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  Carbohydrates-physical properties and functions, Biology tutorial

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:

  • Primary function of carbohydrates is that they provide nutrients to cells like glucose
  • Carbohydrates also act as form by which energy is stored in cells example: glycogen is storage form of energy in animal cell where as cellulose is storage form of energy in plant cells.
  • Carbohydrates function in providing as structural components of cells and tissue example: chitin found in insects and cellulose found in plants.
  • Carbohydrates such as peptidoglycans act as ground substance in connective tissues ( the gelly-like material) which is very significant to proper functioning of tissue).
  • Carbohydrates like Hyaluronic acid can also act as lubricants because of their viscosity in joint.
  • Carbohydrates like Oligosaccharides act as components of glycoprotein where they are engaged cellular/molecular recognition.
  • Carbohydrates like Galactose and fucose also function as antigenic determinant of blood group (ABO) system.
  • Carbohydrates, like Sialic acid serve protecting roles, they shield oligosaccharides of glycoconjugates from action of hydrolytic enzymes.

Physical property of carbohydrates:

  • Carbohydrate like monosaccharides are soluble in water but insoluble in organic solvents example chloroform. Polysaccharides such as cellulose are insoluble in water but will dissolve in ammoniacal solution of cupric salts.
  • Carbohydrates comprising monosaccharides, oligosaccharides and polysaccharides are solid at room temperature.
  • Majority of monosaccharides and few diasaccharideslike sucrose are sweet to taste.
  • Carbohydrates like monosaccharides are colorless crystalline solids.
  • Monosaccharides, diasaccharides and polysaccharides are odorless.

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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