BIOMOLECULES

Chapter 14
Class 12th

“It is the harmonious and synchronous

progress of chemical reactions in body which leads to life”.

BIOMOLECULES
Living systems are made up of various complex molecule
called biomolecules like carbohydrates, proteins,
nucleic acids, lipids, etc. Proteins and carbohydrates
are essential constituents of our food
 Carbohydrates
Carbohydrates may be defined as optically active
polyhydroxy aldehydes or ketones or the
compounds which produce such units on hydrolysis.
examples of carbohydrates are cane sugar, glucose
etc.

Note :- 1) it have a general formula, Cx(H2O)y

For example, the molecular formula of glucose
(C6H12O6) fits into this general formula, C6(H2O)6
but Rhamnose, C6H12O5 is a carbohydrate but
does not fit in this definition
2) Carbohydrates are also called saccharides
(Greek: sakcharon means sugar)

Carbohydrates are classified on the basis

of their behaviour on hydrolysis.
(i) Monosaccharides: A carbohydrate that cannot be hydrolysed
further to give simpler unit of polyhydroxy aldehyde or ketone is
called a monosaccharide. Some common examples are glucose,
fructose, ribose, etc.
(ii) Oligosaccharides: Carbohydrates that yield two to ten
monosaccharide units, on hydrolysis, are called oligosaccharides.
They are further classified as disaccharides, trisaccharides,
tetrasaccharides, etc., depending upon the number of
monosaccharides, they provide on hydrolysis.
Disaccharides gives two monosaccharide units on hydrolysis .
 (iii) Polysaccharides: Carbohydrates which yield a large number of
monosaccharide units on hydrolysis are called polysaccharides.
Some common examples are starch, cellulose, glycogen, gums,
etc. Polysaccharides are not sweet in taste, hence they are also
called non-sugars
Note :- The carbohydrates may also be classified as either reducing
or non-reducing sugars. All those carbohydrates which reduce
Fehling’s solution and Tollens’ reagent are referred to as reducing
sugars. All monosaccharides whether aldose or ketose are reducing
sugars. In disaccharides, if the reducing groups of monosaccharides
i.e., aldehydic or ketonic groups are bonded, these are non-reducing
sugars,

If a monosaccharide contains an aldehyde group,

it is known as an aldose
if a monosaccharide contains a keto group, it is
known as a ketose

Glucose
Glucose occurs freely in nature as well as in the
combined form. It is present in sweet fruits and honey.
Ripe grapes also contain glucose in large amounts.
It is prepared as follows:
1. From sucrose (Cane sugar): If sucrose is boiled
with dilute HCl or H2SO4 in alcoholic solution,
glucose and fructose are obtained in equal amounts
 Structure of Glucose
Glucose is an aldohexose and is also known as
dextrose. Elucidation of glucose structure
Glyceraldehyde contains one asymmetric carbon atom and
exists in two enantiomeric forms as shown below.

Cyclic Structure of Glucose

The glucose explained most of its properties but the
following reactions and facts could not be explained
by this cyclic structure of Glucose.
 The two cyclic forms of glucose differ only in the
configuration of the hydroxyl group at C1, called
anomeric carbon. Such isomers, i.e., α-form and β-
form, are called anomers
The cyclic structure of glucose is more correctly represented by
Haworth structure as given below.

Fructose
Fructose is an important ketohexose. It is obtained
along with glucose by the hydrolysis of disaccharide,
Fructose also has the molecular formula C6H12O6 .It is a
laevorotatory compound. It is appropriately written as D-(–)-
fructose. Its open chain structure is as shown.
Cyclic structures

Haworth structures

Glycosidic linkage.
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 oxygen atom is called glycosidic linkage.
(i) Sucrose : One of the common disaccharides is
sucrose which on hydrolysis gives equimolar mixture
of D-(+)-glucose and D-(-) fructose. these two
monosaccharides are held together by a glycosidic
linkage between C1 of α-D-glucose and C2 of β-D-
fructose.
Invert sugar( important)
Sucrose is dextrorotatory but after hydrolysis gives dextrorotatory glucose and
laevorotatory fructose. Since the laevorotation of fructose (–92.4°) is more than
dextrorotation of glucose (+ 52.5°), the mixture is laevorotatory. Thus, hydrolysis
of sucrose brings about a change in the sign of rotation, from dextro (+) to laevo
(–) and the product is named as invert sugar.
(ii) Maltose: Another disaccharide, maltose is composed of two α-D-glucose
units in which C1 of one glucose (I) is linked to C4 of another glucose unit (II).

(iii) Lactose: It is more commonly known as milk

sugar since this disaccharide is found in milk. It is
composed of β-D- galactose and β-D-glucose. The
linkage is between C1 of galactose and C4 of
glucose

Polysaccharides

Polysaccharides
Polysaccharides contain a large number of
monosaccharide units joined together by glycosidic
linkages. These are the most commonly encountered
carbohydrates in nature
 (i) Starch: Starch is the main storage polysaccharide of plants. High content
of starch is found in cereals, roots and some vegetables. It is a polymer of
α-glucose and consists of two components
a) Amylose is water soluble component which constitutes about 15-20%
of starch. Chemically amylose is a long unbranched chain with 200-1000
α-D-(+)-glucose units together by C1– C4 glycosidic linkage.

b) Amylopectin is insoluble in water and constitutes about 80- 85% of starch. It

is a branched chain polymer of α-D-glucose units in which chain is formed by
C1–C4 glycosidic linkage whereas branching occurs by C1–C6 glycosidic linkage.

(ii) Cellulose: Cellulose occurs in plants and it is the most abundant organic
substance in plant kingdom. It is a main constituent of cell wall of plant cells.
Cellulose is a straight chain polysaccharide composed only of β-D-glucose
units which are joined by glycosidic linkage between C1 of one glucose unit
andC4 of the next glucose unit
(iii) Glycogen: The carbohydrates are stored in animal body as glycogen.
It is also known as animal starch because its structure is similar to
amylopectin and is rather more highly branched. It is present in liver, muscles
and brain. When the body needs glucose, enzymes break the glycogen down
to glucose. Glycogen is also found in yeast and fungi.

Importance of Carbohydrates
1) Carbohydrates are essential for life in both plants and animals. Carbohydrates
are used as storage molecules as starch in plants and glycogen in animals.
2) Cell wall of bacteria and plants is made up of cellulose
3) They provide raw materials for many important industries like textiles,
paper, lacquers and breweries.
4) Two aldopentoses i.e. D-ribose and 2-deoxy-D-ribose are present in nucleic
acids
Proteins
Proteins are the most abundant biomolecules of the living system. They are
required for growth and maintenance of body.
All proteins are polymers of α-amino acids
Amino acids
Amino acids contain amino (–NH2) and carboxyl (–COOH) functional groups.
When the amino group present at alpha position then its called α-amino acids

this side chain may be other functional groups also.

Refer to page no :- 420 (Table 14.2: Natural Amino Acids chapter 14 NCERT)

note :-
1) Except glycine, all other naturally occurring α-amino acids are
optically active, since the α-carbon atom is asymmetric.

2) These exist both in ‘D’ and ‘L’ forms. Most naturally occurring amino acids
have L-configuration. L-Aminoacids are represented by writing the –NH2
group on left hand side

What are essential and non- essential amino acids? Give two e.g. of each ?
The amino acids, which can be synthesized in the body, are known as non-
essential amino acids e.g. Glycine and Alanine.
On the other hand, those which cannot be synthesized in the body and must
be obtained through diet, are known as essential amino acids e.g. Valine and
Lysine

Classification of Amino Acids

1) Amino acids are classified as acidic, basic or neutral depending upon
the number of amino and carboxyl groups in their molecule.
a ) If Equal number of amino and carboxyl groups makes it neutral.
b) More number of amino than carboxyl groups makes it basic.
c) More carboxyl groups as compared to amino groups makes it acidic.
2) Amino acids are usually colourless, crystalline solids. These are
water-soluble, high melting solids.
3) Its generally exist in salts rather than simple amines or carboxylic acids.
This behaviour is due to the presence of both acidic (carboxyl group) and
basic (amino group) groups in the same molecule.
In aqueous solution, the carboxyl group can lose a proton and amino group
can accept a proton, giving rise to 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.

Structure of Proteins
Peptide linkage.
Chemically, peptide linkage is an amide formed between –COOH group and
–NH2 group. This results in the elimination of a water molecule and
formation of a peptide bond i.e. –CO–NH–

The product of the reaction is called a dipeptide because it is made up of two

amino acids. For example, when carboxyl group of glycine combines with the
amino group of alanine we get a dipeptide, glycylalanine
If a third amino acid combines to a dipeptide, the product is called a
tripeptide and So on…..
 A polypeptide with more than hundred amino acid residues, having molecular
mass higher than 10,000u is called a protein.
Note :- However, the distinction between a polypeptide and a protein is
not very sharp. Polypeptides with fewer amino acids are likely to be called
proteins if they ordinarily have a well defined conformation of a protein such
as insulin which contains 51 amino acids.
Proteins
Proteins can be classified into two types on the basis of their molecular shape
(a) Fibrous proteins
When the polypeptide chains run parallel and are held together by
hydrogen and disulphide bonds, then fibre– like structure is formed. Such
proteins are generally insoluble in water. Some common examples are
keratin (present in hair, wool, silk) and myosin (present in muscles), etc
(b) Globular proteins
This structure results when the chains of polypeptides coil around
to give a spherical shape. These are usually soluble in water. Insulin
and albumins are the common examples of globular proteins.

On the basis of structure and shape of proteins can be studied at four different
levels, i.e., primary, secondary, tertiary and quaternary,
Note :- each level being more complex than the previous one.

(i) Primary structure of proteins: Proteins may have one or more polypeptide
chains. Each polypeptide in a protein has amino acids linked with each
other in a specific sequence and it is this sequence of amino acids that is
said to be the primary structure of that proteins.
(ii) Secondary structure of proteins: The secondary structure of protein refers to
the shape in which a long polypeptide chain can exist.
They are found to exist in two different types of structures
α-Helix
α-Helix is one of the most common ways in which a polypeptide chain
forms all possible hydrogen bonds by twisting into a right handed screw
(helix) with the –NH group of each amino acid residue hydrogen bonded to
the C O of an adjacent turn of the helix.
β-pleated sheet
β-pleated sheet structure all peptide chains are stretched out to nearly
maximum extension and then laid side by side which are held together by
intermolecular hydrogen bonds. The structure resembles the pleated folds of
drapery and therefore is known as β-pleated sheet.

α-Helix structure of proteins β-Pleated sheet structure of proteins

(iii) Tertiary structure of proteins: The tertiary structure of proteins represents

overall folding of the polypeptide chains i.e., further folding of the secondary
structure. It gives rise to two major molecular shapes viz. fibrous and globular.

(iv) Quaternary structure of proteins: 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.
what is denaturation of proteins ? What is the effect of denaturation
on the structure of proteins?
Protein found in a biological system with a unique three-dimensional
structure and unique biological activity is called a native protein or
natural conformation of protein but the natural conformation of
protein is lost due to physical change like change in temperature or
chemical change like change in pH. This is called denaturation of
proteins

effect of denaturation on the structure of proteins

During denaturation secondary and tertiary structures are destroyed
but primary structure remains intact. Due to denaturation, the
hydrogen bonds are disturbed., globules unfold and helix get uncoiled
and protein loses its biological activity e.g.
The coagulation of egg white on boiling is a common example of
denaturation. Another example is curdling of milk which is caused due
to the formation of lactic acid by the bacteria present in milk

Enzymes
Refer to chapter 5 surface chemistry in details
Nucleic Acids
The particles which are present in nucleus of the cell, responsible for
heredity, are called chromosomes which are made up of proteins and
another type of biomolecules called nucleic acids
These are mainly of two types
1) The deoxyribonucleic acid (DNA)
2) Ribonucleic acid (RNA)
Note :-
1) On Complete hydrolysis of DNA (or RNA) yields a pentose sugar,
Phosphoric acid and nitrogen containing heterocyclic compounds (called
bases).
2) In DNA molecules, the sugar moiety is β-D-2-deoxyribose whereas in
RNA molecule, it is β-D-ribose.
 What is the difference between Nucleotide and nucleoside

Structure of (a) a nucleoside and (b) a nucleotide

RNA molecules are of three types and they perform different functions.
They are named as
Messenger RNA (m-RNA)
Ribosomal RNA (r-RNA)
Transfer RNA (t-RNA).