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Carbohydrates are defined as polyhydroxy aldehydes or polyhydroxy ketones or substances which
give these on hydrolysis and contain at least one chiral carbon atom. It may be noted here that
aldehydic and ketonic groups in carbohydrates are not present as such but usually exist in
combination with one of the hydroxyl group of the molecule in the form of hemiacetals and hemiketals
respectively.
Classification

The carbohydrates are divided into three major classes depending upon whether or not they undergo
hydrolysis, and if they do, on the number of products formed.

(i) Monosaccharides: The monosaccharides are polyhydroxy aldehydes or polyhydroxy ketones

which cannot be decomposed by hydrolysis to give simpler carbohydrates. Examples are glucose
and fructose, both of which have molecular formula, C6H12O6.

(ii) Oligosaccharides: The oligosaccharides (Greek, oligo, few) are carbohydrates which yield a
definite number (2-9) of monosaccharide molecules on hydrolysis. They include,
(a) Disaccharides, which yield two monosaccharide molecules on hydrolysis. Examples are
sucrose and maltose, both of which have molecular formula, C12H22O11.

(b) Trisaccharides, which yield three monosaccharide molecules on hydrolysis. Example is

raffinose, which has molecular formula, C18H32O16.

(c) Tetrasaccharides, etc.

(iii) Polysaccharides: The polysaccahrides are carbohydrates of high molecular weight which yield
many monosaccharide molecules on hydrolysis. Examples are starch and cellulose, both of which
have molecular formula, (C6H10O5)n.

In general, the monosaccharides and oligosaccharides are crystalline solids, soluble in water and
sweet to taste. They are collectively known as sugars. The polysaccharides, on the other hand, are
amorphous, insoluble in water and tasteless. They are called non-sugars. The carbohydrates may
also be classified as either reducing or non-reducing sugars. All those carbohydrates which have the
ability to reduce Fehling’s solution and Tollen’s reagent are referred to as reducing sugars, while
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others are non-reducing sugars. All monosaccharides and the disaccharides other than sucrose are
reducing sugars.
MONOSACCHARIDES
The monosaccharides are the basis of carbohydrate chemistry since all carbohydrates are either
monosaccharides or are converted into monosaccharides on hydrolysis. The monosaccharides are
polyhydroxy aldehydes or polyhydroxy ketones. There are, therefore, two main classes of
monosaccharides.

1. The Aldoses, which contain an aldehyde group

2. The Ketoses, which contain a ketone group (— —)

The aldoses and ketoses are further divided into sub-groups on the basis of the number of carbon
atoms in their molecules, as trioses, tetroses, pentoses, hexoses, etc. To classify a monosaccharide
completely, it is necessary to specify both, the type of the carbonyl group and the number of carbon
atoms present in the molecule. Thus monosaccharides are generally referred to as aldotrioses,
aldotetroses, aldopentoses, aldohexoses, ketohexoses, etc.
The aldoses and ketoses may be represented by H O CH2OH
the following general formulas. |
C C=O
| |
(CHOH)n (CHOH)n
| |
CH2OH CH2OH
Aldoses Ketoses
(n = 1, 2, 3, 4, 5) (n = 0, 1, 2, 3, 4)

Glucose and
fructose
are
specific
examples
of an
aldose
and a
ketose.

Cyclic Structure of Glucose – Anomers

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We have discussed above that monosaccharides have cyclic hemiacetal or

hemiketal structures. To illustrate, let us first consider the example of D-glucose.
During hemiacetal formation C5 – OH of glucose combines with the C 1 –
aldehydic group. As a result, C1 becomes chiral or asymmetric and thus has two
possible arrangements of H and OH groups around it. In other words, D-glucose
exists in two stereoisomeric forms, i.e., -D-glucose and -D-glucose as shown
below:
In -D-glucose, the OH group at C1 is towards right while in -D-glucose, the OH
group at C1 is towards left. Such a pair of stereoisomers which differ in
configuration only around C1 are called anomers and the C1 carbon is called
Anomeric carbon (or glycosidic carbon. The cyclic structures of
monosaccharides can be better represented by Haworth Projection formulae. To
get such a formula for any monosaccharide (say -and -D-glucose), draw a
hexagon with its oxygen atom at the upper right hand corner. Place all the
groups (on C1, C2, C3 and C4) which are present on left hand side in structures I
and II, above the plane of the ring and all those groups on the right hand side
below the plane of the ring.
The terminal – CH2OH group is always placed above the plane of the hexagon
ring (in D-series). Following the above procedure, Haworth Projection Formulae
for -D-glucose (I) and -D-glucose (II) are obtained as shown below:
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6 CH2OH 6 CH2OH

5 5
H H H OH

H H
4 1 4 1
OH H  OH H
OH OH OH H
3 2 3 2
I II
-D-Glucose -D-Glucose
H OH H OH
or -D(+)-Glucopyranose or -D(+)-Glucopyranose

Cyclic structure of Fructose

Like glucose, fructose also has a cyclic structure. Since fructose contains a keto
group, it forms an intramolecular hemiketal. In the hemiketal formation, C 5– OH
of the fructose combines with C2-keto group. As a result, C2 becomes chiral and
thus has two possible arrangements of CH2OH and OH group around it. Thus,
D-fructose exists in two stereoisomeric forms, i.e., -D-fructopyranose and -D
fructopyranose. However in the combined state (such as sucrose), fructose
exists in furanose form as shown below:

REACTIONS OF GLUCOSE

(a) With HI/P: It undergoes reduction to form n-hexane while with sodium amalgam it forms sorbitol.

n-hexane

sorbitol
(b) With H2O: It forms neutral solution
(c) With Hydroxylamine (NH2OH)
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Glu cos e  NH 2OH  H C = N O H
|
(C H O H )4
|
C H 2O H
G lu c o s e o x im e
(d) With HCN: It form addition product cyanohydrin
Glu cos e  HCN  CN
|
(C H O H )5
|
C H 2O H
G lu c o s e c y a n o h y d rin

(e) Oxidation: Glucose on oxidation with Br 2 gives gluconic acid which on further oxidation with HNO 3
gives glucaric acid
Br2 / H2O HNO3
Glucose   COOH  Strong
   COOH
| oxidation |
(CHOH)4 (CHOH)4
| |
CH2OH COOH
Gluconic acid Glucaric acid

(f) With Tollen reagent and Fehling solution. Glucose forms silver mirror and red ppt. of Cu 2O
respectively.
(g) With acetic anhydride. In presence of pyridine glucose forms pentaacetate.
Glu cos e  5 ( CH 3 CO ) 2O  C H O
C 5H 5N
|
(C H O C O C H 3)4
|
C H 2O C O C H 3
G lu c o s e p e n ta a c e ta te

(h) With phenylhydrazine: it forms glucosazone

Glu cos e  C 6 H 5 NHNH  2  C H 2 O H
C 5H 5N
|
(C H O H )3
|
C = N N H C 6H 5
|
C H = N N H C 6H 5
(G lu c o s a z o n e )

(i) With conc. HCl acid: Glucose gives laevulinic acid

Glu cos e  Conc . HCl  CH 3 CO  CH 2 CH 2  COOH  HCOOH  H 2O
L a e v u lin ic a c id

(j) Glycoside formation: When a small amount of gaseous HCl is passed into a solution of D (+)
glucose in methanol , a reaction takes place that results in the formation of anomeric methyl
acetals.
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C H 2O H

5 1
H O
H
D (  ) Glucose CH
     3 OH H
( Pyranose form) HCl
O H H
H O

H O H O C H 3

M e t h y l  - D - g lu c o p y r a n o s id e
+
M e th y l  - D - g lu c o p y r a n o s id e

Carbohydrate acetals, generally are called glycosides and an acetal of glucose is called
glucoside.
DISACCHARIDES
Carbohydrates which upon hydrolysis give two molecules of the same or different monosaccharides
are called disaccharides. Their general formula is C12H22O11. The three most important disaccharides
are sucrose, maltose, and lactose. Each one of these on hydrolysis with either an acid or an enzyme
gives two molecules of the same or different monosaccharides as shown below:

Disaccharides may also be considered to be formed by a condensation reaction between two

molecules of the same or different monosaccharides with the eliminatioin of a molecule of water. This
reaction involves the formation of an acetal from a hemiacetal and an alcohol – in which one of the
monosaccharides acts as the hemiacetal while the other acts as the alcohol.
Sucrose
It is formed by condensation of one molecule of glucose and one molecule of fructose. Unlike maltose
and lactose, it is non-reducing sugar since both glucose (C 1 - ) and fructose (C2 - ) are connected to
each other through their reducing centres. Its structure is shown below:
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Hydrolysis: (Invert Sugar or Invertose). Hydrolysis of sucrose
with hot dilute acid yields
D-glucose and D-fructose.

Sucrose is dextrorotatory, its specific rotation being +66.5%, D-

glucose is also dextrorotatory, []D = +53°, but D-fructose has a
large negative rotation, []D = -92°. Since D-fructose has a greater
specific rotation than D-glucose, the resulting mixture is
laevorotatory. Because of this the hydrolysis of sucrose is
known as the inversion of sucrose, and the equimolecular mixture
of glucose and fructose is known is invert sugar or invertose.
POLYSACCHARIDES
Polysaccharides are formed when a large number (hundreds to even thousands) of monosaccharide
molecules join together with the elimination of water molecule. Thus, polysaccharides may be
regarded as condensation polymers in which the monosaccharides are joined together by glycosidic
linkages. Some important polysaccharides are:
1. Cellulose 2. Starch
3. Glycogen 4. Gums and
5. Pectins
Starch
It is a polymer of glucose. Its molecular formula is (C6H10O5)n where the value of
n (200 – 1000) varies from source to source. It is the chief food reserve material or storage
polysaccharide of plants and is found mainly in seeds, roots, tubers, etc. Wheat, rice, potatoes, corn,
bananas etc., are rich sources of starch.
Starch is not a single compound but is a mixture of two components – amylose (10 to 20%) and
amylopectin (20 to 80%). Both amylose and amylopectin are polymers of
-D-glucose.
Amylose is a linear polymer of -D-glucose. It contains about 200 glucose units which are linked to
one another through -linkage involving C1 of one glucose unit with C4 of the other as shown below:

Amylopectin, on the other hand, is a highly branched polymer. It consists of a large number (several
branches) of short chains each containing 20-25 glucose units which are joined together through -
linkages involving C1 of one glucose unit with C4 of the other. The C1 of terminal glucose unit in each
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chain is further linked to C6 of the other glucose unit in the next chain through C 1 – C6 -linkage. This
gives amylopectin a highly branched structure as shown below.-

Hydrolysis: Hydrolysis of starch with hot dilute acids or by enzymes gives dextrins of varying
complexity, maltose and finally D-glucose. Starch does not reduce Tollen’s reagent and Fehling’s
solution.
Uses: It is used as a food. It is encountered daily in the form of potatoes, bread, cakes, rice etc. It is
used in coating and sizing paper to improve the writing qualities. Starch is used to treat textile fibres
before they are woven into cloth so that they can be woven without breaking. It is used in manufacture
of dextrins, glucose and ethyl alcohol. Starch is also used in manufacture of starch nitrate, which is
used as an explosive.
AMINO ACIDS
Introduction
(i) Amino acid are organic compound of both an amino group & carboxylic group.
(ii) They are represent by general formula:

(iii) These amino acid are very important because they are the building blocks of protein.
(iv) Protein is the natural polymer moving  - amino acids as monomer.
(v) With the exceptions of glycine. All the other amino acids have chiral carbon & have two optically
active isomers.
(vi) All naturally occurring amino acids are in L – series which on NH2 group on the left as OH
group in L – glyceraldehydes.

Classification
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Amino acid with non – polar side chain

(i) (ii)

(iii) (iv)

(v) (vi)

(vii)

Essential amino acids

(a) These must be supplied to our diet as are not synthesized in body.
(b) Some of them are
(1) Valine (2) Leucine (3) Isoelucine (4) Phenylalanine (5) Arganine (5) Threonine
(6) Tryptophan (7) Methionine (8) Lysine (9) Arginine (10) Histadine

Note: Histidine and arginine are essential i.e. can be syntrhesized but not in quantities sufficient to
permit normal growth.

Non – Essential Amino acid

These amino acids are synthesized in body.
Some of them are
These are as follows:

(1) Glycine (2) Alanine (3) Tyrosine (4) Serine (5) Cystine (6) Pronine (7) Hydroxyprocine
(8) Cysteine (9) Aspartic acid (10) Glutonic acid

Synthesis of  - amino acid

Protein can be hydrolyzed by refluxing with dilute hydrochloric acid to give a mixture of  - amino
acids. The resulting mixture can be separated by
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(a) fractional crystallization.
(b) Fractional distillation of their ester followed by hydrolysis (Fischer’s method)
(c) Selective precipitation as salt with phosphotungstic and picric acid.
(d) Distribution of amino acid between n – butanol saturated with water (Dakin’s method).
(e) Column, paper and gas chromatography.
(f) Electrophoresis.
By animation of  - halo acid

(i)

(ii
)

By Gabriel synthesis

By Strecker Synthesis

(i)

(ii)

Note: Generally the aldehyde is treated with a mixture of ammonium chloride and potassium cyanide
in aqueous solution.
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Physical properties of amino acid

(i) Amino acids are generally, crystalline substance having sweet taste.
(ii) They melt with decomposition at higher temperature (more than 200C).
(iii) They are soluble in water but insoluble in organic solvents.

ZWITTER ION
(i) Amino acids contain both acidic carboxyl group (COOH) and basic amino group in the same
molecules.
(ii) In aqueous solution, the acidic carboxyl group can lose a proton and basic amino group gain a
proton in a kind of internal acid – base reaction.

(iii) The product of this internal reaction is called a Dipolar or a Zwitter ion.
(iv) The Zwitter ion is dipolar, changed but overall electrically neutral and contain both a positive and
negative charge.
(v) Amino acid in the dipolar ion form are amphoteric in nature.
(vi) Depending upon the pH of the solution, the amino acid can donate or accept proton.

ISOELECTRIC POINT
(i) When ionized form of amino acid is placed in an electric field it will migrate towards the opposite
electrode.
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(ii) Depending upon the pH of the medium following three thing may happen.
(a) In acidic medium, the cation move towards cathode.
(b) In basic medium, the anion move towards anode.
(c) The Zwitter ion does not move towards any of the electrodes.
(iii) At a certain pH (i.e. H+ concentration), the amino acid molecules show no tendency to migrate
towards any of the electrodes and exists as a neutral dipolar ion, when placed in electric field in
known as isoelectric point.
(iv) All amino acids do not have the same isoelectric point & it depends upon the nature of
R – linked to - carbon atom.
Isoelectric point of some amino acid

Amino acid Isoelectric point

Neutral amino acids (pH 5.5 to 6.3)
Glycine 5.7
Alanine 6.1
Valine 6.0
Serine 5.7
Threonine 5.6
Acidic amino acids (pH  3)
Aspartic acid 2.8
Glutamic acid 3.2
Basic amino acids (pH  10)
Lysine 9.7
Arginine 10.8

(v) Amino acid have minimum aqueous solubility at isoelectric point.

Chemical Properties
Amino acids show the following characteristic reactions.
1.Reaction of the carboxyl group.
2.Reaction of the amino group.
3.Reaction involving both the carboxyl and the amino group.
Reaction of the carboxyl group

Reaction with base

(i)
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(ii)

Mechanism:

Esterification

Note: HCl first convert the dipolar ion into an acid which is subsequently esterified.
Decarboxylation

Reduction

Reaction with strong acid

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Acetylation

Reaction with Nitrous acid

Note: (i) This reaction from the basis of the “van slyke method” for the estimation of amino acids.
(ii) The nitrogen is evolved (one half comes from the amino acid) quantitatively and its
volume measured.

Reaction with Nitrosyl halide

Reaction with 2, 4 – Dintrofluorobenzene (DNFB)

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Reaction involving both the carboxyl & the amino group

Effect of heat
 - amino acids undergo dehydration on heating (200C) to give diketo piperazines.

PEPTIDES & PROTEINS

Introduction
(i) Proteins are formed by joining the carboxyl group of one amino to the  - amino group of another
acid.
(ii) The bond formed between two amino acids by the elimination of water molecules is called peptide
linkage.

(iii) The product formed by linking amino acid molecules through peptide linkage CO  NH  is
called a peptite.
(iv) When two amino acid combined in this way the resulting product is called a dipeptide.

(v) Peptide are further designated as tri, tetra or penta peptides accordingly as they contain three,
four or five amino acid molecules, same or different.
(vi) In a peptide the amino acid that contains the free amino group is called the N – terminal residue
(written on L.H.S).
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(vii) The amino acid that contain the free carboxyl group is called the C – terminal residue (written on
R.H.S).

(viii) If a large number of  - amino acids (100 to 1000) are joined by peptide bonds the resulting
polyamide is called polypeptide.

(ix) By convention a peptide having molecular weight upto 10,000 is called polypeptide.
(x) While a peptide having a molecular more than 10,000 is called a protein. ‘

Structure of Proteins
(i) Proteins have three dimensional structure.
(ii) There are number of factors which determine the exact shape of proteins.
Structure of Proteins

Primary structure
(i) This type of structure was given by Friedrich Sanger in 1953 in Insulin.
(ii) Primary structure is conformed by single polypeptide chain in a linear manner.
(iii) All amino acid are attached in a straight chain by peptide bond.

Secondary structure
(i) The fixed configuration of polypeptide skeleton is referred to as the secondary structure of
protein.
(ii) It gives information
(a) About the manner in which the protein chain is folded and bent.
(b) About the nature of the bond which stabilizes this structure.
(iii) This structure of protein is mainly of two types
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(A) -Helix
(a) The chain of -amino acid coiled as a right handed screw (called -helix) because of the
formation of hydrogen bond.
(b) The spiral is held together by H-bonds between N–H and C = O group vertically adjacent
to one another.
(c) X-Ray studies have shown that there are approximately 3.6 amino acid unit for each turn
in helix.
(d) Such proteins are elastic i.e., they can be stretched.
(e) On stretching week H-bonds break up and the peptide act like a spring.
(f) The hydrogen bonds are reformed on releasing the tension.
e.g. Myosin, Keratin, Tropomysin.
(B) Beta-pleated sheet
(a) Polypeptide chains are arranged side by side.
(b) The chain are held together by a very large number of hydrogen bond between C = O and
NH of different chains.
(c) These sheets can slide over each other to form a three dimensional structure called a
beta pleated sheet.
e.g. Silk has a beta pleated structure.
Tertiary structure
(i) It refers to the arrangement and interrelationship of the twisted chain of protein into specific
layer or fibres.
(ii) This tertiary structure is maintained by weak interatomic force such as, H-bonds hydrophobic
bond, van der Waals’ force and disulphide bonds (eg Insulin).
e.g. Protein of tobacco mosaic virus (TMV); Myoglobin; Hemoglobin

Quaternary structure
(i) When two or more polypeptide chain united by the force other than covalent bond i.e., not
peptide and disulphide bonds.
(ii) It refers to final three dimensional shape that results from twisting bonding and folding of the
protein helix.
(iii) It is most stable structure.

Classification of Proteins

There are two methods for classifying proteins.

(i) Classification according to Composition
(ii) Classification according to Functions
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Classification according to Composition

Simple proteins
(i) Simple proteins are those which yield only -amino acids upon hydrolysis.
(ii) Simple proteins are composed of chain of amino acid unit only joined by peptide linkage.
Examples are:
Egg (albumin); Serum (globulins); Wheat (Glutelin); Rice (Coryzenin)

Conjugated proteins
(i) Conjugated proteins are those which yield  - amino acids plus a non protein material on
hydrolysis.
(ii) The non protein material is called the prosthetic group.
Example:
Casein in milk (prosthetic group is phosphoric acid); Hemoglobin (prosthetic group is Nucleic
acid); Chlolesterol (prosthetic group – lipid).
According to molecular shape, proteins are further classified into two types.
(A) Fibrous protein
(a) These are made up of polypeptide chain that are parallel to the axis & are held together by strong
hydrogen and disulphide bonds.
(b) They can be stretched & contracted like thread.
(c) They are usually insoluble in water.
Example:
Keratin (hair, wool, silk & nails); Myosin (muscle)
(B) Globular Proteins
(a) These have more or less spherical shape (compact structure).
(b)  - amino helix are tightly held bonding; H – bonds, disulphide bridges, ionic or salt bridges:
Examples:
Albumin (egg)

Classification According to functions

The functional classification includes following groups.

Structural proteins
These are the fibrous proteins such as collogen (skin, cartilage & bones) which hold living system
together.
Blood proteins

(i) The major proteins constituent of the blood are albumin hemoglobin & fibrinogen.
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(ii) There presence contribute to maintenance of osmotic pressure, oxygen transport system & blood
coagulation respectively.

Tests of protein
Biuret test

(i) On adding a dilute of copper sulphite to alkaline solution of protein, a violet colour is developed.

(ii) This test is due to the presence of peptide linkage.

Millon’s test

(a) Millon’s reagent consist of mercury dissolved in nitric acid (forming a mixture of mercuric &
mercurous nitrates).
(b) When millon’s reagent is added to a protein, a white ppt from, which turn brick red on heating.
(c) This test is given by protein which yield tyrosine on hydrolysis (due to the presence of phenolic
group).

Nihydrin test

(i) This test is given by all proteins.

(ii) When protein is boiled with a dilute solution of ninhydrin, a violet colour is produced.

Uses of Proteins

(i) Protein constitute as essential part of our food, meat, eggs, fish, cheese provide protein to human
beings.
(ii) Casein (a milk protein) is used in the manufacture of artificial wool & silk.
(iii) Amino acid needed for medicinal use & feeding experiment, are prepared by hydrolysis of
proteins.
(iv) Gelatin is used in desserts, salad’s, candies bakery goods etc.
(v) Leather is obtained by tanning the protein of animal hides.
(vi) Hemoglobin present in blood is responsible for carrying oxygen and CO2.
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(vii) Hormones control various process.
(viii) Enzymes are the proteins produces by living system & catalyse specific biological reaction.
Example:
Ureases (Urea  CO2 + NH2)
Pepsin (Protein  Amino acid)
Trypsin (Protein  Amino acid)
Carbonic anhydride (H2CO3  H2O + CO2)
Nuclease (RNA, DNA  Nucleotides)
Isoelectric Point
What happens when a solution of an amino acid is placed in an electric field depends upon the acidity
or basicity of solution. In quite alkaline solution.
+ +
– H+ – H+
H
2N-
C H R C
OO –HN
3 .C
HR C
OO –H
3N.C
H R COO
H
OH O
H
(II) (
I) (III)
Anions (II) exceed cations (III), and there is a net migration of amino acid toward the anode. In quite
acidic solution cations (III) are in excess, and there is a net migration of amino acid towards the
cathode. If (II) and (III) are exactly balanced, there is no net migration; under such conditions any one
molecule exists as a positive ion and as a negative ion for exactly the same amount of time and any
small movement in the direction of one electrode is subsequently cancelled by an equal movement
back towards the other electrode. The hydrogen ion concentration of the solution in which a particular
amino acid does not migrate under the influence of an electric field is called the isoelectric point of
that amino acid.

An amino acid shows its lowest solubility in a solution at the isoelectric point, since here there is the
highest concentration of the dipolar ion. As the solution is made more alkaline or more acidic, the
concentration of one of the more soluble ions, II or III increases.

If an amino acid has amino group and one carboxyl group, it has two pK values. The isoelectric point
(PI) of this amino acid has the average value of the both pK values.
We take example of glycine.
H3+N  CH2  COOH H3N+  CH2  COO– + H+ …(1)
Conjugated acid (CA) Dipolar Ion (DI)

At equilibrium
H3N+  CH2  COO– H2N  CH2  COO– + H+ …(2)
DI Conjugated Base (CB)

At equilibrium
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[CA] =

[CB] =
At isoelectric point [CA] = [CB]

Where = conc. of [H+] at isoelectric point.

or, = K 1 K2
or, 2log [Hi+] = log K1 + log K2
or –2 log (Hi+] = - log k1 – logK2
or 2pHi = pK1 + pK2

or pHi =

NUCLEIC ACIDS

Nucleic acids are biologically important polymers which are present in all living cells, also called as
polynucleotides because repeating structural unit is nucleotide.
There are two types of nucleic acids
(i) DNA (deoxyribonucleic acid)
(ii) RNA (Ribonucleic acid)

Composition of nucleic acids

1. A phosphate group
2. Sugar
3. Nitrogenous bases

1. Phosphate Group: Phosphate group in nucleic acid is:

Phosphate group is bonded to a hydroxyl group of sugar.

2. Sugar: Two types of sugars present in nucleic acids.

RNA (D – ribose)
DNA (D – deoxyribose)
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3. Nitrogenous bases: Two types

(i) Purines – Adenine (A) and Guanine (G)

(ii) Pyrimidines – Cytosine (C), thymine (T) and uracil (U)

NUCLEOSIDES AND NUCLEOTIDES

Nucleosides: Nucleoside contains only two basic components of nucleic acids (a pentose sugar and
a nitrogenous base). During their formation, 1- position of pyrimidine or 9 – position of purine moiety is
linked to C1 of sugar (ribose or deoxyribose) by  - linkage.
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General structure of a nucleoside

Depending upon the type of sugar present, nucleosides are of two types:
(i) Ribonucleosides and
(ii) Deoxyribonucleosides
Nucleotides: Nucleotides contains all the three basic components of nucleic acids. Nucleotides are
nucleoside monophosphates.
They are of two types depending upon the type of sugar – Ribonucleotides and Deoxyribonucleotides.
Nucleotide may be represented as follows.

DNA RNA
1. Sugar present in DNA is 2- 1 Sugar is D-ribose.
deoxyribose. .
2. It contains cytosine and thymine as 2 It contains cytosine and uracil as
pyrimidine. . pyrimidine.
3. It has double stranded  - helix 3 It has single stranded  - helix.
structure. .
4. DNA occurs in the nucleus of cell. 4 RNA occurs in cytoplasm of the cell.
.
5. Controls transmission of hereditary 5 It control the synthesis of proteins.
material. .
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Structure of DNA
Primary structure and its double helix: Sequence in which four nitrogen bases are attached to the
sugar phosphate backbone of a nucleotide chain is called primary structure.

Watson and crick in 1953 proposed that DNA polymers form a duplex structure consisting of two
strands of polynucleotide chain coiled around each other in the form of a double helix. Bases of one
strand of DNA are paired with bases on the other strand by means of hydrogen bonding. According to
Chargaff rule – Thymine and adenine can be joined by 2 hydrogen bonds (T = A) while cytosine and
guanine can be joined by three hydrogen bonds.

Structure of RNA is similar to DNA except that it is a single strand structure.

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Biological functions of nucleic acids

Important functions are

1. Replication
Process by which a single DNA molecule produces two identical copies of itself is called replication.
Replication of DNA is an enzyme catalyzed process. In this process, two strands of DNA helix unwind
and each strand serves as a template or pattern for the synthesis of a new strand. Newly synthesized
complementary strand is an exact copy of the original DNA. In this way hereditary characteristics are
transmitted from one cell to another.

Protein Synthesis

(i) Transcription
It is the process of synthesis of RNA (mRNA) by using
DNA as template. This process is similar to replication
process. Differ in following ways.

In transcription, ribose nucleotide assemble along the

uncoiled template instead of deoxyribose nucleotide and
base uracil (U) is substituted for the base thymine (T).

Synthesis of RNA or DNA always takes place in 5 - 3

direction. Process is catalyzed by an enzyme RNA
polymerase. In this way DNA transfers its genetic code
to mRNA.
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After synthesis, RNA detaches from DNA and moves from nucleus to the cytoplasm where it acts as
template for protein synthesis. DNA returns to its double helix structure.
(ii) Translation
It is the process of synthesis of protein. This process is directed by mRNA in the cytoplasm of cell with
the help of tRNA (transfer RNA) and ribosomal particles (RNA – protein complex).
The process occurs with the attachment of mRNA to ribosome particle mRNA then gives the message
of the DNA and dictates the specific amino acid sequence for the synthesis of protein.
4 bases in mRNA act in the form of triplets and each triplet acts as a code for a particular amino acid.
This triplet is called codon. There may be more than one codon for same amino acid. E.g. amino acid
methionine has code AUG while glycine has 4 codons GGU, GGC, GGA, GGG.
These codon expressed in mRNA is read by tRNA carrying anticodon and is translated into an amino
acid sequence. This process is repeated again and again thus proteins are synthesized. After
completion, it is released from ribosome.

Protein synthesis is a fast process and about 20 amino acids are added in one second. It may be
noted that translation is always unidirectional but transcription can sometimes be reversed. (RNA is
copies into DNA) This is called reverse transcription (occurs in Retroviruses).

Genetic Code
Segment of DNA is called gene and each triplet of nucleotides is called a codon that specifies one
amino acid. This relationship between nucleotide triplets and amino acids is called genetic code. E.g.