Introduction to Carbohydrates


• Carbohydrates are polyhydroxy aldehydes or ketones or compounds derived from their hydrolysis.

• Carbohydrates are the most abundant organic constituents of plants.

• They are the major source of chemical energy for living organism (e.g. Sugars & Starch).

• It compassed of carbon, Hydrogen and Oxygen.

• The general molecular formula of carbohydrates is c(H2O). Starch and cellulose are two common carbohydrates.

Role of Carbohydrates:

• Carbohydrates are the major source of energy in animal body.

• Carbohydrates are helpful in the absorption of calcium are phosphorus in younger people.

• They help in the secretin of digestive juices in gastrointestinal tract.

• They help in peristaltic movement of food.

• They maintain Glucose level in plasma.

Classification of Carbohydrates


• Carbohydrates that cannot be hydrolyzed to simpler compound are called Monosaccharide.

• Monosaccharaides have the one sugar molecule.


• Carbohydrates that give two monomeric unit on hydrolysis are called Disaccharides.

• E.g. Maltose, Sucrose, Lactose.

They have two sugar molecules.

1.  Oligosaccharides:-

• Carbohydrates that gives There to (10) ten monosaccharide on hydrolysis are called oligosaccharides.

• E.g: Reffinose, Maltotriose.

• They have two or ten sugar molecules.

1.  Polysaccharides:

• Carbohydrates that give many monosaccharide on hydrolysis are called polysaccharides.

• They have ten or more sugar molecules.

• Polysaccharides are made-up of one or different types of sugars.


1.  Homopolysaccharides:

• The monomeric units are arranged in the form of long chain either unbranched or branched.

• E.g: Starch, Glycogen, Cellulose etc.

2.  Hetrropolysaccharides:

• Hemicellulose is a polymer containing D-xylose, L-arabinoc, D-Galactose, L- Rhamnose, D-Monnose and D-Glucoronic acid

• e.g.: Heparin

3.  Mucopolysaccharides:

On hydrolysis mucopolysaccharides give amino sugars acetylated sugar and uranic acids.

Mucopolysaccharides are important biological substance.

Chemical reactions and Qualitative teste for Carbohydrates:-

1. Dehydration:-

• Carbohydrates on dehydration give for fural or its derivative.

• Concentrated sulphuric acid is used as a dehydrating agent.

Molisch test:

• It is general test for Carbohydrates identification.

• Furfural or its derivative formed during dehydration, react with α-nepenthes to give violet colour.

• In this test concentrated sulphuric acid is used as a dehydrating agent.

2. Reactions of carbonyl group:

1) Benedict’s test –

• Carbohydrate is wanted with alkaline copper sulphates.

• Copper ions get reduced and give red precipitate of cuprous oxide.

Note:- All reducing Sugars give this test positive while sugar like sucrose does not give this test positive.

2) Barfoed’s test:

• This test’s used for the identification of reducing mono-saccharidies.

• In presence of weak acidic condition only mono-saccharidies can reduce copper ion to give red precipitate.

3) Fehling’s Test:-

• Reducing sugar reduces the copper ions present in the Fehling solution so as to give red precipitate.

4) Formation of asazone:

• When reducing Sugar is heated with phenyldyrazine yellow crystalline compound called osazones are formed.

• E.g. D-Glucose +Phenylhydrazine →Glucose (Yellow crystal) Phenyldyrazine + H2O

3. Reduction:-

• The carbonyl group of sugar can be reduced by variety of reagents such as Hydrogen and platinum to an alcohol.

• Such Carbohydrate derivatives are called alditols.

• Important example are sorbitol glycerol rabitol.

4. Oxidation:

• Sugar are oxidation gives acid. The oxidation product depends upon oxidizing agent used in the reaction.

• E.g. Glucose is oxidising to different products with the help of different oxidizing agents.

5. Mucic Acid test:

• This is used for identification of galactose and lactose.

• Galactose or Lactose on oxidation in presence of conc. Nitric acid gives galactosaccharid acid (Mucic Acid).

6. Iodine test:

• Iodine reacts with starch dextrin and Glycogen to form a coloured complex.

  Type of Polysaccharide                                       Colour with Iodine

1                                Starch                                                            Blue

2                                Dextrin                                                          Brown

3                                 Glycogen                                                     Pink

4                                 Amylose                                                       Deep Blue

5                                Amylopectin                                                  Purple

Disease related to Carbohydrate metabolism:-

• Various disorders have been reported due to abnormal metabolism of Carbohydrates.

A) Diabetes mellitus:-

• It is a group of metabolic disorders with a common characteristics feature of hyperglycemia.

• Hyperglycemia in diabetes mellitus due to defect in insulin action insulin secretion or both.

• Diabetes comes from the Greek words “Siphon” and implies that a lot of urine is made.

• The second term Mellitus come from the Latin word “Mel” which means Honey.

Diabetes mellitus is broadly classified into 2 categories.

1.  a) Type 1 Diabetes

2.  b) Type 2 Diabetes

a) Type 1 Diabetes:-

• It is characterized by absolute deficiency of insulin due to distruction of B-cell of pancreas.

• A chronic condition in which the pancreas produce little or no insulin.


• Increase Thrust

• Frequent Urination

• Hunger

• Fatigue

b) Type 2 Diabetes:-

• This this type of diabetes a due to inadequate secretion of insulin by B- cells of pancreas.

• In this type of diabetes the body either does not produce enough insulin.


• Increase thirst

• Frequent Urination hanger

• Fatigue

• Blurred vision.

Note:- Normal range- (70-120mg/dl) High range – (←200mg/dl)

B) Glycosuria:-

• when Glucose is excreted in urine the condition is called as glycosuria.

• Glycosuria occurs due to elevated blood glucose level.

• Glycosuria more common during pregnancy.

C) Galactosemia:-

• Due to deficiency of enzyme galactose 1-phosphate luridly transferees and Galactosemia.

• Galactose cannot be converted in glucose which leads to a condition called as Galactosemia.

• It is characterized by increased galactose level in circulation and urine.

• Accumulated galactose into galactic which responsible for development of cataract.


• Jaundice

• Hepatospicenmegaly

• Mental retardation, etc.

D) Fructose intolerance:-

• One of the very normal hexose sugars of fruits (i.e. Fructose) gets normally metabolised to give energy and CO2 but defective metabolism of fructose developed in blood, disorder known as fructose in tolerance.

• This disease occurs when calls on the surface of the intestine are not able to break down fructose efficiently.

E) Glycogen storage disease:-

• The metabolic abnormalities related with glycogen synthesis is termed as glycogen storage disease.

• Glycogen is main source of energy when an enzyme is missing glycogen can build up in the liver or glycogen may not form properly.


• Not growing fast enough

• Low blood sugar

• Weak muscles.

Biochemical importance of Carbohydrates:-

1. Carbohydrates are important constituents of the cell structures in the form of glycolipids, glycoprotein, heparin, cellulose, starch, Glycogen.

2. Carbohydrates serie as an important source and store of energy.

• Carbohydrates are important basic material for many organic compounds like- Amino acids, Nucleic acid, Lipids.

1. Carbohydrates are important row material for the industrial production of products like – Glucose, maltose, alcohol, Acids etc.

For PDF Notes Click on Download Button

Leave a Comment