NCERT Solutions for Class 12 Chemistry Chapter 10 Biomolecules

NCERT
SOLUTIONS
CLASS - 12th

aglase .co

Class : 12th
Subject : Chemistry
Chapter : 14
Chapter Name : Biomolecules

Q14.1 Glucose or sucrose are soluble in water but cyclohexane or benzene (simple six membered
ring compounds) are insoluble in water. Explain.

Answer. A glucose molecule contains ve —OH groups while a sucrose molecule contains eight —
OH groups. Thus, glucose and sucrose undergo extensive H- bonding with water. Hence, these are
soluble in water. But cyclohexane and benzene do not contain —OH groups. Hence, they cannot
undergo H-bonding with water and as a result, are insoluble in water.

Page : 420 , Block Name : Intext Questions

Q14.2 What are the expected products of hydrolysis of lactose?

Answer. Lactose is composed of ß-D galactose and 3-D glucose. Thus, on hydrolysis, it gives 3-D
galactose and ß-D glucose.

C12 H22 O11 + H2 O C6 H12 O6 C6 H12 O6
⟶ +
Lactose D − (+) - Glucose D − (+) − Galactose

Page : 420 , Block Name : Intext Questions

Q14.3 How do you explain the absence of aldehyde group in the pentaacetate of D-glucose?

Answer. D-glucose reacts with hydroxylamine (NH OH)to form an oxime because of the presence
2

of aldehydic (—CHO) group or carbonyl carbon. This happens as the cyclic structure of glucose
forms an open chain structure in an aqueous medium, which then reacts with NH OH to give an
2

oxime.

But pentaacetate of D-glucose does not react with NH OH. This is because pentaacetate does not
2

form an open chain structure.

Page : 420 , Block Name : Intext Questions

Q14.4 The melting points and solubility in water of amino acids are generally higher than that of
the corresponding halo acids. Explain.

Answer. Both acidic (carboxyl) as well as basic (amino) groups are present in the same molecule of
amino acids. In aqueous solutions, the carboxyl group can lose a proton and the amino group can
accept a proton, thus giving rise to a dipolar ion known as a zwitter ion.

Due to this dipolar behaviour, they have strong electrostatic interactions within them and with
water. But halo-acids do not exhibit such dipolar behaviour.
For this reason, the melting points and the solubility of amino acids in water is higher than those
of the corresponding halo-acids.

Page : 425 , Block Name : Intext Questions

Q14.5 Where does the water present in the egg go after boiling the egg?

Answer. When an egg is boiled, the proteins present inside the egg get denatured and coagulated.
After boiling the egg, the water present in it is absorbed by the coagulated protein through H-
bonding.

Page : 425 , Block Name : Intext Questions

Q14.6 Why cannot vitamin C be stored in our body?

Answer. Vitamin C cannot be stored in our body because it is water soluble. As a result, it is readily
excreted in the urine.

Page : 431 , Block Name : Intext Questions

Q14.7 What products would be formed when a nucleotide from DNA containing thymine is
hydrolysed?

Answer. When a nucleotide from the DNA containing thymine is hydrolyzed, thymine ß-D-2-
deoxyribose and phosphoric acid are obtained as products.

Page : 431 , Block Name : Intext Questions

Q14.8 When RNA is hydrolysed, there is no relationship among the quantities of different bases
obtained. What does this fact suggest about the structure of RNA?

Answer. A DNA molecule is double-stranded in which the pairing of bases occurs. Adenine always
pairs with thymine, while cytosine always pairs with guanine. Therefore, on hydrolysis of DNA, the
quantity of adenine produced is equal to that of thymine and similarly, the quantity of cytosine is
equal to that of guanine. But when RNA is hydrolyzed, there is no relationship among the
quantities of the different bases obtained. Hence, RNA is single-stranded.

Page : 431 , Block Name : Intext Questions

Q14.1 What are monosaccharides?

Answer. Monosaccharides are carbohydrates that cannot be hydrolysed further to give simpler
units of polyhydroxy aldehyde or ketone. Monosaccharides are classi ed 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 classi ed 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.

Q14.2 What are reducing sugars?

Answer. Reducing sugars are carbohydrates that reduce Fehling's solution and Tollen's reagent. All
monosaccharides and disaccharides, excluding sucrose, are reducing sugars.

Q14.3 Write two main functions of carbohydrates in plants.

Answer. Two main functions of carbohydrates in plants are:
(i) Polysaccharides such as starch serve as storage molecules.
(ii) Cellulose, a polysaccharide, is used to build the cell wall.

Q14.4 Classify the following into monosaccharides and disaccharides. Ribose, 2-deoxyribose,
maltose, galactose, fructose and lactose

Answer. Monosaccharides: Ribose, 2-deoxyribose, galactose, fructose Disaccharides: Maltose,
lactose

Q14.5 What do you understand by the term glycosidic linkage?

Answer. 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 fructose, are joined
together by a glycosidic linkage.

Q14.6 What is glycogen? How is it different from starch?

Answer. 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.

Q14.7 What are the hydrolysis products of (i) sucrose and (ii) lactose?

Answer.

Q14.8 What is the basic structural difference between starch and cellulose?

Answer.

Q14.9 What happens when D-glucose is treated with the following reagents?
(i) HI
(ii) Bromine water
(iii) HNO 3

Answer.

Q14.10 Enumerate the reactions of D-glucose which cannot be explained by its open chain
structure.

Answer. (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 — α and ß. The (m.p. = 419 K) crystallises from a
concentrated solution of glucose at 303 K and the ß-form (m.p = 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.

Q14.11 What are essential and non-essential amino acids? Give two examples of each type.

Answer. 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

Q14.12 De ne the following as related to proteins
(i) Peptide linkage
(ii) Primary structure
(iii) Denaturation.

Answer. (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 speci c 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.

Q14.13 What are the common types of secondary structure of proteins?

Answer. There are two common types of secondary structure of proteins:
(i) 𝛼-helix structure
(ii) ß-pleated sheet structure
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 (𝛼-helix)

ß-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.

Q14.14 What type of bonding helps in stabilising the cc helix structure of proteins?

Answer. 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.

Q14.15 Differentiate between globular and brous proteins.

Answer.

Q14.16 How do you explain the amphoteric behaviour of amino acids?

Answer. In aqueous solution, the carboxyl group Of an amino acid can lose a proton and the amino
group can accept a proton to give a dipolar ion known as zwitter ion.
Therefore, in zwitter ionic form, the amino acid can act both as an acid and as a base.

Thus, amino acids show amphoteric behaviour.

Q14.17 What are enzymes?

Answer. Enzymes are proteins that catalyse biological reactions. They are very speci c in nature

and catalyse only a particular reaction for a particular substrate. Enzymes are usually named after
the particular substrate or class of substrate and sometimes after the particular reaction. For
example, the enzyme used to catalyse the hydrolysis of maltose into glucose is named as maltase.
C12 H22 O11 2C6 H12 O6
⟶
Maltose Glucose

Again, the enzymes used to catalyse the oxidation of one substrate with the simultaneous
reduction of another substrate are named as oxidoreductase enzymes.
The name of an enzyme ends with ‘-ase'.

Q14.18 What is the effect of denaturation on the structure of proteins?

Answer. 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.

Q14.19 How are vitamins classi ed? Name the vitamin responsible for the coagulation of blood.

Answer. On the basis of their solubility in water or fat, vitamins are classi ed into two groups.
(i) 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
(ii) Water-soluble vitamins: Vitamins that are soluble in water belong to this group.
For example: B group vitamins (Bl, 82, 86, 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.

Q14.20 Why are vitamin A and vitamin C essential to us? Give their important sources.

Answer. The de ciency of vitamin A leads to xerophthalmia (hardening of the cornea of the eye)
and night blindness. The de ciency of vitamin C leads to scurvy (bleeding gums). The sources of
vitamin A are sh liver oil, carrots, butter, and milk. The sources of vitamin C are citrus fruits,
amla, and green leafy vegetables.

Q14.21 What are nucleic acids? Mention their two important functions.

Answer. Nucleic acids are biomolecules found in the nuclei of all living cells, as one of the
constituents of chromosomes. There are mainly two types of nucleic acids — deoxyribonucleic acid

(DNA) and ribonucleic acid (RNA). Nucleic acids are also known as polynucleotides as they are
long-chain polymers of nucleotides.
Two main functions of nucleic acids are:
(i) DNA is responsible for the transmission of inherent characters from one generation to the next.
This process of transmission is called heredity.
(ii) Nucleic acids (both DNA and RNA) are responsible for protein synthesis in a cell. Even though
the proteins are actually synthesised by the various RNA molecules in a cell, the message for the
synthesis of a particular protein is present in DNA.

Q14.22 What is the difference between a nucleoside and a nucleotide?

Answer. A nucleoside is formed by the attachment of a base to 1' 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

Q14.23 The two strands in DNA are not identical but are complementary. Explain.

Answer. In the helical structure of DNA, the two strands are held together by hydrogen bonds
between speci c pairs of bases. Cytosine forms hydrogen bond with guanine, while adenine forms
hydrogen bond with thymine. As a result, the two strands are complementary to each other.

Q14.24 Write the important structural and functional differences between DNA and RNA.

Answer.

Q14.25 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)