Chapter 14: Respiration in Plants – Long Answer Type Questions

CBSE Class 11 Biology – Respiration in Plants | Long Answer Type Questions with Answers (NCERT Based)

Course & Examination Details

Course: CBSE Class 11 Biology
Unit: Unit IV – Plant Physiology
Chapter: Chapter 14 – Respiration in Plants
Prescribed Textbook: NCERT
Board: CBSE

CBSE Board Examination Significance

• Long answer questions usually carry 5 marks
• Test conceptual depth, sequential explanation, and clarity
• Answers must strictly follow NCERT terminology and processes

Section A: Glycolysis (Q1–Q8)

Q1. Define respiration in plants and explain its importance.

Ans:
Respiration in plants is the enzymatic oxidation of organic food substances such as glucose to release energy in the form of ATP. This energy is essential for growth, cell division, repair, active transport, and biosynthesis. Respiration also provides metabolic intermediates used in synthesis of amino acids, fats, and nucleotides. Unlike photosynthesis, respiration occurs continuously in all living plant cells, day and night. Without respiration, plants cannot maintain cellular organisation, growth, or survival, making it a vital life process.

Q2. Describe glycolysis and its significance in respiration.

Ans:
Glycolysis is the first stage of respiration in which one molecule of glucose is partially oxidised into two molecules of pyruvic acid through a series of enzyme-controlled reactions. It occurs in the cytoplasm and does not require oxygen. Glycolysis yields a net gain of two ATP molecules and two NADH molecules. It is significant because it is a universal pathway found in all living organisms and links aerobic and anaerobic respiration. It also supplies intermediates for various biosynthetic pathways.

Q3. Explain the steps and energy yield of glycolysis.

Ans:
Glycolysis occurs in two phases: the preparatory phase and the pay-off phase. In the preparatory phase, glucose is phosphorylated using two ATP molecules to form fructose-1,6-bisphosphate. In the pay-off phase, this compound splits into two three-carbon molecules, which are oxidised to pyruvic acid. Four ATP molecules and two NADH molecules are produced. Since two ATP are consumed initially, the net gain is two ATP per glucose molecule.

Q4. What is the fate of pyruvic acid under aerobic conditions?

Ans:
Under aerobic conditions, pyruvic acid enters the mitochondria, where it undergoes oxidative decarboxylation to form acetyl-CoA. This reaction releases one molecule of carbon dioxide and reduces NAD⁺ to NADH. Acetyl-CoA then enters the Krebs cycle for complete oxidation. This step, called the link reaction, connects glycolysis with the Krebs cycle and is essential for efficient energy production.

Q5. Why is glycolysis considered a universal pathway?

Ans:
Glycolysis is considered a universal pathway because it occurs in almost all living organisms, including plants, animals, fungi, bacteria, and anaerobic organisms. It does not require oxygen and takes place in the cytoplasm, making it suitable for both aerobic and anaerobic respiration. Its universality highlights its evolutionary importance as a fundamental energy-yielding pathway.

Q6. Explain substrate-level phosphorylation with reference to glycolysis.

Ans:
Substrate-level phosphorylation is the direct synthesis of ATP by transfer of a phosphate group from a high-energy intermediate to ADP. In glycolysis, ATP is produced directly during conversion of certain intermediates into pyruvate. This process does not involve the electron transport system or oxygen and contributes to ATP generation under both aerobic and anaerobic conditions.

Q7. How does glycolysis contribute to anaerobic respiration?

Ans:
In anaerobic respiration, glycolysis is the only source of ATP. Pyruvate formed during glycolysis is converted into ethanol or lactic acid, regenerating NAD⁺ required for continuation of glycolysis. This allows ATP production even in absence of oxygen, enabling cells to survive anaerobic conditions.

Q8. Why is glycolysis important for plant metabolism?

Ans:
Glycolysis provides ATP and reducing power required for cellular activities. It also supplies metabolic intermediates used in synthesis of amino acids, lipids, and other biomolecules, making it essential for overall plant metabolism.

Section B: Krebs Cycle (Q9–Q15)

Q9. Describe the Krebs cycle and mention its site.

Ans:
The Krebs cycle is a cyclic pathway in which acetyl-CoA is completely oxidised to carbon dioxide with the release of energy. It occurs in the mitochondrial matrix. Acetyl-CoA combines with oxaloacetic acid to form citric acid, which undergoes a series of reactions, releasing CO₂ and producing NADH, FADH₂, and ATP. These reduced coenzymes later generate ATP through the electron transport system.

Q10. Explain the link reaction between glycolysis and Krebs cycle.

Ans:
The link reaction converts pyruvic acid produced during glycolysis into acetyl-CoA. This occurs in the mitochondria and involves removal of carbon dioxide and reduction of NAD⁺ to NADH. Acetyl-CoA then enters the Krebs cycle for complete oxidation, linking cytoplasmic glycolysis to mitochondrial respiration.

Q11. Why is the Krebs cycle called an amphibolic pathway?

Ans:
The Krebs cycle is amphibolic because it performs both catabolic and anabolic functions. While it breaks down acetyl-CoA to release energy, its intermediates are used in synthesis of amino acids, fatty acids, chlorophyll, and nucleotides. Thus, it integrates energy production and biosynthesis.

Q12. Describe the energy yield of the Krebs cycle.

Ans:
For each acetyl-CoA molecule, the Krebs cycle produces three NADH, one FADH₂, and one ATP molecule. Since one glucose produces two acetyl-CoA molecules, the cycle operates twice per glucose. These reduced coenzymes contribute significantly to ATP production during oxidative phosphorylation.

Q13. Why does Krebs cycle stop in absence of oxygen?

Ans:
Although Krebs cycle does not use oxygen directly, it depends on oxygen for regeneration of NAD⁺ and FAD through the electron transport system. Without oxygen, reduced coenzymes accumulate, halting the cycle.

Q14. Explain the central role of Krebs cycle in metabolism.

Ans:
The Krebs cycle links carbohydrate, fat, and protein metabolism. Its intermediates are used for synthesis of various biomolecules, making it the central metabolic pathway of the cell.

Q15. State two significances of the Krebs cycle.

Ans:
It releases maximum energy from glucose and provides metabolic intermediates for biosynthesis.

Section C: Electron Transport System & Fermentation (Q16–Q25)

Q16. Describe the electron transport system (ETS).

Ans:
ETS is a series of electron carriers located on the inner mitochondrial membrane. Electrons from NADH and FADH₂ pass through carriers to oxygen, releasing energy. This energy pumps protons across the membrane, creating a gradient used by ATP synthase to synthesise ATP.

Q17. What is oxidative phosphorylation? Explain its significance.

Ans:
Oxidative phosphorylation is ATP synthesis using energy released during electron transport to oxygen. It produces maximum ATP and is the most efficient energy-yielding stage of respiration.

Q18. Why is oxygen essential for aerobic respiration?

Ans:
Oxygen acts as the final electron acceptor, allowing continuous electron flow and ATP production. Without oxygen, ETS stops.

Q19. Explain fermentation in plants.

Ans:
Fermentation is anaerobic respiration where glucose is partially oxidised. In plants, alcoholic fermentation converts pyruvate into ethanol and CO₂, producing only two ATP molecules.

Q20. Why is fermentation less efficient than aerobic respiration?

Ans:
Glucose is incompletely oxidised, releasing less energy and producing only two ATP molecules.

Q21. What is respiratory quotient (RQ)?

Ans:
RQ is the ratio of CO₂ evolved to O₂ consumed and indicates the type of respiratory substrate.

Q22. Explain the amphibolic nature of respiration.

Ans:
Respiration provides energy and metabolic intermediates for biosynthesis, functioning both catabolically and anabolically.

Q23. Compare aerobic and anaerobic respiration.

Ans:
Aerobic respiration produces 36 ATP using oxygen, while anaerobic respiration produces only 2 ATP without oxygen.

Q24. Why is respiration a stepwise process?

Ans:
Stepwise reactions ensure controlled energy release and efficient ATP synthesis.

Q25. Explain the importance of respiration in plant growth.

Ans:
Respiration supplies energy and intermediates required for growth, development, maintenance, and reproduction in plants.

Best Suited For

• CBSE Class 11 Annual Examinations
• NCERT-based 5-mark answer practice
• Conceptual clarity and structured writing