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.
BiomoleculesBiomolecules
BiomoleculesBiomolecules
Biomolecules
BiomoleculesBiomolecules
BiomoleculesBiomolecules
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”.
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 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
14.114.1
14.114.1
14.1
CarbohydratesCarbohydrates
CarbohydratesCarbohydrates
Carbohydrates
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412Chemistry
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 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
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
14.1.2.1 Glucose
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413 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
393 K; 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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414Chemistry
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 10). 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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415 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 α and β. The α-form of glucose (m.p. 419 K) is obtained by
crystallisation from concentrated solution of glucose at 303 K while
the β-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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416Chemistry
(the aldehyde carbon before cyclisation). Such isomers, i.e., α-form
and β-form, are called anomers. The six membered cyclic structure
of glucose is called pyranose structure (α– or β–), in analogy with
pyran. Pyran is a cyclic organic compound with one oxygen atom
and five carbon atoms in the ring. The cyclic structure of glucose is
more correctly represented by Haworth structure as given below.
Fructose is an important ketohexose. It is obtained along with glucose
by the hydrolysis of disaccharide, sucrose. It is a natural
monosaccharide found in fruits, honey and vegetables. In its pure
form it is used as a sweetner. It is also an important ketohexose.
Fructose also has the molecular formula C
6
H
12
O
6
and
on the basis of its reactions it was found to contain a
ketonic functional group at carbon number 2 and six
carbons in straight chain as in the case of glucose. It
belongs to D-series and is a laevorotatory compound.
It is appropriately written as D-(–)-fructose. Its open
chain structure is as shown.
It also exists in two cyclic forms which are obtained
by the addition of —OH at C5 to the (
) group. The ring, thus formed
is a five membered ring and is named as furanose with analogy to the
compound furan. Furan is a five membered cyclic compound with one
oxygen and four carbon atoms.
Structure
of Fructose
The cyclic structures of two anomers of fructose are represented by
Haworth structures as given.
14.1.2.2 Fructose
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417 Biomolecules
You have already read that disaccharides on hydrolysis with dilute
acids or enzymes yield two molecules of either the same or different
monosaccharides. 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.
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.
(i) Sucrose: One of the common disaccharides is sucrose which on
hydrolysis gives equimolar mixture of D-(+)-glucose and D-(-) fructose.
14.1.3
Disaccharides
These two monosaccharides are held together by a glycosidic
linkage between C1 of α-D-glucose and C2 of β-D-fructose. Since
the reducing groups of glucose and fructose are involved in
glycosidic bond formation, sucrose is a non reducing sugar.
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). The free aldehyde group can be
produced at C1 of second glucose in solution and it shows reducing
properties so it is a reducing sugar.
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418Chemistry
(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. Free aldehyde group may be produced at C-1 of glucose
unit, hence it is also a reducing sugar.
Polysaccharides contain a large number of monosaccharide units joined
together by glycosidic linkages. These are the most commonly
encountered carbohydrates in nature. They mainly act as the food
storage or structural materials.
(i) Starch: Starch is the main storage polysaccharide of plants. It is
the most important dietary source for human beings. High content
of starch is found in cereals, roots, tubers and some vegetables. It
is a polymer of α-glucose and consists of two components—
Amylose and Amylopectin. 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
held together by C1– C4 glycosidic linkage.
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.
14.1.4
Polysaccharides
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419 Biomolecules
(ii) Cellulose: Cellulose occurs exclusively in plants and it is the most
abundant organic substance in plant kingdom. It is a predominant
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 and
C4 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.
Carbohydrates are essential for life in both plants and animals. They
form a major portion of our food. Honey has been used for a long time
as an instant source of energy by ‘Vaids’ in ayurvedic system of
medicine. Carbohydrates are used as storage molecules as starch in
plants and glycogen in animals. Cell wall of bacteria and plants is
made up of cellulose. We build furniture, etc. from cellulose in the form
14.1.5
Importance of
Carbohydrates
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420Chemistry
of wood and clothe ourselves with cellulose in the form of cotton fibre.
They provide raw materials for many important industries like textiles,
paper, lacquers and breweries.
Two aldopentoses viz. D-ribose and 2-deoxy-D-ribose (Section
14.5.1, Class XII) are present in nucleic acids. Carbohydrates are found
in biosystem in combination with many proteins and lipids.
14.1 Glucose or sucrose are soluble in water but cyclohexane or
benzene (simple six membered ring compounds) are insoluble in
water. Explain.
14.2 What are the expected products of hydrolysis of lactose?
14.3 How do you explain the absence of aldehyde group in the
pentaacetate of D-glucose?
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Proteins are the most abundant biomolecules of the living system.
Chief sources of proteins are milk, cheese, pulses, peanuts, fish, meat,
etc. They occur in every part of the body and form the fundamental
basis of structure and functions of life. They are also required for
growth and maintenance of body. The word protein is derived from
Greek word, “proteios” which means primary or of prime importance.
All proteins are polymers of α-amino acids.
Amino acids contain amino (–NH
2
) and carboxyl (–COOH) functional
groups. Depending upon the relative position of amino group with
respect to carboxyl group, the amino acids can be
classified as α, β, γ, δ and so on. Only α-amino
acids are obtained on hydrolysis of proteins. They
may contain other functional groups also.
All α-amino acids have trivial names, which
usually reflect the property of that compound or
its source. Glycine is so named since it has sweet taste (in Greek glykos
means sweet) and tyrosine was first obtained from cheese (in Greek, tyros
means cheese.) Amino acids are generally represented by a three letter
symbol, sometimes one letter symbol is also used. Structures of some
commonly occurring amino acids along with their 3-letter and 1-letter
symbols are given in Table 14.2.
1. Glycine H Gly G
2. Alanine – CH
3
Ala A
3. Valine* (H
3
C)
2
CH- Val V
4. Leucine* (H
3
C)
2
CH-CH
2
- Leu L
Name of the Characteristic feature Three letter One letter
amino acids of side chain, R symbol code
Table 14.2: Natural Amino Acids
14.2.1 Amino
Acids
R
CH COOH
NH
2
a-amino acid
(R = side chain)
COOH
H
2
N H
R
14.214.2
14.214.2
14.2
ProteinsProteins
ProteinsProteins
Proteins
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421 Biomolecules
5. Isoleucine* H
3
C-CH
2
-CH- Ile I
|
CH
3
6. Arginine* HN=C-NH-(CH
2
)
3
- Arg R
|
NH
2
7. Lysine* H
2
N-(CH
2
)
4
- Lys K
8. Glutamic acid HOOC-CH
2
-CH
2
- Glu E
9. Aspartic acid HOOC-CH
2
- Asp D
O
||
10. Glutamine H
2
N-C-CH
2
-CH
2
- Gln Q
O
||
11. Asparagine H
2
N-C-CH
2
- Asn N
12. Threonine* H
3
C-CHOH- Thr T
13. Serine HO-CH
2
- Ser S
14. Cysteine HS-CH
2
- Cys C
15. Methionine* H
3
C-S-CH
2
-CH
2
- Met M
16. Phenylalanine* C
6
H
5
-CH
2
- Phe F
17. Tyrosine (p)HO-C
6
H
4
-CH
2
- Tyr Y
18. Tryptophan*
–CH
2
N
H
Trp W
19. Histidine* His H
20. Proline Pro P
* essential amino acid, a = entire structure
Amino acids are classified as acidic, basic or neutral depending upon
the relative number of amino and carboxyl groups in their molecule.
Equal number of amino and carboxyl groups makes it neutral; more
number of amino than carboxyl groups makes it basic and more
carboxyl groups as compared to amino groups makes it acidic. The
amino acids, which can be synthesised in the body, are known as non-
essential amino acids. On the other hand, those which cannot be
synthesised in the body and must be obtained through diet, are known
as essential amino acids (marked with asterisk in Table 14.2).
14.2.2
Classification of
Amino Acids
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Amino acids are usually colourless, crystalline solids. These are
water-soluble, high melting solids and behave like 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.
Except glycine, all other naturally occurring α-amino acids are
optically active, since the α-carbon atom is asymmetric. These exist
both in ‘D’ and ‘L’ forms. Most naturally occurring amino acids have
L-configuration. L-Aminoacids are represented by writing the –NH
2
group
on left hand side.
You have already read that proteins are the polymers of α-amino acids
and they are connected to each other by peptide bond or peptide
linkage. Chemically, peptide linkage is an amide formed between
–COOH group and –NH
2
group. The reaction between two molecules of
similar or different amino acids, proceeds through
the combination of the amino group of one molecule
with the carboxyl group of the other. This results in
the elimination of a water molecule and formation of
a peptide bond –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. A tripeptide contains three amino acids linked by two peptide
linkages. Similarly when four, five or six amino acids are linked, the respective
products are known as tetrapeptide, pentapeptide or hexapeptide,
respectively. When the number of such amino acids is more than ten, then
the products are called polypeptides. A polypeptide with more than hundred
amino acid residues, having molecular mass higher than 10,000u is called
a protein. 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 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.
14.2.3 Structure
of Proteins
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423 Biomolecules
Fig. 14.1:
α
-Helix
structure of proteins
Fig. 14.2:
β
-Pleated sheet structure of
proteins
(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.
Structure and shape of proteins can be studied at four different
levels, i.e., primary, secondary, tertiary and quaternary, 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 protein.
Any change in this primary structure i.e., the sequence
of amino acids creates a different protein.
(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 viz. α-helix and
β-pleated sheet structure. These structures arise due
to the regular folding of the backbone of the polypeptide
chain due to hydrogen bonding between
and
–NH– groups of the peptide bond.
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 as shown in Fig.14.1.
In 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.
(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. The main forces which
stabilise the 2° and 3° structures of proteins are
hydrogen bonds, disulphide linkages, van der Waals
and electrostatic forces of attraction.
(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.
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424Chemistry
Fig. 14.3: Diagrammatic representation of protein structure (two sub-units
of two types in quaternary structure)
A diagrammatic representation of all these four structures is
given in Figure 14.3 where each coloured ball represents an
amino acid.
Fig. 14.4: Primary,
secondary, tertiary
and quaternary
structures of
haemoglobin
Protein found in a biological system with a unique three-dimensional
structure and biological activity is called a native protein. When a
protein in its native form, is subjected to physical change like change
in temperature or chemical change like change in pH, the hydrogen
bonds are disturbed. Due to this, globules unfold and helix get uncoiled
and protein loses its biological activity. This is called denaturation of
14.2.4
Denaturation of
Proteins
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425 Biomolecules
protein. During denaturation secondary and tertiary structures are
destroyed but primary structure remains intact. 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.
Intext QuestionsIntext Questions
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14.4 The melting points and solubility in water of amino acids are generally
higher than that of the corresponding halo acids. Explain.
14.5 Where does the water present in the egg go after boiling the egg?
Life is possible due to the coordination of various chemical reactions in
living organisms. An example is the digestion of food, absorption of
appropriate molecules and ultimately production of energy. This process
involves a sequence of reactions and all these reactions occur in the
body under very mild conditions. This occurs with the help of certain
biocatalysts called enzymes. Almost all the enzymes are globular
proteins. Enzymes are very specific for a particular reaction and for a
particular substrate. They are generally named after the compound or
class of compounds upon which they work. For example, the enzyme
that catalyses hydrolysis of maltose into glucose is named as
maltase
.
12 22 11 6 12 6
Maltase
Maltose
G lucose
C H O 2 C H O
→
Sometimes enzymes are also named after the reaction, where they
are used. For example, the enzymes which catalyse the oxidation of
one substrate with simultaneous reduction of another substrate are
named as oxidoreductase enzymes. The ending of the name of an
enzyme is -ase.
Enzymes are needed only in small quantities for the progress of a reaction.
Similar to the action of chemical catalysts, enzymes are said to reduce
the magnitude of activation energy. For example, activation energy for
acid hydrolysis of sucrose is 6.22 kJ mol
–1
, while the activation energy is
only 2.15 kJ mol
–1
when hydrolysed by the enzyme, sucrase. Mechanism
for the enzyme action has been discussed in Unit 5.
It has been observed that certain organic compounds are required in
small amounts in our diet but their deficiency causes specific diseases.
These compounds are called vitamins. Most of the vitamins cannot be
synthesised in our body but plants can synthesise almost all of them,
so they are considered as essential food factors. However, the bacteria
of the gut can produce some of the vitamins required by us. All the
vitamins are generally available in our diet. Different vitamins belong
to various chemical classes and it is difficult to define them on the
basis of structure. They are generally regarded as organic compounds
required in the diet in small amounts to perform specific
biological functions for normal maintenance of optimum growth
14.3.1 Mechanism
of Enzyme
Action
14.4 Vitamins14.4 Vitamins
14.4 Vitamins14.4 Vitamins
14.4 Vitamins
14.3 Enzymes14.3 Enzymes
14.3 Enzymes14.3 Enzymes
14.3 Enzymes
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426Chemistry
and health of the organism. Vitamins are designated by alphabets
A, B, C, D, etc. Some of them are further named as sub-groups e.g. B
1
,
B
2
, B
6
, B
12
, etc. Excess of vitamins is also harmful and vitamin pills
should not be taken without the advice of doctor.
The term “Vitamine” was coined from the word vital + amine since
the earlier identified compounds had amino groups. Later work showed
that most of them did not contain amino groups, so the letter ‘e’ was
dropped and the term vitamin is used these days.
Vitamins are classified into two groups depending upon their solubility
in water or fat.
(i) Fat soluble vitamins: Vitamins which are soluble in fat and oils
but insoluble in water are kept in this group. These are vitamins A,
D, E and K. They are stored in liver and adipose (fat storing) tissues.
(ii) Water soluble vitamins: B group vitamins and vitamin C are soluble
in water so they are grouped together. Water soluble vitamins must
be supplied regularly in diet because they are readily excreted in
urine and cannot be stored (except vitamin B
12
) in our body.
Some important vitamins, their sources and diseases caused by
their deficiency are listed in Table 14.3.
14.4.1
Classification of
Vitamins
Fish liver oil, carrots,
butter and milk
Yeast, milk, green
vegetables and cereals
Milk, eggwhite, liver,
kidney
Yeast, milk, egg yolk,
cereals and grams
Meat, fish, egg and
curd
Citrus fruits, amla and
green leafy vegetables
Exposure to sunlight,
fish and egg yolk
Xerophthalmia
(hardening of cornea of
eye)
Night blindness
Beri beri (loss of appe-
tite, retarded growth)
Cheilosis (fissuring at
corners of mouth and
lips), digestive disorders
and burning sensation
of the skin.
Convulsions
Pernicious anaemia
(RBC deficient in
haemoglobin)
Scurvy (bleeding gums)
Rickets (bone deformities
in children) and osteo-
malacia (soft bones and
joint pain in adults)
1. Vitamin A
2. Vitamin B
1
(Thiamine)
3. Vitamin B
2
(Riboflavin)
4. Vitamin B
6
(Pyridoxine)
5. Vitamin B
12
6. Vitamin C
(Ascorbic acid)
7. Vitamin D
Sl. Name of Sources Deficiency diseases
No. Vitamins
Table 14.3: Some important Vitamins, their Sources and their
Deficiency Diseases
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427 Biomolecules
8. Vitamin E
9. Vitamin K
Vegetable oils like wheat
germ oil, sunflower oil,
etc.
Green leafy vegetables
Increased fragility of
RBCs and muscular
weakness
Increased blood clotting
time
Every generation of each and every species resembles its ancestors in
many ways. How are these characteristics transmitted from one
generation to the next? It has been observed that nucleus of a living
cell is responsible for this transmission of inherent characters, also
called heredity. The particles 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, the deoxyribonucleic acid (DNA) and ribonucleic acid
(RNA). Since nucleic acids are long chain polymers of nucleotides, so
they are also called polynucleotides.
14.514.5
14.514.5
14.5
Nucleic Acids
Nucleic Acids
Nucleic AcidsNucleic Acids
Nucleic Acids
James Dewey Watson
Born in Chicago, Illinois, in 1928, Dr Watson received his Ph.D.
(1950) from Indiana University in Zoology. He is best known for
his discovery of the structure of DNA for which he shared with
Francis Crick and Maurice Wilkins the 1962 Nobel prize in
Physiology and Medicine. They proposed that DNA molecule takes
the shape of a double helix, an elegantly simple structure that
resembles a gently twisted ladder. The rails of the ladder are
made of alternating units of phosphate and the sugar deoxyribose;
the rungs are each composed of a pair of purine/ pyrimidine bases. This
research laid the foundation for the emerging field of molecular biology. The
complementary pairing of nucleotide bases explains how identical copies of
parental DNA pass on to two daughter cells. This research launched a revolution
in biology that led to modern recombinant DNA techniques.
Complete hydrolysis of DNA (or RNA) yields a pentose sugar, phosphoric
acid and nitrogen containing heterocyclic compounds (called bases). In
DNA molecules, the sugar moiety is β-D-2-deoxyribose whereas in
RNA molecule, it is β-D-ribose.
14.5.1 Chemical
Composition
of Nucleic
Acids
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428Chemistry
Uracil (U)
Thymine (T)
Cytosine (C)
DNA contains four bases viz. adenine (A), guanine (G), cytosine (C)
and thymine (T). RNA also contains four bases, the first three bases are
same as in DNA but the fourth one is uracil (U).
A unit formed by the attachment of a base to 1′ position of sugar is
known as nucleoside. In nucleosides, the sugar carbons are numbered
as 1′, 2′, 3′, etc. in order to distinguish these from the bases
(Fig. 14.5a). When nucleoside is linked to phosphoric acid at 5′-position
of sugar moiety, we get a nucleotide (Fig. 14.5).
14.5.2 Structure
of Nucleic
Acids
Fig. 14.5: Structure of (a) a nucleoside and (b) a nucleotide
Nucleotides are joined together by phosphodiester linkage between
5′ and 3′ carbon atoms of the pentose sugar. The formation of a typical
dinucleotide is shown in Fig. 14.6.
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429 Biomolecules
A simplified version of nucleic acid chain is as shown below.
Fig. 14.6: Formation of a dinucleotide
Fig. 14.7: Double strand helix structure for DNA
Information regarding the sequence of nucleotides in the chain
of a nucleic acid is called its primary structure. Nucleic acids
have a secondary structure also. James Watson and Francis Crick
gave a double strand helix structure for DNA (Fig. 14.7). Two
nucleic acid chains are wound about each other and held together
by hydrogen bonds between pairs of bases. The two strands are
complementary to each other because the hydrogen bonds are
formed between specific pairs of bases. Adenine forms hydrogen
bonds with thymine whereas cytosine forms hydrogen bonds
with guanine.
In secondary structure of RNA single stranded helics is present
which sometimes foldsback on itself. RNA molecules are of three
types and they perform different functions. They are named as
messenger RNA (m-RNA), ribosomal RNA (r-RNA) and transfer
RNA (t-RNA).
Sugar Phosphate Sugar Phosphate
Base
Sugar
Base
n
Base
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430Chemistry
Har Gobind Khorana
DNA Fingerprinting
It is known that every individual has unique fingerprints. These occur at the tips of
the fingers and have been used for identification for a long time but these can be
altered by surgery. A sequence of bases on DNA is also unique for a person and
information regarding this is called DNA fingerprinting. It is same for every cell and
cannot be altered by any known treatment. DNA fingerprinting is now used
(i) in forensic laboratories for identification of criminals.
(ii) to determine paternity of an individual.
(iii) to identify the dead bodies in any accident by comparing the DNA’s of parents or
children.
(iv) to identify racial groups to rewrite biological evolution.
DNA is the chemical basis of heredity and may be regarded as the reserve
of genetic information. DNA is exclusively responsible for maintaining
the identity of different species of organisms over millions of years. A
DNA molecule is capable of self duplication during cell division and
identical DNA strands are transferred to daughter cells. Another important
function of nucleic acids is the protein synthesis in the cell. Actually, the
proteins are synthesised by various RNA molecules in the cell but the
message for the synthesis of a particular protein is present in DNA.
Hormones are molecules that act as intercellular messengers. These
are produced by endocrine glands in the body and are poured directly
in the blood stream which transports them to the site of action.
In terms of chemical nature, some of these are steroids, e.g., estrogens
and androgens; some are poly peptides for example insulin and
endorphins and some others are amino acid derivatives such as
epinephrine and norepinephrine.
Hormones have several functions in the body. They help to maintain
the balance of biological activities in the body. The role of insulin in keeping
the blood glucose level within the narrow limit is an example of this
function. Insulin is released in response to the rapid rise in blood glucose
level. On the other hand hormone glucagon tends to increase the glucose
level in the blood. The two hormones together regulate the glucose level
in the blood. Epinephrine and norepinephrine mediate responses to
external stimuli. Growth hormones and sex hormones play role in growth
and development. Thyroxine produced in the thyroid gland is an iodinated
derivative of amino acid tyrosine. Abnormally low level of thyroxine leads
14.5.3 Biological
Functions
of Nucleic
Acids
Har Gobind Khorana, was born in 1922. He obtained his M.Sc.
degree from Punjab University in Lahore. He worked with Professor
Vladimir Prelog, who moulded Khorana’s thought and philosophy
towards science, work and effort. After a brief stay in India in
1949, Khorana went back to England and worked with Professor
G.W. Kenner and Professor A.R.Todd. It was at Cambridge, U.K.
that he got interested in both proteins and nucleic acids. Dr Khorana shared the
Nobel Prize for Medicine and Physiology in 1968 with Marshall Nirenberg and Robert
Holley for cracking the genetic code.
14.6
14.6
14.6
14.6
14.6
Hormones
Hormones
Hormones
Hormones
Hormones
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431 Biomolecules
to hypothyroidism which is characterised by lethargyness and obesity.
Increased level of thyroxine causes hyperthyroidism. Low level of iodine
in the diet may lead to hypothyroidism and enlargement of the thyroid
gland. This condition is largely being controlled by adding sodium iodide
to commercial table salt (“Iodised” salt).
Steroid hormones are produced by adrenal cortex and gonads (testes
in males and ovaries in females). Hormones released by the adrenal cortex
play very important role in the functions of the body. For example,
glucocorticoids control the carbohydrate metabolism, modulate
inflammatory reactions, and are involved in reactions to stress. The
mineralocorticoids control the level of excretion of water and salt by the
kidney. If adrenal cortex does not function properly then one of the results
may be Addison’s disease characterised by hypoglycemia, weakness and
increased susceptibility to stress. The disease is fatal unless it is treated by
glucocorticoids and mineralocorticoids. Hormones released by gonads are
responsible for development of secondary sex characters. Testosterone is
the major sex hormone produced in males. It is responsible for development
of secondary male characteristics (deep voice, facial hair, general physical
constitution) and estradiol is the main female sex hormone. It is responsible
for development of secondary female characteristics and participates in
the control of menstrual cycle. Progesterone is responsible for preparing
the uterus for implantation of fertilised egg.
Intext QuestionsIntext Questions
Intext QuestionsIntext Questions
Intext Questions
14.6 Why cannot vitamin C be stored in our body?
14.7 What products would be formed when a nucleotide from DNA containing
thymine is hydrolysed?
14.8 When RNA is hydrolysed, there is no relationship among the quantities of different
bases obtained. What does this fact suggest about the structure of RNA?
SummarySummary
SummarySummary
Summary
Carbohydrates are optically active polyhydroxy aldehydes or ketones or molecules which
provide such units on hydrolysis. They are broadly classified into three groups —
monosaccharides, disaccharides and polysaccharides. Glucose, the most important
source of energy for mammals, is obtained by the digestion of starch. Monosaccharides
are held together by glycosidic linkages to form disaccharides or polysaccharides.
Proteins are the polymers of about twenty different
αα
αα
α-amino acids
which are
linked by peptide bonds. Ten amino acids are called essential amino acids because
they cannot be synthesised by our body, hence must be provided through diet. Proteins
perform various structural and dynamic functions in the organisms. Proteins which
contain only α-amino acids are called simple proteins. The secondary or tertiary
structure of proteins get disturbed on change of pH or temperature and they are not
able to perform their functions. This is called denaturation of proteins. Enzymes are
biocatalysts which speed up the reactions in biosystems. They are very specific and
selective in their action and chemically all enzymes are proteins.
Vitamins are accessory food factors required in the diet. They are classified as
fat soluble (A, D, E and K) and water soluble (Β group and C). Deficiency of vitamins
leads to many diseases.
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432Chemistry
Nucleic acids are the polymers of nucleotides which in turn consist of a base,
a pentose sugar and phosphate moiety. Nucleic acids are responsible for the transfer
of characters from parents to offsprings. There are two types of nucleic acids —
DNA and RNA. DNA contains a five carbon sugar molecule called 2-deoxyribose
whereas RNA contains ribose. Both DNA and RNA contain adenine, guanine and
cytosine. The fourth base is thymine in DNA and uracil in RNA. The structure of
DNA is a double strand whereas RNA is a single strand molecule. DNA is the
chemical basis of heredity and have the coded message for proteins to be synthesised
in the cell. There are three types of RNA — mRNA, rRNA and tRNA which actually
carry out the protein synthesis in the cell.
14.1 What are monosaccharides?
14.2 What are reducing sugars?
14.3 Write two main functions of carbohydrates in plants.
14.4 Classify the following into monosaccharides and disaccharides.
Ribose, 2-deoxyribose, maltose, galactose, fructose and lactose.
14.5 What do you understand by the term glycosidic linkage?
14.6 What is glycogen? How is it different from starch?
14.7 What are the hydrolysis products of
(i) sucrose and (ii) lactose?
14.8 What is the basic structural difference between starch and cellulose?
14.9 What happens when D-glucose is treated with the following reagents?
(i) HI (ii) Bromine water (iii) HNO
3
14.10 Enumerate the reactions of D-glucose which cannot be explained by its
open chain structure.
14.11 What are essential and non-essential amino acids? Give two examples of
each type.
14.12 Define the following as related to proteins
(i) Peptide linkage (ii) Primary structure (iii) Denaturation.
14.13 What are the common types of secondary structure of proteins?
14.14 What type of bonding helps in stabilising the α-helix structure of proteins?
14.15 Differentiate between globular and fibrous proteins.
14.16 How do you explain the amphoteric behaviour of amino acids?
14.17 What are enzymes?
14.18 What is the effect of denaturation on the structure of proteins?
14.19 How are vitamins classified? Name the vitamin responsible for the
coagulation of blood.
14.20 Why are vitamin A and vitamin C essential to us? Give their important sources.
14.21 What are nucleic acids? Mention their two important functions.
14.22 What is the difference between a nucleoside and a nucleotide?
14.23 The two strands in DNA are not identical but are complementary. Explain.
14.24 Write the important structural and functional differences between DNA
and RNA.
14.25 What are the different types of RNA found in the cell?
ExercisesExercises
Exercises
Exercises
Exercises
2020-21
