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NCERT Class 12 Chemistry Textbook Chapter 14 With Answer PDF Free Download
Chapter 14: Biomolecules
A living system grows, sustains, and reproduces itself. The most amazing thing about a living system is that it is composed of non-living atoms and molecules.
The pursuit of knowledge of what goes on chemically within a living system falls in the domain of biochemistry.
Living systems are made up of various complex biomolecules like carbohydrates, proteins, nucleic acids, lipids, etc.
Proteins and carbohydrates are essential constituents of our food. These biomolecules interact with each other and constitute the molecular logic of life processes.
In addition, some simple molecules like vitamins and mineral salts also play an important role in the functions of organisms.
The structures and functions of some of these biomolecules are discussed in this Unit.
Carbohydrates are primarily produced by plants and form a very large group of naturally occurring organic compounds.
Some common examples of carbohydrates are cane sugar, glucose, starch, etc. Most of them have a general formula, Cx(H2O)y, and were considered hydrates of carbon from where the name carbohydrate was derived.
For example, the molecular formula of glucose (C6H12O6) fits into this general formula, C6(H2O)6.
But all the compounds which fit into this formula may not be classified as carbohydrates.
For example, acetic acid (CH3COOH) fits into this general formula, C2(H2O)2 but is not a carbohydrate. Similarly, rhamnose, C6H12O5 is a carbohydrate but does not fit in this definition.
A large number of their reactions have shown that they contain specific functional groups. Chemically, the carbohydrates may be defined as optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis.
Some of the carbohydrates, which are sweet in taste, are also called sugars. The most common sugar, used in our homes is named sucrose whereas the sugar present in milk is known as lactose.
Monosaccharides are further classified on the basis of the number of carbon atoms and the functional group present in them.
If a monosaccharide contains an aldehyde group, it is known as an aldose and if it contains a keto group, it is known as a ketose.
The number of carbon atoms constituting the monosaccharide is also introduced in the name as is
evident from the examples given in Table 14.1
Structure of Glucose
Glucose is aldohexose and is also known as dextrose. It is the monomer of many of the larger carbohydrates, namely starch, and cellulose.
It is probably the most abundant organic compound on earth. It was assigned the structure given below on the basis of the following evidence:
- Its molecular formula was found to be C6H12O6
- On prolonged heating with HI, it forms n-hexane, suggesting that all
the six carbon atoms are linked in a straight chain.
You have already read that disaccharides on hydrolysis with dilute acids or enzymes yield two molecules of either the same or different monosaccharides.
The two monosaccharides are joined together by an oxide linkage formed by the loss of a water molecule.
Such a linkage between two monosaccharide units through an oxygen atom is called glycosidic linkage.
In disaccharides, if the reducing groups of monosaccharides i.e., aldehydic or ketonic groups are bonded, these are non-reducing sugars, e.g., sucrose. On the other hand, sugars in which these functional groups are free, are called reducing sugars, for example, maltose and lactose.
(i) Sucrose: One of the common disaccharides is sucrose which on hydrolysis gives an equimolar mixture of D-(+)-glucose and D-(-) fructose.
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NCERT Solutions Class 12 Chemistry Chapter 14 Biomolecules
Q 1. What are monosaccharides?
Monosaccharides known as simple sugars comprise one sugar unit that cannot be further broken down into simple sugars.
We can classify a monosaccharide on the basis of the number of carbon atoms and the functional group present in them.
The monosaccharide which contains an aldehyde group is termed aldoses and those which have a keto group are called ketoses.
Depending on the number of carbon atoms present in a monosaccharide it is further classified as trioses, tetroses, pentoses, hexoses, and heptoses.
For example, we can call an aldose which contains 3 carbon atoms as aldotriose, and a keto which contains 3 carbon atoms as ketotriose.
Q 2. What are reducing sugars?
Those type of carbohydrates that reduces Fehling’s solution and Tollen’s reagent are termed reducing sugars.
Q 3. Write two main functions of carbohydrates in plants.
The two main functions of carbohydrates in a plant are: (a) Polysaccharides like starch act as storage molecules. (b) Cellulose is used to build the cell wall, and it is a polysaccharide
Q 4. Classify the following into monosaccharides and disaccharides. Ribose, 2-deoxyribose, maltose, galactose, fructose and lactose.
Monosaccharides: 2-deoxyribose, galactose, Ribose, fructose
Disaccharides: lactose, Maltose
Q 6. What is glycogen? How is it different from starch?
Glycogen, also termed animal starch, is found only in animals. It is a polysaccharide.
Both Glycogen and starch are the main sources of glucose that provides energy to humans that are later converted into carbohydrates.
They differ in structure. Starch comprises a chain and a branched compound whereas glycogen is composed of a single molecule and is branched.
Biomolecules NCERT With Solutions PDF Free Download