Chapter 9: Biomolecules – Case-Based / Source-Based Questions with Answers

CBSE Class 11 Biology Chapter 9 Biomolecules: 25 Case-Based & Source-Based Questions with Answers (NCERT)

Course Details (Systematic Format)

Course: CBSE Class 11 Biology
Unit: III – Cell Structure and Function
Chapter: 9 – Biomolecules
Based on: NCERT Syllabus
Examination: CBSE Board Examination (Class XI)
Assessment Focus: Conceptual clarity, chemical nature, structures, application-based understanding

Section A: Carbohydrates (Case-Based Questions Q1–Q6)

Case 1:

A student tests two plant samples using iodine solution. Sample A turns blue-black, while Sample B shows no colour change.

Q1. Identify the carbohydrate present in Sample A. Explain the basis of the test.
Answer:
Sample A contains starch. Iodine reacts with the helical structure of amylose in starch, forming a blue-black complex. This colour change confirms the presence of starch, a storage polysaccharide in plants.

Q2. Why does Sample B not show a colour change with iodine?
Answer:
Sample B lacks starch. Carbohydrates like glucose, sucrose, or cellulose do not form a complex with iodine, so no colour change is observed.

Case 2:

Two polysaccharides, X and Y, are made of glucose. X is digestible by humans, while Y is not.

Q3. Identify polysaccharides X and Y. Justify your answer.
Answer:
X is starch and Y is cellulose. Starch has α-1,4 glycosidic bonds that human enzymes can digest, whereas cellulose has β-1,4 bonds which humans lack enzymes to break.

Q4. Mention one biological role each of X and Y.
Answer:
Starch acts as an energy storage molecule in plants, while cellulose provides structural strength to plant cell walls.

Case 3:

A disaccharide breaks down into glucose and fructose on hydrolysis.

Q5. Name the disaccharide and describe its glycosidic linkage.
Answer:
The disaccharide is sucrose. It has an α-1,2 glycosidic bond between glucose and fructose.

Q6. Why is sucrose considered a non-reducing sugar?
Answer:
Sucrose lacks a free aldehyde or ketone group because both are involved in bond formation, making it non-reducing.

Section B: Proteins (Case-Based Questions Q7–Q12)

Case 4:

Egg white becomes solid when heated.

Q7. Which biomolecule is affected here and what phenomenon occurs?
Answer:
Proteins are affected, and the phenomenon is denaturation. Heat disrupts the protein’s secondary and tertiary structures, causing coagulation.

Q8. Does denaturation break peptide bonds? Explain.
Answer:
No, denaturation does not break peptide bonds. It only disrupts weak bonds like hydrogen bonds responsible for folding.

Case 5:

A protein loses its function due to a change in amino acid sequence.

Q9. Which level of protein structure is altered initially?
Answer:
The primary structure is altered, which affects higher-level folding and protein function.

Q10. Why is amino acid sequence critical for protein function?
Answer:
The sequence determines folding patterns and active sites, directly influencing the protein’s biological activity.

Case 6:

Two proteins differ in solubility and function—one is structural, the other enzymatic.

Q11. Identify the types of proteins involved.
Answer:
The structural protein is fibrous, while the enzymatic protein is globular.

Q12. Give one example of each type.
Answer:
Fibrous protein: keratin; Globular protein: enzymes such as amylase.

Section C: Lipids (Case-Based Questions Q13–Q17)

Case 7:

A lipid sample remains solid at room temperature.

Q13. What type of fatty acids are present?
Answer:
Saturated fatty acids are present, as they have no double bonds and remain solid at room temperature.

Q14. Mention one source of such lipids.
Answer:
Butter and animal fats are common sources of saturated fatty acids.

Case 8:

Cell membranes remain flexible even at low temperatures.

Q15. Which lipid contributes to this property?
Answer:
Cholesterol contributes to membrane fluidity and stability at varying temperatures.

Q16. Name the major lipid component forming membrane bilayers.
Answer:
Phospholipids form the basic bilayer structure of cell membranes.

Case 9:

Seeds store large amounts of energy-rich molecules.

Q17. Identify the lipid involved and its function.
Answer:
Triglycerides are stored in seeds and provide energy during germination.

Section D: Nucleic Acids (Case-Based Questions Q18–Q22)

Case 10:

A nucleic acid contains uracil instead of thymine.

Q18. Identify the nucleic acid.
Answer:
The nucleic acid is RNA.

Q19. Mention one structural difference between this nucleic acid and DNA.
Answer:
RNA contains ribose sugar and is usually single-stranded, unlike DNA.

Case 11:

During DNA replication, adenine pairs with thymine.

Q20. What type of bonding is involved in this pairing?
Answer:
Hydrogen bonding is involved in complementary base pairing.

Q21. Why is complementary base pairing important?
Answer:
It ensures accurate replication and transmission of genetic information.

Case 12:

A genetic material controls protein synthesis in a cell.

Q22. Name the molecule responsible and its role.
Answer:
DNA stores genetic information and directs protein synthesis via RNA.

Section E: Enzymes & Metabolism (Case-Based Questions Q23–Q25)

Case 13:

An enzyme works best at pH 7 but loses activity at extreme pH.

Q23. Explain the reason for loss of activity.
Answer:
Extreme pH alters enzyme structure, disrupting the active site and causing denaturation.

Case 14:

A metabolic pathway stops when the end product accumulates.

Q24. Identify this regulatory mechanism.
Answer:
The mechanism is feedback inhibition, where the end product inhibits an earlier enzyme.

Case 15:

A cell requires immediate energy for active transport.

Q25. Which molecule supplies this energy and why?
Answer:
ATP supplies energy because it releases energy rapidly by breaking high-energy phosphate bonds.

✅ Academic Relevance

✔ Strictly aligned with NCERT Class 11 Biology
✔ Ideal for CBSE case-based and competency-based questions
✔ Enhances application-oriented understanding of Biomolecules