NCERT Textbook: Biomolecules | NCERT Textbooks (Class 6 to Class 12) - CTET & State TET 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
• define the biomolecules like
carbohydrates, proteins and
nucleic acids;
• classify carbohydrates, proteins,
nucleic acids and vitamins on the
basis of their structures;
• explain the difference between
DNA and RNA;
• appreciate the role of biomolecules
in biosystem.
Objectives
“It is the harmonious and synchronous progress of chemical
reactions in body which leads to life”.
14
Unit Unit Unit Unit Unit
14
Carbohydrates are primarily produced by plants and form a very large
group of naturally occurring organic compounds. Some common
examples 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. 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,
14.1 14.1 14.1 14.1 14.1 Carbohydrates Carbohydrates Carbohydrates Carbohydrates Carbohydrates
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
• define the biomolecules like
carbohydrates, proteins and
nucleic acids;
• classify carbohydrates, proteins,
nucleic acids and vitamins on the
basis of their structures;
• explain the difference between
DNA and RNA;
• appreciate the role of biomolecules
in biosystem.
Objectives
“It is the harmonious and synchronous progress of chemical
reactions in body which leads to life”.
14
Unit Unit Unit Unit Unit
14
Carbohydrates are primarily produced by plants and form a very large
group of naturally occurring organic compounds. Some common
examples 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. 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,
14.1 14.1 14.1 14.1 14.1 Carbohydrates Carbohydrates Carbohydrates Carbohydrates Carbohydrates
404 Chemistry
C:\Chemistry-12\Unit-14.pmd    28.02.07
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 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,
sucrose on hydrolysis gives one molecule each of glucose and fructose
whereas maltose gives two molecules of glucose only.
(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.
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.
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 14.1
14.1.1
Classification of
Carbohydrates
14.1.2
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 14.1: Different Types of Monosaccharides
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
• define the biomolecules like
carbohydrates, proteins and
nucleic acids;
• classify carbohydrates, proteins,
nucleic acids and vitamins on the
basis of their structures;
• explain the difference between
DNA and RNA;
• appreciate the role of biomolecules
in biosystem.
Objectives
“It is the harmonious and synchronous progress of chemical
reactions in body which leads to life”.
14
Unit Unit Unit Unit Unit
14
Carbohydrates are primarily produced by plants and form a very large
group of naturally occurring organic compounds. Some common
examples 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. 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,
14.1 14.1 14.1 14.1 14.1 Carbohydrates Carbohydrates Carbohydrates Carbohydrates Carbohydrates
404 Chemistry
C:\Chemistry-12\Unit-14.pmd    28.02.07
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 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,
sucrose on hydrolysis gives one molecule each of glucose and fructose
whereas maltose gives two molecules of glucose only.
(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.
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.
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 14.1
14.1.1
Classification of
Carbohydrates
14.1.2
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 14.1: Different Types of Monosaccharides
405 Biomolecules
C:\Chemistry-12\Unit-14.pmd    28.02.07
I 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
H
2
SO
4
 in alcoholic solution, glucose and fructose are obtained in
equal amounts.
+
H
12 22 11 2 6 12 6 6 12 6
C H O HO CHO +CHO +??? ?
      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 105n 2 6 126
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 = 0) 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 CHO
(CH )
4
OH OH
(CH )
4
OH OH
CH
2
OH OH
CH
2
OH OH
Br water
2
COOH COOH
Gluconic acid
CHO CHO
(CH )
4
OH OH
CH
2
OH OH
14.1.3
Preparation of
Glucose
14.1.4
Structure of
Glucose
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
• define the biomolecules like
carbohydrates, proteins and
nucleic acids;
• classify carbohydrates, proteins,
nucleic acids and vitamins on the
basis of their structures;
• explain the difference between
DNA and RNA;
• appreciate the role of biomolecules
in biosystem.
Objectives
“It is the harmonious and synchronous progress of chemical
reactions in body which leads to life”.
14
Unit Unit Unit Unit Unit
14
Carbohydrates are primarily produced by plants and form a very large
group of naturally occurring organic compounds. Some common
examples 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. 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,
14.1 14.1 14.1 14.1 14.1 Carbohydrates Carbohydrates Carbohydrates Carbohydrates Carbohydrates
404 Chemistry
C:\Chemistry-12\Unit-14.pmd    28.02.07
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 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,
sucrose on hydrolysis gives one molecule each of glucose and fructose
whereas maltose gives two molecules of glucose only.
(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.
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.
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 14.1
14.1.1
Classification of
Carbohydrates
14.1.2
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 14.1: Different Types of Monosaccharides
405 Biomolecules
C:\Chemistry-12\Unit-14.pmd    28.02.07
I 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
H
2
SO
4
 in alcoholic solution, glucose and fructose are obtained in
equal amounts.
+
H
12 22 11 2 6 12 6 6 12 6
C H O HO CHO +CHO +??? ?
      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 105n 2 6 126
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 = 0) 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 CHO
(CH )
4
OH OH
(CH )
4
OH OH
CH
2
OH OH
CH
2
OH OH
Br water
2
COOH COOH
Gluconic acid
CHO CHO
(CH )
4
OH OH
CH
2
OH OH
14.1.3
Preparation of
Glucose
14.1.4
Structure of
Glucose
406 Chemistry
C:\Chemistry-12\Unit-14.pmd    28.02.07
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.
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 may be remembered that ‘D’
and ‘L’ have no relation with the optical activity of the compound.
The meaning of D– and L– notations is given as follows.
The letters ‘D’ or ‘L’ before the name of any compound indicate the
relative configuration of a particular stereoisomer. 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.
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
• define the biomolecules like
carbohydrates, proteins and
nucleic acids;
• classify carbohydrates, proteins,
nucleic acids and vitamins on the
basis of their structures;
• explain the difference between
DNA and RNA;
• appreciate the role of biomolecules
in biosystem.
Objectives
“It is the harmonious and synchronous progress of chemical
reactions in body which leads to life”.
14
Unit Unit Unit Unit Unit
14
Carbohydrates are primarily produced by plants and form a very large
group of naturally occurring organic compounds. Some common
examples 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. 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,
14.1 14.1 14.1 14.1 14.1 Carbohydrates Carbohydrates Carbohydrates Carbohydrates Carbohydrates
404 Chemistry
C:\Chemistry-12\Unit-14.pmd    28.02.07
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 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,
sucrose on hydrolysis gives one molecule each of glucose and fructose
whereas maltose gives two molecules of glucose only.
(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.
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.
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 14.1
14.1.1
Classification of
Carbohydrates
14.1.2
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 14.1: Different Types of Monosaccharides
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I 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
H
2
SO
4
 in alcoholic solution, glucose and fructose are obtained in
equal amounts.
+
H
12 22 11 2 6 12 6 6 12 6
C H O HO CHO +CHO +??? ?
      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 105n 2 6 126
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 = 0) 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 CHO
(CH )
4
OH OH
(CH )
4
OH OH
CH
2
OH OH
CH
2
OH OH
Br water
2
COOH COOH
Gluconic acid
CHO CHO
(CH )
4
OH OH
CH
2
OH OH
14.1.3
Preparation of
Glucose
14.1.4
Structure of
Glucose
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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.
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 may be remembered that ‘D’
and ‘L’ have no relation with the optical activity of the compound.
The meaning of D– and L– notations is given as follows.
The letters ‘D’ or ‘L’ before the name of any compound indicate the
relative configuration of a particular stereoisomer. 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.
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All those compounds which can be chemically correlated to (+) isomer
of glyceraldehyde are said to have D-configuration whereas those which
can be correlated to (–) isomer of glyceraldehyde are said to have
L—configuration. For assigning 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 it is
assigned D-configuration. For this comparison, the structure is written
in a way that most oxidised carbon 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 2,4-DNP
test, 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 ß. The a-form of glucose (m.p. 419 K) is obtained by
crystallisation from concentrated solution of glucose at 303 K while
the ß-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.
14.1.5 Cyclic
Structure
of Glucose