Classification of Carbohydrates

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Chapter: Pharmacognosy and Phytochemistry : Drugs Containing Carbohydrates and Derived Products

The term ‘monosaccharides’ is employed for such sugars that on hydrolysis yield no further, lower sugars. The general formula of monosaccharides is Cn H2n On. The monosaccharides are subdivided as bioses, trioses, tetroses, pentoses, hexoses, heptoses, depending upon the number of carbon atoms they possess.


CLASSIFICATION

 

 

Monosaccharides

 

The term ‘monosaccharides’ is employed for such sugars that on hydrolysis yield no further, lower sugars. The general formula of monosaccharides is Cn H2n On. The monosaccharides are subdivided as bioses, trioses, tetroses, pentoses, hexoses, heptoses, depending upon the number of carbon atoms they possess.

 

Bioses

 

They contain two carbon atoms. They do not occur free in nature.

 

Trioses

 

They contain three carbon atoms, but in the form of phosphoric esters, for example, glyceraldehydes.

 

Tetroses

 

They contain four carbon atoms, for example, erythrose, threose, etc.

 

 

Pentoses

 

They are very common in plants and are the products of hydrolysis of polysaccharides like hamicelluloses, mucilages and gums, for example, ribose, arabinose and xylose.

 


 

Hexoses

 

They are monosaccharides containing six carbon atoms and are abundantly available carbohydrates of plant kingdom. They are further divided into two types: aldoses and ketoses. They may be obtained by hydrolysis of polysaccharides like starch, insulin, etc.

 

Depending upon the type of product of hydrolysis these are further classified as Pentosans and Hexosans. Xylan is pentosan, whereas starch, insulin and cellulose are the examples of hexosans.

Cellulose is composed of glucose units joined by β-1, 4 linkages, whereas starch contains glucose units connected with α- 1, 4 and α- 1, 6 units. Polyuronides, gums and

 

Aldoses        :    Glucose, mannose, galactose

 

Ketoses         :   Fructose and sorbose



 

 

Heptoses

 

They contain seven carbon atoms, vitally important in the photosynthesis of plant and glucose metabolism of animals and are rarely found accumulated in plants, for example, glucoheptose and manoheptose.

 

Disaccharides

 

Carbohydrates, which upon hydrolysis yield two molecules of monosaccharides, are called as disaccharides.


 

 

Trisaccharides

 

As the name indicates, these liberate three molecules of monosaccharides on hydrolysis.

 


 

Tetrasaccharides

 

Stachyose, a tetrasaccharide, yields on hydrolysis, four molecules of monosaccharide, found in manna.

 

Polysaccharides

 

On hydrolysis they give an indefinite number of monosaccharides. By condensation, with the elimination of water, polysaccharides are produced from monosaccharides.

Depending upon the type of product of hydrolysis these are further classified as Pentosans and Hexosans. Xylan is pentosan, whereas starch, insulin and cellulose are the examples of hexosans.

 

Cellulose is composed of glucose units joined by β-1, 4 linkages, whereas starch contains glucose units connected with α- 1, 4 and α- 1, 6 units. Polyuronides, gums and mucilages are the other pharmaceutically important poly-saccharide derivatives.

 


 

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