CBSE Class 12 - Chapter 14

1. CH – 14
BIOMOLECULES

2. Biomolecules are the organic compounds which form the basis of
life, i.e., they build up the living system and responsible for their
growth and maintenance.
The sequence that relates biomolecules to living organism is -
Biomolecules → Organelles → Cells → Tissues → Organs → Living
organism.
The complex organic substances like carbohydrates, proteins etc
which combine in a specific manner to produce living systems and
maintain it.

3. Carbohydrates :
➢ They are polyhydroxy-aldehydes or ketones or substances which give
these substances on hydrolysis and contain at least one chiral atom.
➢ They have general formula of C x (H2O)y
➢ Rhamnose, deoxyribose, rhamnohexose do not obey this formula but
are carbohydrates.
➢ Optically active polyhydroxy aldehydes (aldoses) or ketones (ketoses)
or compounds which on hydrolysis give these units are known as
carbohydrates. They are also called saccharides
(Latin Saccharum = sugar) due to sweet taste of simpler members.

4. Types of Carbohydrates :
Depending upon their behaviour towards hydrolysis, carbohydrates can be of following
three types –
*Monosaccharide
*Oligosaccharide
*Polysaccharide
Monosaccharides:
✓ These are simplest carbohydrate which can’t be hydrolyzed further into smaller compounds.
✓ They are called as aldose or ketose depending upon whether they have aldehyde or ketone
group.
✓ Depending upon the number of carbon atoms present they are called as triose, tetrose etc.
Contd…

5. Oligosaccharides:
These carbohydrates on hydrolysis give 2 to 10 molecules of monosaccharides.
They are further of few types:
•Disaccharides (C12H22o11): On hydrolysis, they give 2 molecules of
monosaccharides which are held together by Glycosidic linkage. Example:
sucrose etc.
•Trisaccharides (C18H32o16): On hydrolysis, they form three molecules of
monosaccharides. Example: raffinose
•Tetra-saccharides: (C24H42o21): Such as stachyose which gives four
monosaccharides on hydrolysis.

6. Polysaccharides:
These are the carbohydrates which on hydrolysis, yield more than ten
monosaccharides molecules. Example: Starch, Cellulose, Glycogen etc.
Difference between Sugars and Non-sugars : ***
Carbohydrates (monosaccharides and disaccharides) having sweet taste,
crystalline and water soluble are collectively known as sugars. e.g. Glucose,
Fructose, Rhamnose, Sucrose, Lactose, Maltose etc.
Carbohydrates (Polysaccharides) which are tasteless, amorphous and insoluble in
water are non-sugars. e.g. Starch, Cellulose, Glycogen etc.

7. Difference between Reducing and Non-reducing sugars : ***
Carbohydrates reducing Fehling reagent or Tollen’s reagent are termed as
reducing carbohydrates due to the presence of free aldehydic or ketonic group
e.g., All monosaccharides and disaccharides (except sucrose).
Carbohydrates which do not reduce such reagents due to absence of free
aldehydic or ketonic group are known as non-reducing carbohydrates. e.g.,
sucrose and polysaccharides.

8. Glucose (C6H12O6) :
Dextrose (Aldohexose), grape sugar, corn sugar, blood sugar; monomer of
many of the larger carbohydrates like starch, cellulose etc.
Preparation of Glucose :
i) By hydrolysis of starch with hot dil mineral acids.
ii) By hydrolysis of sucrose with hot dil mineral acids.

9. Structure of Glucose :
➢ Reaction with HI:
**Reaction shows the presence of 6 – carbons linked in a straight chain.
• i) Reaction with hydroxylamine (NH2OH):
Contd..

10. • ii) Reaction with hydrogen cyanide (HCN):
➢ Oxidation : (a) – By Br2/H2O : Mild oxidizing agent
Contd..
Reactions (i) and (ii)
show the presence
of carbonyl group
>C=O in Glucose.
Glucose gets oxidised to six
carbon carboxylic acid (gluconic
acid) indicates that the carbonyl
group is present as an aldehydic
group.

11. (b) – By conc.HNO3 : Strong oxidizing agent
➢ Reaction with Acetic anhydride (Acetylation Reaction):
Reaction indicates the
presence of a primary alcoholic
(–OH) group in glucose.
Formation of Glucose
pentaacetate shows
the presence of five –
OH groups (one 10 and
four 20.

12. Open chain structure(Fisher model):
Glucose is correctly named as
D(+)-glucose. ‘D’ before the
name of glucose represents the
configuration whereas ‘(+)’
represents dextrorotatory nature
of the molecule.
*** ‘D’ and ‘L’ have no relation with the optical activity of the compound but
represents configuration.

13. D- and L- ???
•Glyceraldehyde (for reference) contains one asymmetric carbon atom
and exists in two enantiomeric forms.
These two optical isomers differ in
configuration around any other C
atom other than C1 atom.

14. ***Cyclic Structure of Glucose:
There are following three reactions of glucose which cannot be explained by its
open chain structure :
(1) Despite having the aldehydic group, Glucose doesn’t undergo 2,4-DNP test,
Schiff’s test and doesn’t form the hydrogen sulphite addition product of NaHSO₃
(2) The pentaacetate of glucose doesn’t react with hydroxylamine. This indicates
that free -CHO group is absent from glucose molecule.
Glucose is found to exist in two different crystalline forms which are named as
α and β.
The α-form of glucose (m.p. 419 K) is obtained by crystallization from
concentrated solution of glucose at 303 K while the β-form (m.p. 423 K) is
obtained by crystallization from hot and saturated aqueous solution at 371 K.

15. Anomers :
In α-D-glucose, the OH group at C1 is towards right while in β-D-glucose, the OH
group at C1 is towards left. Such a pair of stereoisomers which differ in
configuration only around C1 are called anomers and the C1 carbon is
called Anomeric carbon (or glycosidic carbon).

16. Haworth Ring Structures of Glucose and Fructose -
Glucose Fructose
6- membered ring –
5-carbons & 1-oxygen
5- membered ring –
4-carbons & 1-oxygen
NCERT Exercise : 14.1 to 14.10

17. Proteins:
Introduction of proteins -
•They are high weight polymers.
•They are polyamides that contain C, H, N, O and S.
•Proteins are derived from alpha amino carboxylic acid monomers.
•A simple protein may contain hundred even thousands of amino acid units.
•In living organisms twenty alpha amino acids occur which combine to form
different protein molecules.

18. Amino acids:
•A simple amino acid can be represented as R-CH(NH)2COOH (carboxy group
and amino group is present in it).
Alpha amino acids
•The acid in which -NH2 group is present at carbon atom adjacent to the -COOH
group are called α-amino acids.
•Alpha amino acids are the building blocks of proteins. The α-carbon of all
amino acids is chiral (except Glycine-Optically inactive), hence all amino acids
exhibit stereoisomerism that is existence of D and L types of structures.
•All the naturally occurring amino acids, belong to L form category and
laevorotatory.
•In L- amino acid -NH2 group lies left to the chiral carbon as shown :
H2N-CH2R (C0OH) L –amino acid Contd…

19. Classification of Amino Acids
➢ Depending upon the relative position of amino group with respect to carboxyl
group, the amino acids can be classified as α, β, γ, δ and so on.
➢ Depending upon the relative number of amino and carboxyl groups in their
molecule, Amino acid may be categorized as:
o Neutral: These are the amino acids containing equal number of -NH2 and
-COOH groups.
o Acidic: These are the amino acids containing more -COOH groups than
-NH2 groups.
o Basic: These are the amino acids containing more -NH2 groups than -COOH
groups.

20. Types of amino acids
The amino acids are of two types (Depending upon their synthesis):
•Essential
•Non essential
Essential amino acids
•The amino acids that can’t be made in our body and must be supplied from
outside.
•The essential amino acids are 10 in number out of all.
•The lack of these amino acids in diet can cause lot of diseases like
kwashiorkor (the disease in which water balance in body is disturbed).
Non essential amino acids
•They are those that can be synthesized in our body.
•Out of total 20 amino acids the 10 can be synthesized in the body.

21. Zwitter ion***
• Amino acid contains both basic acidic (carboxyl group) and basic (amino group)
groups in the same molecule. In aqueous solution, the carboxyl group
- COOH loses a proton which is accepted by the -NH2 group which results in the
formation of a dipolar ion known as Zwitter ion. This is neutral but contains both
positive and negative charges.
• ***In zwitter ionic form, amino acids show amphoteric behaviour as they
react both with acids and bases.

22. Peptides
Peptides are condensation products of two or more amino acids.
Peptide Linkage
Peptide linkage is an amide linkage formed by condensation of two amino acids
involving −NH2 group of one amino acid and the −COOH group of the other amino
acid with the loss of water.
Classification of Peptides:
• On the basis of number of amino acids undergoing the
condensation, the peptides can be classified as:
o Dipeptide: It is a peptide composed of two amino-
acid residues.
o Tripeptide: It is a peptide composed of three amino-
acid residues.
o Polypeptide: It is a peptide composed of more than
ten amino-acid residues.

23. Protein
Peptide having molecular mass higher than 10,000 u is called a protein.
(Insulin – 51 aminoacids)
Classification of Proteins
• On the basis their molecular shape, proteins can be classified as:
o Fibrous proteins: The polypeptide chains run parallel and are held
together by hydrogen and disulphide bond forming a fibre like structure.
They are generally insoluble in water.
For example: Keratin (present in hair, wool, silk) and myosin (present in
muscles), etc.
o Globular proteins: The polypeptide chains coil around to give a spherical
shape. These are soluble in water.
For example: Insulin and albumins.

24. Structure of Proteins
• Primary structure: The sequence of amino acids is said to be the primary
structure of a protein.
• Secondary structure: This structure of a protein refers to the shape in
which a long polypeptide chain can exist.
There are two secondary structures of proteins:
o α-Helix: In α-Helix, the polypeptide chain coil itself into a righthanded
screw with the −NH group attached to the −C=O of an adjacent turn of
the helix by the intramolecular hydrogen bonding.
It generally exists when R- group is large
o β-Helix: In this structures, the peptide chains are stretched out to
nearly maximum extension and then laid side by side, held together by
intermolecular hydrogen bonds.
It generally exists when R- group is small.
Contd…

25. • Tertiary structure: The tertiary structure of proteins represents overall
folding and superimposition of the polypeptide chains, i.e., further folding
of the secondary structure giving rise to a ball shape or spheroidal shape
structure.
It is stabilised by covalent, ionic, hydrogen and disulphide bonds.
• Quaternary structure: Some of the proteins are composed of two or
more polypeptide chains referred to as sub-units. The spatial arrangement
of these subunits with respect to each other is known as quaternary
structure.

26. Denaturation of Proteins***
• The most stable conformation of a protein at a given temperature and the pH
is known as its native state.
• The loss of biological activity of proteins when a protein in its native form, is
subjected to physical change like change in temperature or chemical change
like change in pH, is called denaturation of protein.
• For example: (i) Coagulation of egg white on boiling,
(ii) curdling of milk which is caused due to the formation of lactic
acid by the lactobacillus bacteria present in milk.
NCERT Ex. – 14.11 to 14.16, 14.18

27. Nucleic acids:
•Nucleic acids are the polymers in which nucleotides are monomers. These are
biomolecules present in nuclei of all living cells in the form of nucleoproteins .They
are also called as polynucleotides .
They help in the role of transmission of hereditary characters and synthesis of
proteins.
Each nucleotide consists of 3 parts:
•A pentose sugar
•A nitrogenous base
•A phosphate group
•The nitrogenous base and a
pentose sugar are called as
nucleoside.

28. Types of Nucleic Acids
The nucleic acids are of two types:
(i) Deoxyribonucleic acid (DNA)
(ii) Ribonucleic acid (RNA)
• DNA is mainly localised in the nucleus, within the chromosome. While small
amount is present in cytoplasm. RNA is also present in the nucleus as well as
cytoplasm.
• DNA is mainly used in protein synthesis involving RNA and also a major
source of genetic information.
• DNA contains deoxyribose while RNA contains ribose.

29. DNA
•It occurs in nucleus of cell. It has double stranded helical structure.
DNA contains:
•Deoxyribose sugar
•Nitrogenous bases :
Purines {adenine(A) and guanine(G) }
Pyrimidines {thiamine(T) and cytosine(C)}
•A phosphate group (Phosphoric acid)

30. RNA
•It occurs in cytoplasm of cell. It has a single strand helical structure.
It consist of:
•Ribose sugar
•Nitrogenous base
Purines: adenine(A) and guanine(G)
Pyrimidines: cytosine(C) and uracil(U)
•A phosphate group

31. ***Note that Purines and Pyrimidines are linked together by hydrogen bonds
•Adenine always bond with thiamine by two hydrogen bonds or vice versa.
(A=T or T=A)
•Cytosine always pairs with guanine by three hydrogen bonds or vice versa.
(C≡G or G≡C)
***Monomers of DNA or RNA i.e. nucleotides are linked together through
Phosphodiester linkage.
***Helices of DNA or RNA are linked together through Hydrogen bonds.

32. Types of RNA & their Function
1.Messenger RNA (m-RNA) It is produced in the nucleus and carries genetic
information from the nucleus to the cytoplasm of a cell for the synthesis of
proteins.
2.Transfer RNA (t-RNA) It is found in cytoplasm. Its function is to collect amino
acids from cytoplasm for protein synthesis.
3.Ribosomal RNA (rRNA) combine with proteins in the cytoplasm to form
ribosomes, which act as the site of protein synthesis and has the enzymes
needed for the process.

33. ***Structural difference between RNA and DNA

34. ***
Two strands of DNA are not identical but complementary to each other.
DNA consists of two strands of nucleic acid chains coiled around each other in the
form of a double helix.The base of one strand of DNA are paired with bases on
other strand by means of hydrogen bonding.
This hydrogen bonding is very specific as the bases can only base pair in a
complementary manner. Adenine pairs with only thymine via 2 hydrogen bonds
and guanine pairs with cytosine through 3 hydrogen bonds.
Thus, the two strands of DNA are complementary to each other in the sense that
the sequence of bases in one strand automatically determines that of the other.
So the DNA strands cannot be identical, but they are complementary to each
other.
NCERT Ex. 14.21 to 14.25