carbon atom are said to experience anomerism example of anomers are α and β - Dglucose. When-OH group on anomeric carbon atom is down or below plane, it is an alpha (α) anomer while -OH group is up or above plane, it is beta (β) anomer.
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