This article provides you the revision notes on Class 12 Chemistry: Chapter - Biomolecules, to give you a quick glance of the chapter. In this particular part, you will learn about carbohydrates and some of its important forms like glucose, sucrose, starch, etc. These quick notes are prepared strictly according to the latest CBSE syllabus for Class 12th Chemistry.
The main topics covered in this part are:
o Chemical reactions
• Glycosidic linkage
Biomolecules are the organic molecules that are involved in the maintenance and metabolic processes of living organisms. Thus, they build up the living system and are responsible for their growth and maintenance.
Some of the common biomolecules are:
• Nucleic acids
• ATP molecules
• Carbohydrates are polyhydroxy aldehydes or ketones or the compounds which upon hydrolysis produce polyhydroxy aldehydes or ketones.
• They are optically active due to the presence of chiral ‘C’.
• They are also called saccharides (From Latin word Saccharum = sugar) due to sweet taste.
Classification of Carbohydrates
Depending upon the reaction of carbohydrates with water, they can be classified into the following three types:
1. Monosaccharides: These are the carbohydrates which cannot get hydrolyzed into simpler molecules.
For example glucose,fructose, galactose etc.
2. Disaccharides: These are the carbohydrates that yield two to ten monosaccharide units, on hydrolysis.
For example sucrose, lactose, maltose.
3. Polysaccharides: These are the high molecular mass carbohydrates which on hydrolysis yield a large number of monosaccharides.
For example starch, cellulose, glycogen, gums, etc.
Note: Monosaccharides and oligosaccharides are crystalline solids, soluble in water and sweet in taste. These are known as Sugars. Polysaccharides are amorphous, insoluble in water and tasteless. There are called Non-sugars.
Based upon reducing and non-reducing properties, carbohydrates can be divided into the following two categories:
1. Reducing sugars: These are the carbohydrates which contain free aldehydic or ketonic group and reduces Fehling's solution or Tollen's reagent.
For example: Maltose, lactose.
2. Non Reducing Sugars: These are the carbohydrates which do not contain free functional group and do not reduce Fehling’s or Tollen’s reagent.
For example: Sucrose.
Some important carbohydrates are described below:
It is a monosaccharide’s with molecular formula C6H12O6. It is present in sweet fruits and in honey.
Preparation of Glucose:
(i) From sucrose, C12H22O11:
(ii) From starch, C6H10O5:
Structure of Glucose:
• Open chain structure (Fisher model):
These two optical isomers differ in configuration around any other C atom other than C1 atom.
In D-Glucose, −OH group on first chiral `C’ from the bottom is on right hand.
In L-Glucose, −OH group points to the left of chiral carbon.
• Cyclic Structure:
Chemical Reactions of Glucose
• Reaction with HI:
• Reaction with hydroxylamine, NH4OH:
• Reaction with bromine water:
• Acetylation reaction:
• Reaction with Fehling’s solution:
• Reaction with Tollen’s reagent:
• Reaction with Phenylhydrazine, C6H5NHNH2:
It is one of the common disaccharides, which on hydrolysis gives equimolar mixture of D (+) -glucose and D (−) fructose.
The ether linkage combining two monosaccharides is known as glycosidic linkage.
For example: In sucrose, the glycosidic linkage is present between glucose and fructose.
• Main sources of starch are wheat, maize, rice, potatoes and barley.
• It is a white amorphous powder, in-soluble in cold water.
• It is a polymer of α-glucose and consists of two components- Amylose and Amylopectin.
o Amylose: It is a linear polymer of α‒ glucose. It is water soluble and gives blue colour with iodine solution.
o Amylopectin: It is a branched chain polymer of α‒ glucose. It is water in-soluble and does not give blue colour with iodine solution.
• Cellulose is a linear polymer of β-glucose. It consists of bundles of polymeric chains which are held together by hydrogen-bonding.
• Main sources are wood (45 –50%), cotton (90 – 95%). It is the main constituent of cell walls.
• It is colourless amorphous solid that decomposes on heating.
• It does not reduce Tollen's reagent or Fehling's solution.
• It cannot be digested by human stomach.
• Cell wall of bacteria and plants is made up of cellulose
• Cellulose forms a number of important products, such as:
o Cellotape used for packaging.
o Viscose rayon used in textile industry.
o Gun cotton used as an explosive.