class-12 chemistry chapter-14 biomolecules

Class-12

Chemistry

Chapter-14

Biomolecules

Biomolecules

Biomolecules are the most essential organic molecules, which are involved in the

maintenance and metabolic processes of living organisms. These non-living molecules

are the actual foot-soldiers of the battle of sustenance of life. They range from small

molecules such as primary and secondary metabolites and hormones to large

macromolecules like proteins, nucleic acids, carbohydrates, lipids etc.

Types of Biomolecules

There are four major classes of Biomolecules –

Carbohydrates, Proteins, Nucleic acids and Lipids.

1 Carbohydrates:

Chemically, the carbohydrates may be defined as optically active polyhydroxy

aldehydes or ketones or the compounds which produce such units on hydrolysis. Some

of the carbohydrates, which are sweet in taste, are also called sugars. The most

common sugar, used in our homes is named as sucrose whereas the sugar present in

milk is known as lactose. Carbohydrates are also called saccharides (Greek: sakcharon

means sugar).

Classification of Carbohydrates:

Carbohydrates are classified on the basis of their behaviour on hydrolysis. They have

been broadly divided into following three groups.

(i) Monosaccharides: A carbohydrate that cannot be hydrolysed further to give simpler

unit of polyhydroxy aldehyde or ketone is called a monosaccharide. About 20

monosaccharides are known to occur in nature. Some common examples are glucose,

fructose, ribose, etc.

(ii) Oligosaccharides: Carbohydrates that yield two to ten monosaccharide units, on

hydrolysis, are called oligosaccharides. They are further classified as disaccharides,

trisaccharide’s, tetra saccharides, etc., depending upon the number of

monosaccharides, they provide on hydrolysis.

(iii) Polysaccharides: Carbohydrates which yield a large number of monosaccharide

units on hydrolysis are called polysaccharides. Some common examples are starch,

cellulose, glycogen, gums, etc. Polysaccharides are not sweet in taste; hence they are

also called non-sugars.

The carbohydrates may also be classified as either reducing or nonreducing sugars. All

those carbohydrates which reduce Fehling’s solution and Tollens’ reagent are referred

to as reducing sugars.

All monosaccharides whether aldose or ketose are reducing sugars.

In disaccharides, if the reducing groups of monosaccharides i.e., aldehydic or ketonic

groups are bonded, these are non-reducing sugars e.g. sucrose.

On the other hand, sugars in which these functional groups are free, are called

reducing sugars, for example, maltose and lactose.

Monosaccharide:

Classification of Monosaccharide:

Monosaccharides are classified on the basis of the functional group present in

them.

Aldose:If a monosaccharide contains an aldehyde group, it is known as an aldose.

Ketose:if it contains a keto group, it is known as a ketose.

Monosaccharides are further classified on the basis of number of carbon atoms

present in them.

Number of carbon atoms constituting the monosaccharide is also introduced in the

name:

I Glucose:

Glucose is a simple sugar with six carbon atoms and one aldehyde group.

This monosaccharide has a chemical formula C6H12O6.

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It is also known as dextrose. It is referred to as aldohexose as it contains 6 carbon

atoms and an aldehyde group.

It is present in sweet fruits and honey. Ripe grapes also contain glucose in large

amounts.

Preparation of Glucose:

1. From sucrose (Cane sugar):

If sucrose is boiled with dilute HCl or H2SO4 in alcoholic solution, glucose and fructose

are obtained in equal amounts.

2. From starch:

Commercially glucose is obtained by hydrolysis of starch by boiling it with dilute H2SO4

at 393 K under pressure.

Structure of Glucose:

On the basis of the following evidence it was assigned the structure illustrated above:

It has a molecular formula of C6H12O6.

When HI is heated for a long time, n-hexane is formed which indicates that all

the six carbon atoms are linked in a straight chain.

The oxime is formed when glucose reacts with hydroxylamine and cyanohydrins

on the addition of hydrogen cyanide to it. This reaction can confirm the

presence of the carbonyl group in glucose.

On the reaction of glucose with a mild oxidizing agent like bromine water, the

glucose gets oxidized to a carboxylic acid that contains six carbon atoms. This

indicates that the carbonyl group is present as an aldehyde group.

The presence of -OH group is confirmed after the acetylation of glucose with

acetic acid which gives glucose pentaacetate.

Glucose as well as gluconic acid both yields dicarboxylic acid and saccharic acid

on oxidation with nitric acid. The presence of primary alcohol is indicated by

this.

Fischer projection

For Glucose:

When the hydroxyl groups on carbons 4 and 5 are to the right side of the Fischer

projection, glucose is D- configuration. When the hydroxyl groups on carbons 4 and 5

are to the left side of the Fischer projection, glucose is L-sugar.

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Cyclic Structure of Glucose:

The structure (I) of glucose explained most of its properties but the following reactions

and facts could not be explained by this structure.

1. Despite having the aldehyde group, glucose does not give 2,4

and it does not form the hydrogen sulphite

2. The pentaacetate of glucose does not react with hydroxylamine indicating the

absence of free —CHO group.

3. Glucose is found to exist in two different crystalline forms which are named as α and

β. The α-form of glucose (m.p. 419 K) is obtained by crystallisation from concentrated

solution of glucose at 303 K while the β

from hot and saturated aqueous solution at 371 K.

This behaviour could not be explained b

was proposed that one of the

cyclic hemiacetal structure. It was found that glucose forms a six

which —OH at C-5 is involved in ring formation.

group and also existence of glucose in two forms as shown below.


and facts could not be explained by this structure.

Despite having the aldehyde group, glucose does not give 2,4-DNP test, Schiff’s test

hydrogen sulphite addition product with NaHSO3.

The pentaacetate of glucose does not react with hydroxylamine indicating the

CHO group.

. Glucose is found to exist in two different crystalline forms which are named as α and

orm of glucose (m.p. 419 K) is obtained by crystallisation from concentrated

solution of glucose at 303 K while the β-form (m.p. 423 K) is obtained by crystallisation

from hot and saturated aqueous solution at 371 K.

This behaviour could not be explained by the open chain structure (I) for glucose. It

was proposed that one of the —OH groups may add to the —CHO group and form a

cyclic hemiacetal structure. It was found that glucose forms a six-membered ring in

5 is involved in ring formation. This explains the absence of



DNP test, Schiff’s test

addition product with NaHSO3.

The pentaacetate of glucose does not react with hydroxylamine indicating the

. Glucose is found to exist in two different crystalline forms which are named as α and

orm of glucose (m.p. 419 K) is obtained by crystallisation from concentrated

form (m.p. 423 K) is obtained by crystallisation

y the open chain structure (I) for glucose. It

CHO group and form a

membered ring in

This explains the absence of —CHO


What is essentially the difference between

meant by pyranose structure of glucose?

Answer

α- glucose and β- glucose are two cyclic hemiacetal forms of glucose which differ only

in the configuration of hydroxyl group (

called anomers. The six-membered cyclic structure of glucose is called pyranose

structure.

Pyranose structure of glucose:

of its resemblance with pyran is called pyranose form.

α-D-glucose and β-D-glucose are stereoisomers, they differ in 3

configuration of atoms/groups at one or more positions.

Structure of Fructose:

Fructose also has the molecular formula C

found to contain a ketonic functional group at carbon number 2 and six carbons in

straight chain as in the case of glucose. It belongs to D

compound. It is appropriately written as D

What is essentially the difference between α - glucose and β - glucose? What is

meant by pyranose structure of glucose?

are two cyclic hemiacetal forms of glucose which differ only

in the configuration of hydroxyl group (-OH) at anomeric carbon. Such isomers are

membered cyclic structure of glucose is called pyranose

glucose: The six membered ring contains oxygen atom because

of its resemblance with pyran is called pyranose form.

glucose are stereoisomers, they differ in 3-dimensional

configuration of atoms/groups at one or more positions.

Fructose also has the molecular formula C6H12O6 and on the basis of its reactions it was


straight chain as in the case of glucose. It belongs to D-series and is a laevorotatory

compound. It is appropriately written as D-(–)-fructose.

glucose? What is

are two cyclic hemiacetal forms of glucose which differ only

OH) at anomeric carbon. Such isomers are

membered cyclic structure of glucose is called pyranose

The six membered ring contains oxygen atom because

dimensional

and on the basis of its reactions it was


es and is a laevorotatory

Cyclic Structure of Fructose:

It also exists in two cyclic forms which are obtained by the addition of —OH at C5 to

the keto group. The ring, thus formed is a five membered ring and is named as

furanose with analogy to the compound furan. Furan is a five membered cyclic

compound with one oxygen and four carbon atoms.

Proteins

Proteins are the complex organic substances which are the basis of protoplasm and are

found in all living organisms.The word protein is derived from Greek word, “proteios”

which means primary or of prime importance.

Chief sources of proteins are milk, cheese, pulses, peanuts, fish, meat, etc.

They occur in every part of the body and form the fundamental basis of structure and

functions of life. They are also required for growth and maintenance of body.

All proteins are polymers of α-amino acids.Like peptides the amino acid unit in

proteins are held up by peptide (-CONH-) linkage. In fact, polymeric products of amino

acids with molecular mass up to 10000 are called polypeptides while those having

molecular mass more than 10000 are considered as proteins.

Amino Acids

Amino acids contain amino (–NH2) and carboxyl (–COOH) functional groups. Depending

upon the relative position of amino group with respect to carboxyl group, the amino

acids can be classified as α, β, γ, δ and so on.

Of the various amino acids, the α-amino acids are quite important because they are

the building blocks of peptides and proteins. In α-amino acids, the amino group is

present on the α-carbon atom (i.e., C atom next to COOH group).

Inα-amino acid group R- is different for different amino acids.

All α-amino acids have trivial names, which usually reflect the property of that

compound or its source. Glycine is so named since it has sweet taste (in Greek glykos

means sweet) and tyrosine was first obtained from cheese (in Greek, tyros means

cheese.) Amino acids are generally represented by a three-letter symbol, sometimes

one letter symbol is also used.

Classification of Amino Acids

Amino acids are classified as acidic, basicor neutral depending upon the relative

number of amino and carboxyl groups in their molecule.

Acidic amino acids:

The amino acids which contain more number of carboxyl groups as compared to amino

groups are known as acidic amino acids. Example; glutamic acid and aspartic acid.

Basic amino acids:

The amino acids which contain more number of amino groups as compared to carboxyl

groups are known as basic amino acids. Example; lysine

Neutral amino acids:

The amino acids containingequal number of amino andcarboxyl groups are known as

neutral amino acids. Example; Alanine.

Essential and Non-Essential amino acids

Essential amino acids:

The amino acids, which cannot be synthesised in the body and must be obtained

through diet, are known as essential amino acids.

Non-Essential amino acids:

The amino acids, which can be synthesised in the body, are known as non-essential

amino acids.

Structure of amino acids:

Amino acids are usually colourless, crystalline solids. These are water-soluble, high

melting solids and behave like salts rather than simple amines or carboxylic acids.

This behaviour is due to the presence of both acidic (carboxyl group) and basic (amino

group) groups in the same molecule. In aqueous solution, the carboxyl group can lose a

proton and amino group can accept a proton, giving rise to a dipolar ion known as

zwitter ion. This is neutral but contains both positive and negative charges.

In zwitter ionic form, amino acids show amphoteric behaviour as they react both with

acids and bases.

In acidic solution amino acids exist as cations and migrate towards cathode in an

electric field whereas in basic solutions they exist as anions and migrate towards

anode. At the intermediate pH, however, they exist as zwitter ion (a dipolar ion) and

do not migrate towards either electrode. This pH is known as the isoelectric point of

the ∝-amino acid.

Except glycine, all other naturally occurring

α-carbon atom is asymmetric. These exist both in ‘D’ and ‘L’ forms. Most naturally

occurring amino acids have L

the –NH2 group on left hand side.

Structure of Proteins

Proteins are the polymers of α

peptide bond or peptide linkage

between –COOH group and

similar or different amino acids, proceeds through the combination of the amino group

of one molecule with the carboxyl group of the other. This results in the elimination of

a water molecule and formation of a peptide bond

reaction is called a dipeptide because it is made up of two amino acids. For example,

when carboxyl group of glycine combines with the amino group of alanine, we get a

dipeptide, glycylalanine.


Except glycine, all other naturally occurring α-amino acids are optically active, since the

carbon atom is asymmetric. These exist both in ‘D’ and ‘L’ forms. Most naturally

occurring amino acids have L-configuration. L-Amino acids are represented by writing

NH2 group on left hand side.

roteins are the polymers of α-amino acids and they are connected to each other by

peptide linkage. Chemically, peptide linkage is an

COOH group and –NH2 group. The reaction between two molecules of

ar or different amino acids, proceeds through the combination of the amino group


a water molecule and formation of a peptide bond –CO–NH–. The product of the

is called a dipeptide because it is made up of two amino acids. For example,



amino acids are optically active, since the

carbon atom is asymmetric. These exist both in ‘D’ and ‘L’ forms. Most naturally

Amino acids are represented by writing

amino acids and they are connected to each other by

. Chemically, peptide linkage is an amide formed

group. The reaction between two molecules of

ar or different amino acids, proceeds through the combination of the amino group


. The product of the

is called a dipeptide because it is made up of two amino acids. For example,


Tripeptide

If a third amino acid combines to a dipeptide, the produ

tripeptide contains three amino acids linked by two peptide linkages

Polypeptides

When the number of amino acids is more than ten, then the products are called

polypeptides.

protein

A polypeptide with more than hundred amino

higher than 10,000u is called a protein.

Classification of Proteins

Proteins can be classified into two types on the basis of their molecular shape.

Fibrous proteins: When the polypeptide chains run parallel and a

hydrogen and disulphide bonds, then fibre

generally insoluble in water. Some common examples are keratin (present in hair,

wool, silk) and myosin (present in muscles), etc.

(b) Globular proteins: This structure results when the chains of polypeptides coil

around to give a spherical shape. These are usually soluble in water. Insulin and

albumins are the common examples of globular proteins

Structure of Proteins

(i) Primary structure of proteins

chains. Each polypeptide in a protein has amino acids linked with each other

If a third amino acid combines to a dipeptide, the product is called a

tripeptide contains three amino acids linked by two peptide linkages


A polypeptide with more than hundred amino acid residues, having molecular mass

higher than 10,000u is called a protein.

Proteins can be classified into two types on the basis of their molecular shape.

When the polypeptide chains run parallel and a

hydrogen and disulphide bonds, then fibre– like structure is formed. Such proteins are


wool, silk) and myosin (present in muscles), etc.

This structure results when the chains of polypeptides coil


albumins are the common examples of globular proteins.

Primary structure of proteins: Proteins may have one or more polypeptide


ct is called a tripeptide. A

tripeptide contains three amino acids linked by two peptide linkages.


acid residues, having molecular mass

Proteins can be classified into two types on the basis of their molecular shape. (a)

When the polypeptide chains run parallel and are held together by

like structure is formed. Such proteins are


This structure results when the chains of polypeptides coil


roteins may have one or more polypeptide


in a specific sequence and it is this sequence of amino acids that is said to be

the primary structure of that protein.

(ii) Secondary structure of proteins: The secondary structure of protein refers to

the shape in which a long polypeptide chain can exist. They are found to exist

in two different types of structures viz. α-helix and β-pleated sheet structure.

These structures arise due to the regular folding of the backbone of the

polypeptide chain due to hydrogen bonding between and –NH– groups of the

peptide bond

α-Helix is one of the most common ways in which a polypeptide chain forms all

possible hydrogen bonds by twisting into a right handed screw (helix) with the –NH

group of each amino acid residue hydrogen bonded to the C O of an adjacent turn

of the helix.

In β-structure all peptide chains are stretched out to nearly maximum extension

and then laid side by side which are held together by intermolecular hydrogen

bonds. The structure resembles the pleated folds of drapery and therefore is known

as β-pleated sheet

(iii) Tertiary structure of proteins: The tertiary structure of proteins represents

overall folding of the polypeptide chains i.e., further folding of the secondary

structure. It gives rise to two major molecular shapes viz. fibrous and

globular. The main forces which stabilise the 2° and 3° structures of proteins

are hydrogen bonds, disulphide linkages, van der Waals and electrostatic

forces of attraction.

(iv) Quaternary structure of proteins: Some of the proteins are composed of two

or more polypeptide chains referred to as sub-units. The spatial arrangement

of these subunits with respect to each other is known as quaternary

structure.

Denaturation of Proteins

The most stable conformation of a protein at a given temperature and the pH is

known as its native state.

The loss of biological activity of proteins when a protein in its native form, is

subjected to physical change like change in temperature or chemical change like

change in pH, is called denaturation of protein.

For example: (i) Coagulation of egg white on boiling, (ii) curdling of milk which is

caused due to the formation of lactic acid by the lactobacillus bacteria present in

milk.

Nucleic acids

The particles in nucleus of the cell, responsible for heredity, are called chromosomes

which are made up of proteins and another type of biomolecules called nucleic acids.

These are mainly of two types, the deoxyribonucleic acid (DNA) and ribonucleic acid

(RNA). Since nucleic acids are long chain polymers of nucleotides, so they are also

called polynucleotides.

They help in the role of transmission of hereditary characters and synthesis of

proteins.

Chemical Composition of Nucleic Acids

Each nucleotide consists of 3 parts:

A pentose sugar

A nitrogenous base

A phosphate group

DNA molecules, the sugar moiety is β-D-2-deoxyribose whereas in RNA molecule, it is

β-D-ribose.

DNA contains four nitrogenous bases viz. adenine (A), guanine (G), cytosine (C) and

thymine (T)

RNA also contains four bases;

one is uracil (U).

Structure of Nucleic Acids:

A unit formed by the attachment of a base to 1

nucleoside

bases; the first three bases are same as in DNA but the fourth

A unit formed by the attachment of a base to 1′ position of sugar is known as

the first three bases are same as in DNA but the fourth

′ position of sugar is known as

Nucleotides are joined together by phosphodiester linkage between 5

atoms of the pentose sugar. The

A simplified version of nucleic acid chain is as shown below.

Information regarding the sequence of nucleotides in the chain of a nucleic acid is

called its primary structure.

Secondary structure of Nucleic acid

James Watson and Francis Crick gave a double strand helix structure for DNA Two

nucleic acid chains are wound about each other and held together by hydrogen bonds

Nucleotides are joined together by phosphodiester linkage between 5

atoms of the pentose sugar. The formation of a typical dinucleotide

A simplified version of nucleic acid chain is as shown below.


Secondary structure of Nucleic acid



Nucleotides are joined together by phosphodiester linkage between 5′ and 3′ carbon

formation of a typical dinucleotide




between pairs of bases. The two strands are complementary to each other because

the hydrogen bonds are formed between specific pairs of bases. Adenine forms

hydrogen bonds with thymine whereas cytosine forms hydrogen bonds with guanine.

RNA molecules are of three types and they perform different functions. They are

named as messenger RNA (m-RNA), ribosomal RNA (r-RNA) and transfer RNA (t-RNA)

The Functions of Nucleic Acids

Nucleic acids are responsible for the transmission of inherent characters from

parent to offspring.

They are responsible for the synthesis of protein in our body

DNA fingerprinting is a method used by forensic experts to determine paternity.

It is also used for the identification of criminals. It has also played a major role in

studies regarding biological evolution and genetics.

NCERT INTEXT QUESTIONS

Solution 1

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Solution 3

Solution 4

Solution 5

Solution 7

Solution 8

NCERT EXERCISES

Page No. 423-424

Q 1. What are monosaccharides?

Ans:

Monosaccharides known as simple sugars comprise of one sugar unit that cannot be

further broken down into simple sugars.

We can classify a monosaccharide on the basis of the number of carbon atoms and the

functional group present in them. The monosaccharide which contains an aldehyde

group is termed as aldoses and those which have keto group are called ketoses.

Depending on the number of carbon atoms present in a monosaccharide it is further

classified as trioses, tetroses, pentoses, hexoses, and heptoses. As for example, we can

call an aldose which contains 3 carbon atoms as aldotriose and a keto which contains 3

carbon atoms as ketotriose.

Q 2. What are reducing sugars?

Ans:

Those type of carbohydrates which reduces the Fehling’s solution and Tollen’s reagent

are termed as reducing sugars.

Q 3. Write two main functions of carbohydrates in plants.

Ans:

The two main functions of carbohydrate in a plant are:

(a) Polysaccharides like starch act as a storage molecule.

(b) Cellulose is used to build the cell wall, and it is a polysaccharide

Q 4. Classify the following into monosaccharides and disaccharides.

Ribose, 2-deoxyribose, maltose, galactose, fructose and lactose.

Ans:

Monosaccharides: 2-deoxyribose, galactose, Ribose, fructose

Disaccharides: lactose, Maltose

Q 7. What are the hydrolysis products of (i) sucrose and (ii) lactose?

Ans:

(a) The hydrolysis of sucrose will give one molecule of alpha/α -D glucose and one

molecule of beta/β –D fructose.

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Q 9. What happens when D-

(i) HI (ii) Bromine water (iii) HNO3

Ans:

(a) After heating a D-glucose with HI for a long time,

(b) After treating a D-glucose with

(c) After treating with HNO _{3}HNO3

-glucose is treated with the following reagents?

(i) HI (ii) Bromine water (iii) HNO3

glucose with HI for a long time, n-hexane is formed.

glucose with Br _{2}Br2 water, D- gluconic acid is produced.

HNO _{3}HNO3 , D – glucose get oxidised to give saccharic acid.

glucose is treated with the following reagents?

hexane is formed.

gluconic acid is produced.

glucose get oxidised to give saccharic acid.

Q 10. Enumerate the reactions of D

chain structure.

Ans:

(i) The pentaacetate of glucose does not react with hydroxylamine. This shows that a

free −CHO group is not present in glucose.

(ii) Aldehydes form the hydrogen sulphite additional product by giving 2,4

Schiff’s test and react with NaHSO

(iii) Glucose is available in two crystalline forms

form (m.p. = 419 K) crystallises from a concentrated solution of glucose at 30

the beta/β -form (m.p = 423 K) crystallises from a hot and saturated aqueous solution

at 371 K. This behaviour can’t be explained by the open chain structure of glucose.

Q 11. What are essential and non

type.

Ans:

Those amino acids which are required by the human body are called essential amino

acids, but these cannot be produced inside the human body. They must be taken from

any external source like food. As for example leucine and valine.

Those acids which are required by the human body but

produced inside the body are called non

alanine.

Q 12. Define the following as related to proteins

(i) Peptide linkage (ii) Primary structure (iii) Denaturation.

Ans:

(a) Primary Structure

We can refer to the specific sequence, in which various amino acids are present if we

talk about the primary structure of the protein. Like the sequence of linkage between

amino acid in a polypeptide chain.

Enumerate the reactions of D-glucose which cannot be explained by its open


−CHO group is not present in glucose.

(ii) Aldehydes form the hydrogen sulphite additional product by giving 2,4

NaHSO4. But glucose does not undergo these reactions.

(iii) Glucose is available in two crystalline forms alpha/α – and beta

form (m.p. = 419 K) crystallises from a concentrated solution of glucose at 30

form (m.p = 423 K) crystallises from a hot and saturated aqueous solution


What are essential and non-essential amino acids? Give two examples of e



any external source like food. As for example leucine and valine.

Those acids which are required by the human body but these types of acids

produced inside the body are called non – essential amino acids. Example: glycine and

. Define the following as related to proteins

ry structure (iii) Denaturation.



ide chain.

be explained by its open


(ii) Aldehydes form the hydrogen sulphite additional product by giving 2,4 – DNP test,

. But glucose does not undergo these reactions.

beta/β. The alpha/α –

form (m.p. = 419 K) crystallises from a concentrated solution of glucose at 303 K and

form (m.p = 423 K) crystallises from a hot and saturated aqueous solution


essential amino acids? Give two examples of each



these types of acids can be

essential amino acids. Example: glycine and



The amino acids are arranged in a different sequence in each of the proteins. A little

difference in the sequence of the arrangements will create a completely different

protein.

(b) Peptide Linkage

A peptide linkage is an amide which is formed by the elimination of a water molecule

between the –COOH group of one molecule of amino acid and −NH2 group of another

molecule of the amino acid.

(c) Denaturation

A protein has a unique 3 – dimensional structure and a unique biological activity inside

a biological system. In these types of circumstances, proteins are called a native

protein. Whenever we put a native protein into a physical change like change in

temperature or any chemical changes like change in pH, then there its H – bonds are

disturbed or changes.

This result in the unfolding of the globules ad uncoils the helix. And the consequences

of this change are that the protein results in the loss of its biological activity. This loss

of biological activity by the protein is called denaturation. During this process, no

changes are encountered in primary structure whereas tertiary and secondary

structures will be destroyed.

Example for denaturation, proteins is the coagulation of egg white when an egg is

boiled.

Q 13. What are the common types of secondary structure of proteins?

Ans:

Secondary structures of proteins are of two types:

(a) alpha/α – helix structure

(b) beta/β – pleated sheet structures.

alpha/α – helix structure

In this structure, the −NH group of an amino acid residue forms H-bond with the

Group of the adjacent turn of the right – handed screw (\alphaα – helix ).

\betaβ – pleated sheet structures.

This structure is called so because it looks like the pleated folds of drapery. In this

structure, the peptide chains are laid side by side after stretching out near to the

maximum extension. The intermolecular hydrogen bond keeps the peptide chain

together.

Q 14. What type of bonding helps in stabilising the α-helix structure of proteins?

Ans:

The H-bonds formed between the −NH group of each amino acid residue and the

Group of the adjacent turns of the \alphaα -helix help in stabilising the helix.

Q 15. Differentiate between globular and fibrou

Ans:

Globular protein

1.

The polypeptide chain in this

protein is folded around itself,

giving rise to a spherical structure.

2. It is usually soluble in water.

3.

Fibrous proteins are usually used

for structural purposes. For

example, keratin is present in nails

and hair; collagen in tendons; and

myosin in muscles.

Q 16. How do you explain the amphoteric behaviour of amino acids?

Ans:

In the presence of water or aqueous solution, the

lose a proton and the amino group can accept a proton to give a dipolar ion known as

zwitterion.

Therefore, the amino acid can act both as an acid and as a base, in the presence of

zwitterionic form.

So The amino acid show amphoteric behaviour.

Differentiate between globular and fibrous proteins.

Fibrous protein

The polypeptide chain in this

protein is folded around itself,

giving rise to a spherical structure.

1

It is a fibre-like structure formed

by the polypeptide chain. These

are the proteins which are

together by strong hydrogen and

disulphide bonds.

It is usually soluble in water. 2. It is usually soluble in water.

Fibrous proteins are usually used

for structural purposes. For

example, keratin is present in nails

tendons; and

3.

All enzymes are globular proteins.

Some hormones such as insulin

are also globular proteins.

. How do you explain the amphoteric behaviour of amino acids?

In the presence of water or aqueous solution, the carboxyl group of an amino acid can



d show amphoteric behaviour.

Fibrous protein

like structure formed

by the polypeptide chain. These

are the proteins which are held

together by strong hydrogen and

disulphide bonds.

It is usually soluble in water.

All enzymes are globular proteins.

Some hormones such as insulin

are also globular proteins.

. How do you explain the amphoteric behaviour of amino acids?

carboxyl group of an amino acid can



Q 18. What is the effect of denaturation on the structure of proteins?

Ans:

The outcome of denaturation, helixes get uncoiled and globules get unfolded. There

would be no change in the primary structure of the protein while the secondary and

the tertiary structure gets destroyed. We can say that the secondary and the tertiary –

structured proteins are changed into primary – structured proteins. Also, because of

the loss of secondary and the tertiary structure the enzymes loses its activity.

Q 21. What are nucleic acids? Mention their two important functions.

Ans:

It is a molecule which is found as one of the constituents of chromosomes which is

found in the nuclei of all the living cells.

Nucleic acid can be categorised into two categories: ribonucleic acid (RNA) and

deoxyribonucleic acid (DNA).

Nucleic acids are long-chain polymers of nucleotides, so they are also known as

polynucleotides.

The functions of Nucleic acid

(i) It is responsible for heredity. In heredity, there is a transfer of inherent characters

from one generation to another. And this process is held by the DNA.

(ii) The protein cell synthesis is held by the Nucleic acid (both RNA and DNA). The

protein synthesis is majorly done by the various RNA molecules in a cell while DNA

contains the message for the synthesis of a specific protein.

Q 22. What is the difference between a nucleoside and a nucleotide?

Ans:

A Nucleotide is formed by the combination of all the three basic components of nucleic

acids (i.e., base, a pentose sugar, and phosphoric acid).

Therefore, Nucleotide = Base + Sugar + Phosphoric acid

On the other hand, A nucleoside is formed by the attachment of a base to 1’ position

of the sugar.

Nucleoside = Sugar + Base

Q 23. The two strands in DNA are

Ans:

In the helical structure of DNA, the hydrogen bond holds the two strands between

specific pairs of bases. Adenine forms a hydrogen bond with thymine, while cytosine

forms a hydrogen bond with guanine. So,

complementary for each other.

Q 24. Write the important structural and functional differences between DNA and

RNA.

Ans:

The difference on the basis of their functions is:

DNA

1 DNA is the chemical

basis of heredity.

The differences on the basis of their structures are as follows:

DNA

1

The sugar moiety in DNA

molecules is beta/β -D-2

deoxyribose.

2

DNA contains uracil (U).

It does not contain

thymine (T).


The two strands in DNA are not identical but are complementary. Explain.



forms a hydrogen bond with guanine. So, as its result, the two strands acts as a

complementary for each other.

Write the important structural and functional differences between DNA and

The difference on the basis of their functions is:

RNA

1 RNA is not responsible for

heredity.

The differences on the basis of their structures are as follows:

RNA

The sugar moiety in DNA

2 1 The sugar moiety in RNA

molecules is beta/β -D-ribose.

contains uracil (U).

2 RNA does not contain uracil (U).

It contains thymine (T).


not identical but are complementary. Explain.



as its result, the two strands acts as a

Write the important structural and functional differences between DNA and

RNA is not responsible for

ribose.

RNA does not contain uracil (U).

3 The helical structure of

DNA is double-stranded. 3

The helical structure of RNA is

single-stranded.

Question 25:

What are the different types of RNA found in the cell?

Answer

(i) Messenger RNA (m-RNA)

(ii) Ribosomal RNA (r-RNA)

(iii) Transfer RNA (t-RNA)

class-12 chemistry chapter-14 biomolecules

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