1126 JEE Main Chemistry

Note Ketotriose are not considered as a monosaccharide

Classification of Carbohydrates or
because it is optically inactive.
Saccharides However, the naturally occurring monosaccharides are
On the basis of number of simplest molecules (monomers) pentoses (5C-atoms) and hexoses (6C-atoms). Out of
produced upon hydrolysis, carbohydrates are classified as these, the most common are ribose, 2-deoxyribose,
follows glucose (aldoses) and fructose (ketose).
Carbohydrates
H—C O H—C O

H—C—OH H—C—H

Monosaccharides Oligosaccharides Polysaccharides H—C—OH H—C—OH

Cannot be hydrolysed (Oligos-few) Hydrolysed H—C—OH H—C—OH
to give simpler unit of Yield 2.9 to give
polyhydroxy aldehyde monosaccharides on more than
CH2OH CH2OH
or ketone. hydrolysis. 9-monosaccharides.
D-(–)-ribose D-(–)-2-deoxyribose
Crystalline solids, soluble in water and Amorphous solids,
sweet in taste and collectively called insoluble in water, H—C O CH2OH
sugars. tasteless are called
non-sugars. H—C—OH C O
On the basis of reducing activity, carbohydrates can be HO—C—H HO—C—H
classiffied as follows
(i) Reducing sugars These are contain free functional H—C—OH H—C—OH
group and reduce Fehling’s solution and Tollen’s H—C—OH H—C—OH
reagent. These include all the monosaccharides.
(ii) Non-reducing sugars These have bonded CH2OH CH2OH
functional groups and cannot reduce Fehling’s D-(+)-glucose D-(–)-fructose
solution or Tollen’s reagent. These include
oligosaccharides and polysaccharides.
Configuration of Carbohydrates
Monosaccharides : General Features Glyceraldehyde is a simplest saccharide exists in two
These are the simplest carbohydrates which cannot be enantiomeric forms.
hydrolysed to smaller molecules. All these are H—C O H—C O
polyhydroxy compounds containing either an aldehydic D-means —OH
(—CHO) group, i.e. aldoses (the aldehydic group being H—C—OH HO—C—H
on right side
monovalent is present at the end of the chain) or a CH2OH CH2OH
ketonic ( C==O) group, i.e. ketoses (the ketonic group is
D-(+)-glyceraldehyde L-means —OH on left side.
present next to terminal carbon atom).
The simplest monosaccharides are trioses such as These two forms serve as a reference point for
glyceraldehyde (aldose), dihydroxyacetone (ketose) and designating and drawing all other monosaccharides.
aldotetrose such as erythrose and threose. These monosaccharides having the same configuration as
CHO CH 2OH of D-glyceraldehyde at the asymmetric carbon, which
½ ½ is adjacent to — CH 2OH group but the most distant
H — C — OH C == O from carbonyl group (aldehyde or ketone group) are
½ ½ designated as D-form and those having the same
CH 2OH CH 2OH configuration as of L-glyceraldehyde are designated as
D- (+)-glyceraldehyde Dihydroxyacetone L-forms. The natural glucose and fructose exists in
(an aldotriose) (a ketotriose) D-forms.
CHO CHO
Epimers
H—C—OH HO—C—H
A more selective term, epimer is used to designate
H—C—OH H—C—OH diastereomers that differ in configuration at only one
chiral centre.
CH2OH CH2OH
D-(–)-erythrose D-(–)-threose
Bimolecules 1127

Thus, glucose is epimeric with D-(+)-mannose and Some important monosaccharides are as follows
D-(+)-galactose as shown below
CHO CHO CHO 1. Glucose (C6 H12O6 )
Glucose is a monosaccharide, aldohexose and reducing
H—C—OH H—C—OH HO—C—H
sugar. It is found in ripe grapes (hence named
HO—C—H HO—C—H HO—C—H grape-sugar), honey and most sweet fruits. It is also a
normal constituent of blood and found in the urine of
HO—C—H H—C—OH H—C—OH diabetics.
H—C—OH H—C—OH H—C—OH The blood normally contains 65 to 110 mg of glucose per
100 mL (hence named blood sugar). In the combined state,
CH2OH CH2OH CH2OH it occurs in glucosides, disaccharides and polysaccharides.
D-(+)-galactose D-(+)-glucose D-(+)-mannose
Epimers of glucose Methods of Preparation
(i) From sucrose Cane sugar (sucrose) on acid hydrolysis
Ring Structure of Monosaccharides in the presence of alcohol, gives a mixture of glucose
The ring form of monosaccharides is favoured in aqueous and fructose.
solution. Structures of pentoses and hexoses are cyclic H+
involving five or six membered rings containing an C12H 22O11 + H 2O ¾® C6H12O6 +C6H12O6
Sucrose Glucose Fructose
oxygen atom.
Glucose being less soluble in alcohol than fructose
The five membered ring containing an oxygen atom separates out by crystallising on cooling (fractional
because of its similarity with furan is called the furanose crystallisation).
form and the six membered ring containing an oxygen
atom because of its similarity with pyran is called the (ii) From starch Glucose is obtained on a large scale by
pyranose form. the hydrolysis of starch with dilute H 2SO4 or dilute
HCl under pressure.
H+
(C6H10O5 )n + nH 2O ¾¾¾¾¾¾® nC6H12O6
393 K, 2-3 atm
O O Starch Glucose
Furan Pyran After neutralisation with CaCO3 and filteration,
During ring formation, reaction between an aldehyde and filterate is decolourised by boiling with animal charcoal
an alcohol forms a hemiacetal. A hemiketal is an analogous and then concentrated under reduced pressure and
finally crystallised.
product formed by reaction of a ketone with an alcohol.
(iii) From lactose or maltose
Formation of a hemiacetal or a hemiketal,
H+
OR C12H 22O11 + H 2O ¾® C6H12O6 + C6H12O6
H+
C==O + ROH ¾® C Lactose Glucose Galactose
R¢ OH H+
C12H 22O11 + H 2O ¾® 2C6H12O6
(where, R¢ = H or R) Maltose Glucose
If aldehyde (i.e. R¢ = H) ¾® hemiacetal,
Physical Properties of Glucose
If ketone (i.e. R¢ = R ) ¾® hemiketal
l Glucose is a white crystalline solid (m.p. 146°C). It
As a result of ring formation also occurs in the form of monohydrate, C6H12O6 × H 2O
Carbon number 1-(C1) becomes asymmetric (chiral) and (m.p. 86°C).
hence, monosaccharides exist in two stereoisomeric forms, l Glucose is readily soluble in water, sparingly soluble
a and b-form. In the a-form, the —OH at C1 is towards right in alcohol and insoluble in ether.
while in the b-form, the —OH at C1 is towards left.
l It is optically active and dextrorotatory (hence named
H OH HO H
dextrose). It shows mutarotation (The change in
C— —OH group C— specific rotation of an optically active compound in
position
O O solution with time, to an equilibrium value is called
a-position b-position mutarotation)
A pair of stereoisomers which differ in configuration only l It has a very sweet taste but about three fourth as
around C1 carbon are called anomers and the C1 is called sweet as sucrose (cane-sugar).
the anomeric carbon (or glycosidic carbon).
1128 JEE Main Chemistry

H
Chemical Reactions of Glucose
CH N.NHC6H5
(i) Reduction C O
H
(a) On reduction with NaBH 4 or Na-Hg, glucose yields H2N. NH. C6H5 C
CHOH OH
sorbitol. –H2O
(CHOH)3
CHO CH 2OH (CHOH)3
½ NaBH4 or ½ CH2OH
(CHOH)4 + 2[H] ¾¾¾¾¾® (CHOH)4 CH2OH
Na-Hg /H2O Glucose Glucose phenyl hydrazone
½ ½
CH 2OH CH 2OH
Glucose Sorbitol
CH NH CH—NH
(b) On reduction with HI and red P, it gives a mixture NH.C6H5
of n-hexane and 2-iodohexane. C6H5NH. NH2 C O C—O—H
–H2O –C6H5NH2 (CHOH)
Red P/HI (CHOH)3 3
C6H12O6 ¾¾¾¾® CH3CH 2CH 2CH 2CH 2CH3
Glucose D n -hexane
CH2OH CH2OH
+
Imino ketone Hydrogen bonded
CH3CHICH 2CH 2CH 2CH3 intermediate
2-iodohexane CH N.NH.C6H5
CH NH
(ii) Reaction with hydroxylamine Glucose forms glucose
C N.NHC6H5 C6H5NH. NH2 C N.NH.C6H5
oxime with hydroxylamine.
–NH3 (CHOH)3
CHO CH == NOH (CHOH)3
½ NH2OH ½
(CHOH)4 ¾¾¾® (CHOH)4 CH2OH CH2OH
–H2O
½ ½ Glucosazone
CH 2OH CH 2OH (yellow crystalline solid)
Glucose Glucose oxime
Osazone formation is given by only a-hydroxy
(iii) Reaction with hydrogen cyanide An addition product, aldehyde and a-hydroxy ketones. (glucose, fructose
glucose cyanohydrin is formed. and mannose form the same osazone, i.e. glucosazone).
HO CN Glucosazone is a yellow crystalline solid, sparingly
CHO CH soluble in water and has sharp melting point. Due to
these properties, it is used to identify glucose.
(CHOH)4 HCN (CHOH)4 (v) Oxidation
CH2OH CH2OH (a) With bromine water, glucose gives gluconic acid.
Glucose Glucose cyanohydrin CHO COOH
½ Br2-H2O ½
(iv) Reaction with phenyl hydrazine When treated with (CHOH)4 + [O] ¾¾¾¾® (CHOH)4
equimolar quantities of phenyl hydrazine, glucose ½ ½
yields a phenyl hydrazone. CH 2OH CH 2OH
H H Glucose Gluconic acid
(b) Glucose on oxidation with Fehling’s solution and
C O C N.NH.C6H5 Tollen’s reagent gives coloured precipitate and
+ H2N.NH.C6H5 gluconic acid.
(CHOH)4 –H2O (CHOH)
4
CHO COOH
CH2OH CH2OH
½ ½
Glucose Glucose phenyl (CHOH)4 + Cu2+ or Ag+ ¾® (CHOH)4 + Cu2O ¯
hydrazone
½ ½ Red ppt.
However, when glucose is warmed with excess of CH 2OH CH 2OH or
phenyl hydrazine a crystalline product, glucosazone Glucose Gluconic acid
Ag ¯
is formed. Silver mirror
Bimolecules 1129

(c) On oxidation with strong acids like nitric acid, (x) Reaction with metallic hydroxides Glucose reacts
glucose gives glucaric (saccharic) acid. with metallic hydroxides like Ca(OH)2 , Ba(OH)2 ,
COOH Sr(OH)2 etc., to form metallic glucosates, which are
CHO
½ ½ soluble in water.
HNO3
(CHOH)4 + 3[O] ¾¾¾® (CHOH)4 C6H11O5 — OH + H O — Ca — OH ¾®
D
½ ½
CH 2OH COOH C6H11O5 — O — Ca — OH + H 2O
Glucose Glucaric acid
(saccharic acid) Calcium glucosate
(xi) Reaction with periodic acid Periodic acid splits
(vi) Reaction with acetyl chloride Glucose reacts with
glucose into formic acid and formaldehyde.
acetyl chloride to form glucose penta-acetate.
CHO CHO CHO
½ ZnCl 2 ½ ½ HIO 4 or H5 IO 6
(CHOH)4 + 5CH3COCl ¾¾® (CHOCOH3 )4 + 5HCl (CHOH)4 ¾¾¾¾¾¾® 5HCOOH + HCHO
Formic acid
½ ½ ½ Formaldehyde

CH 2OH CH 2OCOCH3 CH 2OH

Glucose Glucose Glucose
pentaacetate
(xii) Fermentation Glucose when fermented by zymase,
(vii) Reaction with PCl5 Glucose reacts with PCl5 to yields ethanol.
form penta-chloroglucose.
CHO CHO Zymase
C6H12O6 ¾¾¾® 2C2H5OH + 2CO2 ­
½ ½ Glucose Ethanol
(CHOH)4 + 5PCl5 ¾® (CHCl)4 + 5POCl3
½ ½ + 5HCl (xiii) Dehydration On strong heating or on treating with
CH 2OH CH 2Cl warm conc. H2 SO4 , glucose is dehydrated to give a
Glucose Penta-chloroglucose black mass (sugar carbon) or black carbon.
(glucose pentachloride)
Conc. H2SO 4
(viii) Reaction with methanol D-(+)-glucose treated C6H12O6 ¾¾¾¾¾® 6C + 6H 2O
with methanol in presence of dry HCl gas, reacts Glucose Black carbon
with its one mole only and yields monomethyl ether (xiv) Reaction with alkalies When warmed with conc.
which is actually a mixture of a- and b- forms alkali, glucose first turns yellow, then brown and
indicating that one of the —OH group is different finally gives a resinous mass.
from others.
Dry HCl A dilute solution of glucose when warmed with
C6H11O5 — OH + H OCH3 ¾¾¾® C6H11O5OCH3 + H 2O dilute alkali is converted into an optically inactive
a- and b-methyl
glucoside solution of D-(+)-glucose, D-(+)-mannose and
D-(–)-fructose. This is known as Lobry-de-Bruyn-van
H OCH3 CH3O H
Ekenstein rearrangement,
C C
H O
(CHOH)3 O (CHOH)3 O C
CH CH HO—C—H
H O H OH
CH2OH CH2OH (CHOH)3
C C
a-methyl glucoside b-methyl glucoside
H—C—OH – C—OH CH2OH
(ix) Reaction with conc. HCl On treatment with conc. OH
D-(+)-mannose
HCl, glucose forms hydroxyl methyl furfural which (CHOH)3 (CHOH)3
further produces laevulic acid. H
CH2OH CH2OH
CH—–CH H—C—OH
+ 3H2O D-(+)-glucose Enediol
Conc. HCl
C6H12O6 C C CHO C O
OHC O
Hydroxyl methyl furfural (CHOH)3

CH2OH
CH3COCH2CH2COOH + H2O
D-(–)-fructose
Laevulic acid
1130 JEE Main Chemistry

Structure of Glucose l Even though an aldehyde group is present, the glucose

does not react with NaHSO3 , NH3 , 2,4-DNP and
l The molecular formula of glucose is found to be Grignard reagent.
C6H12O6. l Glucose does not respond to Schiff’s reagent test.
l Formation of 2-iodohexane and n-hexane (Reaction l Glucose penta-acetate does not react with
(i)-b) indicates that six carbon atoms in glucose are
hydroxylamine. It indicates the absence of free
present in a straight chain. ¾ CHO group in glucose.
l Formation of glucose penta acetate Reaction-(vi) l Glucose exists in two stereoisomeric forms, i.e. a and b
indicates the presence of five ¾ OH groups. Since, it anomers.
exists as a stable compound, five ¾ OH groups l a-glucose (m.p. 146°C) with specific rotation +111° is
should be attached to different carbon atoms. obtained by crystallising glucose from alcohol or acetic
l Formation of glucose oxime and glucose cyanohydrin acid solution whereas b-glucose (m.p. 150°C) with
(reaction (iii) and (iv)) respectively confirms the specific rotation 19.2° is obtained by crystallising
presence of carbonyl group in glucose. glucose from pyridine solution.
l Formation of gluconic acid in presence of mild
l An aqueous solution of glucose shows mutarotation,
oxidising agent like Br2/H 2O (Reaction (ii)-a) indicates i.e. if either of the two forms is dissolved in water and
that glucose contains an aldehydic group. Since allowed to stand, the specific rotation of the solution
aldehydic group is monovalent, it must be present on changes gradually until a final value of +52.5° is
the end of the chain. reached. It means that the specific rotation gradually
decreases from +111° to +52.5° in case of a-glucose
On the basis of point discussed above, it was suggested and increases from +19.2° to +52.5° in case of
that glucose has an open chain structure. b-glucose. This phenomena is known as mutarotation.
The open chain structure, for the first time, was l Cyclic structure of glucose This behaviour could not be
proposed by Baeyer. It contains one ¾ CHO group, explained by the open chain structure for glucose. It
one 1° alcoholic group and four 2° alcoholic groups as, was proposed that one of the ¾ OH groups may add to
OH OH OH OH OH H the ¾ CHO group to form a cyclic hemiacetal
structure.
H—C—C*—C*—C*—C* C O
According to Fischer, glucose forms a stable cyclic
H H H H H Aldehyde group hemiacetal between ¾ CHO group and the ¾ OH
group of the fifth carbon atom in pyranose structure.
One 1°
alcoholic Four 2° alcoholic H OH H O HO H
group groups 1C 1C 1C
(C* = asymmetric centres)
Chiral centers present in glucose molecule 2 2 2
CHOH CHOH CHOH
There are four dissimilar chiral carbon atoms in the 3 3 3
CHOH O CHOH CHOH O
molecule, but a definite configuration to these
asymmetric centres has not been assigned. 4 4 4
CHOH CHOH CHOH
The configuration of D-glucose was proved by 5 5 5
CH CHOH CH
Emil Fischer.
1 6 6 6
H O CH2OH CH2OH CH2OH
C
2
a-D-(+)-glucose open-chain b-D-(+)-glucose
H—C—OH [a]D = 111° [a]D = 52.5° [a]D = 19.2°
3 (36%) (0.02%) (64%)
HO—C—H
4
Fischer projections are not the best way to show the
H—C—OH structure of a glucose. The cyclic structure of glucose is
5 more correctly represented by Haworth projection formula.
H—C—OH 6 6 6
6
CH2OH CH2OH CH2OH
CH2OH H
5 O OH H
5 OH H
5 OH
O
D-(+)-glucose 4 H 4 H 4 H
OH H 1 OH H OH H 1
HO3 H HO3 1 H HO3 OH
2 2 2
Evidence Against Open Chain Structure H OH H OH H OH
However, there are some evidences which do not support b-D-(+)-glucose D-(+)-glucose (open chain) a-D-(+)-glucose
the open chain structure of glucose. These are (64%) (0.02%) (36%)
Haworth projection of glucose
Bimolecules 1131

The two cyclic hemiacetal forms of glucose differ only in Chemical Properties
the configuration of the hydroxyl group at C-1 called
The chemical reactions of fructose are almost similar to
anomeric carbon. Such isomers, i.e. a-form and b-form
are called anomers. those of glucose. The only different behaviour are
oxidation and reduction. These are as follows
Sametimes, glucose is illustrated as a chair form
because it is a more accurate representation of (i) Reduction On reduction with NaBH 4 or Na-Hg/H 2O,
bond-angles of the molecule, (Reeves, 1950). fructose forms a mixture of sorbitol and mannitol.
H H CH2OH CH2OH CH2OH
6 6
HO—
4
CH2OH HO—
4
CH2OH C O H—C—OH HO—C—H
5 O O + 2[H] +
H 5 2
H
1 1 (CHOH)3 (CHOH)3 (CHOH)3
H 2 —H H —OH
3 3
HO— HO HO— HO CH2OH CH2OH CH2OH
OH H Sorbitol Mannitol
Glucose
H H
a-anomer b-anomer (stable) Note The reduction of glucose with NaBH4 forms D-sorbitol
Anomer forms of glucose while that of fructose forms a mixture of D-sorbitol and
In b-anomer, the glycosidic hydroxyl group is equatorial D-mannitol (they differ in configuration).
and as a general rule, the configuration with greatest (ii) Oxidation Fructose is not oxidised by mild oxidising
number of large groups in equatorial orientation is the agent like Br2 water. However, when oxidised with
most stable form. nitric acid, fructose is converted into a mixture of
Thus, b-anomer predominates in the equilibrium mixture. trihydroxy glutaric, glycollic and tartaric acids.
The boat form of glucose is unstable. COOH
Oxidative
cleavage
2. Fructose (CHOH)2 + CO2 + H2O
CH2OH at C1-C2
The important ketohexose is D-(–)-fructose (also known as C O CH2OH
laevulose). In the free state, it is present along with HNO3 Trihydroxy
honey and most sweet fruits (berries, melons etc.), hence (CHOH)3 glutaric acid
Oxidative COOH
named as fruit sugar. CH2OH
CH2OH cleavage
In the combined state, fructose is present in sucrose and + (CHOH)2 + H2O
Glucose at C2-C3 COOH
insulin. It is the sweetest monosaccharide. Glycollic acid CH2OH
Some methods of preparation of fructose are Tartaric acid
(i) By hydrolysis of cane sugar with dilute acids. Since a mixture of acids each containing fewer
H+ , D carbon atoms than fructose is obtained, the carbonyl
C12H22O11 + H2O ¾¾® C6 H12O6 + C6 H12O6
Cane sugar
group in fructose must be a ketonic group.
D-(+)- glucose D-(–)-fructose
The solution having equal molecules of D-glucose and (iii) Reducing nature Unlike ketones, fructose can reduce
D-fructose is termed as invert sugar and the process is Fehling’s solution and Tollen’s reagent. This is
known as inversion. probably due to formation of an equilibrium mixture
(ii) By calcium fructosate of glucose, mannose and fructose in alkaline solution.
C6 H11 O5 —O—Ca—OH + CO2 ¾® C6 H12O6 + CaCO3 (iv) Action of conc. HCl
Fructose HCl
(iii) From insulin C6H12O6 ¾¾® H3C × CO × CH 2 × CH 2 × COOH + HCOOH
HCl Glucose Laevulinic acid Formic acid
(C6 H10 O5 )n + nH2O ¾¾® nC6 H12O6
+ H 2O
(Insulin is a polysaccharide which occurs in dahlia Structure of Fructose
tubers and Jerusalem artichokes).
Some major structure of fructose are discussed below
Physical Properties 1. Open Chain Structure
l Fructose is colourless crystalline compound
The open chain structure of fructose may be represented
(m.p. 102°).
as below
l It is soluble in water and insoluble in benzene and * * *
ether. CH 2 — C— CH — CH — CH — CH 2
½ ½½ ½ ½ ½ ½
l It is less soluble in water than glucose. Like glucose, O OH OH OH OH
OH
it also shows mutarotation.
1132 JEE Main Chemistry

The structure contains three asymmetric carbon atoms Oligosaccharides : General Features
and eight optically active forms but only six are
known. The configuration of D-(–)-fructose is Molecules of these carbohydrates are made up of a small
1 number of monosaccharide units joined together by
CH2OH
æ ½ ½ ö
2
C O ç ÷
glycosidic bonds ç — C— O — C— ÷ . Glycosidic bonds are
3 ç ½ ½ ÷ø
HO—C—H è
4 established during the condensation of monosaccharides.
H—C—OH The process is called dehydration.
5
H—C—OH These are further classified into disaccharides,
6 trisaccharides, tetrasaccharides etc., depending upon the
CH2OH number of monosacharides, they peroxide on hydrolysis.
D-(–)-fructose
Some important oligosaccharides are as follows
2. Cyclic Structure
Fructose also has a cyclic structure in aqueous
1. Sucrose (C12 H 22O11 )
solution, i.e. intramolecular hemiketal. It also exhibit It is commercial sugar, which is also called cane sugar
mutarotation. because most of it is obtained from sugarcane (16-20%). It
1 is also present in sugarbeet (10-15%), pineapple (10-12%),
CH2OH 1 apricot banana, mango and honey. It is formed by
1
HOH2C 2 OH 2 HO 2 CH2OH condensation of one molecule each of glucose and fructose.
C C O C
6 6
3 3 3 CH2OH CH2OH
HO—C—H HO—C—H HO—C—H 5 5
H O H H O H
4 O 4 4 O H H
H—C—OH H—C—OH H—C—OH 4 1 4 1
OH H OH H
5 5 5 HO 3 OH HO
H—C—OH H—C—OH H—C—OH 2 3 2
6
CH2
6
CH2OH 6 OH OH Glucosidic H HO
CH2 a-D-(–)-gluco linkage
a-D-(–)-fructose a-D-(–)-fructose b-D-(–)-fructose pyranose
[a]D = –21° [a]D = –92° [a]D = –133° + O
–H2O
Cyclic structure fructose 6 O 6 O
HOH2C OH HOH2C
3. The Haworth Projection Formulae 5
H HO
2 5
H HO
2

The cyclic structure of D-(–)-fructose is represented as, H 4 CH2OH H 4 CH2OH

3 1 3 1
H H OH H OH H
6 1 6
H O CH H O OH b-D-(–)-fructofuranose Sucrose
2OH
5 H 2 5 H 2 Glycosidic linkage representation in sucrose molecules
H HO H HO
HO OH HO CH2OH
4 3 4 3 1 Properties
OH H OH H
a-D-(–)-fructopyranose b-D-(–)-fructopyranose
l It is a colourless, odourless, crystalline substance
having m.p. 180°C. It is very soluble in water but
However, in the combined state (such as in sucrose) insoluble in alcohol and ether. It is dextro-rotatory with
fructose exists in the furanose form. a specific rotation + 66.5°.
6 O 1 6 O l On hydrolysis with dilute acids or invertase or sucrose,
HOH2C CH2OH HOH2C OH
it gives an equimolar mixture of D-(+)-glucose and
5 2 5 2
H HO H HO D-(–)-fructose.
H 4 OH H 4 CH2OH
3 3 1 Acid or
C12H 22O11 + H 2O ¾¾¾® C6H12O6 + C6H12O6
OH H OH H enzyme
(+)-sucrose D-(+)-glucose D-(–)-fructose
a-D-(–) fructofuranose b-D-(–) fructofuranose [ a ]D = +66.5° [ a ]D = + 52.7° [ a ]D = –92.4°
In solution, fructose exists as an equilibrium mixture Since, D-(–)-fructose has a greater specific rotation
of 70% fructopyranose and about 22% fructofuranose than D-(+)-glucose, the resulting mixture is
as well as small amounts of the three other forms, laevorotatory. Because of this, the hydrolysis of cane sugar
including the acylic structure (i.e. D-(+)-glucose, is known as the inversion of cane sugar and the mixture is
D-(+)-mannose and D-(–)-fructose) known as invert sugar.
Bimolecules 1133

Due to the presence of fructose, invert sugar is sweeter 3. Maltose (C12 H 22O11 )
than sucrose. Invert sugar is used to coat chocolate.
It is obtained by partial hydrolysis of starch by diastase
Sweetening power of common sugars : Fructose > enzyme present in malt, i.e. sprouted barley seeds
Invert sugar > Sucrose > Glucose > Maltose > Lactose (hence named maltose or malt sugar).
l On heating (200°C), sucrose melts and on cooling Diastase
forms a glassy yellow solid known as barley sugar. 2(C6H10O5 )n + nH 2O ¾¾¾® nC12H 22O11
When heated above its melting point, it loses water Maltose
and gives a brown substance known as caramel. On
further heating sucrose gets charred to almost pure
Properties
carbon (sugar charcoal). l Maltose is a white crystalline solid (with m.p.
l Sucrose on acetylation gives sucrose octaacetate. This 160°-165°C), soluble in water and dextrorotatory.
shows the presence of eight —OH groups in sucrose. 6 6
CH2OH CH2OH
l Sucrose does not form oxime or osazone. It shows the 5 5
H O H H O H
absence of C ==O group.
4
H 1 4
H 1
l Sucrose does not reduces Tollen’s reagent and OH H OH H
Fehling’s solution. It shows the absence of —CHO HO OH HO OH
3 2 3 2
group in sucrose. H OH H OH
l Fermentation In the presence of yeast, it yields a-D-(+)-glucopyranose a-D-(+)-glucopyranose
ethanol and carbon dioxide. a-glycosidic –H2O
Invertase C H O + C H O linkage
C12H22O11 + H2O ¾¾¾¾® 6 12 6 6 12 6 6 6
Glucose Gructose CH2OH CH2OH
Zymase
C6 H12O6 ¾¾¾® 2C2H5 OH + CO2 5 O H 5 O H
Glucose Ethanol H H
4
H 1 4
H 1
l With conc. H 2SO4 , sucrose loses water to give sugar OH H OH H
HO O OH
charcoal (charring of sugar). 3 3
2 2
C12H22O11 + H2SO4 ¾® 12C + 11H2O H OH H OH
C + 2H2SO4 ¾® 2SO2 + CO2 + 2H2O Representation of a-1,4-glycosidic linkage
A small amount of sulphurdioxide is also observed l Maltose is a reducing sugar. It reduces Fehling’s
due to reduction of the acid. solution Tollen’s reagent, it forms an oxime and an
osazone and undergoes mutarotation. It indicates that
2. Lactose (C12 H 22O11 ) at least one aldehyde group is free in it.
It occurs in the milk of all animals (milk-sugar). It is a (Free aldehyde group can be produced at C-1 of second
white crystalline solid (with m.p. 203°C), soluble in water glucose in solution which shows reducing property).
and is dextrorotatory. It is hydrolysed by dilute acid or
enzyme lactose, to an equimolar mixture of D-(+)-glucose Example 1. What are the functional groups present in the
and D-(+)-galactose. Lactose is a reducing sugar, forms structure of maltose ? (JEE Main 2020)
an oxime and osazone and undergoes mutarotation. It (a) One ketal and one hemiketal
gets hydrolysed by emulsin also, an enzyme which (b) One acetal and one ketal
specifically hydrolyses b-1,4-glycosidic linkage. (c) One acetal and one hemiacetal
6 (d) Two acetals
CH2OH H OH
5 3 2 Sol. (c) The functional groups present in the structure of maltose is
HO O OH HO OH one acetal and one hemiacetal. It is illustrated in following structure.
H OH H
4 1 + 4 1 CH2OH Acetal CH2OH
OH H H
H
3 2
H H
5 OH O H O
H H H
H OH CH2OH H H
6
b-D-(+)-galactose
b-D-(+)-glucose OH H OH H
b-glycosidic linkage –H2O HO O OH
CH2OH H OH
O H OH H OH Hemiacetal
HO O OH
H OH Maltose
H
OH H H
H H H
O H
Polysaccharides : General Features
It composed of large number of monosaccharides units
H OH CH2OH
(+)-lactose joined together by glycosidic linkages. These are used as
Representation of b-1,4-glycosidic linkage
food storage or structural material.
1134 JEE Main Chemistry

Some important polysaccharide are as follows branched chain polymer of a-D-glucose units in which chain is
formed by C1-C4 glycosidic linkage whereas branching occurs by
1. Starch C1-C6 glycosidic linkage.
It is a polymer of a-glucose. It is found in cereals, 6
roots, tubers etc. It is the most important dietary CH2OH CH2OH CH2OH
source. It consists of two cmponents, i.e. amylose H O H H O H H O H
5
and amylopectin. Amylose is water soluble H H H
4 1 4 1 4 1
component which constitutes about 15-20% of – OH H OH H OH H –
O O O O
starch. It is a long unbranched chain with 3 2
200-10000 a-D-( + )-glucose units held by C1-C4 H OH H OH H OH
glycosidic linkage. a-link a-link

Whereas amylopectin is insoluble in water and Amylose

constitutes about 80-85% of starch. It is a
CH2OH CH2OH
H O H H O H

4 H 1 4
H 1
– OH H OH H a-link
O O

H OH H OH
O Branch at C6

6
CH2OH CH2 CH2OH

H O H H 5 O H H O H
4 H 1 4
H 1 4 H 1
– OH H OH H OH H –
O O O O

H OH H OH H OH
a-link a-link

Amylopectin
Representation of glycosidic linkage in amylopectin and amylose
2. Cellulose 3. Glycogen
It occur exclusively in plants. It is predominant
constitutent of cell wall of plant cells. It is a It is known as animal starch because its structure is similar to
straight chain polysaccharide composed of amylopectin and is more highly branched. It is stored in animal
b-D-glucose units joined by glycosidic linkage body and present in liver, muscles and brain. In need of glucose,
between C1 of the glucose and C4 of next glycose. enzymes break the glycogen down to glucose. It is also found in
yeast and fungi.
HOH2C

O
O
B. a-Amino Acids
Bifunctional organic compounds containing carboxylic and
OH amino group either at the same carbon atom or at the nearby
HOH2C O carbon atoms are called amino acids. These are the monomers
of proteins. Usually, amino acids have primary amino group but
O
OH proline is a secondary amine.
Natural proteins can be broken down into about 20 different
OH a-amino acids (19 a-amino acids and 1 a-imino acid). These
HOH2C O molecules differ in the nature of the R-group attached to the
O alpha carbon. R-group can be
OH
l An aliphatic side chain
OH b-links l A hydroxyl group containing side chain
O l A sulphur atom containing side chain
l A side chain containing acidic (carboxylic) group or amides
OH group
Cellulose l A side chain containing basic groups
Bimolecules 1135
l A side chain containing aromatic ring
Amino acids, their Symbols and Structures
Group I Side Chains with Aliphatic Carbon Chains

H3N CH COO– H3N CH COO– H3N CH COO– H3 N CH COO– H3N CH COO–

H CH3 CH CH2 CH

H3 C CH3 CH CH2 CH3

H3C CH3 CH3
Glycine (Gly) Alanine (Ala) Valine (Val) Leucine (Leu) Isoleucine (Ile)
Group II-Side Chains With —OH Groups Group III-Side Chains With S Atoms
– –
H3N CH COO H3 N CH COO H3 N CH COO– H3 N CH COO–

CH2 CH CH2 CH2

OH OH CH3 SH CH2

S CH2
Serine (Ser) Threonine (Thr) Cysteine (Cys) Methionine (Met)
Group IV-Side Chains With Acid Groups or their Amides

H3N CH COO– H3 N CH COO– H3N CH COO– H3 N CH COO–

CH2 CH2 CH2 CH2

C C CH2 CH2
O O– O NH2 C C
O O– O NH2

Aspartate (Asp) Asparagine (Asn) Glutamate (Glu) Glutamine (Gln)

Group V-Side Chains Contain Basic Groups

H3N CH COO– H3 N CH COO– H3 N CH COO– H3 N CH COO–

CH2 CH2 CH2 CH2
CH2 CH2 CH2 C CH
CH2 CH2 CH OH HN NH
NH H3 N CH2 H3N CH2 C
H2 N C NH2 H

Arginine (Arg) Lysine (Lys) Hydroxylysine (Hyl) Histidine (His)

Group VI-Side Chains Contain Aromatic Rings

H3 N CH COO– H3N CH COO– H3 N CH COO–

CH2 CH2 CH2

CH
OH N
H
Phenylalanine (Phe) Tyrosine (Tyr) Tryptophan (Trp)
Group VII-Imino Acids
H H
–
H2N C COO H2N C COO–

H2 C CH2 H2C CH2

CH2 C

OH OH
Proline (Pro) 4-hydroxyproline (Hyp)
1136 JEE Main Chemistry

Stereochemistry of a-Amino Acids Example 2. Which of the following is not an essential

amino acid? (JEE Main 2020)
With exception of glycine, all common naturally occurring
a-amino acids have asymmetric a-carbon atom. The (a) Leucine (b) Valine (c) Lysine (d) Tyrosine
chiral amino acids found within naturally occurring Sol. (d) Tyrosine is not an essential amino acid and remaining all
proteins have only one enantiomeric form, which has the are essential amino acid.
following configuration.
Amino acids which are synthesised by the body, are called
COO– non-essential. On the otherhand, those amino acids which cannot
+
H3 N H be synthesised in the human body and are supplied in the form of
diet are called essential amino acids the ten essential amino acids.
R
Valine, leucine, isoleucine, arginine lysine, threonine,
This configuration is S in all cases except for cysteine. methionine, phenylalanine, trpropham, histidine are essential
amino acid.
D and L-Configuration for a-amino Acids
It refers to the configuration of the a-carbon regardless of Example 3. Thiol group is present in (JEE Main 2016)
the number of asymmetric carbons in the molecule. (a) cystine (b) cysteine (c) methionine (d) cytosine
D-amino acid has an amino group on right and hydrogen NH2
on left when —COOH group is up and side chain is at Sol. (b)
Cystine HO S COOH
lower position in a Fischer projection of the a-carbon C S
whereas a L-amino acid has amino group on left.
O NH2
Naturally occurring amino acids have L-configuration. COOH
Cysteine
COO– COO– HS
+ + NH2
H3N H H NH3
O
CH2OH CH2OH
S C
L-serine or (S) serine D-serine or (R) serine Methionine
H3C OH
NH2
Classification
NH2
I. On the basis of relative number of amino and carboxyl
groups present, amino acids are classified as follows N
Cytosine
l Acidic amino acids These are contain more number
O N
of carboxylic groups than amino groups, e.g. asp, glu H
etc.
l Basic amino acids These are contain more number of Zwitter Ion Structure
amino groups than carboxyl groups, e.g. lys, arg etc. l Amino acids contain the —COOH group, which is
l Neutral amino acids These are contain equal acidic and the —NH 2 group which is basic. In the
number of amino group and carboxyl group, e.g. gly, solid state, an amino acid ordinarily exists as a
ala etc. Zwitter ion or as a dipole ion, formed by the transfer
II. On the basis of requirement in human diet, amino of a proton from a —COOH group to an —NH 2 group.
acids are classified into following types R R
Human body can synthesis 10 out of 20 amino acids ½ ½
H 2N — C— COOH ¾® H3N + — C— COO-
l

found in proteins, these are called non-essential or

½ ½
dispensable amino acids while the remaining ten, H H
which the human body cannot synthesis, are called Amino acid Zwitter ion form
essential or indispensable amino acids. l Modern researches have proved that the acidic
l These essential amino acids are required for the properties of amino acids are due to —NH3+ group
growth of the body and their deficiency causes (which can donate a proton) and basic properties are
disease like kwashiorkor. O
l The essential amino acids are : Arginine, Valine, ½½
Methionine, Leucine, Threonine, Phenylalanine, due to the — C— O– group (which can accept a
Histidine, Isolucine, Lysine and Tryptophan. proton) in a Zwitter ion or ‘inner salt’.
Bimolecules 1137

l A Zwitter ion behaves like a polar molecule. Within 2.34 + 9.69

The isoelectric point, pI = = 6.02
the molecule, there is a positive charge at the 2
nitrogen atom of the amino group and a negative The pI of an amino acid that has an ionisable side chain
charge at the oxygen atom of the carboxyl group. is the average of the pK a values of the similarly ionising
Overall, the Zwitter ion has no net charge. group.

Isoelectric Point C. Proteins

In aqueous solution, Zwitter ions are stable only over a The term protein is derived from the Greek word
certain pH range. At high H+ ion concentration (low pH), ‘proteios’ means ‘first rank’ because the proteins are
the —COO- group picks up a proton and forms a cation ranked first amongst the natural polymer essential for
(due to the presence of NH3+ group) with a positive the growth and maintenance of life. These are the
charge. At low H+ ion concentration (high pH), the NH3+ polymers of amino acids (specifically a-amino acids) and
group loses a proton and forms an anion (due to the make up to 15% by mass of our body. The chief sources of
presence of —COO- group) with a negative charge. proteins are pulses, cheese, milk, egg, peanut, fish, meat
etc. The protein content of these sources are
H+
NH3+ — CH 2COOH ¬¾¾¾ NH3+ — CH 2 — COO– Source Protein percentage
Cation Less pH Zwitter ion
Soyabean (pulse) - 54%
¾¾¾® NH 2 — CH 2 — COO–
More pH Meat - 22%
Anion
Note The amino acid can never exist as an uncharged Egg - 14%
compound at any pH. In fairly acidic medium, it exists as Wheat - 12%
cation while in fairly basic medium it exists as an anion. Milk - 4%
At physiological pH (» 7.3 ), an amino acid exists as a Examples of Some Proteins and their Biological Functions
dipolar ion called Zwitter ion or inner salt.
The point (pH range) at which the amino acid molecule Class of proteins Example Function
has equal positive and negative charges is called the Structural proteins Collagen Connective tissue
isoelectric point. At this point, the amino acids do not Enzymes DNA Replicate and repair DNA
migrate in an electric field. All amino acids do not have polymerase

the same isoelectric point. Neutral amino acids have Transport protein Haemoglobin Transportation of respiratory
gases (O2 and CO2 )
isoelectric point from pH 5.5 to 6.3 (e.g. glycine = 6.1).
Contractile protein Active myosin Responsible for contraction of
Acidic amino acids have isoelectric point around 3 muscles
(e.g. aspartic acid = 3) and basic amino acids have Protective proteins Antibodies Complex with foreign proteins
isoelectric points from pH 7.6 to 10.8 (e.g. Lysine = 9.7) Hormones Insulin Regulate glucose metabolism
Amino acids usually shows their lowest solubility in a Toxins Snake venom Incapcitate prey
solution at the isoelectric point. Since, there is a highest
concentration of dipolar ion. This property has been used Proteins are important components of most foods. In the
in the separation of different amino acids obtained from digestive system, proteins are broken down into small
the hydrolysis of proteins. molecules called a-amino acids. These molecules are
reassembled in cells to form other proteins required by
a-amino acids have protonated —NH3+ group which
the body.
exerts a strong electron withdrawing inductive effect
(-I effect) and therefore, increases acid strength. That’s
why the carboxylic acid groups of the amino acids are so
Features of Proteins
much more acidic (pK a = 2) than a carboxylic acid such as
l A pure protein is tasteless, odourless and colourless.
acetic acid (pK a = 4.76 ). If an amino acid has amino group Chromoproteins are coloured. Most of proteins are
and one carboxyl group such as alanine, it has two pK a hydrophilic. They do not have sharp melting point.
values. l All proteins on partial hydrolysis give peptide of
The isoelectric point of this amino acid has the average varying molecular masses, which on complete
value of the both pK a values. hydrolysis gives a-amino acids.
Hydrolysis Hydrolysis
H3C—CH—COOH ¬ pKa = 2.34 Proteins ¾¾¾® Peptides ¾¾¾¾® a-amino-acids
+ l All the proteins are laevorotatory due to the presence
NH3 pKa = 9.69 of asymmetric carbon in a-amino acids.
1138 JEE Main Chemistry

Peptide Bond and Protein Structure Structure of Proteins

Amino acids may be joined together by an amide linkage On the basis of different configuration or conformations,
called peptide linkage (—CO—NH—). A water molecule there are mainly four types of structure of protein.
is always eliminated in forming a peptide linkage. These are as follows
The parts of amino acids in a peptide (after liberation of
water molecules) are called amino acid residues. 1. Primary Structure
The number and sequence of amino acids in the
R R
½ ½ polypeptide chain constitute the primary structure. It
H 2N — C — CO OH + H —N — C — COOH ¾® shows how the atoms in protein molecule are joined to
½ ½ ½ one another through covalent bonds to form chains.
H H H Peptide bond Fredrick Sanger, in first time determine the
sequence of amino acid sequence of a protein (i.e. insulin)
and for this work he was awarded Nobel prize (in 1958).
R O H R The nature and the sequence of the amino acids determine
½ ½½ ½ ½ the three dimensional structure and properties of proteins.
H 2N — C — C—N — C — COOH
½ ½ Determination of sequence of amino acids in a
H H peptide chain. It can be determined either by analysing
Dipeptide molecule the products of partial hydrolysis or by end group analysis.
l Peptide are amides formed by the condensation of (i) Partial hydrolysis In partial hydrolysis method,
amino group of one a-amino acid with the carboxyl dilute acids or enzymes are used to break the
group of another molecule through peptide linkage, polypeptide chain into small fragments. By knowing
with the elimination of a molecule of water. the structures of these small fragments, the sequence
of amino acids in a polypeptide chain is determine.
l The molecule derived from two amino acids
For example, partial hydrolysis of a tetra peptide
containing a single peptide linkage is called a
containing Ala, Gly, Phe and Val yields a tripeptide
dipeptide, that derived from three amino acids is Gly–Phe–Val and a dipeptide Ala–Gly.
termed as a tripeptide. The peptides having 2-10
Hydrolysis
amino acid residues are called oligopeptides while Tetrapeptide ¾¾¾¾® Gly–Phe–Val + Ala–Gly
those with greater than 10 amino acid residues are
Since, dipeptide shows that Ala is linked to Gly, the
called polypeptides. amino acids in the tetrapeptide are linked in the
l Polypeptide with molecular weight greater than following sequence.
10,000 u is termed as a protein. Proteins generally Ala—Gly—Phe—Val
have more than 70 amino acid residues, but 1. 2.
a polypeptide with fewer a-amino acids may also
Cleavage at 1. gives tripeptide and at 2. gives dipeptide.
called a protein if it has a well defined conformation
characteristic of a protein such as insulin (contains (ii) Terminal residue analysis or end group
51 amino-acids). analysis Peptide structures are written in such a
way that the amino group is at the left and carboxy
l Polypeptides are amphoteric in nature because of the group is at the right. Hence, the amino end is called
presence of terminal ammonium and carboxylate ions the N-terminal and carboxy end is called
as well as the ionised side chains of amino acid C-terminal. Amino groups of all amino acids except
residues. Therefore, like a-amino acids, they N-terminal amino acid and carboxy group of all amino
neutralise both acids as well as bases and acids except C-terminal amino acid are involved in
possess isoelectric point. At isoelectric points, amide bond formation.
polypeptides have least solubility and hence can be In other words, amino group is free at the N-terminal
separated. end and carboxy group is free at the C-terminal end.
O O
Left hand side Right hand side
Composition of Proteins
H2N—CH—C—NH—CH—C—NH---CH—COOH
An approximate composition of proteins is as follows
Carbon - 50 - 53% Hydrogen - 6 - 7% R R¢ R¢¢ C-terminal end
Oxygen - 23 - 25% Sulphur - 1% N-terminal end

Nitrogen - 16 - 17% Hydrogen - 6 - 7% The primary structure of proteins dissolved in water

is not disrupted by heating above 80°C. The difference
Other elements such as phosphorus in nucleoproteins, in chemical and biological properties of various
iron in haemoglobin and iodine in thyroid are also present. proteins and peptides arise due to the difference in
their primary structure. e.g. In haemoglobin (blood
Bimolecules 1139

protein) which carries oxygen, there are 574 amino l Although, the hydrogen bond is fairly weak, their
acid units in a definite sequence but the replacement large number stabilises the structure. The imino
of only one a-amino acid results in defective acid, proline along with amino acids glycine,
haemoglobin. This is the cause of a disease, called serine do not fit into the normal a-helix. They
sickle cell anaemia. disrupt the a-helical structure and cause sharp
In the patients suffering from this disease, the bends in the direction of the chain. They are called
defective haemoglobin precipitates causing the cells to helix-breakers. a-helix is found in both fibrous and
sickle shaped and sometimes even makes them burst globular proteins. Fibrous proteins such as a-keratin
leading ultimately to death. Normal haemoglobin in hair, nail, wool, skin, beaks, claws and myosin in
has–Val–His–Leu–Thr–Pro–Glu–Lys. muscle have a-helix structure.
On the other hand, sickle cell haemoglobin structure. l Globular proteins also contain segments of a-helix,
because of the a-helical structure, human hair fibres
Val–his–Leu–Thr–Pro–Val–Glu–Lys structure.
are stretchable and elastic to some extent. When hair
is stretched, the H-bonds are broken but when the
2. Secondary Structure stretching force is removed, the H-bonds reformed
Most of the long polypeptide chain are folded or coiled to again.
produce specific three-dimensional structures. These are
(ii) b-conformation (b-pleated sheet) it results from
called secondary structure and give idea about shape hydrogen bonding between two peptide chains. In this
of the conformation of the protein molecule. Depending conformation, the polypeptide chains lie side by side
upon the size of R groups, three major types of secondary in a zig-zag manner with alternate —R groups on the
structure (a-helix, b-conformation, b-pleated sheet and same side situated at fixed distances apart. The
triple-helix) have been identified. chains may be parallel or anti-parallel.
H Hydrogen bond In a parallel chain b-pleated sheet, the N-atoms point
H in the same direction, while in the antiparallel chain
H2 N C C b-pleated sheet, alternate chains are oriented in the
N
R1 O same direction. The anti-parallel structure permits
H H H H C R2 maximum hydrogen bonding. The b-conformation is
C C N
found in fibrous proteins.
N
R4
l Keratin protein (present in hair) has parallel
H O
C C b-pleated sheet structure while the silk protein,
C H
C N fibroin has anti-parallel b-pleated sheet structure.
O O R5
H R3 H H
H C CCH CCH
C C N O
C Hydrogen
H R7 O H R8 bond
H C N
N N
C C
R6 N C
O
a-helix
R groups
a-helix structure of proteins HCC CCH

These are as follows

C N
(i) a-helix In a-helix, the size of R groups is quite large
and H-bonds are formed between the C == O of one C N
amino acid and the ( N—H) of the fourth amino Polypeptide
CCH Hydrogen CCH
acid residue in the chain. Only right handed a- helix bond

exists in nature, since it is more stable than left C N

handed helix. In this form, there are 3.6 amino acid
residues per complete turn. The rise along the central C
N
axis is 1.5 Å per residue.
l The structure is stabilised by intramolecular hydrogen HCC CCH
bonds between an amide hydrogen ( N—H group)
Extended b-pleated (anti-parallel)
and the carbonyl oxygen ( C ==O group) of the
polypeptide strands
fourth amino acid residue away in the peptide chain. b-pleated structure
C ==O - - - H — N
1140 JEE Main Chemistry

(iii) Triple helix In this structure, three loosely coiled heterogeneous quaternary structure, e.g.
helical polypeptide chains can wind around each other haemoglobin which consists of two a chains and two b
to form a stiff cable. It is very strong and relatively rigid. chains.
The triple helix is found mainly in collagen, the major Polypeptide chain
structural protein of skin, bones, teeth, tendons and
cartilage. It can be seen that the triple helix structure is e
em
more extended and stabilised by hydrogen bonds Ha

between the chains, while in a-helix, it is between the

amino acid residues in the same chain.

Myoglobin

Ha
em
–N

e
–C a-helix

Tertiary and quaternary

structures of myoglobin
Triple helix
A protein may have the different secondary structures Classification of Proteins
through out its length. Some parts may have a-helix Proteins can be classified on the basis of their chemical
structure, while other may have b-pleated sheet composition and functions or on the basis of their physical
structure. Some parts of the chain may even have no properties.
secondary structure at all. These structureless parts l Based on the molecular structure and the function
are called random coils and the arrangement is they perform, proteins can be further classified as
called random coil arrangement. This type of (i) Fibrous proteins These are long and thread like
structure is flexible, changing and statistically molecules and tend to lie side by side to form fibres.
random. Synthetic polylysine exists as a random coil. They are held together by intermolecular
H-bonding and hence insoluble in water,
3. Tertiary Structure e.g. keratin (in hair, skin, nails, horn and wool),
The tertiary structure of a protein has its three fibroin (in silk) and myosin (in muscle).
dimensional shape that arises from further foldings of its (ii) Globular proteins These are folded with
polypeptide chains, foldings superimposed on the coils of spheroidal shapes. The folding takes place in such a
the a -helixes. These folding brings together active way that the lipophilic parts are turned inward and
amino acids, which are otherwise scattered along the hydrophilic parts tend to move towards the surface,
chain, and may form a distinctive cavity or cleft in Hydrogen bonding is chiefly intramolecular. Hence,
which the substrate is bound. they are soluble in water, acids, bases and salts,
In proteins consisting of a single polypeptide chain, the e.g. albumin (in eggs), haemoglobin (in blood), all
tertiary structure determines the overall shape of the enzymes and most hormones (insulin).
molecule. So, proteins are called fibrous proteins when l On the basis of their physical properties, proteins can
they form thin, long threads and globular proteins, if be classified as,
very compact. (i) Simple proteins These are give only amino acids
The tertiary structure is specific to a given amino acid on hydrolysis, e.g. albumin, globulins etc.
sequence and is called the native shape of the protein. (ii) Conjugated proteins These are contain a
Thus, primary structure of a protein dictates its tertiary non-protein part (prosthetic group). The prosthetic
structure. group in a protein controls its biological functions.
The common prosthetic group in the proteins are
4. Quaternary Structure tabulated below
It is shown by proteins containing more than one
Examples of Some Conjugated Proteins
polypeptide chain. Two or more polypeptide chains may
associate to give rise to the quaternary structure. These Name of protein Prosthetic group
are held together by non-covalent forces such as Glyco-protein Sugars (carbohydrates)
hydrogen bonds, electrostatic interactions and van der Nucleo-protein Nucleic acid
Waals’ interactions. Lipo-protein Lipid
If the protein consists of identical units it is said to Phospho-proteins Phosphoric acid
have a homogeneous quaternary structure. Chromo-protein Pigments having metals Cu, Fe
If the units are dissimilar, the protein is said to have a (haemoglobin)
Bimolecules 1141

(iii)Derived proteins These are obtained by the Hence, mercury and lead are poisonous to the human
hydrolysis of higher proteins with acids, alkalies or system as they denature proteins in the body.
enzymes. The first aid treatment for a person who has ingested
e.g. Peptones, proteases and denatured proteins. a heavy metal is to give a large dose of egg whites or
milk, both of which are rich in proteins. The proteins
Denaturation of Proteins in them form complexes with the heavy metals in the
A protein that is in a biologically inactive form is said to stomach, temporarily preventing absorption of the
be in a denatured state. The conformational change that metals into the blood. The patient should later be
produces this state is called denaturation of proteins. given an emetic to get rid of the poison.
Denatured proteins usually forms large aggregates that
are precipitated from solution. This process is called Test for Proteins
coagulation. With the help of following tests, presence of proteins can
During denaturation no peptide bonds are broken, i.e. be detected by these methods.
chemical composition or primary structure remains (i) Biuret test Protein is gently warmed with
unaffected. 10% solution of NaOH and then a drop of dil. CuSO4
solution is added. Formation of reddish-violet
colour indicates the presence of peptide linkage
O
½½
(— C— NH—).
This test is also given by the compound biuret,
obtained by urea on heating.
(ii) Xanthoprotic test Certain proteins give yellow
Disulphide bridges colour with conc. HNO3 . This yellow colour is same
which is formed on skin when skin comes in contact
with conc. HNO3 .
The relatively weak, non-covalent interactions are
(iii) Millon’s test When Millon’s reagent (a solution of
disrupted in the denaturation of a protein.
mercurous and mercuric nitrates in nitric acid
Some following factors can bring about the denaturation containing some nitrous acid) is added to a protein
of proteins are solution, a white precipitate is obtained. On heating,
(i) Heat When you fry or boil an egg, you bring about a it turns to red precipitate or colour.
denaturation of egg proteins. That’s the reason why (iv) Ninhydrin test When a protein is boiled with a
bacteria are destroyed at high temperatures that exist dilute solution of ninhydrin, a violet colour is
in an autoclave is that their proteins are denatured. obtained. This test is given by all proteins.
(ii) Exposure to organic solvents Organic solvents (v) Nitroprusside test When sodium nitroprusside is
such as ethanol and rubbing alcohol can denature added to proteins containing —SH group, a violet
proteins. Alcohol is rubbed on the skin, before an colour is obtained.
injection to kill surface bacteria by denaturing
bacteria proteins and preventing infection.
5. Enzymes
(iii) Exposure to acid and bases Strong acids and
Kuhne (1878) coined the term enzyme. Buchner in
bases can denature proteins by disrupting salt bridges
(1897, 1903) isolated enzyme (including zymase from yeast
and hydrogen bonds. Prolonged treatment with strong
for the first time). Sunner found that enzymes are
acids will bring about hydrolysis of peptide bonds of a
proteinaceous. He crystallised the first enzyme, urease
protein.
from jack bean.
(iv) Exposure to salts of heavy metal ions Cation of
An enzyme is a specialised protein produced within
metals such as Hg2+ , Ag+ and Pb2+ interfere with the
an organism which is capable of catalysing a
disulphide bonds and salt bridges that stabilise the specific chemical reaction. Since, the enzymes act as
protein structure and bring about denaturation. catalyst, they are sometimes referred to as biocatalysts.
1142 JEE Main Chemistry

A catalyst alters the rate of a chemical reaction, (v)Isomerases These are catalyse reactions which bring
usually without undergoing any change itself. In this about intramolecular rearrangement of atoms in
respect an enzyme differs from a normal catalyst. The substrates.
enzyme may participate in a reaction by combining (vi) Ligases (Synthetases) These are catalyse reactions in
with the substrate. Ultimately, it is set free. which the pyrophosphate bond of ATP is broken down and
Some examples of enzymes along with the reaction, linkage takes place between two molecules. These
catalyse are given in tabulated form below enzymes form the following bonds : C—O, C—S, C—N and
Enzymes and their Reaction Catalyse C—C.
Enzymes Reaction catalyse Types of Reactions Shown by Enzymes
Amylase Starch to n glucose Enzymes Reactions
Maltase Maltose to (2) glucose Oxidoreductases Oxidation-reduction reactions
Lactase Lactose to glucose + galactose Transferases Group transfer reactions
Invertase Sucrose to glucose + fructose Hydrolases Hydrolytic reactions (addition of H2O)
Pepsin Proteins to amino acids Lyases Addition or loss of groups to double bonds
Trypsin Polypeptides to a-amino acids Isomerases Isomeration reactions
Urease Urea to ammonia + CO2 Ligases Synthesis by condensation of two groups requiring ATP
Nuclease DNA, RNA to nucleotides
Carbonic anhydrase H2CO3 to H2O + CO2
DNA polymerase Deoxyribonucleotide triphosphate to DNA
In another system of classification, the name of the enzyme is
RNA polymerase Ribonucleotide triphosphate to RNA
derived from its substrate.
e.g. Carbohydrases, proteases, dehydrogenases, oxidases,
decarboxylases, hydrases, isomerases, transferases, amidases
Nomenclature of Enzymes and esterases.
Enzymes are generally named by adding ase to the
root indicating the substrate on which the enzyme Mechanism of Enzyme Action
acts. This system was provided by Duclau (X) Lock and key and induced fit models both explain the
(1883). Thus, fumarase catalyses the conversion of enzyme specificity and its mechanism.
fumaric acid to malic acid.
In 1894, Fischer suggested a lock and key concept to
Classification of Enzymes explain the working of an enzyme. According to this
mechanism, an enzyme catalysed reaction involves the
The International Union of Biochemistry
following steps.
report of 1962 (revised in 1964) contains a scheme
for the classification of enzymes. Enzymes have been Step 1 Binding of enzyme (E ) to the substrate (S) to form an
divided into following six groups, viz. enzyme substrate complex.
(i) Oxidoreductases These are include a large E + S ¾® (ES )
number of enzymes (221 are listed). These are Step 2 Product formation in the complex.
bring about the main energy yielding reactions (ES ) ¾® EP
Enzyme substrate Enzyme product
of living tissue. Oxidoreductases include complex
oxidases and dehydrogenases. It act by
Step 3 Release of the product from the enzyme.
transferring electrons and hydrogen ions.
EP ¾® E + P
(ii) Transferases These are concerned with the Enzyme-product Enzyme Product
transfer of a group of atoms from one molecule to Key Enzyme
another. Oxidoreductases and transferases
together represent over half the enzymes
known. Substrate Enzyme Substrate
E+S ES complex
(iii) Hydrolases Complex molecules undergo cleavage,
and the elements of H 2O are added across the bond
cleaved by the action of hydrolases.
(iv) Lyases These may work in two ways.
A group of atoms may be removed from the Product
substrate leaving double bonds, or groups may Enzyme Substrate Enzyme Enzyme
molecule molecule substrate
be added to double bonds without hydrolysis, complex
oxidation or reduction. The enzymes act on the Lock and key model of enzyme action
following bonds :
In induced fit mechanism, the active site undergoes a
C—C, C — O, C — N, C — S and C — X
change in its conformation in presence of a substrate to allow
Bimolecules 1143

a better fit between the active site and the substrate. It Some enzymes require a loosely bound cation such
means that enzymes are highly specific for the reaction as Mg2+ .
that they catalyse. It is shown in below
Dn
Substrates

Reaction rate

Reaction rate
Enzyme substrate Product
Active site complex
Enzyme

+
Enzyme
4 6 8 10
Temperature C pH D
Features of Enzymes (C) Effect of temperature on the rate of enzymatic reaction. On
represents the point of thermal denaturation of the enzyme
Some important features of enzymes are discussed below (44-45°C). (D) Effect of pH on the rate of enzyme reaction.
(i) Specificity Enzymes show striking specificity.
These catalyse have specific reactions of specific (iv) Concentration Enzyme concentration affect the rate
substrates. Some enzymes are so specific that they of a reaction. If the substrate concentration is
catalyse only one type of substrate molecule. increased, the rate of enzyme reaction also increases.
Beyond a certain point, however, the enzyme becomes
e.g. The enzyme chymotrypsin catalyses the
saturated with substrate molecules. Further increase
hydrolysis of acetyl, L-phenyl-alanine methyl ester,
in reaction velocity occurs only if the enzyme
but is inert to the D isomer.
concentration is increased.
(ii) Required in small amount Only one enzyme can For example, during starvation the supply of the
catalyse a large number of substrate molecules. In substrate (glucose) decreases and glycolysis is
other words, enzymes are required in very small depressed. Conversely, increase in glucose
amounts. concentration accelerates the rate of reaction upto the
(iii) Activation energy Enzymes alter the speed of a point when enzyme is saturated with glucose.
chemical reaction. They lower the energy of activation (v) Inhibitors Certain compounds (e.g. drugs, poisons)
of a reaction, thus enabling it to occur at ordinary combine with an enzyme but do not serve as substrates.
physiological temperatures. They block reaction by the enzyme and function as
inhibitors. The inhibitors usually resemble the
A.E. A.E. substrate in structure. The enzyme and the inhibitor
Free energy

Free energy

form an enzyme-inhibitor complex which is inactive.

Enzyme
inhibitor
complex

A Course of reaction B
Enzyme Inhibitor
Change in activation energy (A.E.) Inhibition
(A) Reaction without enzyme, (B) Reaction with enzyme. Mechanism of enzyme action inhibition.
l Inhibition may be competitive or non-competitive.
Factors Affecting Enzyme Activity In competitive inhibition, both inhibitor and
(i) Effect of temperature Enzyme action is greatly substrate molecules compete for binding with the
affected by temperature. If the temperature is enzyme. If the inhibitor is in sufficiently high
increased by 10°C, the rate of most chemical concentration, it displaces the substrate molecules. For
reactions is doubled. However, at 40-60°C, there is example, sulphanilamide is a competitive inhibitor for a
loss of enzyme activity because denaturation of bacterial enzyme that incorporates p-amino benzoic acid
proteins occurs at this temperature. into folic acid.
(ii) pH At optimum pH, the activity of enzyme is l Competitive inhibition can be reversed by increasing
maximum. For most enzymes, the effective pH the concentration of the substrate. In
range is 4–9. Below and beyond these limits, non-competitive inhibition, the inhibitors (poisons)
denaturation of enzymes takes place. The optimum react with the various functional groups of the
pH for pepsin is 2.0 and for trypsin is 8.0. enzyme. They inhibit the normal reactions catalysed
by the enzyme and result in death. Non-competitive
(iii) Ions Enzyme activity is affected by H + ion inhibition cannot be reversed by increasing the
concentration and other ionic concentrations. concentration of the substrate.
1144 JEE Main Chemistry

Note Provitamins are the biologically inactive compounds

6. Vitamins which have almost similar structure as vitamins and can
These are essential dietary factors required by an be converted into active vitamins.
organism in minute quantities. These are not utilised in
cell building or as energy source but they act as catalysts Example 4. Match the following :
(coenzymes) in biological processes. (i) Riboflavin (A) Beri-beri
Note Vitamins are not synthesised in the body and (ii) Thiamine (B) Scurvy
hence should be supplied in diet. Their deficiency causes (iii) Pyridoxine (C) Cheilosis
specific diseases (avitaminosis). (iv) Ascorbic acid (D) Convulsions
(a) (i)-(C), (ii)-(D), (iii)-(A), (iv)-(B) (JEE Main 2020)
Classification of Vitamins (b) (i)-(C), (ii)-(A), (iii)-(D), (iv)-(B)
These are classified into two groups (c) (i)-(D), (ii)-(B), (iii)-(A), (iv)-(C)
(i) Fat soluble vitamins These are vitamin A, D, E (d) (i)-(A), (ii)-(D), (iii)-(C), (iv)-(B)
and K.
Sol. (b) (i) Riboflavin - (C) Cheilosis (ii) Thiammine - (A) Beri-beri
(ii) Water soluble vitamins These are vitamin B and
vitamin C. Water soluble vitamins must be supplied (iii) Pyridoxine - (D) Convulsions (iv) Ascorbic acid - (B) Scurvy
regularly in diet because they are excreted in urine Thus, the correct option is (b).
and cannot be stored in our body (except B12 ).
l Vitamins catalyse biological reactions in very low Nucleic Acids
concentration, therefore the daily requirement of Nucleic acids are present in all living organisms, whether
any vitamin for any individual is extremely small. plants, animals or virus. These are generally associated
with proteins (in eukaryotes) to form nucleoproteins.
l The sources and diseases caused by the deficiency of
These are responsible in the biosynthesis of proteins and
different vitamins are tabulated below
for the transmission of heredity characters. The genetic
Vitamins : Sources and Deficiency Diseases information coded in nucleic acids helps to know the
structure of all proteins including enzymes and all
Name of vitamin Sources Deficiency disease
metabolic activities of living organisms.
Vitamin A (Retinol or Fish liver oil Xerophthalamia
Axerophthol, C20H30O) (hardening of cornea) Classification of Nucleic Acids
Vitamin B1 (Thiamine or Green vegetables, Beri-Beri (disease of
Aneurin, C12H18N4 SOCl 2 ) milk nervous system)
There are two types of nucleic acids, deoxyribose nucleic
acid (DNA) and ribose nucleic acid (RNA).
Vitamin B2 (Riboflavin or Vegetables, milk, Dark red tongue
Lactoflavin, C17 H20N4 O6 ) liver, egg white, dermatitis, cheilosis DNA is found predominantly in the nucleus, whereas RNA
kidney is predominant in the cytoplasm. DNA is the genetic
Vitamin B3 (Pantothenic All food, more in Dermatities in cocks, material of most organisms, including many viruses.
acid, C9H17 O5N) yeast, liver, retarded body and Some viruses, however, have RNA as their genetic
kidneys tomatoes mental growth
material.
Vitamin B 5 (Nicotinic acid Fresh meat, liver, Pellagra, dermatities
or Niacin,
(C5H4 N — COOH)
fish, cereals,
milk, pulses
1. Deoxyribose Nucleic Acid (DNA)
Vitamin B6 (Pyridoxine or Grams, molasses, Severe dermatitis, It is present in the cells of all plants, animals,
adermin, C8H11 O3N) egg yolk meat convulsions prokaryotes and in a number of viruses. In prokaryotes
Vitamin H (Biotin, Yeast, liver, Dermatitis, loss of (e.g. Escherichia coil , a bacterium), the genetic material
C10H16N2O3S) kidney milk hair and paralysis consists of a single giant molecule of DNA without any
Vitamin B12 Liver of ox, sheep, Pernicious anaemia associated proteins. DNA is present mainly in the
(Cyanocobalamine, pig, fish etc. chromosomes. It has also been reported in cytoplasmic
C63H88O4 N14 PCo)
organ cells like mitochondria and chloroplasts. The DNA
Vitamin C (Ascorbic acid, Citrus fruits, Scurvy of most plants and animals and many viruses (polyoma
C6H8O6 ) green vegetables
virus, small-pox virus, bacteriophages T 2, T 4 and T 6) is
Vitamin K (K1 and K2 ) Cereals, leafy Hemorrhagic
double stranded. In bacteria and in higher plants and
(Phylloquinone, C13H46O2 ) vegetables conditions,
increased blood animals both DNA and RNA are present. Viruses usually
clotting time contain either DNA or RNA.
Vitamin D (Ergocalciferol, Synthesized in Rickets with
C28H44 O) skin cells in the osteomalacia, soft and Structure of DNA
presence of fragile teeth The widely accepted molecular model of DNA is the
sunlight, liver,
egg yolk, meat double helix structure, proposed by Watson and
and milk Crick (1953). The DNA molecule consists of two helically
twisted strands. Each strand consists of alternating
Bimolecules 1145

molecules of deoxyribose (a pentose sugar) and phosphate The four atoms of the ring are numbered 1¢ , 2¢ , 3¢ and
groups. 4’. The carbon atom of —CH 2 is numbered 5¢.
The two strands are intertwined in a clockwise direction, 5¢
HOH2C O OH
i.e. in a right hand helix, and run in opposite directions.
Each successive nucleotide turns 36 degrees in the 4¢ H H 1¢
horizontal plane. The width of the DNA molecule is 20 Å. H H
3¢ 2¢
The twisting of the strands result in the formation of OH H
deep and shallow spiral grooves. 2-deoxyribose sugar
20Å (ii) Nitrogenous bases are of two types, viz, pyrimidines
5¢ 3¢ Sugar phosphate chains
and purines. The pyrimidines are single ring
Base pairs compounds, with nitrogen in position 1 and 3 of a
G C 3.4 Å 6-membered benzene ring.
A T
T A G C The two most common pyrimidines of DNA are
A T S S
cytosine (C) and thymine (T). The purines are
34Å

P P double ring compounds. A purine molecule consists of

C G a 5-membered imidazole ring joined to a pyrimidine
5¢ direction
T A
G C S S
T A ring at positions 4 and 5. The two most common
C G P purines of DNA are adenine (A) and guanine (G).
P
C G The structure of these bases are follows :
S S
T A
G C P NH2 O O
A T P
T A C G H3C H
3¢

S S N N N—H
P
A T O O O
T A A T N N N
S S
C G
G C H H H
Cytosine (C) Thymine (T) Uracil
(a pyrimidine base
5' 3' Pyrimidines present in RNA)
Double helix structure of DNA NH2 O
N N H
Chemical Composition of Nucleic Acids N N
Nucleic acids are biopolymers made of nucleotides joined
together to form a long chain. Hence, these are called N N N N NH2
polynucleotide. Each nucleotide consists of the pentose
sugar, deoxyribose, a phosphate group and nitrogenous H H
Adenine (A) Suanine (G)
base which may be either a purine or pyrimidine.
Deoxyribose and a nitrogenous base together form a Purines
nucleoside.
l Two purines would occupy too much space, while two
A nucleoside and a phosphate together form a nucleotide.
pyrimidines would occupy too little. Because of the
Nucleoside = deoxyribose + nitrogenous base
purine-pyrimidine pairing, the total number of
Nucleotide = deoxyribose + nitrogenous base +phosphate purines in a double-stranded DNA molecule is equal
= nucleoside + phosphate to the total number of pyrimidines.
Nucleotide may be represented as, l Thus, A / T = 1 and G/C = 1
Base l or, A + G = C + T (Chargaff’s rule).
O Sugar (iii) Phosphate group In the DNA strand, the
O phosphate groups alternate with deoxyribose. Each
HO—P phosphate group is joined to carbon atom 3 of one
OH deoxyribose and to carbon atom 5 of another.
(i) Deoxyribose is a pentose sugar with five carbon Thus, each strand has a 3 end and a 5 end. The two
atoms. Four of the five carbon atoms plus a single atom strands are oriented in opposite direction. The 3 end
of oxygen form a five-membered ring. The fifth carbon of one strand corresponds to the 5 end of the other.
atom is outside the ring and forms a part of a —CH 2 Consequently the oxygen atoms of deoxyribose point
group. in opposite directions.
1146 JEE Main Chemistry

Example 5. Which one of the following bases is not present with the same untill these are required. These RNA +
in DNA ? (JEE Main 2014) protein spherical balls are called informosomes.
(a) Quinoline Termination codon AAUAAA sequence
Poly (A) sequence
(b) Adenine Cap (AUG) (UAA) (20-200 nucleotides)
(c) Cytosine 5¢ 3¢
(d) Thymine Non coding region1 Coding region Non coding region 2
(10-100 nucleotides) (~1600 nucleotides) (50-150 nucleotides)
Sol. (a) DNA contains four nitrogenous pyramidine bases, Structure of m-RNA
adenine, guanine, cytosine, thymine. While quinoline is an
alkaloid, hence, it is not present in DNA. (ii) r-RNA It makes 80% of total cellular RNA. These
RNA is the basic constituent of ribosomes. It is
2. Ribose Nucleic Acid (RNA) developed from r-DNA in the case of prokaryotes
It is the genetic material of mainly viruses. It can be while in the case of eukaryotes, it is developed from
single stranded or double stranded. the nucleolar organiser region of chromosome.
The various r-RNAs present in different units of
Chemical Composition of RNA ribosome are as follows
RNA, like DNA is also a polymer of nucleotide which in Prokaryotes (70 S)
turn obtained from nucleoside, chemical which when
combines with phosphate. 30 S ¾®16 S r-RNA; 50 S ¾® 23 S and 5 S r-RNA
Here, the point of difference is that the sugar present in Eukaryotes (80S)
nucleoside is ribose sugar instead of deoxyribose sugar. 40S ¾®18 S r-RNA; 60 S ¾® 28-29 S, 5.85 S, 5 S
Nucleoside = ribose + nitrogenous base r-RNA
Nucleotide = ribose + nitrogenous base + phosphate Chloroplast and Mitochondria (55 S)
These are classified into 30S ¾® 12-13 S r-RNA; 40 S ¾®16-17 S r-RNA + 5 S
(i) Ribose The pentose sugar of RNA has an identical Structure of r-RNA is
structure with deoxyribose sugar except that there is
an —OH group instead of H on carbon atom 2¢ . Unpaired
5¢ bases
HOH2C O OH
4¢ H H 1¢
H H
3¢ 2¢
OH OH Coiled
Paired bases region
Ribose sugar
Uncoiled region
(ii) Nitrogenous base are divided into two types : Structure of r-RNA
pyrimidines and purines. Purine bases are same as
(iii) t-RNA It makes 10-20% of total cellular RNA with
that in DNA but pyrimidine bases are cytosine (C)
sedimentation coefficient of 3.8 S. These RNA
and uracil (U). (In RNA uracil replaces thymine).
contains 73-93 nucleotides with in the structure given
(iii) Phosphate is same as DNA. below
Note There are viruses, called retroviruses, in which Amino
information flows from RNA to DNA. The virus that causes acid Amino acid
binding site
ACC

AIDS is a retrovirus. Synthetase

site
Types of RNA TYc - arm
RNA are divided into three types.
Formed
m - RNA ¾¾¾® During cleavage
Ribosome
Formed D-arm recognition site
RNA¾®r - RNA ¾¾¾® At the end of cleavage Variable arm
Anticodon arm
Formed Anticodon
t - RNA ¾¾¾® During gastrulation site
(i) m-RNA It makes 3–5% of total cellular RNA. Structure of t-RNA
The m-RNA comes out with proteins into the
cytoplasm and normally swim as spherical balls along
Bimolecules 1147

The above structure is called clover leaf model and was (ii) DNA molecules can control the synthesis of proteins
fully worked out by Holley et al. of yeast alanine t-RNA. in an exact and specific way. Synthesis of a
The function of t-RNAs is to align the required amino polypeptide chain is controlled by a particular gene.
acids according to the nucleotide sequence of m-RNA. The gene, which is almost always a segment of a DNA
strand, transcribes an m-RNA which acts as an
intermediate in conveying information from the
The Genetic Code sequence of amino acids in the polynucleotide chain.
Nucleic acids control heredity on molecular level. The Each amino acid is specified by a sequence of three
double helix of DNA is reponsible for the hereditary bases known as codon of m-RNA.
information of the organisms. The information is stored
Each t-RNA molecules has a sequence of three
as the sequence of bases along the polynucleotide chain.
bases known as anticodon, which reads a codon of
DNA preserve the hereditary informations and use it.
m-RNA. t-RNA molecules thus serve as adaptors in
It done these things through two properties protein synthesis by reading of m-RNA codons in a
(i) DNA molecules can duplicate themselves (replication). sequence.

Practice Exercise
ROUND I Topically Divided Problems
Carbohydrates Which can be used to make distinction between an
1. The two functional groups present in a typical aldose and a ketose ?
carbohydrate are (a) I, II and III (b) II and III (c) I only (d) II only
(a) — OH and —COOH (b) —CHO and —COOH 6. If a-D-glucopyranose is reacted with acetic
(c) C == O and —OH (d) —OH and —CHO anhydride at 373 K, the major product is the
b-isomer of the pentaacetate. It is attributed to
2. Which is not true for carbohydrates?
(a) isomerisation of a-D into b-D-glucose at 373 K
(a) General formula is CnH2nOn
(b) opening of glucopyranose ring
(b) Glucose is the most common monomer of
(c) Both the statements are correct
carbohydrates
(d) None of the statement is correct
(c) Fructose is the sweetest of all sugars
(d) Do not conjugate with lipids 7. Number of stereo-centers present in linear and cyclic
structures of glucose are respectively (JEE Main 2019)
3. Identify the product ‘ C ’ in the following series of
reactions (a) 4 and 5 (b) 4 and 4 (c) 5 and 4 (d) 5 and 5
HCN
Glucose ¾¾® 2
A ¾¾®
H O
HI
B ¾¾® C
8. When glucose reacts with bromine water the main
product is
(a) heptanoic acid (b) hexanoic acid
(a) acetic acid (b) saccharic acid
(c) a-methyl caproic acid (d) None of these
(c) glyceraldehyde (d) gluconic acid
4. The two forms of D-glucopyranose obtained from 9. Which of the following statements is correct?
the solution of D-glucose are called [JEE Main 2020]
(a) isomer (b) anomer (a) Gluconic acid is obtained by oxidation of glucose
(c) epimer (d) enantiomer with HNO3
5. Consider the following reagents (b) Gluconic acid is a dicarboxylic acid
(c) Gluconic acid can form cyclic (acetal/hemiacetal)
I. Br 2 water II.
structure
Tollen’s reagent
(d) Gluconic acid is a partial oxidation product of glucose
III. Fehling’s solution