Chapter 14: Biomolecules Class 12 Chemistry NCERT Solutions

NCERT Solutions for Class 12 Chemistry Chapter 14

Biomolecules – Exercise Questions with Answers

Question 14.1: Glucose or sucrose are soluble in water, but cyclohexane and benzene are insoluble in water. Explain.

Answer:
The solubility follows the principle “like dissolves like.” Glucose and sucrose have multiple hydroxyl (-OH) groups, allowing them to form hydrogen bonds with water, making them soluble. Cyclohexane and benzene are non-polar molecules without hydroxyl groups and thus cannot form hydrogen bonds, rendering them insoluble in water.

Question 14.2: What are the expected products of hydrolysis of lactose?

Answer:
Lactose is composed of D-(+)-glucose and D-(+)-galactose. Upon hydrolysis, lactose yields one molecule of glucose and one molecule of galactose.

Question 14.3: How do you explain the absence of aldehyde group in the pentaacetate of D-glucose?

Answer:
In the cyclic form of glucose, the aldehyde group reacts with the hydroxyl group to form a hemiacetal. When glucose is treated with acetic anhydride, all hydroxyl groups, including the one at C-1, are acetylated, preventing the formation of an open-chain structure. Thus, the aldehyde group is absent in the pentaacetate form of glucose.

Question 14.4: The melting points and solubility in water of α-amino acids are generally higher than those of corresponding haloacids. Explain.

Answer:
α-Amino acids are dipolar in nature, leading to strong intermolecular dipolar interactions and hydrogen bonding, which result in higher melting points and greater solubility in water. Haloacids lack such strong dipolar character and hydrogen bonding ability, leading to lower melting points and solubility.

Question 14.5: Where does the water present in the egg go after boiling the egg?

Answer:
During boiling, proteins in the egg denature and coagulate, trapping the water molecules within the protein matrix via hydrogen bonding. The water does not evaporate but remains absorbed in the solidified protein network.

Question 14.6: Why cannot Vitamin C be stored in our body?

Answer:
Vitamin C is water-soluble, and any excess is excreted through urine. Therefore, it is not stored in the body and must be regularly supplied through the diet.

Question 14.7: Which products would be formed when a nucleotide from DNA containing thymine is hydrolyzed?

Answer:
Hydrolysis of a DNA nucleotide containing thymine yields:
– 2-Deoxyribose (sugar)
– Phosphoric acid (phosphate group)
– Thymine (nitrogenous base)

Question 14.8: When RNA is hydrolyzed, there is no relationship among the quantities of different bases obtained. What does this suggest about the structure of RNA?

Answer:
Unlike DNA, RNA does not follow complementary base pairing rules, as it is mostly single-stranded. Consequently, hydrolysis yields unequal amounts of adenine, guanine, cytosine, and uracil bases, suggesting that RNA lacks the fixed base pairing present in DNA.

NCERT Solutions for Class 12 Chemistry Chapter 14

Biomolecules – Exercise Questions with Answers

Question 14.1: What are monosaccharides?

Answer:
Monosaccharides are the simplest form of carbohydrates that cannot be hydrolyzed into smaller molecules. Their general formula is (CH₂O)ₙ, where n ranges from 3 to 7. They are classified as aldoses (with an aldehyde group) or ketoses (with a keto group). Examples include glucose, fructose, and ribose.

Question 14.2: What are reducing sugars?

Answer:
Reducing sugars have free aldehyde (-CHO) or ketone (>C=O) groups capable of reducing Tollen’s reagent or Fehling’s solution. Examples include glucose and fructose. Non-reducing sugars like sucrose lack such free groups.

Question 14.3: Write two main functions of carbohydrates in plants.

Answer:
– Structural function: Cellulose provides structural support in plant cell walls.
– Energy storage: Starch stores energy in seeds, tubers, and roots.

Question 14.4: 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

Question 14.5: What do you understand by the term glycosidic linkage?

Answer:
A glycosidic linkage is an ether bond between two monosaccharide units formed by the loss of a water molecule, connecting a hydroxyl group of one sugar to the anomeric carbon of another.

Question 14.6: What is glycogen? How is it different from starch?

Answer:
Glycogen is a highly branched polysaccharide storing glucose in animals. It has more frequent branches than starch, making it more compact. Starch, found in plants, consists of amylose (linear) and amylopectin (branched).

Question 14.7: What are the hydrolysis products of (i) sucrose, and (ii) lactose?

Answer:
(i) Sucrose hydrolyzes into glucose and fructose.
(ii) Lactose hydrolyzes into glucose and galactose.

Question 14.8: What is the basic structural difference between starch and cellulose?

Answer:
– Starch: Made of α-D-glucose units linked by α-1,4 and α-1,6 glycosidic bonds.
– Cellulose: Made of β-D-glucose units linked by β-1,4 glycosidic bonds.

Question 14.9: What happens when D-glucose is treated with the following reagents?

Answer:
(i) HI: Reduces D-glucose to n-hexane.
(ii) Bromine water: Oxidizes D-glucose to gluconic acid.
(iii) HNO₃: Oxidizes D-glucose to glucaric acid.

Question 14.10: Enumerate the reactions of D-glucose that cannot be explained with the open-chain structure.

Answer:
– No reaction with 2,4-DNP.
– No reaction with NaHSO₃.
– No restoration of pink color with Schiff’s reagent.
– Pentaacetate of glucose does not react with hydroxylamine.
These observations support its cyclic structure.

Question 14.11: What are essential and non-essential amino acids? Give two examples of each.

Answer:
– Essential amino acids: Cannot be synthesized by the body (e.g., valine, leucine).
– Non-essential amino acids: Synthesized by the body (e.g., glycine, alanine).

Question 14.12: Define the following related to proteins: (i) Peptide linkage (ii) Primary structure (iii) Denaturation

Answer:
(i) Peptide linkage: Bond between the carboxyl group of one amino acid and the amino group of another.
(ii) Primary structure: Specific sequence of amino acids.
(iii) Denaturation: Loss of native protein structure due to temperature or pH changes.

Question 14.13: What are the common types of secondary structure of proteins?

Answer:
– α-helix: Spiral structure stabilized by hydrogen bonds.
– β-pleated sheet: Flat sheet formed by hydrogen bonding between chains.

Question 14.14: What types of bonding help in stabilizing the α-helix structure of proteins?

Answer:
Hydrogen bonding between the carbonyl oxygen of one amino acid and the amide hydrogen of the fourth amino acid stabilizes the α-helix.

Question 14.15: Differentiate between globular and fibrous proteins.

Answer:
– Fibrous proteins: Linear, insoluble in water, provide structural support (e.g., keratin, collagen).
– Globular proteins: Spherical, soluble in water, perform dynamic functions (e.g., enzymes, hormones).

Question 14.16: How do you explain the amphoteric behaviour of amino acids?

Answer:
Amino acids contain both acidic (-COOH) and basic (-NH₂) groups. In aqueous solution, they form zwitterions, allowing them to react with both acids and bases, thus showing amphoteric behavior.

Question 14.17: What are enzymes?

Answer:
Enzymes are biological catalysts that speed up chemical reactions in living organisms. They are highly specific and efficient, functioning optimally at particular temperatures and pH values.

Question 14.18: What is the effect of denaturation on the structure of proteins?

Answer:
Denaturation disrupts hydrogen bonds, altering secondary and tertiary structures, and leading to loss of biological activity. Primary structure remains intact.

Question 14.19: How are vitamins classified? Name the vitamin responsible for coagulation of blood.

Answer:
– Water-soluble vitamins: B-complex, Vitamin C.
– Fat-soluble vitamins: Vitamins A, D, E, and K.
Vitamin K is responsible for blood coagulation.

Question 14.20: Why are vitamin A and vitamin C essential to us? Give their important sources.

Answer:
– Vitamin A: Essential for vision; sources include fish liver oil, carrots, butter.
– Vitamin C: Maintains healthy gums and tissues; sources include citrus fruits, amla, and leafy vegetables.

Question 14.21: What are nucleic acids? Mention their two important functions.

Answer:
Nucleic acids (DNA and RNA) are biomolecules that:
– DNA: Store and transmit genetic information.
– RNA: Help in protein synthesis.

Question 14.22: What is the difference between a nucleoside and a nucleotide?

Answer:
– Nucleoside: Nitrogenous base + Pentose sugar.
– Nucleotide: Nucleoside + Phosphate group.

Question 14.23: The two strands in DNA are not identical but are complementary. Explain.

Answer:
DNA strands are complementary because of specific base pairing: Adenine (A) pairs with Thymine (T) and Guanine (G) pairs with Cytosine (C), but they are not identical sequences.

Question 14.24: Write the important structural and functional differences between DNA and RNA.

Answer:
– Structure: DNA is double-stranded, RNA is single-stranded.
– Sugar: DNA contains deoxyribose; RNA contains ribose.
– Bases: DNA has thymine (T); RNA has uracil (U).
– Function: DNA stores genetic information; RNA helps in protein synthesis.

Question 14.25: What are the different types of RNA found in the cell?

Answer:
– Ribosomal RNA (rRNA): Forms ribosomes.
– Messenger RNA (mRNA): Carries genetic code.
– Transfer RNA (tRNA): Brings amino acids during protein synthesis.