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.