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Biomolecules: Building Blocks




Carbohydrates

These are hydrates of carbon with the molecular formula $(CH_2O)_n$
 

Monosaccharides

These are the molecules having up-to 6 carbon atoms. Its members work as building blocks of large carbohydrates such as cellulose, glycogen. For example: glucose, fructose, galactose
 

Classification of Monosaccharides

1.On the basis of the number of carbon atoms.


2.On the basis of type of functional group

 

Nomenclature

  • If a monosaccharide has 5 carbon atoms with an aldehyde functional group, it is called aldopentose for example ribose.
  • If it has six carbon atoms with a kenotic functional group, it is called ketohexose. For Example : fructose.
  • The monosaccharides (sugars) have two major functional groups i.e. hydroxyl (-OH)  and carbonyl (Aldehyde/ketone).
  • Carbonyl group can easily form hemiacetals with hydroxyl group.
  • If hemiacetal formation occurs within the same molecule, then it is called internal hemiacetal formation, which results in the formation of ring structure as in all pentoses and hexoses.
  • Ring structure can be depicted linearly (Fisher's structure) or ring like structure (Haworth's structure.)
 

Anomeric carbon

The carbon atom carrying the aldehyde/ketonic group (C-1 in glucose C-2 in fructose) is called anomeric carbon because it forms a ring structure and two configurations $\alpha$  and  $\beta$.
Here is the Anomeric formation of D-Glucose.

 

Dissaccharides

The -OH group attached to an anomeric carbon atom of a monosaccharide can easily dehydrate with -OH group attached to another monosaccharide leading to formation of disaccharide. The bond between them is called glycosidic bond. If this reaction occurs between 3 sugar residues then tri-saccharides, (3-20) oligosaccharides and more than 50 residues polysaccharides are formed.

 

Properties of sugar

Sugars with free anomeric carbon are called reducing sugar and can reduce alkaline solution of copper salts giving rise to yellow to red precipitate of cuprous oxide.
To detect the presence of reducing sugar in blood and wine samples in urine, Fehling’s and Benedict’s tests are conducted in a pathologic lab.
 

Non-reducing sugar

In it, the anomeric carbon are involved in N-glycosidic bond and it gives negative benedict's test. For example-sucrose.



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