Unit
Objectives
10
Biomolecules
After studying this Unit, you will be
able to
• explain the characteristics of “It is the harmonious and synchronous progress of chemical
biomolecules like carbohydrates, reactions in body which leads to life”.
proteins and nucleic acids and
hormones;
• classify carbohydrates, proteins, A living system grows, sustains and reproduces itself.
nucleic acids and vitamins on the The most amazing thing about a living system is that it
basis of their structures; is composed of non-living atoms and molecules. The
• explain the difference between pursuit of knowledge of what goes on chemically within
DNA and RNA; a living system falls in the domain of biochemistry. Living
• describe the role of biomolecules systems are made up of various complex biomolecules
in biosystem. 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. Structures and functions of some of these
biomolecules are discussed in this Unit.
10.1 Carbohydrates 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 as 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 as sucrose whereas the sugar present
Rationalised 2023-24
, in milk is known as lactose. Carbohydrates are also called saccharides
(Greek: sakcharon means sugar).
Carbohydrates are classified on the basis of their behaviour on
hydrolysis. They have been broadly divided into following three groups.
10.1.1 (i) Monosaccharides: A carbohydrate that cannot be hydrolysed further
Classification of to give simpler unit of polyhydroxy aldehyde or ketone is called a
Carbohydrates monosaccharide. About 20 monosaccharides are known to occur in
nature. 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. Amongst these the most common are disaccharides.
The two monosaccharide units obtained on hydrolysis of a disaccharide
may be same or different. For example, one molecule of sucrose on
hydrolysis gives one molecule of glucose and one molecule of fructose
whereas maltose gives two molecules of only glucose.
(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.
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.
10.1.2 Monosaccharides are further classified on the basis of number of carbon
Monosaccharides 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. Number of carbon atoms
constituting the monosaccharide is also introduced in the name as is
evident from the examples given in Table 10.1
Table 10.1: Different Types of Monosaccharides
Carbon atoms General term Aldehyde Ketone
3 Triose Aldotriose Ketotriose
4 Tetrose Aldotetrose Ketotetrose
5 Pentose Aldopentose Ketopentose
6 Hexose Aldohexose Ketohexose
7 Heptose Aldoheptose Ketoheptose
10.1.2.1 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:
Preparation of 1. From sucrose (Cane sugar): If sucrose is boiled with dilute HCl or
Glucose H2SO4 in alcoholic solution, glucose and fructose are obtained in
equal amounts.
Chemistry 282
Rationalised 2023-24
, +
C12 H22O11 + H2O
H
→ C6 H12O6 + C6 H12O6
Sucrose Glucose Fructose
2. From starch: Commercially glucose is obtained by hydrolysis of
starch by boiling it with dilute H2SO4 at 393 K under pressure.
+
(C6 H10 O5 )n + nH 2 O H
393 K; 2-3 atm
→ nC6 H12 O6
Starch or cellulose Glucose
Structure of Glucose is an aldohexose and is also known as dextrose. It is the
Glucose monomer of many of the larger carbohydrates, namely starch, cellulose.
It is probably the most abundant organic compound on earth. It was
assigned the structure given below on the basis of the following
evidences:
CHO 1. Its molecular formula was found to be C6H12O6.
(CHOH)4 2. On prolonged heating with HI, it forms n-hexane, suggesting that all
the six carbon atoms are linked in a straight chain.
CH2OH
Glucose
3. Glucose reacts with hydroxylamine to form an oxime and adds a
molecule of hydrogen cyanide to give cyanohydrin. These reactions
confirm the presence of a carbonyl group (>C = O) in glucose.
4. Glucose gets oxidised to six carbon carboxylic acid (gluconic acid)
on reaction with a mild oxidising agent like bromine water. This
indicates that the carbonyl group is present as an aldehydic group.
CHO COOH
Br2 water (CHOH)4
(CHOH)4
CH2OH CH2OH
Gluconic acid
5. Acetylation of glucose with acetic anhydride gives glucose
pentaacetate which confirms the presence of five –OH groups. Since
it exists as a stable compound, five –OH groups should be attached
to different carbon atoms.
283 Biomolecules
Rationalised 2023-24
Objectives
10
Biomolecules
After studying this Unit, you will be
able to
• explain the characteristics of “It is the harmonious and synchronous progress of chemical
biomolecules like carbohydrates, reactions in body which leads to life”.
proteins and nucleic acids and
hormones;
• classify carbohydrates, proteins, A living system grows, sustains and reproduces itself.
nucleic acids and vitamins on the The most amazing thing about a living system is that it
basis of their structures; is composed of non-living atoms and molecules. The
• explain the difference between pursuit of knowledge of what goes on chemically within
DNA and RNA; a living system falls in the domain of biochemistry. Living
• describe the role of biomolecules systems are made up of various complex biomolecules
in biosystem. 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. Structures and functions of some of these
biomolecules are discussed in this Unit.
10.1 Carbohydrates 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 as 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 as sucrose whereas the sugar present
Rationalised 2023-24
, in milk is known as lactose. Carbohydrates are also called saccharides
(Greek: sakcharon means sugar).
Carbohydrates are classified on the basis of their behaviour on
hydrolysis. They have been broadly divided into following three groups.
10.1.1 (i) Monosaccharides: A carbohydrate that cannot be hydrolysed further
Classification of to give simpler unit of polyhydroxy aldehyde or ketone is called a
Carbohydrates monosaccharide. About 20 monosaccharides are known to occur in
nature. 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. Amongst these the most common are disaccharides.
The two monosaccharide units obtained on hydrolysis of a disaccharide
may be same or different. For example, one molecule of sucrose on
hydrolysis gives one molecule of glucose and one molecule of fructose
whereas maltose gives two molecules of only glucose.
(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.
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.
10.1.2 Monosaccharides are further classified on the basis of number of carbon
Monosaccharides 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. Number of carbon atoms
constituting the monosaccharide is also introduced in the name as is
evident from the examples given in Table 10.1
Table 10.1: Different Types of Monosaccharides
Carbon atoms General term Aldehyde Ketone
3 Triose Aldotriose Ketotriose
4 Tetrose Aldotetrose Ketotetrose
5 Pentose Aldopentose Ketopentose
6 Hexose Aldohexose Ketohexose
7 Heptose Aldoheptose Ketoheptose
10.1.2.1 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:
Preparation of 1. From sucrose (Cane sugar): If sucrose is boiled with dilute HCl or
Glucose H2SO4 in alcoholic solution, glucose and fructose are obtained in
equal amounts.
Chemistry 282
Rationalised 2023-24
, +
C12 H22O11 + H2O
H
→ C6 H12O6 + C6 H12O6
Sucrose Glucose Fructose
2. From starch: Commercially glucose is obtained by hydrolysis of
starch by boiling it with dilute H2SO4 at 393 K under pressure.
+
(C6 H10 O5 )n + nH 2 O H
393 K; 2-3 atm
→ nC6 H12 O6
Starch or cellulose Glucose
Structure of Glucose is an aldohexose and is also known as dextrose. It is the
Glucose monomer of many of the larger carbohydrates, namely starch, cellulose.
It is probably the most abundant organic compound on earth. It was
assigned the structure given below on the basis of the following
evidences:
CHO 1. Its molecular formula was found to be C6H12O6.
(CHOH)4 2. On prolonged heating with HI, it forms n-hexane, suggesting that all
the six carbon atoms are linked in a straight chain.
CH2OH
Glucose
3. Glucose reacts with hydroxylamine to form an oxime and adds a
molecule of hydrogen cyanide to give cyanohydrin. These reactions
confirm the presence of a carbonyl group (>C = O) in glucose.
4. Glucose gets oxidised to six carbon carboxylic acid (gluconic acid)
on reaction with a mild oxidising agent like bromine water. This
indicates that the carbonyl group is present as an aldehydic group.
CHO COOH
Br2 water (CHOH)4
(CHOH)4
CH2OH CH2OH
Gluconic acid
5. Acetylation of glucose with acetic anhydride gives glucose
pentaacetate which confirms the presence of five –OH groups. Since
it exists as a stable compound, five –OH groups should be attached
to different carbon atoms.
283 Biomolecules
Rationalised 2023-24
