Health

Carbohydrates : Nomenclature, Classification, Function & Derivatives

By Dr Daya

Posted on February 24, 2017

Carbohydrates : Nomenclature, Classification, Function & Derivatives

Introduction

Carbohydrates are defined as the aldehydic or ketonic derivatives of polyhydroxy alcohols and their polymers having hemiacetal glycosidic linkages.
General formula for carbohydrates – Cn(H2O)n.
Carbohydrates are the main source of energy in the body. Brain cells and RBCs are exclusively depend on carbohydrates (glucose) as the energy source.

Functions

Carbohydrates are for following functions :

Provides energy, i.e. as major source of energy to the body. Their calorific value is 4 kcal per gm.
As structural components of membranes.
As structural basis for DNA and RNA (Ribose/Deoxyribose).
As structural basis for nucleosides and nucleotides.
As source of carbon skeltons for some amino acids.
As basis of some intracellular messenger systems.

Classification & Nomenclature

Carbohydrates are classified into 2 types :

Sugars – Monosaccharides & Oligosaccharides
Polysaccharides

Monosaccharides

Molecules having only one actual or potential sugar group are called monosaccharides.
They cannot be further hydrolysed into smaller units.
They are also called simple sugars.
Monosaccharides are further divided into : Aldoses (Aldo sugars), Ketoses (Keto sugars)

Aldoses (Aldo sugars)

Monosaccharides containing aldehydic group as the functional group are called aldoses.

Ketoses (Keto sugars)

Monosaccharides containing ketonic group as the functional group are called ketoses.

They are classified according to the number of carbon atoms present. Monosaccharides containing three to seven carbon atoms are called trioses, tetroses, pentoses, hexoses and heptoses respectively.
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Reactions of Monosaccharides

Action of acids
Mutarotation
Reducing property
Osazone formation
Action of dilute alkali
Oxidation
Reduction
Glycoside formation.

Oligosaccharides

Oligosaccharides are arbitrarily defined as carbohydrates that contains two to ten monosaccharide units per molecule joined by glycosidic linkages.
On hydrolysis they yield monosaccharides.

Depending upon the number of constituent monosaccharide units, the oligosaccharides are called disaccharides, trisaccharides, etc.
Disaccharides

Maltose
Lactose
Sucrose
Isomaltose

Polysaccharides

Polysaccharides are the polymers of monosaccharide units which are joined in linear or branched chain fashion by glycosidic linkages.

Polysaccharides can be divided into two groups:

Homopolysaccharides : They contain only one type of monosaccharides as the repeating unit and on hydrolysis gives only one type of sugar.
Example: Starch, cellulose, glycogen, dextrins, etc.
Heteropolysaccharides : Heteropolysaccharides are made up of mixed disaccharides repeating units and on hydrolysis gives a mixture of more than one product of monosaccharides and their derivatives of amino sugars and sugar acids.