Class 12 - Chemistry - Biomolecules

Question 14.1.

What are monosaccharides?


Monosaccharides are carbohydrates that cannot be hydrolysed further to give simpler units of Polyhydroxy aldehyde or ketone.

Monosaccharides are classified on the bases of number of carbon atoms and the functional group present in them.

Monosaccharides containing an aldehyde group are known as aldoses

and those containing a keto group are known as ketoses.

Monosaccharides are further classified as trioses, tetroses, pentoses, hexoses,

and heptoses according to the number of carbon atoms they contain.

For example, a ketose containing 3 carbon atoms is called ketotriose

and an aldose containing 3 carbon atoms is called aldotriose.



Question 14.2.

What are reducing sugars?


A reducing sugar is a carbohydrate that is oxidized by a weak oxidizing agent

(an oxidizing agent capable of oxidizing aldehydes but not alcohols, such as the Tollen’s reagent)

in basic aqueous solution. 

The characteristic property of reducing sugars is that, in aqueous medium, they generate one or more compounds containing an aldehyde group.

For Example. 1:  α-D-glucose, which contains a hemiacetal group and,

therefore, reacts with water to give an open-chain form containing an aldehyde group.




Question 14.3.

Write two main functions of carbohydrates in plants?


The importance of carbohydrates in plants:-

(i) Carbohydrates are used as storage molecules as starch in plants.

(ii) The cell wall of the plants is made up of cellulose



Question 14.4.

Classify the following into monosaccharides and disaccharides.

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


Monosaccharides are the simplest units of carbohydrates which cannot be hydrolysed into simpler compounds.

  1. Maltose and lactose are disaccharides.

A disaccharide is a carbohydrate that is formed when two monosaccharides are joined together

and a molecule of water is removed from the structure.

Lactose is a disaccharide formed from the combination of galactose and glucose



Question 14.5.

What do you understand by the term glycosidic linkage?




Glycosidic linkage refers to the linkage formed between two monosaccharide units through an oxygen atom by the loss of a water molecule.

For example, in a sucrose molecule, two monosaccharide units, ∝-glucose and β-fructose, are joined together by a glycosidic linkage.



Question 14.6.

What is glycogen? How is it different from starch?


Glycogen is a carbohydrate (polysaccharide). In animals, carbohydrates are stored as glycogen.

Starch is a carbohydrate consisting of two components - amylose (15 - 20%) and amylopectin (80 - 85%).


However, glycogen consists of only one component whose structure is similar to amylopectin.

Also, glycogen is more branched than amylopectin.



Question 14.7.

What are the hydrolysis products of?

(i) Sucrose and (ii) lactose


  • On hydrolysis, sucrose gives one molecule of -D glucose and one molecule of β- D fructose.


  • The hydrolysis of lactose gives β-D-galactose and β-D-glucose.



Question 14.8.

What is the basic structural difference between starch and cellulose?


Starch consists of two components – amylase and amylopectin. Amylose is a long linear chain of α–D-(+)-glucose units

joined by C1-C4 glycosidic linkage (α -link).


Amylopectin is a branched-chain polymer of ∝-D-glucose units, in which the chain is formed by C1-C4 glycosidic linkage

and the branching occurs by C1-C6 glycosidic linkage.


On the other hand, cellulose is a straight-chain polysaccharide of β-D-glucose units joined by C1-C4 glycosidic linkage (β-link).



Question 14.9.

What happens when D-glucose is treated with the following reagents?

  • HI
  • Bromine water
  • HNO3?


  • When D-glucose is heated and treated with HI for a long period of time, then n-hexane is formed,
  • which shows that all the six-carbon atoms are linked in a straight chain.


  • When D-glucose is treated with Br2 water, D- gluconic acid is produced.


  • On being treated with HNO3, D-glucose get oxidised to give saccharic acid.




Question 14.10.

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


(1) Aldehydes give 2, 4-DNP test, Schiff’s test, and react with NaHSO4 to form the hydrogen sulphite addition product.

However, glucose does not undergo these reactions.

(2) The pentaacetate of glucose does not react with hydroxylamine. This indicates that a free −CHO group is absent from glucose.

(3) Glucose exists in two crystalline forms − andβ. The -form (melting point = 419 K)

crystallises from a concentrated solution of glucose at 303 K and the β-form (melting point = 423 K)

crystallises from a hot and saturated aqueous solution at 371 K.

This behaviour cannot be explained by the open chain structure of glucose.



Question 14.11.

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


Essential amino acids are required by the human body, but they cannot be synthesised in the body.

They must be taken through food. For example: valine and leucine.


Non-essential amino acids are also required by the human body, but they can be synthesised in the body.

For example: glycine, and alanine.



Question 14.12.

Define the following as related to proteins

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


(i) Peptide linkage:

The amide formed between -COOH group of one molecule of an amino acid and -NH2 group of another

molecule of the amino acid by the elimination of a water molecule is called a peptide linkage.


(ii) Primary structure:

The primary structure of protein refers to the specific sequence in which various amino acids are present in it, i.e.,

the sequence of linkages between amino acids in a polypeptide chain.

The sequence in which amino acids are arranged is different in each protein.

A change in the sequence creates a different protein.

(iii) Denaturation:

In a biological system, a protein is found to have a unique 3-dimensional structure and a unique biological activity.

In such a situation, the protein is called native protein.

However, when the native protein is subjected to physical changes

such as change in temperature or chemical changes such as change in pH, its H-bonds are disturbed.

This disturbance unfolds the globules and uncoils the helix.

As a result, the protein loses its biological activity.

This loss of biological activity by the protein is called denaturation.

During denaturation, the secondary and the tertiary structures of the protein get destroyed,

but the primary structure remains unaltered.

One of the examples of denaturation of proteins is the coagulation of egg white when an egg is boiled.



Question 14.13.

What are the common types of secondary structure of proteins?


There are two common types of secondary structure of proteins:

(i) α–helix structure

(ii) β–pleated sheet structure


α–Helix structure

If the size of R-groups is quite large then the intramolecular bonds are formed between the C=O of one amino acid

and the N-H group of the forth amino acid residue in the chain.

This causes the polypeptide chain to coil up into a spiral structure called right handed α-helix structure.


β-pleated sheet structure:

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

In this structure, all the peptide chains are stretched out to nearly the maximum extension and then laid side by side.

These peptide chains are held together by intermolecular hydrogen bonds.



Question 14.14.

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


The H-bonds formed between the −NH group of each amino acid residue and the –C=O group of the adjacent turns of the -helix help in stabilising the helix.



Question 14.15.

Differentiate between globular and fibrous proteins?


Fibrous Proteins

Globular Proteins

1. It is a fibre-like structure formed by the polypeptide chain. These proteins are held together by strong hydrogen and disulphide bonds.

The polypeptide chain in this protein is folded around itself, giving rise to a spherical structure.

2. It is usually insoluble in water.

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.

All enzymes are globular proteins. Some hormones such as insulin are also globular proteins.




Question 14.16.

How do you explain the amphoteric behaviour of amino acids?


Amino acid has both acidic group (C=O) and basic group (N-H).

Thus in aqueous solution, the carboxyl group can lose a proton and the basic group (amine group) can accept a proton.

In this way, it forms Zwitter ion which can act in both ways i.e., acidic as well as basic. Hence amino acids are amphoteric in nature.



Question 14.17.

What are enzymes?


Enzymes are naturally occurring simple conjugate proteins acting as specific catalysts in all processes.

In contrast to ordinary chemical catalyst, it loses activity by pH or temperature change.


For example: - the enzyme used to catalyse the hydrolysis of maltose into glucose is named as maltase.


C12H22O11 --> 2C6H12O6

Maltose          Glucose

Enzymes are highly specific, i.e., a particular enzyme catalyses a specific reaction. For example, urase attacks on urea.

This specific action is due to active sites present in the enzyme molecule (E) that fits into substrate (S)

and forms E-S complex which changes into product P and E.

Enzymes increase the speed of reactions. They can catalyze several million of reactions per second.



Question 14.18.

What is the effect of denaturation on the structure of proteins?


As a result of denaturation, globules get unfolded and helixes get uncoiled.

Secondary and tertiary structures of protein are destroyed, but the primary structures remain unaltered.

It can be said that during denaturation, secondary and tertiary-structured proteins get converted into primary-structured proteins.

Also, as the secondary and tertiary structures of a protein are destroyed, the enzyme loses its activity.



Question 14.19.

How are vitamins classified? Name the vitamin responsible for the coagulation of blood?


On the basis of their solubility in water or fat, vitamins are classified into two groups.

  • Fat-soluble vitamins: Vitamins that are soluble in fat and oils, but not in water, belong to this group. For example: Vitamins A, D, E, and K
  • Water-soluble vitamins: Vitamins that are soluble in water belong to this group.
  • For example: B group vitamins (B1, B2, B6, B12, etc.) and vitamin C However, biotin or vitamin H is
  • neither soluble in water nor in fat. Vitamin K is responsible for the coagulation of blood.

Vitamin H (Biotin) as an exception, it is neither soluble in water nor in fat



Question 14.20.

Why are vitamin A and vitamin C essential to us? Give their important sources?


The deficiency of vitamin A leads to xerophthalmia (hardening of the cornea of the eye) and night blindness.

The deficiency of vitamin C leads to scurvy (bleeding gums).

The sources of vitamin A are fish liver oil, carrots, butter, and milk. The sources of vitamin C are citrus fruits, amla, and green leafy vegetables.




Question 14.21.

What are nucleic acids? Mention their two important functions.


Nucleic acids are Biomolecules which are found in the nuclei of all living cells,

inform of nucleoproteins or chromosomes (proteins containing nucleic acids as the prosthetic group).

Nucleic acids are of two types: – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

Nucleic acids are also known as Polynucleotide as they are long- chain polymers of nucleotides.

The two important functions of nucleic acids are listed below:-

(i) DNA which is responsible for the transference of hereditary effects from one generation to another,

which is due to their property of replication during cell division as a result of which two identical DNA strands are transferred to the daughter cells.

(ii) Nucleic acids (both DNA and RNA) are responsible for synthesis of all proteins needed for the growth and maintenance of our body.

Actually, the proteins are synthesised by various RNA molecules in the cell but the message for the synthesis of a particular protein is given by DNA molecules.



Question 14.22.

What is the difference between a nucleoside and a nucleotide?


A nucleoside is formed by the attachment of a base to position of sugar.

Nucleoside = Sugar + Base


On the other hand, all the three basic components of nucleic acids (i.e., pentose sugar, phosphoric acid, and base) are present in a nucleotide.

Nucleotide = Sugar + Base + Phosphoric acid



Structure of (a) a nucleoside and (b) a nucleotide



Question 14.23.

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



In the helical structure of DNA, the two strands are held together by hydrogen bonds between specific pairs of bases.

Cytosine from hydrogen bond with guanine, while adenine forms hydrogen bond with thymine,

As a result, the two strands are complementary to each other.

DNA consists of two strands of nucleic acid chains coiled around each other in the form of a double helix.

The base of one strand of DNA is paired with bases on other strand by means of hydrogen bonding.

This hydrogen bonding is very specific as the bases can only base pair in a complementary manner.

Adenine pairs with only thymine via 2 hydrogen bonds and guanine pairs with cytosine through 3 hydrogen bonds.

Thus, the two strands of DNA are complementary to each other in the sense that the sequence of bases in one strand automatically

determines that of the other. So the DNA stands cannot be identical, but they are complementary to each other.




Question 14.24.

Write the important structural and functional differences between DNA and RNA?


The structural differences between DNA and RNA are as follows:



1.   The sugar moiety in DNA molecules is β-D-2 deoxyribose.

The sugar moiety in RNA molecules is β-D-ribose.

2.   DNA contains uracil (U). It does not contain thymine (T).

RNA contains thymine (T). It does not contain uracil (U).

3.   The helical structure of DNA is double-stranded.

The helical structure of RNA is single-stranded.

The functional differences between DNA and RNA are as follows:



1. DNA is the chemical basis of heredity.

RNA is not responsible for heredity.

2. Proteins are synthesised by RNA molecules in the cells.

DNA molecules do not synthesise proteins, but transfer coded message for the synthesis of proteins in the cells.


Question 14.25.

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


The different types of RNA found in the cell are listed below:-

(i) Messenger RNA (m-RNA)

It carries the genetic message code from the DNA to ribosomes.

It is produced by the DNA; m-RNA is also single stranded and constitutes about 15% of total RNA.

(ii) Ribosomal RNA (r-RNA)

It is found in the ribosomes and it is usually associated with protein to form the ribosomes.

It is synthesised in the nucleus by DNA. It is single stranded, comprising about 80% of total RNA. It is metabolically stable.

(iii) Transfer RNA (t-RNA)

It is synthesised in nucleus by DNA. It is also called soluble RNA. It is single stranded.

There are 20 different kinds of t-RNA and each type has specificity for a particular amino acid.

It constitutes about 5% of total RNA. It has very short life.

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