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Current Category » Bio-chemistry

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:

Aldoses:

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

Ketoses:

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.

 

Current Category » Bio-chemistry