NCERT Textbook: Biomolecules | Chemistry Class 12 - NEET PDF Download

 Page 1


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.  Structures and functions of some of these
biomolecules are discussed in this Unit.
Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules
After studying this Unit, you will be
able to
• explain the characteristics of
biomolecules like carbohydrates,
proteins and nucleic acids and
hormones;
• classify carbohydrates, proteins,
nucleic acids and vitamins on the
basis of their structures;
• explain the difference between
DNA and RNA;
• describe the role of biomolecules
in biosystem.
Objectives
“It is the harmonious and synchronous progress of chemical
reactions in body which leads to life”.
10
Unit Unit Unit Unit Unit
10
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, C
x
(H
2
O)
y
, and were considered as hydrates
of carbon from where the name carbohydrate was derived. For example,
the molecular formula of glucose (C
6
H
12
O
6
) fits into this general formula,
C
6
(H
2
O)
6
. But all the compounds which fit into this formula may not be
classified as carbohydrates. For example acetic acid (CH
3
COOH) fits into
this general formula, C
2
(H
2
O)
2
 but is not a carbohydrate. Similarly,
rhamnose, C
6
H
12
O
5
 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
10.1 10.1 10.1 10.1 10.1 Carbohydrates Carbohydrates Carbohydrates Carbohydrates Carbohydrates
Reprint 2024-25
Page 2


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.  Structures and functions of some of these
biomolecules are discussed in this Unit.
Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules
After studying this Unit, you will be
able to
• explain the characteristics of
biomolecules like carbohydrates,
proteins and nucleic acids and
hormones;
• classify carbohydrates, proteins,
nucleic acids and vitamins on the
basis of their structures;
• explain the difference between
DNA and RNA;
• describe the role of biomolecules
in biosystem.
Objectives
“It is the harmonious and synchronous progress of chemical
reactions in body which leads to life”.
10
Unit Unit Unit Unit Unit
10
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, C
x
(H
2
O)
y
, and were considered as hydrates
of carbon from where the name carbohydrate was derived. For example,
the molecular formula of glucose (C
6
H
12
O
6
) fits into this general formula,
C
6
(H
2
O)
6
. But all the compounds which fit into this formula may not be
classified as carbohydrates. For example acetic acid (CH
3
COOH) fits into
this general formula, C
2
(H
2
O)
2
 but is not a carbohydrate. Similarly,
rhamnose, C
6
H
12
O
5
 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
10.1 10.1 10.1 10.1 10.1 Carbohydrates Carbohydrates Carbohydrates Carbohydrates Carbohydrates
Reprint 2024-25
282 Chemistry
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.
(i) Monosaccharides: A carbohydrate that cannot be hydrolysed further
to give simpler unit of polyhydroxy aldehyde or ketone is called a
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.
Monosaccharides are further classified on the basis of 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. Number of carbon atoms
constituting the monosaccharide is also introduced in the name as is
evident from the examples given in Table 10.1
10.1.1
Classification of
Carbohydrates
10.1.2
Monosaccharides
3 Triose Aldotriose Ketotriose
4 Tetrose Aldotetrose Ketotetrose
5 Pentose Aldopentose Ketopentose
6 Hexose Aldohexose Ketohexose
7 Heptose Aldoheptose Ketoheptose
Carbon atoms General term Aldehyde Ketone
Table 10.1: Different Types of Monosaccharides
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
H
2
SO
4
 in alcoholic solution, glucose and fructose are obtained in
equal amounts.
Preparation of
Glucose
10.1.2.1 Glucose
Reprint 2024-25
Page 3


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.  Structures and functions of some of these
biomolecules are discussed in this Unit.
Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules
After studying this Unit, you will be
able to
• explain the characteristics of
biomolecules like carbohydrates,
proteins and nucleic acids and
hormones;
• classify carbohydrates, proteins,
nucleic acids and vitamins on the
basis of their structures;
• explain the difference between
DNA and RNA;
• describe the role of biomolecules
in biosystem.
Objectives
“It is the harmonious and synchronous progress of chemical
reactions in body which leads to life”.
10
Unit Unit Unit Unit Unit
10
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, C
x
(H
2
O)
y
, and were considered as hydrates
of carbon from where the name carbohydrate was derived. For example,
the molecular formula of glucose (C
6
H
12
O
6
) fits into this general formula,
C
6
(H
2
O)
6
. But all the compounds which fit into this formula may not be
classified as carbohydrates. For example acetic acid (CH
3
COOH) fits into
this general formula, C
2
(H
2
O)
2
 but is not a carbohydrate. Similarly,
rhamnose, C
6
H
12
O
5
 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
10.1 10.1 10.1 10.1 10.1 Carbohydrates Carbohydrates Carbohydrates Carbohydrates Carbohydrates
Reprint 2024-25
282 Chemistry
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.
(i) Monosaccharides: A carbohydrate that cannot be hydrolysed further
to give simpler unit of polyhydroxy aldehyde or ketone is called a
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.
Monosaccharides are further classified on the basis of 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. Number of carbon atoms
constituting the monosaccharide is also introduced in the name as is
evident from the examples given in Table 10.1
10.1.1
Classification of
Carbohydrates
10.1.2
Monosaccharides
3 Triose Aldotriose Ketotriose
4 Tetrose Aldotetrose Ketotetrose
5 Pentose Aldopentose Ketopentose
6 Hexose Aldohexose Ketohexose
7 Heptose Aldoheptose Ketoheptose
Carbon atoms General term Aldehyde Ketone
Table 10.1: Different Types of Monosaccharides
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
H
2
SO
4
 in alcoholic solution, glucose and fructose are obtained in
equal amounts.
Preparation of
Glucose
10.1.2.1 Glucose
Reprint 2024-25
283 Biomolecules
+
H
12 22 11 2 6 12 6 6 12 6
C H O H O C H O + C H O + --- ?
      Sucrose   Glucose Fructose
2. From starch: Commercially glucose is obtained by hydrolysis of
starch by boiling it with dilute H
2
SO
4
 at 393 K under pressure.
+
H
6 10 5 n 2 6 12 6
393K; 2-3  atm
(C H O ) + nH O nC H O ------- ?
    Starch or cellulose     Glucose
Glucose is an aldohexose and is also known as dextrose. It is the
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:
1. Its molecular formula was found to be C
6
H
12
O
6
.
2. On prolonged heating with HI, it forms n-hexane, suggesting that all
the six carbon atoms are linked in a straight chain.
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
(CH )
4
OH
(CH )
4
OH
CH
2
OH
CH
2
OH
Br water
2
COOH
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.
Structure of
Glucose
CHO
(CH )
4
OH
CH
2
OH
Glucose
Reprint 2024-25
Page 4


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.  Structures and functions of some of these
biomolecules are discussed in this Unit.
Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules
After studying this Unit, you will be
able to
• explain the characteristics of
biomolecules like carbohydrates,
proteins and nucleic acids and
hormones;
• classify carbohydrates, proteins,
nucleic acids and vitamins on the
basis of their structures;
• explain the difference between
DNA and RNA;
• describe the role of biomolecules
in biosystem.
Objectives
“It is the harmonious and synchronous progress of chemical
reactions in body which leads to life”.
10
Unit Unit Unit Unit Unit
10
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, C
x
(H
2
O)
y
, and were considered as hydrates
of carbon from where the name carbohydrate was derived. For example,
the molecular formula of glucose (C
6
H
12
O
6
) fits into this general formula,
C
6
(H
2
O)
6
. But all the compounds which fit into this formula may not be
classified as carbohydrates. For example acetic acid (CH
3
COOH) fits into
this general formula, C
2
(H
2
O)
2
 but is not a carbohydrate. Similarly,
rhamnose, C
6
H
12
O
5
 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
10.1 10.1 10.1 10.1 10.1 Carbohydrates Carbohydrates Carbohydrates Carbohydrates Carbohydrates
Reprint 2024-25
282 Chemistry
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.
(i) Monosaccharides: A carbohydrate that cannot be hydrolysed further
to give simpler unit of polyhydroxy aldehyde or ketone is called a
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.
Monosaccharides are further classified on the basis of 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. Number of carbon atoms
constituting the monosaccharide is also introduced in the name as is
evident from the examples given in Table 10.1
10.1.1
Classification of
Carbohydrates
10.1.2
Monosaccharides
3 Triose Aldotriose Ketotriose
4 Tetrose Aldotetrose Ketotetrose
5 Pentose Aldopentose Ketopentose
6 Hexose Aldohexose Ketohexose
7 Heptose Aldoheptose Ketoheptose
Carbon atoms General term Aldehyde Ketone
Table 10.1: Different Types of Monosaccharides
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
H
2
SO
4
 in alcoholic solution, glucose and fructose are obtained in
equal amounts.
Preparation of
Glucose
10.1.2.1 Glucose
Reprint 2024-25
283 Biomolecules
+
H
12 22 11 2 6 12 6 6 12 6
C H O H O C H O + C H O + --- ?
      Sucrose   Glucose Fructose
2. From starch: Commercially glucose is obtained by hydrolysis of
starch by boiling it with dilute H
2
SO
4
 at 393 K under pressure.
+
H
6 10 5 n 2 6 12 6
393K; 2-3  atm
(C H O ) + nH O nC H O ------- ?
    Starch or cellulose     Glucose
Glucose is an aldohexose and is also known as dextrose. It is the
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:
1. Its molecular formula was found to be C
6
H
12
O
6
.
2. On prolonged heating with HI, it forms n-hexane, suggesting that all
the six carbon atoms are linked in a straight chain.
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
(CH )
4
OH
(CH )
4
OH
CH
2
OH
CH
2
OH
Br water
2
COOH
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.
Structure of
Glucose
CHO
(CH )
4
OH
CH
2
OH
Glucose
Reprint 2024-25
284 Chemistry
6. On oxidation with nitric acid, glucose as well as gluconic acid both
yield a dicarboxylic acid, saccharic acid. This indicates the presence
of a primary alcoholic (–OH) group in glucose.
CHO
(CH )
4
OH
CH OH
2
Oxidation
(CH )
4
OH
CH OH
2
COOH
(CH )
4
OH
COOH
COOH
Oxidation
Saccharic
acid
Gluconic
acid
The exact spatial arrangement of different —OH groups was given
by Fischer after studying many other properties. Its configuration is
correctly represented as I. So gluconic acid is represented as II and
saccharic acid as III.
CHO
H OH
OH H
H OH
H OH
CH
2
OH
I
COOH
H OH
OH H
H OH
H OH
CH
2
OH
II
COOH
H OH
OH H
H OH
H OH
COOH
III
Glucose is correctly named as D(+)-glucose. ‘D’ before the name
of glucose represents the configuration whereas ‘(+)’ represents
dextrorotatory nature of the molecule. It should be remembered that
‘D’ and ‘L’ have no relation with the optical activity of the compound.
They are also not related to letter ‘d’ and ‘l’ (see Unit 6). The meaning
of D– and L– notations is as follows.
The letters ‘D’ or ‘L’ before the name of any compound indicate the
relative configuration of a particular stereoisomer of a compound with
respect to configuration of some other compound, configuration of
which is known. In the case of carbohydrates, this refers to their
relation with a particular isomer of glyceraldehyde. Glyceraldehyde
contains one asymmetric carbon atom and exists in two enantiomeric
forms as shown below.
(+) Isomer of glyceraldehyde has ‘D’ configuration. It means that when
its structural formula is written on paper following specific conventions
which you will study in higher classes, the –OH group lies on right hand
side in the structure.  All those compounds which can be chemically
correlated to D (+) isomer of glyceraldehyde are said to have D-
configuration whereas those which can be correlated to ‘L’ (–) isomer of
glyceraldehyde are said to have L—configuration. In L (–) isomer –OH
group is on left hand side as you can see in the structure. For assigning
Reprint 2024-25
Page 5


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.  Structures and functions of some of these
biomolecules are discussed in this Unit.
Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules Biomolecules
After studying this Unit, you will be
able to
• explain the characteristics of
biomolecules like carbohydrates,
proteins and nucleic acids and
hormones;
• classify carbohydrates, proteins,
nucleic acids and vitamins on the
basis of their structures;
• explain the difference between
DNA and RNA;
• describe the role of biomolecules
in biosystem.
Objectives
“It is the harmonious and synchronous progress of chemical
reactions in body which leads to life”.
10
Unit Unit Unit Unit Unit
10
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, C
x
(H
2
O)
y
, and were considered as hydrates
of carbon from where the name carbohydrate was derived. For example,
the molecular formula of glucose (C
6
H
12
O
6
) fits into this general formula,
C
6
(H
2
O)
6
. But all the compounds which fit into this formula may not be
classified as carbohydrates. For example acetic acid (CH
3
COOH) fits into
this general formula, C
2
(H
2
O)
2
 but is not a carbohydrate. Similarly,
rhamnose, C
6
H
12
O
5
 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
10.1 10.1 10.1 10.1 10.1 Carbohydrates Carbohydrates Carbohydrates Carbohydrates Carbohydrates
Reprint 2024-25
282 Chemistry
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.
(i) Monosaccharides: A carbohydrate that cannot be hydrolysed further
to give simpler unit of polyhydroxy aldehyde or ketone is called a
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.
Monosaccharides are further classified on the basis of 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. Number of carbon atoms
constituting the monosaccharide is also introduced in the name as is
evident from the examples given in Table 10.1
10.1.1
Classification of
Carbohydrates
10.1.2
Monosaccharides
3 Triose Aldotriose Ketotriose
4 Tetrose Aldotetrose Ketotetrose
5 Pentose Aldopentose Ketopentose
6 Hexose Aldohexose Ketohexose
7 Heptose Aldoheptose Ketoheptose
Carbon atoms General term Aldehyde Ketone
Table 10.1: Different Types of Monosaccharides
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
H
2
SO
4
 in alcoholic solution, glucose and fructose are obtained in
equal amounts.
Preparation of
Glucose
10.1.2.1 Glucose
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283 Biomolecules
+
H
12 22 11 2 6 12 6 6 12 6
C H O H O C H O + C H O + --- ?
      Sucrose   Glucose Fructose
2. From starch: Commercially glucose is obtained by hydrolysis of
starch by boiling it with dilute H
2
SO
4
 at 393 K under pressure.
+
H
6 10 5 n 2 6 12 6
393K; 2-3  atm
(C H O ) + nH O nC H O ------- ?
    Starch or cellulose     Glucose
Glucose is an aldohexose and is also known as dextrose. It is the
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:
1. Its molecular formula was found to be C
6
H
12
O
6
.
2. On prolonged heating with HI, it forms n-hexane, suggesting that all
the six carbon atoms are linked in a straight chain.
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
(CH )
4
OH
(CH )
4
OH
CH
2
OH
CH
2
OH
Br water
2
COOH
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.
Structure of
Glucose
CHO
(CH )
4
OH
CH
2
OH
Glucose
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284 Chemistry
6. On oxidation with nitric acid, glucose as well as gluconic acid both
yield a dicarboxylic acid, saccharic acid. This indicates the presence
of a primary alcoholic (–OH) group in glucose.
CHO
(CH )
4
OH
CH OH
2
Oxidation
(CH )
4
OH
CH OH
2
COOH
(CH )
4
OH
COOH
COOH
Oxidation
Saccharic
acid
Gluconic
acid
The exact spatial arrangement of different —OH groups was given
by Fischer after studying many other properties. Its configuration is
correctly represented as I. So gluconic acid is represented as II and
saccharic acid as III.
CHO
H OH
OH H
H OH
H OH
CH
2
OH
I
COOH
H OH
OH H
H OH
H OH
CH
2
OH
II
COOH
H OH
OH H
H OH
H OH
COOH
III
Glucose is correctly named as D(+)-glucose. ‘D’ before the name
of glucose represents the configuration whereas ‘(+)’ represents
dextrorotatory nature of the molecule. It should be remembered that
‘D’ and ‘L’ have no relation with the optical activity of the compound.
They are also not related to letter ‘d’ and ‘l’ (see Unit 6). The meaning
of D– and L– notations is as follows.
The letters ‘D’ or ‘L’ before the name of any compound indicate the
relative configuration of a particular stereoisomer of a compound with
respect to configuration of some other compound, configuration of
which is known. In the case of carbohydrates, this refers to their
relation with a particular isomer of glyceraldehyde. Glyceraldehyde
contains one asymmetric carbon atom and exists in two enantiomeric
forms as shown below.
(+) Isomer of glyceraldehyde has ‘D’ configuration. It means that when
its structural formula is written on paper following specific conventions
which you will study in higher classes, the –OH group lies on right hand
side in the structure.  All those compounds which can be chemically
correlated to D (+) isomer of glyceraldehyde are said to have D-
configuration whereas those which can be correlated to ‘L’ (–) isomer of
glyceraldehyde are said to have L—configuration. In L (–) isomer –OH
group is on left hand side as you can see in the structure. For assigning
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285 Biomolecules
the configuration of monosaccharides, it is the lowest asymmetric carbon
atom (as shown below) which is compared. As in (+)   glucose, —OH on
the lowest asymmetric carbon is on the right side which is comparable
to (+) glyceraldehyde, so (+) glucose is assigned D-configuration. Other
asymmetric carbon atoms of glucose are not considered for this
comparison. Also, the structure of glucose and glyceraldehyde is written
in a way that most oxidised carbon (in this case –CHO)is at the top.
CHO
H OH
OH H
H OH
H OH
CH
2
OH
D–(+) – Glucose
CHO
CH
2
OH
H OH
D– (+) – Glyceraldehyde
The structure (I) of glucose explained most of its properties but the
following reactions and facts could not be explained by this structure.
1. Despite having the aldehyde group, glucose does not give Schiff’s
test and it does not form the hydrogensulphite addition product with
NaHSO
3
.
2. The pentaacetate of glucose does not react with hydroxylamine
indicating the absence of free —CHO group.
3. Glucose is found to exist in two different crystalline forms which are
named as a and b. The a-form of glucose (m.p. 419 K) is obtained by
crystallisation from concentrated solution of glucose at 303 K while
the b-form (m.p. 423 K) is obtained by crystallisation from hot and
saturated aqueous solution at 371 K.
This behaviour could not be explained by the open chain structure
(I) for glucose. It was proposed that one of the —OH groups may add
to the —CHO group and form a cyclic hemiacetal structure. It was
found that glucose forms a six-membered ring in which —OH at C-5
is involved in ring formation. This explains the absence of —CHO
group and also existence of glucose in two forms as shown below.
These two cyclic forms exist in equilibrium with open chain structure.
The two cyclic hemiacetal forms of glucose differ only in the
configuration of the hydroxyl group at C1, called anomeric carbon
Cyclic
Structure
of Glucose
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