Classification, Structure and Properties of Carbohydrates


Classification, Structure and Properties of Carbohydrates

Classification of Carbohydrates:

Carbohydrates are classified in to three major classes on the basis of complexity and behavior on hydrolysis
1) Mono saccharides
2) Oligosaccharides
3) Polysaccharides

1) Mono saccharides:

Simple sugars are cannot be hydrolyzed into smaller units. Depending upon no. of carbon in a unit, mono saccharides are subdivided into a dioses to decoses. More common subclasses of mono saccharides are:


Aldotrioses e.g. Glycerose,
Aldotertroses e.g. Erythrose,
Aldopentoses e.g. Ribose,
Aldohexoscs e.g. Glucose,  Galatose
Aldoheptose e.g. Glucoheptose.


Ketotrioses e.g Dihydroxyacetone,
Ketotetroses e.g Erythrulose,
Ketopentoses e.g Ribulose,
Ketohexoses e.g. Fructose,
Ketoheptose e.g. Scdoheptulose.
Properties of Mono saccharides:
1) Muta-rotation
2) Glycoside Formation
3) Reducing Power
4) Reduction
5) Oxidation with mild and strong oxidizing agent
6) Methylation / Esterification
7) Dehydration
8) Form osazone with phenyl hydrazine.

2. Oligosaccharide: Oligosaccharides are polymers of mono saccharides containing two to ten residues accumulate in vacuole while polysaccharides in plastids, they are classified as

a) Disaccharides: Yield two mono saccharides on hydrolysis.

i) Reducing Disaccharides: e.g. Maltose (Glucose + Glucose), Lactose (Galactose + Glucose), other examples are Isomaltose, cellobiose.

ii) Non Reducing Disaccharides: Sucrose (Glucose + Fructose)

b) Tri saccharides: e.g. Raffinose      (Glucose + Fructose + Galactose) found in cotton seed and sugar beet.
c) Tetra saccharides: Yield 4 mono saccharides on hydrolysis e.g. stachyose (Glucose + Fructose + Galactose + Galactose) (only tetra saccharide known to exist in plant).
3. Polysaccharides: Polysaccharides are polymeric anhydrides of mono saccharides. The long chain polymers are either straight chain or branched. They are also called glycanes.

Classification of Polysaccharides:

1) On the Basis of Function:

a) Storage e.g. Starch, glycogen          
b) Structural e.g. Cellulose, Pectin

2) On the Basis of Composition:

a) Homo polysaccharides
b) Hetero polysaccharides.

a) Homo Polysaccharides:

On hydrolysis gives single monosaccharide units

i) Pentosan: Contains pentoses (C5 H8 04).
ii) Hexosans: Contains hexoses (C6 H10 05) subdivided in to
                                            A) Glucosans: Polymer or glucose e.g. Starch, Glycogen
                                            B) Fructosans: Polymer or fructose e.g. Inulin
                                            C) Galactans: Polymer of galactose e.g. Galactan
                                            D) Mannans: Polymer of mannose e.g. Mananas.

b) Hetero Polysaccharide:  e.g. Hyaluronic acid, Chondroitin sulphates.

A) Gum: Consist of arabinose, rhamnose, galactose and glucoronic acid.
B) Agar: The sulphuric acid esters of galactans consists of galactose, galactouronic acid.
C) Pectins: Fundamental unit is pectic acid, consist of arabinosc, galactose, galactouronic acid.

Functions of Polysaccharides:

1) They serve as structural components of the cells
2) They serve as stored form of energy
3) They serve as nutrient.

Structure and Properties of Starch: Consist of two components, Amyloses and Amylopectin. Amylose is a long chain polysaccharides containing α – D glucose molecules linked by 1- 4 glycosidic linkages, produce blue colour with iodine. Amylopectin is a branched chain polysaccharides consisting α –D glucose molecules linked by 1- 4 glycosidic linkage and branches by 1-6, linkage produce purplish colour with iodine and forms a gel with hot water.

Cellulose: It is structural polysaccharide found in cell walls of plants, made up of long chains of a-D-Glucose molecules linked by 1-4 linkages, no branching, yield on hydrolysis crystalline D-glucose.

Pectin: Present in apple, lemon, form gel with sugar sole, contains galactouronic acid, galactose and arabinose.