Appeared in: 2026 (2 sets)
Living systems are made up of various complex biomolecules like carbohydrates, proteins, nucleic acids, lipids, etc. Proteins and carbohydrates are essential constituents of our food. In addition, some simple molecules like vitamins and mineral salts also play an important role in the functions of organisms. All proteins are polymers of $\alpha$-amino acids. Proteins can be classified into two types on the basis of their molecular shape — Fibrous and Globular proteins. Vitamins are accessory food factors required in the diet. They are classified as fat-soluble and water-soluble. Deficiency of vitamins leads to many diseases. Nucleic acids are the polymers of nucleotides which in turn consist of a base, a pentose sugar and phosphate moiety. There are two types of nucleic acids — DNA and RNA. Nucleic acids are responsible for the transfer of characters from parents to offsprings.
(a) Write the name of basic building units of proteins and nucleic acids. How can you differentiate between Fibrous and Globular proteins on the basis of their structures ?
(b) (i) What products would be formed when a nucleotide from DNA containing thymine is hydrolyzed ?
OR
(b) (ii) Write one structural difference between DNA and RNA.
(c) Give one example each of a fat-soluble vitamin and a water-soluble vitamin.
✓ Verified CBSE Marking Scheme
Official Evaluation Steps & EL10 Insight
CBSE Class 12 Chemistry: Biomolecules Marking Scheme
(a) Basic Building Units and Protein Classification [1.5 Marks]
- Step 1 (Identification): [0.5 Mark] The basic building units of proteins are $\alpha$-amino acids, and the basic building units of nucleic acids are nucleotides.
- Step 2 (Structural Differentiation): [1.0 Mark]
- Fibrous Proteins: Polypeptide chains run parallel and are held together by hydrogen and disulphide bonds, forming a fibre-like structure. They are generally insoluble in water (e.g., keratin).
- Globular Proteins: Polypeptide chains fold around themselves to give a spherical shape. These are usually soluble in water (e.g., insulin, albumin).
(b) (i) Hydrolysis of DNA Nucleotide [1.0 Mark]
- Step 1 (Reaction Components): [0.5 Mark] A nucleotide consists of a nitrogenous base, a pentose sugar, and a phosphate group.
- Step 2 (Final Products): [0.5 Mark] Upon complete hydrolysis, a DNA nucleotide containing thymine yields:
Thymine (base) + 2-deoxy-D-ribose (sugar) + Phosphoric acid ($H_3PO_4$).
OR (b) (ii) Structural Difference between DNA and RNA [1.0 Mark]
- Step 1 (Comparison): [1.0 Mark] DNA contains 2-deoxy-D-ribose sugar and the nitrogenous base thymine, whereas RNA contains D-ribose sugar and the nitrogenous base uracil.
(c) Examples of Vitamins [1.5 Marks]
- Step 1 (Fat-soluble): [0.75 Mark] Example: Vitamin A (or Vitamin D, E, K).
- Step 2 (Water-soluble): [0.75 Mark] Example: Vitamin C (or Vitamin B-complex).
💡 EL10 Concept Insight: Think of proteins like a ball of yarn; fibrous proteins are like straight, organized threads, while globular proteins are like the yarn tangled into a neat, round ball. Just like a Lego set, DNA and RNA are built from specific blocks (nucleotides) that, when broken down, reveal the exact pieces—sugar, phosphate, and base—used to construct them.