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BIOMOLECULES
A living system grows, sustains and reproduces itself. The most amazing characteristic of the
living system is that it is composed of several non-living (lifeless) substances which are present
in their cells in a very complex but highly organised form. These are called biomolecules. Thus,
biomolecules are the complex lifeless molecules which build up living organisms and are
required for their growth, maintenance and ability to reproduce. These form the basis of life.
Some common examples are carbohydrates, proteins, enzymes, nucleic acids, lipids, amino
acids, fats, etc. These biomolecules interact with each other and constitute the molecular logic of
life processes.
The branch of science which deals with the study of biomolecules and their role in living systems
is called biochemistry.
These biomolecules interact with each other in a specific manner to produce life. Many of these
biomolecules are polymers. For example, starch, proteins, nucleic acids are condensation
polymers of simple sugars, amino acids and nucleotides respectively. In addition, some simple
molecules like vitamins and mineral salts also play an important role in the functions of
organisms.
CARBOHYDRATES
Carbohydrates are a class of naturally occurring organic compounds of carbon, hydrogen and
oxygen which are primarily produced by plants. These are formed in plants by a process known
as photosynthesis.
The common examples are glucose, fructose, cellulose, sucrose, starch, etc.
In the earlier days, the carbohydrates were regarded as the hydrates of carbon with the general
formula Cx(H2O)y. For example, carbohydrates such as glucose (C6H12O6), fructose (C6H12O6),
sucrose (C12H22O11) satisfied this definition. However, this definition could not hold good due to
the following reasons:
(i) A number of compounds such as rhamnose (C6H12O5), deoxyribose (C5H10O4), etc. are known
which are carbohydrates by their chemical behaviour but do not obey this formula.
(ii) There are other compounds like formaldehyde (CH2O), acetic acid (C2H4O2) etc. which do
not behave like carbohydrates but have the formula of hydrates of carbon.

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(iii) Carbon is not known to form hydrates. A large number of their reactions have shown that
these contain polyhydric aldehydes, polyhydric ketones and large polymeric substances which
can be broken down to polyhydric aldehydes or ketones.
Therefore, these days carbohydrates are defined as optically active polyhydroxy aldehydes or
polyhydroxy ketones or the compounds which produce such compounds on hydrolysis.
Some of the carbohydrates, which are sweet in taste, are also called sugars. The most common
sugar that we use is named as sucrose and sugar present in milk is known as lactose. The
carbohydrates are also called saccharides (Greek Sakcharon meaning sugar).
Classification of Carbohydrates
Carbohydrates, in general, may be classified into two classes :
(i) Sugars. These are crystalline substances which are sweet and water soluble. For example,
glucose, fructose and cane sugar.
(ii) Non-sugars. These are tasteless, insoluble in water and amorphous. For example, starch,
cellulose, etc.
However, these days carbohydrates are systematically classified into three principal classes :
(i) Monosaccharides. These are the simplest carbohydrates which cannot be hydrolysed into
simpler compounds. Therefore, they represent the simplest single carbohydrate units. About 20
monosaccharides occur naturally. They
contain up to six carbon atoms. They have the general formula (CH2O)nwhere n = 3–7. The
common examples are : ribose, C5H10O5, glucose, C6H12O6, fructose, C6H12O6, etc.
(ii) Oligosaccharides. These are the carbohydrates which give two to ten monosaccharide
molecules on hydrolysis. These are further classified as disaccharides, trisaccharides,
tetrasaccharides, etc. depending upon the number
of monosaccharide units present in their molecules. For example,
• Disaccharides : Carbohydrates which on hydrolysis give two molecules of the same or
different monosaccharides. For example, sucrose, lactose,
maltose. All these have the molecular formula C12H22O11.
• Trisaccharide : Carbohydrates which on hydrolysis give three molecules of the same or
different monosaccharides. For example, raffinose (C18H32O16).
• Tetrasaccharides : Carbohydrates which on hydrolysis give four molecules of the same or
different monosaccharides. For example, stachyose (C24H42O21).

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(iii) Polysaccharides. These are carbohydrates which are polymeric and can be hydrolysed to
give a large number of monosaccharide units. The common examples are cellulose, starch,
glycogen, etc. The general formula of starch and cellulose is (C6H10O5)n. These get hydrolysed
to give monosaccharides.
D- and L- Designations
The sugars are divided into two families : the D-family and L-family which have definite
configurations. These configurations are represented with respect to glyceraldehyde as the
standard. The glyceraldehyde may be presented by two forms as:

The D-configuration has —OH attached to the carbon adjacent to —CH2OH on right while L-
configuration has —OH attached to the carbon adjacent to —CH2OH on left. The sugars are
called D- or L- depending upon whether the configuration of the molecule is related to D-
glyceraldehyde or L-glyceraldehyde. It has been found that all naturally occurring sugars belong
to D-series. e.g., D-glucose, D-ribose and D-fructose.
Glucose
Glucose occurs in nature in free as well as in the combined form. It is present in sweet fruits and
honey. Ripe grapes contain about 20% of glucose and that is why it is also known as grape
sugar. In the combined form, glucose occurs in abundance in cane sugar and polysaccharides
such as starch and cellulose.
Preparation of Glucose
1. From Sucrose (Cane sugar)
When sucrose is boiled with dilute HCl or H2SO4 in alcoholic solution, glucose and fructose are
obtained in equal amounts.

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2. From Starch
Glucose is produced commercially by the hydrolysis of starch by boiling it with dilute H2SO4 at
393 K under pressure of 2–3 atm.

In this process, an aqueous solution of starch obtained from corn is acidified with dil H 2SO4. It is
then heated under 2–3 atm pressure steam in an autoclave. When the hydrolysis is complete, the
liquid is neutralised with sodium carbonate to a pH of 4–5. The resulting solution is concentrated
under reduced pressure to get the crystals of glucose.
Structure of glucose
Glucose is an aldohexose. It is also known as dextrose. It is monomer of many of the larger
carbohydrates such as starch, glycogen, cellulose, etc. It is probably the most abundant
compound on the earth. It has one aldehyde group (–CHO), one primary alcoholic group (–
CH2OH) and four secondary alcoholic groups (–CHOH).
This structure was assigned on the basis of the following evidences :
1. Molecular formula
The molecular formula of glucose has been found to be C6H12O6 .
2. Straight chain structure
(i) When aqueous solution of glucose is treated with sodium amalgam (Na/Hg) or sodium
borohydride, it is reduced to sorbitol (or glucitol), a hexahydric alcohol.

(ii) Prolonged heating with hydriodic acid and red phosphorus at 100°C gives a mixture of n-
hexane and 2-iodohexane.

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The formation of n-hexane suggests that all the six carbon atoms in glucose are arranged in a
straight chain structure of glucose.
3. Presence of five hydroxyl (–OH) groups
(i) Acetylation. Glucose reacts with acetic anhydride in the presence of anhydrous zinc chloride
to form glucose pentaacetate (or penta acetyl glucose). This is known as acetylation of glucose.

The formation of penta-acetyl derivative confirms the presence of five –OH groups in glucose
molecule. We know that the presence of two or more OH groups on the same carbon atom makes
the molecule unstable. Now, since glucose exists as a stable compound, this shows that five —
OH groups should be attached to different carbon atoms.
4. Presence of an aldehyde (–CHO) group
(i) Reaction with hydrogen cyanide. Like aldehydes, glucose reacts with hydrogen cyanide
forming cyanohydrin.

(ii) Action with hydroxylamine. Glucose reacts with hydroxylamine, NH2OH to form glucose
oxime.

These reactions suggest that glucose contains a carbonyl ( >C=O) group.

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5. Oxidation of glucose. The oxidation of glucose occurs as:

(i) 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 (–CHO) group.

(ii) Strong oxidising agents like nitric acid oxidise both the terminal groups (–CHO and –
CH2OH) of glucose to give the dibasic acid, saccharic acid (also known as glucaric acid). This
indicates the presence of a primary alcoholic (—OH) group in glucose.

On the basis of above reactions, Fischer assigned an open chain structure of glucose shown
below as structure I. Similarly, gluconic acid is represented as II and saccharic acid as III.

Cyclic Structure of D-Glucose

The open chain structure of glucose explained most of its properties. However, it could not
explain the following facts.
1. Despite having an aldehydic (–CHO) group, glucose does not undergo certain characteristic
reactions of aldehydes. For example,

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(a) Glucose does not react with sodium bisulphite (NaHSO3) to form addition
product.
(b) Glucose does not react with ammonia.
(c) Glucose does not give Schiff’s test and 2,4-DNP test like other aldehydes.
2. Glucose reacts with hydroxylamine (–NH2OH) to form an oxime but glucose
pentaacetate does not react with hydroxylamine. This shows that —CHO group is not present
in glucose pentaacetate.
3. D (+)-Glucose exists in two stereoisomeric forms i.e., a-D-glucose and β-D-glucose. These
two forms are crystalline and have different melting points and optical rotations. When glucose
was crystallized from a concentrated solution at 303K, it gave α-form of glucose having melting
point 419 K (or 146°C) and [α]D = + 111°. On the other hand, the β-form of glucose is obtained
on crystallisation of glucose from a hot saturated solution at a temperature above 371 K. The β-
form of glucose has melting point 423 K (or 150°C) and [α]D = + 19.2°.
4. Mutarotation. When either of the two forms of glucose (α-D-glucose and β-D-glucose) are
dissolved in water and allowed to stand, these get slowly converted into other form and a
equilibrium mixture of both α-D-glucose (about 36%) and β-D-glucose (about 64%) is formed.

The formation of equilibrium mixture can be explained as :

The α-D-glucose has a specific rotation of +111° while β-D-glucose has a specific rotation of
+19.2°. When α-form is dissolved in water, its specific rotation falls until a constant value of
+52.5° is reached. On the other hand, when β-form is dissolved in water, its specific rotation
increases and becomes constant at + 52.5°.
This spontaneous change in specific rotation of an optically active compound with time to
an equilibrium value is called mutarotation. (Latin, muto means to change). Thus, there is an
equilibrium mixture of α- and β-forms in the solution.

5. Glucose forms isomeric methyl glucosides. When glucose is heated with methanol in the
presence of dry hydrogen chloride gas, it gives two isomeric monomethyl derivatives known as

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methyl α-D-glucoside (m.p. = 438 K or 165°C) and methyl β-D-glucoside (m.p. = 380 K or
107°C).

These two glucosides do not reduce Fehling’s solution and also do not react with hydrogen
cyanide or hydroxylamine indicating that the free –CHO group is not present.
Since only one molecule of methanol is used for the formation of methyl glucoside, these must
be hemiacetals. These results show that glucose does not have open chain form structure. Like
glucose, the open chain structures of other monosaccharides (aldoses and ketoses) show similar
limitations.
Cyclic Structures of Monosaccharides
The monosaccharides give the characteristic reactions of alcohols and carbonyl group (aldehydes
and ketones). It has been found that these monosaccharides exist in the form of cyclic structures.
We know that aldehydes and ketones react with the hydroxyl group to form hemiacetals and
acetals, as

Monosaccharides contain a number of —OH groups and an aldehyde or a keto group. Therefore,
they can undergo intramolecular reaction (within the molecule) to form hemiacetals which result
in cyclic structures. In cyclization, the —OH groups (generally of C5 or C4 in aldoses and C5 or
C6 in ketoses) combine with the aldehyde or keto groups. As a result, cyclic structures of five or
six membered rings containing one oxygen atom are formed. For example, glucose forms a ring
structure. It forms a six membered ring of five carbon atoms and one oxygen atom.
Cyclic Structure of Glucose
Anomers
Glucose forms a hemiacetal between the –CHO group and the –OH group on the C5 atom. As a
result, of cyclization, C1 becomes asymmetric (chiral) and the newly formed –OH group may be
either on the left or on the right in Fischer projection formulae. This results in the formation of

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two isomers which differ in the orientation of H and –OH groups around C1 atom. These isomers
are known as α-D-glucose and β-D-glucose.
The isomer having the hydroxyl group (–OH) on the right is called α-Dglucose and the isomer
having the hydroxyl group (–OH) on the left is called β-D-glucose.
Such pairs of optical isomers which differ in the configuration only around C1 atom are
called anomers.
These two forms are not mirror images of each other and, hence, are not enantiomers. The C1
carbon is known as anomeric carbon or glycosidic carbon.

The above representations are called Fischer projection formulae. The formation of two methyl
glucosides by reaction of glucose with methanol can be explained as :

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Pyranose Structures
The structures of α-D-glucose and β-D-glucose may be drawn in a simple six membered ring
form called pyranose structures. These resemble pyran which is a six membered heterocyclic
ring containing five carbon atoms and one oxygen atom. These structures were suggested by
Haworth and are known as Haworth projection formulae or pyranose structures.

Fructose (Fruit sugar) C6H12O6

Fructose occurs in fruits and is called fruit sugar. It is also present in honey and sweet fruits
alongwith glucose. In the combined state, it is also present in disaccharide (sucrose) and
polysaccharide (inulin). It is obtained alongwith glucose by hydrolysis of cane sugar with dilute
H2SO4.

Structure of Fructose
Its molecular formula is C6H12O6. On the basis of its
reactions, it has been established that fructose contains a keto
group at C–2 and the six carbon atoms are arranged in a
straight chain as in case of glucose. It belongs to D-series and
is a laevorotatory compound. It is also called laevulose.
Therefore, it is written as D–(–)–fructose. It is pentahydroxy
ketone and its open chain structure is shown ahead:

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Cyclic Structure
Like glucose, fructose also has a cyclic structure. The hemiacetal is formed by the intramolecular
combination of C2 keto group and —OH group of C6 atom. As a result, C2 atom becomes
asymmetric and, therefore, D-fructose has two possible isomers as α- D–(–)–fructose and β-D–(–
–fructose which differ in the arrangement of CH2OH and OH groups around C2. These are
shown below:

The above structures may be written in the Haworth forms as pyranose ring structures as :

In the free state, D-fructose exists as a six membered ring or as pyranose ring. However, in the
combined state as a component of disaccharides, it exists in the furanose form (5-membered
hemiketal). This structure is similar to furan ring which is a five membered heterocyclic ring
with one oxygen atom. The furanose structure can be obtained by internal ketal formation by
combining keto group (of C2) and —OH group of C5 as shown below :

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These structures may also be represented as :

DISACCHARIDES
Disaccharides are the carbohydrates which on hydrolysis give two same or different
monosaccharides. Their general formula is C12H22O11. The important members belonging to
disaccharides are sucrose, maltose and lactose.

1. Sucrose
It is the most common disaccharide and is widely distributed in plants particularly sugar cane
and sugar beet. It is manufactured either from cane sugar or sugar beet. The sugar obtained from
sugar beet is called beet sugar. It is a colourless, crystalline and sweet substance. It is very
soluble in water and its aqueous solution is dextrorotatory having [α]D = + 66.5°. On hydrolysis
with dilute acids or enzyme invertase, cane sugar gives equimolar mixture of D–(+)–glucose and
D-(–)-fructose.

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So, sucrose is dextrorotatory but after hydrolysis gives dextrorotatory glucose and laevorotatory
fructose. D–(–)–fructose has a greater specific rotation than D–(+)–glucose. Therefore, the
resultant solution upon hydrolysis is laevorotatory in nature with specific rotation of (– 39.9°).
Since there is change in the sign of rotation from dextro before hydrolysis to laevo after
hydrolysis, the reaction is called inversion reaction and the mixture (glucose and fructose) is
called invert sugar.
Sucrose solution is fermented by yeast when the enzyme invertase hydrolyses the sucrose to
glucose and fructose and enzyme zymase converts these monosaccharides to ethanol (ethyl
alcohol).

Sucrose is composed of D-glucose and β-D-fructose. These units are held together by α, β-
glycosidic linkage between C1 of the glucose unit (pyranose ring) and C2 of the fructose unit
(furanose ring). This structure was proposed by Haworth (1927).

2. Maltose
It is known as malt sugar. It is the principal disaccharide obtained by the partial hydrolysis of
starch by diastase, an enzyme present in malt (sprouted barley seeds)

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Structure
On hydrolysis, one mole of maltose gives two moles of α-D-glucose. It is a reducing sugar. It is
composed of two α-D-glucose units which are condensed together through C1 of one unit and C4
of the other unit. Both glucose units are in pyranose form.

3. Lactose
Lactose occurs in milk and, therefore, it is also called milk sugar.
Structure
Lactose on hydrolysis with dilute acid gives equimolar mixture of β-Dglucose and β-D-
galactose. It is a reducing sugar. Therefore, it is composed of β-D-glucose and β-D-galactose
units. These units are held together by glycosidic linkage between C1 of galactose and C4 of the
glucose unit. Lactose gets hydroysed by emulsin, an enzyme which specifically hydrolyses β-
glycosidic linkages.

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Reducing and non-reducing sugars

Reducing sugars. The saccharides which reduce Fehling’s solution and Tollen’s reagent are
called reducing sugars. The reducing sugars contain groups which can be easily oxidised by
Fehling solution or Tollen’s reagent. For example, they contain the following characteristic
groups :
(i) Reducing sugars contain α-hydroxyaldehyde or α-hydroxy ketone groups :

(ii) Reducing sugars contain cyclic hemiacetal or hemiketal groups. In aqueous solutions these
hemiacetals or hemiketals exist in equilibrium with relatively small concentration of non-cyclic
aldehydes or α-hydroxy ketones having a free —CHO or —CO group.

Non-reducing sugars. The saccharides which do not reduce Fehling’s solution or Tollen’s
reagent are called non-reducing sugars.
These do not contain free aldehydic or ketonic group with –OH group on the carbon adjacent to
carbonyl group. They contain stable acetal or ketal structures. Their cyclic structures cannot be
opened into an open chain form having a free carbonyl group.
Examples. All monosaccharides contain free –CHO or —C=O group and are reducing sugars.
For example, D-glucose or D-fructose.
Among the disaccharides maltose and lactose are reducing sugars because in one of the
monosaccharide units there is a hemiacetal group that can be opened to give free —CHO group.
Sucrose is non-reducing because the reducing groups of glucose and fructose are involved in
glycosidic bond formation.

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POLYSACCHARIDES
These are neutral polymeric compounds in which hundreds or even thousands of monosaccharide
units are joined by glycosidic linkages. They have the general formula (C6H10O5)n, where n has
very large value. They are colourless, tasteless and are insoluble in water. They play very
important role in plant and animal life as food storage and structural role. They are usually made
up of pentoses or hexoses. The important polysaccharides are cellulose, starch, glycogen and
dextrins.
However, starch and cellulose are most important of the polysaccharides.
1. Starch (amylum), (C6H10O5)n
It is the main storage polysaccharide of plants. It is an important dietary source for human
beings. It occurs in all plants, particularly in their seeds, roots, tubers, etc. The main sources are
wheat, rice, maize, potatoes, barley and sorghum. It occurs in the form of granules, which vary in
size and shape depending upon their plant source. Starch is a white powder, insoluble in cold
water. Its solution gives blue colour with iodine solution. The blue colour disappears on heating
and reappears on cooling. Starch is hydrolysed with dilute acids or enzymes and breaks down to
molecules of variable complexity (n > n’) and finally gives D-glucose.

Starch is a non-reducing saccharide. It does not reduce Fehling's solution or Tollen's reagent. It
also does not form an osazone indicating that all hemiacetal hydroxyl groups of glucose units
(C1) are not free but are linked with glycosidic linkages.
Starch is a polymer of α-D-glucose and consists of two components : water soluble component
amylose (15–20%) and water insoluble component amylopectin (80–85%).

(i) Amylose. It is a water soluble fraction. It is a linear polymer of α-D-glucose. It contains about
200–1000 α-D-glucose units which are linked to one another through α-glycosidic linkage
involving C1 of one glucose and C4 of the next as shown below :

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(ii) Amylopectin. It is water insoluble fraction. It is a highly branched chain polymer which does
not give blue colour with iodine. It consists of a large number (several hundreds) of short chains
of 25–30 D-glucose units. In this case, the main chain involves α-linkages between C1 of one α-
D-glucose unit and C4 of the other. The C1 of terminal glucose in each chain is further linked to
C6 of the other glucose unit in the next chain through C1–C6 α-linkage. This gives highly
branched structure.

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2. Cellulose (C6H10O5)n
It is also major structural polysaccharide in higher plants where it constitutes the bulk of cell
wall. It is probably the most abundant organic substance found in plant kingdom. Over 50% of
the total organic matter in the living world is cellulose. Dry leaves contain 10–20% cellulose,
wood contains 50% and cotton contains 90% cellulose. Cellulose forms the fibrous component of
plant cell walls. Cellulose does not reduce Fehling solution or Tollen's reagent. It does not form
osazone and is not fermented by yeast. It is not hydrolysed so readily as starch, but on heating
with dilute sulphuric acid under pressure gives D-glucose. Structurally, cellulose is a straight
chain polysaccharide composed of only β-D-glucose units, which are joined by β-glycosidic
linkages between C1 of one glucose unit and C–4 of the next glucose unit. The chains are
arranged to form bundles and are held together by hydrogen bonds between glucose units of
adjacent strands. Its molecular mass is in the range of 50,000 – 500,000 (about 300–2500 D-
glucose units).
Cellulose is an industrially important compound. It finds uses in textiles, paper and plastic
industries. When treated with a wide variety of chemicals, it forms many useful products,
celluloid, rayon, gun cotton (an explosive), cellulose acetate (plastics and wrapping films),
methyl cellulose (fabric sizing, pastes and cosmetics), ethyl cellulose (plastic coats and films),
etc.

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3. Glycogen (C6H10O5)n
Glycogen is a polysaccharide of α-D-glucose. The carbohydrates are stored in animal body as
glycogen. Just as glucose is stored in plants as starch, it is stored as glycogen in liver, muscles
and brain of human beings. It serves as a reserve carbohydrate. When the body needs glucose
during strenous exercise or fasting, the enzymes break down glycogen to provide glucose.
Glycogen is also present in yeast and fungi.
Structurally, glycogen is a condensation polymer of α-D-glucose. It is known as animal starch
because its structure is similar to amylopectin. The only difference between glycogen and
amylopectin is that amylopectin chains consist of 20–25 glucose units but glycogen chains are
shorter because they consist of 10–14 glucose units. Glycogen is more highly branched than
amylopectin.
IMPORTANT FUNCTIONS OF CARBOHYDRATES
The important functions of carbohydrates are :
1. 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 medicines.
2. Carbohydrates (with the exception of cellulose) work as body fuels and act as the main source
of energy. For example, slow oxidation of glucose by a series of steps provides energy for living
organisms :

Polysaccharides first undergo hydrolysis to give glucose which then supplies the energy. Starch
and sugars get hydrolysed to glucose by the enzymes present in the various juices secreted by
different organs in the human and animal digestive systems.

3. The carbohydrates act as storage of energy for the functioning of living organisms. In case of
emergency like illness or fasting, they supply energy. Starch is major food reserve in plants and
glycogen in animals.

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4. They form structural materials for cells. For example, cellulose is present in the cell walls of
the plant cells.
5. Carbohydrates provide raw materials for many important industries such as textiles, papers,
lacquers, breweries, etc.
6. The monosaccharides D-ribose and 2-deoxy-D-ribose are present in nucleic acids. These
nucleic acids control the transmission of hereditary effects from one generation to another and
also biosynthesis of proteins.
7. The monosaccharide ribose is an essential component of adenosine triphosphate (ATP)
which acts as energy currency of the cells during metabolism of carbohydrates, proteins and fats.
It is called energy currency of the cells because a part of chemical energy obtained by the
oxidation of biomolecules such as carbohydrates, lipids, etc. is stored in the cells in the form of
ATP which in turn carries out all the cellular functions.

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Proteins
Proteins are high molecular mass complex biomolecules of amino acids present in all living
cells. The chief source 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. The
name proteins is derived from the Greek word proteios which means primary or of prime
importance. These are so named because proteins are vital chemical substances essential for
growth and maintenance of life. They are present almost in all the living cells of plants and
animals. The protoplasm of plant or animal cells contains 10–20% proteins. The important
proteins required for our body are
• enzymes : as biocatalysts to catalyse biochemical reactions,
• hormones : to regulate various body functions,
• antibodies : to protect the body against toxic substances and infections,
• transport proteins : to carry different substances in the blood to various tissues of the body,
• structural proteins : structural elements of the cells and tissues,
• contractile proteins : to help in the contraction of muscles and other cells etc.
All proteins contain the elements carbon, hydrogen, oxygen, nitrogen and sulphur. Some of these
may also contain phosphorus, iodine and traces of metals such as iron, copper, zinc, manganese,
etc. All proteins on partial hydrolysis give peptides of varying molecular masses which on
complete hydrolysis give α-amino acids.

Thus, chemically proteins are condensation polymers (polyamides) in which the monomer
units are α-amino acids.

AMINO ACIDS
α-Amino acids are building They are represented by the general formula :
blocks of proteins. Amino
acids are organic compounds
containing both an amino
group and carboxyl group.

The amino (—NH2) group may be attached to any carbon atom other than that of carboxyl (—
COOH) group. They are referred to as α, β, γ depending upon whether the amino group is

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present on α, β or γ carbon atom relative to carboxyl group. Nearly all the naturally occurring
amino acids are α-amino acids, i.e., containing amino group on the α (adjacent) carbon atom to
carboxyl group.
Nomenclature of Amino Acids
Although amino acids can be named according to IUPAC system, they are generally known by
their common names or trivial names. For example, NH2CH2COOH is better known as glycine
rather than α-amino acetic acid or 2-amino ethanoic acid. The trivial names are derived either
from the name of the source or the name of some characteristic property of that amino acid. For
example, glycine is so named because it has sweet taste (in greek glykos means sweet) and
tyrosine was first obtained from cheese (in Greek tyros means cheese).
For the sake of simplicity, each amino acid has been given an abbreviation which generally
consists of the first three letters or one letter symbols of the common name. For example, the
simplest α-amino acid is glycine, NH2CH2COOH. It may be abbreviated as Gly. Sometimes one
letter symbols are also used. For example, glycine is represented by G. Similarly, alanine
CH3CH(NH2)COOH may be represented as Ala or A.
Classification of α-Amino acids—Neutral, acidic or basic amino acids
Amino acids can be broadly classified as acidic, basic, or neutral amino acids depending upon
the relative number of amino and carboxyl groups in their molecules.
Neutral amino acids contain equal number of amino and carboxyl groups. For example, amino
acids such as glycine, alanine, valine, etc. are neutral amino acids.
Acidic amino acids contain more number of carboxyl groups than amino groups. For example,
aspartic acid, glutamic acid which contain two –COOH groups and one –NH2 group are acidic
amino acids.
Basic amino acids contain more number of amino groups than carboxyl groups. For example,
lysine, arginine, and histidine, which contain two –NH2 groups and one –COOH groups are basic
amino acids.
Essential and non-essential amino acids
Certain amino acids can be made by our bodies and, therefore, we do not require them in our
diet. These are called non-essential amino acids. The human body can synthesise 10 out of 20
amino acids found in proteins. Therefore, other must be supplied to our diet and these are called
essential amino acids. The 10 essential amino acids are valine, leucine, isoleucine, arginine,

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lysine, threonine, methionine, phenylalanine, tryptophan and histidine. These essential amino
acids are required for the growth of our body and lack of these essential amino acids in diet can
cause diseases such as kwashiorkor.

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PROPERTIES AND STRUCTURE OF α-AMINO ACIDS

Physical Properties of α-Amino acids
Amino acids are usually colourless, crystalline solids. These are soluble in water and have high
melting points. Therefore, they behave like salts rather than simple amines or carboxylic acids.
This behaviour is due to the presence of both acidic and amino group in the same molecule.
Therefore, the amino acid exist as dipolar ion called a zwitter ion. It has positive as well as
negative ends within the same molecule. In the formation of zwitter ion, the proton goes from the
carboxyl group to the amino group. The zwitter ion structure of α-amino acid may be written as :

The dipolar structure is also called internal salt. All α-amino acids exist largely in dipolar ionic
forms.
Acidic and basic character of amino acids according to dipolar ion structure :
On the basis of dipolar ion structure, the acidic and basic reactions of amino acids may be
represented as :
(i) When the solution of amino acid is made acidic or an acid is added to amino acid, —COO–
accepts the proton and gets converted to carboxyl substituent (–COOH). Therefore, the basic
character is due to —COO– group.

(ii) When an alkali is added to amino acid, —NH3 + group releases the proton and changes to
amino (NH2) group. Therefore, the acidic character is due to —NH3+ group.

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ISOELECTRIC POINT OF AMINO ACIDS

Amino acids have zwitter ion structure, NH+ 3CHRCOO– and we expect that their aqueous
solutions would be neutral. However, aqueous solutions of neutral amino acids are slightly acidic
because the acidic character of –NH3+ group is more than the basic character of –COO– groups.
When we place the aqueous solution of an amino acid in an electric field, its behaviour will
depend upon the acidity or basicity of the solution. In alkaline solution, an amino acid exists as a
negative ion (II) and migrates towards anode under the influence of electric field. On the other
hand, in acidic solution, it exists as a positive ion (III) and migrates towards cathode under the
influence of electric field. However, at a certain hydrogen ion concentration (pH), the dipolar ion
exists as a neutral ion and does not migrate to either electrode. In this solution, the structures II
and III are exactly balanced and therefore, there is no net migration of amino acid.
The pH or hydrogen ions concentration of the solution at which a particular amino acid
does not migrate under the influence of an electric field is called isoelectric point of that
amino acid.
The isoelectric point depends on other functional groups in the amino acid. The neutral amino
acids have the isoelectric points in the range of PH 5.5 to 6.3 (e.g., glycine, PH = 6.1). For acidic
amino acids, isoelectric point lies between PH 3.2 – 3.5 (e.g., aspartic acid, PH = 3.0) while for
basic amino acids, it lies between PH 7.6 – 10.8 (e.g., lysine, PH = 9.7). At isoelectric point, the
amino acids have the least solubility in water and this property is used for the separation of
different amino acids obtained from the hydrolysis of proteins. Amino acids form salts with acids
and bases. Their chemical properties are similar to primary amines and carboxylic acids.

PEPTIDES AND PROTEINS

Peptides are compounds formed by the condensation of two or more same or different α-amino
acids. The condensation occurs between amino acids with the elimination of water. In this case,
the carboxyl group of one amino acid and amino group of another amino acid gets condensed
with the elimination of water molecule. The resulting —CO—NH— linkage is called a peptide
linkage or peptide bond.

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Alternatively, the amino group of glycine may react with

carboxyl group of alanine resulting in the formation of a
different dipeptide, analylglycine (Ala-Gly). In both these
dipeptides i.e., glycylalanine or analylglycine, there are
free functional groups at both ends. These groups can
further react with the appropriate groups of other amino
acids forming tri, tetra, pentapeptides and so on.
Polypeptides
If a large number of α-amino acids (hundreds to thousands) are joined by peptide bonds, the
resulting polyamide is called polypeptide as shown below :

It is clear from the above structure that each polypeptide chain has a free amino group (—NH2)
at one end and the free carboxyl group (—COOH) at the other end. The amino group end is
called amino or N-terminal end while the end having free —COOH group is called C-terminal
end. The structure is generally written with N-terminal end to the left and C-terminal end to the
right. The name of the peptide is written from the names of the amino acids as they appear from
left to right starting from N-terminal amino acid. The suffix-ine in the name of the amino acid is
replaced by –yl (as glycine to glycyl, alanine to alanyl, etc.) for all amino acids except C–
terminal acid. Generally, polypeptides are written with three letter abbreviation or one letter
abbreviation for amino acids. For example, the tripeptide formed by glycine, alanine and serine
is written as :

Depending upon the number of amino acids residues per molecule, the peptides are called
dipeptide, tripeptide, polypeptide, etc. The formation of peptide bonds can continue until a

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molecule containing several hundred thousands amino acids is formed. Relatively shorter
peptides are called oligopeptides, while larger polymers are called polypeptides or proteins. By
convention, a peptide having molecular mass upto 10,000 u is called polypeptide, while a
peptide having a molecular mass more than 10,000 u is called a protein. However, the
distinction between a polypeptide and a protein is not very sharp. A polypeptide with a fewer α-
amino acids may also be called a protein if it has a well defined conformation of a protein such
as insulin which contains 51 amino acids. Polypeptides are amphoteric because of the presence
of terminal ammonium and carboxylate ions as well as the ionized side chains of amino acid
residues. Therefore, they behave as acids or bases and have an isoelectric point at which they are
frequently least soluble and have the greatest tendency to aggregate. Most of the toxins
(poisonous substances) in animal venoms and in plant sources are polypeptides. A derivative of
dipeptide aspartyl phenylalanine methyl ester (aspartame) is about 100 times as sweet as sucrose
and is used as sugar substitute as sweeting agent.

PROTEINS
Proteins are complex nitrogenous molecules which are essential for the growth and maintenance
of life. These perform a wide variety of biological functions. Proteins are the constituents of cells
and, therefore, are present in
all living bodies. The molecular masses of proteins are very high. Structurally, roteins are long
polymers of amino acids linked by peptide (—CO—NH—) bonds.
CLASSIFICATION OF PROTEINS
A : Classification of proteins on the basis of molecular structure
Proteins can be classified into two broad classes on the basis of molecular structure as :
(i) Fibrous proteins (ii) Globular proteins
(i) Fibrous proteins. These types of proteins consist of linear thread like molecules which tend
to lie side by side to form fibres. The molecules are held together at many points by hydrogen
bonds or disulphide bonds. These are usually insoluble in water. The common examples of
fibrous proteins are keratin, in skin, hair, nails and wool, collagen in tendons, fibroin in silk,
myosin in muscle, etc. These proteins serve as the main structural materials of animal tissues.

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(ii) Globular proteins. In this type of proteins, the molecules are folded together into compact
units forming, spheroidal shapes. The peptide chains in globular proteins are also held by
hydrogen bonds but these forces are
comparatively weak. These are soluble in water or aqueous solutions of acids, bases or salts. The
common examples of globular proteins are albumin, insulin, etc.
B. Classification of proteins on the basis of hydrolysis products
Based on the type of products formed on hydrolysis, the proteins may be classified as :
(i) Simple proteins. These are proteins which give amino acids only on hydrolysis with acids or
enzymes. The important examples are : albumins, globulins, glutalins, prolamines, keratin, etc.
(ii) Conjugated proteins. These are proteins which on hydrolysis give a non-protein part and α-
amino acids. Thus, these are formed by the combination of simple proteins with some non-
proteinous substance. The non-proteins part is called prosthetic group and it controls the
biological functions of the protein. The common prosthetic groups in the proteins are :

(iii) Derived Proteins. They are the products of partial hydrolysis of simple or conjugated
proteins. For example, proteoses, peptones, polypeptides, etc.
STRUCTURE OF PROTEINS
Proteins are biopolymers containing a large number of amino acids joined together through
peptide bonds having three dimensional (3D) structures. The structure of proteins is very
complex. The protein structure and shape
can be usually studied at four different levels i.e., primary, secondary, tertiary and quaternary
structures. These are discussed as follows :
1. Primary structure
Proteins may have one or more polypeptide chains. Each polypeptide in a protein has amino
acids linked with each other in a specific sequence. This sequence of amino acids is said to be
the primary structure of that protein.

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Thus, the sequence in which the amino acids are linked in one or more polypeptide chains
of a protein is called the primary structure of protein as shown below :

The primary structure is usually determined by its successive hydrolysis with enzymes or
mineral acids. The amino acid sequence of a protein determines its function and is critical to its
biological activity. Frederick Sangar determined the primary structure of a protein (insulin) for
the first time in 1953. The importance of primary structure of a protein lies in the fact that even a
change of one amino acid can change drastically the properties of the entire protein. It also
creates a different protein. For example, a normal haemoglobin has 574 amino acid units and
changing just one amino acid in the sequence results in defective haemoglobin found in patients
suffering from sickle cell anemia.

Normal haemoglobin
—Val—His—Leu—Thr—Pro—Glu—Glu—Lys—
Sickle cell haemoglobin
—Val—His—Leu—Thr—Pro—Val —Glu—Lys—
In the patients suffering from sickle cell anemia, the defective haemoglobin in red blood cells
precipitates causing the cells to sickle and sometimes even burst leading ultimately to the death.

2. Secondary structure
The secondary structure gives the manner in which the polypeptide chains are folded or
arranged. Therefore, it gives the shape or conformation of the protein molecule. This arises
from the plane geometry of the peptide bond and hydrogen bond between the > C == O and N—
H groups of different peptide bonds. Pauling and Corey investigated the structures of many
proteins with the help of X-rays patterns. It was observed that there are two common types of
structures.

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(i) α-Helix structure

It is the most common form in which a polypeptide chain
forms all possible types of 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 group of an
adjacent turn of the helix as shown in Fig This is called α-
helix. The stability of the structure is due to the hydrogen
bonding between —NH and >C==O groups of peptide
bonds.Therefore, a structure having maximum hydrogen
bonding shall be stable and favoured. The α-helix structure is
also known as 3.613 helix. This represents that each turn of the
helix contains approximately 3.6 amino acids and a 13-
member ring is formed by hydrogen bonding. The helix is
held in its shape primarily by hydrogen bonds between one
amide group and carbonyl group which is 3.6 amino acids
units away.
(ii) β-pleated sheet structure
This was also proposed by Linus Pauling and co-workers in 1951. In this structure, all
polypeptide chains are stretched out to nearly maximum extension and then laid side by side in a
zig-zag manner to form a flat sheet. Each chain is held to the two neighbouring chains by
hydrogen bonds. These sheets are stacked one upon another to form a three dimensional
structure called β-pleated sheet structure. Two types of pleated sheets are possible. The
polypeptide chains may run parallel i.e., the adjacent chains run in the same direction or may be
antiparallel i.e.,the adjacent chains run in the opposite direction.

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3. Tertiary structure
The tertiary structure arises due to folding, coiling and
bending of polypeptide chains producing three-dimensional
structures. This structure gives the overall shape of proteins.
In other words, the tertiary structure of a protein gives the
overall folding of polypeptide chains i.e., further folding of
the secondary structure. Two major molecular shapes found
are fibrous and globular. These are already discussed. The
fibrous proteins such as silk collagen and α-keratins have
large helical content and have rod-like rigid shape and are
insoluble in water. On the other hand, in globular proteins
such as haemoglobin the polypeptide chains consist partly of
helical sections which are folded about the random cuts to
give at a spherical shape. Perutz and Kendrew determined the
tertiary structure of haemoglobin and myoglobin through X-
rays determination and were awarded Noble Prize in 1962.
The main forces which stabilise the secondary and tertiary
structures of proteins are hydrogen bonds, disulphide
linkages, van der Waals and electrostatic forces of attraction.
4. Quaternary Structure
Many proteins exist as a single polypeptide chain but there are some proteins which exist as
assemblies of two or more polypeptide chains called sub-units or protomers. These sub-units
may be identical or different. These are held together by non-covalent forces such as hydrogen
bonds, electrostatic interactions and van der Waal's interactions. The quaternary structure
refers to the determination of the number of sub-units and their arrangement in an aggregate
protein molecule. The best known example of a protein possessing quaternary structure is
haemoglobin which transports oxygen from the lungs to the cells and carbon dioxide from the
cells to the lungs through the blood stream. It is an aggregate of four polypeptide chains or sub-
units, two identical alpha chains (each containing 141 amino acid residues) and two identical
beta chains (each containing 146 amino acid residues). These four sub-units lie more or less at

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the vertices of a regular tetrahedron. Each polypeptide chain carries a heme group
(ironprotoporphyrin complex) at its end.

FORCES THAT STABILIZE PROTEIN STRUCTURES

The following types of forces stabilize the protein structure :
1. Hydrogen bonding. These are weak forces and arise between a partially positive hydrogen
and a partially negative atom such as oxygen, fluorine or nitrogen on the same or different
molecule.
2. Ionic bonding. Ionic bonding can take place between an ionic and cationic side chains
resulting side chain cross linking.
3. Covalent bonding. The most common form of inter-chain bonding is the disulphide bond
formed between the sulphur atoms of two cysteine residues. The insulin consists of two
polypeptide chains linked together by covalent
bonding.
4. Hydrophobic bonding. Many amino acid residues have hydrophobic (water hating) side
chains. Proteins in aqueous solutions fold so that most of the hydrophobic chains become
clustered inside the folds. The polar side chains which are hydrophilic (water-loving) lie on the
outside or the surface of the protein.

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These forces are shown below :

NATIVE STATE AND DENATURATION OF PROTEINS

Native state. The most energetically stable state of a protein is
called its native state. In other words, native state corresponds
to the proteins found in a biological system with a unique three
dimensional structure (or configuration) and biological activity.
Denaturation of proteins. A process that changes the physical
and biological properties of proteins without affecting the
chemical composition of a protein is called denaturation. The
denaturation is caused by certain physical change like change
in temperature or a chemical change like change in pH,
presence of salts or certain chemical agents. Due to this,
globules unfold and helix get uncoiled. As a result, the protein
molecule uncoils from an ordered and specific conformation
into a more random conformation and protein precipitates from
solution. Changes in pH have the greatest disruptive effect on
hydrogen bonding and salt bridges in proteins.
1. The most common example of denaturation of protein is the coagulation of albumin present in
the white of an egg. Proteins present in egg white areglobular and soluble. When an egg is boiled
in water, the globular proteins
present in it change to a rubber like insoluble mass. This is irreversible denaturation and the
protein cannot return to its original state.
2. Curdling of milk is another example of denaturation of proteins. It is caused due to the
formation of lactic acid by the bacteria present in milk.

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ENZYMES
The enzymes are biological catalysts produced by living cells which catalyze the biochemical
reactions in living organisms. Chemically enzymes are naturally occurring simple or conjugate
proteins. Some enzymes may be
non-proteins also. Almost all enzymes are globular proteins. Without enzymes, the living
processes would be very slow to sustain life. For example, without the presence of enzymes in
our digestive tract, it would take about 50 years to digest a single meal. All enzymes are proteins.
About 3000 enzymes have been identified. The enzymes differ from other types of catalysts in
being highly selective and specific. The enzymes are generally named after the compound or
class of compound upon which they work. For example, the enzyme which catalyses the
hydrolysis of maltose into glucose is named as maltase.
Sometimes the 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.
The enzymes facilitates the biochemical reactions by providing alternative lower activation
energy path and, therefore, increases the rate of reactions. At present about 3000 enzymes have
been recognized by the Internal Union of Biochemistry. However, only about 10% (i.e., 300) are
commercially available.
Properties of Enzymes
The important characteristics of enzymes are :
1. High efficiency. Enzymes increase the speed of reactions up to 10 million times as compared
to the uncatalysed reactions. This is because the enzyme reduces the magnitude of activation
energy. For example, the activation energy of acid hydrolysis of sucrose is 6.22 kJ mol–1 while
the activation energy is only 2.15 kJ mol–1 when hydrolysis is carried out by the enzyme
sucrase.
2. Extremely small quantities. Extremely small quantities of enzymes as small as millionth of a
mole—can increase the rate of reaction by factors of 103 to 106.
3. Specificity. The enzymes are highly specific in nature. Almost every biochemical reaction is
controlled by its own specific enzymes. For example, maltase catalyses the hydrolysis of
maltose. No other enzyme can catalyse its

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hydrolysis.
4. Optimum temperature and pH. The enzymes are active at moderate temperature (about
37°C) and pH (around 7).
5. Control of activity of enzymes. The action of enzymes are controlled by various mechanisms
and are inhibited by various organic and inorganic molecules.
Coenzymes
In some cases, most active enzymes are associated with some non-protein components required
for their activity. These are called prosthetic groups. The prosthetic group which is covalently
attached with the enzyme molecule
is known as cofactor. The prosthetic groups which get attached to the enzyme at the time of
reaction are known as coenzymes. These are generally metal ions or small organic molecules.
The common metal ions are Zn, Mg, Mn, Fe, Cu, Co, Mo, K and Na. Several coenzymes are
derived from vitamins : such as thiamine, niacin, riboflavin, etc. In some cases, the enzyme
activity can be reduced or inhibited by the presence of certain compounds known as enzyme
inhibitors.
Mechanism : Enzyme Catalysed Reactions
The various steps involved in the enzyme catalysed reaction
are given below :
(i) Binding of the enzyme (E) to substrate (S) to form a
complex.
E + S → ES
ES is called the enzyme-substrate complex.
(ii) Product formation in the complex.
ES → EP
where EP is a complex of enzyme and product.
(iii) Release of product from the enzyme-product complex.
EP → E + P
The catalytic property of enzymes is present at certain
specific regions on their surfaces. These are called active sites
or catalytic sites.

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Applications of Enzymes
1. Enzyme deficiencies and prevention of diseases. The deficiencies of enzyme in living
system cause many diseases. Some of these are given below :
(i) The deficiency of phenylalanine hydroxylase enzyme causes a congenital disease called
phenyl-ketone urea. This disease causes accumulation of compounds in the body which results
into severe brain damage and retardation in children. This can be prevented by a diet with low
phenylalanine content.
(ii) Deficiency of enzyme tyrosinase causes albinism. Due to deficiency of tyrosinase sufficient
melanin (a pigment which gives colour to the skin or hair) is not produced. Therefore, the
persons and animals suffering from this
disease have white skin or hair. These diseases can be prevented by the supply of enzymes
through diet.
2. Curing diseases. Certain enzymes are also useful for treating heart diseases. An enzyme
streptokinase is used to dissolve blood clot to prevent heart attacks.
3. Industrial applications. The enzymes are widely used in industrial processes. For example,
enzymes are used
(i) in breweries for the manufacture of beer, wine, etc. by the fermentation of carbohydrates.
(ii) in food processing industries for preparing sweet, syrup, etc.
(iii) in the production of cheese by coagulation of milk.
HORMONES
Hormones are the chemical substances which are produced in the ductless glands in the body.
These are carried to different parts of the body by the blood stream and control various body
functions. Because of the action of hormones as communication among cells, they are called
chemical messengers.
In mammals, the secretion of hormones is controlled by the anterior lobe of the pituitary
gland present at the base of the brain. These hormones are transported to other glands such as
adrenal cortex, thyroid and sex glands to stimulate the production of other hormones.
Classification of Hormones
Based upon the structure of hormones, these are classified into three main types :
(i) Steroid hormones. These hormones, contain a steroid nucleus which is based on a four-ring
network consisting of three cyclohexane rings and one cyclopentane ring. These are mostly

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secreted by testes and adrenal cortex of males and are called sex hormones or adrenal cortex
hormones. The common examples are testosterone, dihydrotestosterone and androgens. During
puberty, these stimulate the male sex characteristics. In females, estrogens are female sex
hormones which are produced in ovaries and are responsible for development of female sex
characteristics during puberty.
(ii) Protein or polypeptide hormones. These hormones contain a peptide chain. The common
examples are oxytoxin, vasopressin and insulin.
(iii) Amine hormones. These are water soluble compounds which have amino group and are
structurally derived from amino acids. The common examples are adrenaline and thyroxine.

Hormones Organ of Secretion Functions

1. Testosterone Testes Regulates the development and normal functioning
of male sex organs.
2. Estrogen Ovary Control the development and normal functioning of
female sex organs.
3. Progesterone Corpus luteum Controls the development and maintenance of
pregnancy.
4. Cortisone Adrenal cortex Regulate the metabolism of fats, proteins and
carbohydrates; control the balance of water and
minerals in the body.
5. Oxytocin Posterior pituitary Controls the contraction of the uterus after child
gland birth and releases milk from the mammary glands.
6. Vasopressin Pituitary gland Controls the reabsorption of water in the kidney.
7. Insulin Pancreas Controls the metabolism of glucose, maintains
glucose level in the blood.
8. Adrenaline or Adrenal Increases pulse rate and controls blood pressure. It
Epinephrine medulla releases glucose from liver glycogen and fatty acids
from fats in emergency.
9. Thyroxine Thyroid gland Controls metabolism of carbohydrates, lipids and
proteins.

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VITAMINS
These are organic compounds which cannot be produced by the body and must be supplied in
small amounts in diet to perform specific biological functions for the normal health, growth and
maintenance of body. The condition of vitamin deficiency is known as avitaminoses. Excess
intake of these vitamins may cause hypervitaminoses.
Vitamins are generally classified into two broad types based on their solubility.
 The vitamins which are soluble in water are called water soluble vitamins.
For example, vitamins B group (B-complex), vitamin C.
 The vitamins which are soluble in fats are called fat soluble vitamins.
For example, vitamins, A, D, E and K.
Vitamin Chemical Name Deficiency Disease Sources of Vitamin
Xerophthalmia i.e., Cod liver oil, shark liver oil,
A Retinol
hardening of cornea of eye or carrot, rice polishing, liver,
night blindness. kidney, butter, milk, etc.
Beri-beri (loss of appetite, Milk, rice, yeast, nuts, eggs, green
B1 Thiamine
retarded growth); disease of vegetables, liver, kidney.
nervous system.
Glossitis (dark red tongue), Turnip, milk, eggs, yeast,
B2 Riboflavin dermatitis and cheilosis vegetables, liver, kidney.
(fissuring at corners of
mouth and lips).
B3 Niacin pellagra
Dermatitis and convulsions. Yeast, milk, meat, fish, egg yolk,
B6 Pyridoxine
whole cereal, grams.
Pernicious anaemia (RBC Meat, eggs, liver of ox, sheep,
B12 Cyanocobalamine deficiency in haemoglobin), pig, fish, curd, etc.
inflammation of tongue and
mouth.
Scurvy (bleeding of gums), Citrus fruits like orange, lemon,
C Ascorbic acid
pyorrhea (loosening and amla, tomato, green
bleeding of teeth). vegetables.

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Rickets (bone deformities in Milk, egg yolk, cod liver oil,

D Ergocalciferol children) and osteomalacia exposure to sunlight.
(soft bones and joint pains in
adults).
Sterility Oils like cotton seed oil, soyabean
E Tocoferol
oil, wheat gram oil, sunflower oil.
Haemophilia (haemorrhagic Cereals, green leafy vegetables.
K Phylloquinone
condition), increased blood
clotting time.
Dermatitis, loss of hair and Yeast, liver, kidney and milk.
H Biotin
paralysis.
Low order of immunity of Chloroplasts of green plants and
Q10 Coenzyme
body against many diseases. mitochondria of animals.
NUCLEIC ACIDS
Nucleic acids are biologically important polymers which are present in all living cells. They play
an important role in the development and reproduction of all forms of life. They direct the
synthesis of proteins and are responsible for the transfer of genetic information i.e., the
hereditary characteristics. The repeating units of nucleic acids are called nucleotides. Therefore,
the nucleic acids are also regarded as polynucleotides.
There are two types of nucleic acids :
(i) DNA (deoxyribonucleic acid) (ii) RNA (ribonucleic acid).
A nucleotide consists of three chemical components :
(i) A nitrogen containing heterocyclic base
(ii) a five carbon sugar
(iii) a phosphate group.
1. Nitrogen containing heterocyclic base. There are two different types of heterocyclic
nitrogeneous bases. These are known and purines and pyrimidines. Pyrimidines have a single
heterocyclic ring while purines have two fused rings. The heterocycles present in nucleic acid are
substituted forms of these compounds. The common examples are
(i) adenine (A) and guanine (G) are substituted purines
(ii) cytosine (C), thymine (T) and uracil (U) are substituted pyrimidines.

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2. Sugars. There are two types of sugars

present in nucleic acids. The sugar present in
RNA is β-D-ribose and in DNA is β-D-2
deoxyribose as shown

3. A phosphate group. These are responsible

for the linkage in nucleic acid polymers. The
phosphate group is bonded to a hydroxyl group
of sugar The phosphate group in nucleic acid is

Nucleosides and Nucleotides

Nucleosides
The molecules in which one of the nitrogen bases (purine or pyrimidine) is bonded with a sugar
molecule is called nucleoside.
Base + Sugar = Nucleoside

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The purine or pyrimidine bases are attached to position 1' of pentoses through N-glycosidic
linkages.

Nucleotides
When the phosphate group is attached to the nucleoside, the compound formed is called
nucleotide. In other words, a nucleotide is a phosphate ester of nucleoside and consists of a
purine or pyrimide base, the 5-carbon sugar and one or more phosphate groups.
Base + Sugar + Phosphate = Nucleotide

In nucleotides, the sugar rings are attached to

the nitrogen atom of the heterocyclic ring by
a bond between C1 atom of sugar and
nitrogen atom of heterocyclic ring. This
linkage is called N-glycosidic bond. The
phosphate group is bonded to a hydroxyl
group of sugar.
Nucelotides are joined together by
phosphodiester linkages between 5' and 3'
carbon atoms of pentose sugar.

Therefore, the consecutive joining of sugar unit of one nucleotide to the phosphate group of next
nucleotide results in a long chain polymer called nucleic acid. A nucleic acid chain is commonly
abbreviated by a one letter code with the 5'- end of the chain written on the left side. For
example, a tetranucleotide having adenine, cytosine, guanine and thymine bases from 5' end to 3'
end is written as ACGT. The backbone of the nucleic acid consists of alternating sugar and
phosphate bonds. For simplicity, the bases are represented by their respective symbols, the

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phosphate bond is represented by the symbol 'P' and sugar is drawn according to simple Fischer
projection. For example, the tetranucleotide ACGT can be written as :

Thus, the nucleic acid backbone consists of alternate sugar-phosphate residues. One of the four
nitrogen base residues is attached to each sugar unit on this backbone. The nucleic acid backbone

STRUCTURE OF DNA
1. Primary Structure of DNA
The sequence of nucleotides in the chain of nucleic acid is called its
primary structure. It was found by E. Chargaff that the base composition
in DNA varied from one species to other species. However, in all cases, the
amount of adenine was equal to that of thymine (A = T) and the amounts of
cytosine was equal to that of guanine (C = G). This is also known as
Chargaff rule. In other words,the total amount of purines was equal to that
of pyrimidines i.e., A + G = C + T. But the ratio AT/CG varied
considerably between species. For example the AT/CG ratio is 1.52 in man
and about 0.93 in E. Coli.
Watson and Crick proposed that DNA polymers have double helical
structure, which explained not only the base equivalence (A = T and G =
C) but other properties of DNA especially, its duplication in a living cell
(called replication). This double helical structure of DNA consists of two
right handed helical polynucleotide chains coiled around the same central
axis. The two strands are antiparallel i.e., their 5' → 3' phosphodiester
linkages run in opposite directions.

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lecturer in Chemistry
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These two strands are marked 5' and 3′ which indicate that the free hydroxyl groups of the
terminal deoxyribose units are present at 5′ and 3′ positions respectively. Therefore, a linear
polymer has a free 5′-hydroxyl group at one end and a free 3′-hydroxyl group at other end. The
nucleotides making up each strand of DNA are connected by phosphate ester bonds. This forms
the backbone of each DNA strand from which the bases extend. The bases (purines and
pyrimidines) are stacked inside the helix in planes perpendicular to the helical axis. It is like a
stack of flat plates held together by two ropes of sugarphosphate polymeric backbone running
along outside of stack. The bases project towards each other within this structure, while the sugar
and phosphate components form a structural framework on the outside of the duplex. The bases,
thus are the steps of the spiral staircase and the sugar phosphate framework is the railing. Such a
structure is called double helix structure. The order in which bases occur is called the base
sequence.
2. Secondary structure of DNA
The two strands are held together by hydrogen bonds. This hydrogen bonding is very specific
because the structures of bases permit only one mode of pairing. For example, guanine is
hydrogen bonded to cytosine and adenine to thymine. Thymine and adenine can be joined by
two hydrogen bonds while cytosine and guanine can be joined by three hydrogen bonds.
This has been shown in Fig. 8. No other combination of four bases is possible because these do
not lead to strong hydrogen bonds.

The two strands of DNA are said to be complementary to each other in the sense that the
sequences of bases in one strand automatically determines that of other. For example, whenever,
adenine (A) appears in one strand, a thymine (T) appears opposite to it in the other strand. The
diameter of double helix is 2 nm and the double helical structure repeats at interval of 3.4 nm
when it completes one turn. This one turn corresponds to ten base pairs. DNA helices can be

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lecturer in Chemistry
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right handed as well as left handed. The β-conformation of DNA having right handed helices is
most stable. On heating the two strands of DNA separate from each other and this process is
known as melting. When these two strands are cooled they again hybridize. This process is
called annealing. The temperature at which the two strands completely separate is known as its
melting temperature (Tm). This is specific for each specific sequence.
Structure of RNA
The structure of RNA is similar to that of DNA except that it is a single strand structure.
Sometimes, they fold back on themselves to form a double helix structure. RNA molecules are of
three types and they perform different functions; They are named as :
(i) messenger RNA : m–RNA
(ii) ribosomal RNA : r–RNA
(iii) transfer RNA : t–RNA
BIOLOGICAL FUNCTIONS OF NUCLEIC ACIDS
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 million's of years. The important biological functions of nucleic acids are :
1. Replication
It is the property of a molecule to synthesise another molecule. DNA has a
unique property to duplicate or replicate itself i.e., it can bring about the
synthesis of another DNA molecule. Replication of DNA is an enzyme
catalysed process. In this process, at the time of cell division (mitosis), the
two strands of DNA double helix partly unwind and each strand serves as
a template or pattern for the synthesis of a new DNA molecule (strand).
Due to unique specificity of base pairing, the newly synthesised
complementary strand in each case is an exact copy of the originally
separated from it. As a result, two double stranded DNA molecules are
formed called two daughter DNA molecules. One of the strand comes
from the parent DNA molecule and the other is newly synthesised. Each
DNA is exact replica of the parent. In this way, hereditary effects are
transmitted from one cell to another. It may be noted that DNA replication
follows the base pairing rules by which A pairs with T and G pairs with C.

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lecturer in Chemistry
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This replication can easily be understood. Suppose a segment along a double helix is :

When this double helix uncoils, then it forms two strands as :

Each strand can act as a template to build identical double helices. The complements to the two
strands are :

These two double helices are identical to each other and to the first double helix. Thus, the
original double helix is repeated itself. The DNA replication is semi-conservative i.e., only half
of the parental DNA is conserved and only one strand is synthesized. DNA replication takes
place only in the 5′→ 3′ direction.

2. Protein Synthesis
DNA molecules also perform an important function of synthesising proteins, which serve as
machinery of the living cell. In this process, the genetic information coded in DNA in the form
of specific base sequences is translated and expressed in the form of sequence of amino acids
which result in the synthesis of specific proteins which perform various functions 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 coded in DNA. Protein synthesis is a fast process and
about 20 amino acids are added in one second. For example, silk has the major component
fibroin protein. A single fibroin gene makes 104 copies of its m-RNA and each m-RNA produces
105 molecules of fibroin protein amounting to a total of 10 9 molecules of protein per cell in a
period of 4 days.

S. SAI KRISHNA
lecturer in Chemistry