CBSE Class 11 Biology – Respiration in Plants | Short 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 Relevance

• Short answer questions test concept clarity, sequencing, and explanation
• Commonly asked in unit tests, term exams, and annual exams
• Answers must be precise, NCERT-terminology based, and well-structured

Section A: Glycolysis (Questions 1–15)

Q1. Define respiration in plants and state its significance.

Ans:
Respiration in plants is the enzymatic oxidation of organic food materials to release energy in the form of ATP. It is significant because ATP produced during respiration provides energy for growth, repair, active transport, and biosynthesis. Respiration also supplies metabolic intermediates required for synthesis of proteins, fats, and nucleic acids, thus maintaining cellular activities.

Q2. What is glycolysis? Mention its site and importance.

Ans:
Glycolysis is the first step of respiration in which one glucose molecule is partially oxidised into two molecules of pyruvic acid. It occurs in the cytoplasm and does not require oxygen. Glycolysis is important because it is a universal pathway present in all organisms and provides ATP, NADH, and intermediates for other metabolic processes.

Q3. Why is glycolysis called the EMP pathway?

Ans:
Glycolysis is called the EMP pathway because it was described by Embden, Meyerhof, and Parnas. These scientists explained the stepwise enzymatic reactions involved in the breakdown of glucose into pyruvate. The pathway forms the foundation of both aerobic and anaerobic respiration in living organisms.

Q4. Explain the preparatory phase of glycolysis.

Ans:
The preparatory phase of glycolysis involves phosphorylation of glucose using ATP. Glucose is converted into fructose-1,6-bisphosphate through a series of reactions. This phase consumes two ATP molecules and prepares the glucose molecule for splitting into two three-carbon compounds in the subsequent steps.

Q5. Describe the pay-off phase of glycolysis.

Ans:
In the pay-off phase of glycolysis, fructose-1,6-bisphosphate is split into two three-carbon molecules, which are further oxidised to form pyruvic acid. This phase results in the formation of four ATP molecules and two NADH molecules, leading to a net gain of two ATP molecules.

Q6. What is the net energy yield of glycolysis?

Ans:
During glycolysis, four ATP molecules are produced and two ATP molecules are consumed, resulting in a net gain of two ATP molecules per glucose. Additionally, two molecules of NADH are formed, which can produce ATP during electron transport in aerobic respiration.

Q7. What is substrate-level phosphorylation?

Ans:
Substrate-level phosphorylation is the direct synthesis of ATP from ADP by transfer of a phosphate group from a phosphorylated intermediate. It occurs during glycolysis and Krebs cycle and does not involve the electron transport system or oxygen.

Q8. Explain the fate of pyruvic acid under aerobic conditions.

Ans:
Under aerobic conditions, pyruvic acid enters the mitochondria and is converted into acetyl-CoA by the pyruvate dehydrogenase complex. During this conversion, carbon dioxide is released and NAD⁺ is reduced to NADH. Acetyl-CoA then enters the Krebs cycle for complete oxidation.

Q9. Why is glycolysis considered an anaerobic pathway?

Ans:
Glycolysis is considered an anaerobic pathway because it does not require oxygen for its reactions. It can occur in both aerobic and anaerobic organisms and continues even in the absence of oxygen, making it essential for energy production under anaerobic conditions.

Q10. State two significances of glycolysis.

Ans:
Glycolysis provides ATP required for cellular activities and produces pyruvate that enters aerobic respiration. It also supplies important metabolic intermediates used in biosynthesis of amino acids, fats, and other organic compounds.

Q11. What links glycolysis and Krebs cycle?

Ans:
Acetyl-CoA acts as the link between glycolysis and the Krebs cycle. Pyruvic acid produced during glycolysis is converted into acetyl-CoA before entering the Krebs cycle for complete oxidation.

Q12. How many pyruvate molecules are formed from one glucose molecule?

Ans:
One molecule of glucose is broken down into two molecules of pyruvic acid during glycolysis.

Q13. Why is glycolysis called a universal pathway?

Ans:
Glycolysis is called a universal pathway because it occurs in almost all living organisms, including plants, animals, fungi, bacteria, and even under anaerobic conditions.

Q14. Does glycolysis occur in mitochondria? Explain.

Ans:
No, glycolysis occurs in the cytoplasm of the cell. Mitochondria are involved in later stages of respiration, such as the Krebs cycle and electron transport system.

Q15. How does glycolysis contribute to anaerobic respiration?

Ans:
In anaerobic respiration, glycolysis provides ATP and pyruvate. Pyruvate is converted into ethanol or lactic acid, regenerating NAD⁺ required for continuation of glycolysis.

Section B: Krebs Cycle (Questions 16–30)

Q16. Define Krebs cycle and mention its site.

Ans:
The Krebs cycle is a cyclic pathway in which acetyl-CoA is completely oxidised into carbon dioxide with the release of energy. It occurs in the mitochondrial matrix and is a major energy-releasing stage of aerobic respiration.

Q17. What is the link reaction?

Ans:
The link reaction involves conversion of pyruvic acid into acetyl-CoA inside the mitochondria. It releases carbon dioxide and produces NADH, linking glycolysis with the Krebs cycle.

Q18. Name the first stable compound of Krebs cycle.

Ans:
Citric acid is the first stable compound formed when acetyl-CoA combines with oxaloacetic acid at the beginning of the Krebs cycle.

Q19. How many CO₂ molecules are released per turn of Krebs cycle?

Ans:
Two molecules of carbon dioxide are released per turn of the Krebs cycle for each acetyl-CoA molecule oxidised.

Q20. What are the energy-rich molecules produced in Krebs cycle?

Ans:
The Krebs cycle produces three NADH, one FADH₂, and one ATP molecule per acetyl-CoA, which contribute to ATP synthesis during electron transport.

Q21. Why is Krebs cycle called amphibolic?

Ans:
Krebs cycle is amphibolic because it functions in both catabolism and anabolism. While it breaks down acetyl-CoA to release energy, its intermediates are used for synthesis of amino acids, fats, and other compounds.

Q22. How does oxygen indirectly affect Krebs cycle?

Ans:
Oxygen is required to regenerate NAD⁺ and FAD by accepting electrons in the electron transport system. Without oxygen, Krebs cycle stops due to accumulation of reduced coenzymes.

Q23. Why is Krebs cycle considered the central metabolic pathway?

Ans:
It connects carbohydrate, fat, and protein metabolism. Intermediates of Krebs cycle are used in multiple biosynthetic pathways, making it a central metabolic hub.

Q24. What is the ATP yield per turn of Krebs cycle?

Ans:
One ATP molecule is produced per turn of the Krebs cycle through substrate-level phosphorylation.

Q25. What is the role of oxaloacetic acid in Krebs cycle?

Ans:
Oxaloacetic acid combines with acetyl-CoA to form citric acid and is regenerated at the end of the cycle to continue the process.

Q26. Name one biosynthetic role of Krebs cycle intermediates.

Ans:
Krebs cycle intermediates are used in synthesis of amino acids, fatty acids, and chlorophyll.

Q27. Why does Krebs cycle not occur under anaerobic conditions?

Ans:
In absence of oxygen, NADH and FADH₂ cannot be oxidised back to NAD⁺ and FAD, causing Krebs cycle to stop.

Q28. How many times does Krebs cycle operate per glucose molecule?

Ans:
Krebs cycle operates twice per glucose molecule because one glucose produces two molecules of acetyl-CoA.

Q29. State two significances of Krebs cycle.

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

Q30. Where are Krebs cycle enzymes located?

Ans:
Most Krebs cycle enzymes are located in the mitochondrial matrix.

Section C: ETS, Fermentation & RQ (Questions 31–50)

Q31. What is electron transport system (ETS)?

Ans:
ETS is a series of electron carriers located on the inner mitochondrial membrane that transfer electrons from NADH and FADH₂ to oxygen, releasing energy for ATP synthesis.

Q32. What is oxidative phosphorylation?

Ans:
Oxidative phosphorylation is the synthesis of ATP using energy released during electron transport from reduced coenzymes to oxygen.

Q33. Why is oxygen essential for aerobic respiration?

Ans:
Oxygen acts as the final electron acceptor in ETS, allowing continuous flow of electrons and ATP production.

Q34. Why is ETS the main ATP-producing stage?

Ans:
Most ATP molecules are produced during ETS through oxidative phosphorylation using energy from electron transfer.

Q35. What happens if ETS is inhibited?

Ans:
ATP synthesis stops, leading to energy deficiency and eventual cell death.

Q36. Define fermentation.

Ans:
Fermentation is anaerobic respiration in which glucose is partially broken down to release energy without oxygen.

Q37. Describe alcoholic fermentation in plants.

Ans:
In alcoholic fermentation, pyruvate is converted into ethanol and carbon dioxide with regeneration of NAD⁺.

Q38. Why is fermentation less efficient than aerobic respiration?

Ans:
Glucose is incompletely oxidised, producing only two ATP molecules.

Q39. What is respiratory quotient (RQ)?

Ans:
RQ is the ratio of carbon dioxide released to oxygen consumed during respiration.

Q40. What is RQ value for carbohydrates?

Ans:
The RQ value for carbohydrates is 1.

Q41. What does RQ value indicate?

Ans:
RQ indicates the type of respiratory substrate used by the plant.

Q42. Why is respiration essential for plant growth?

Ans:
Respiration supplies ATP and intermediates required for growth, development, and maintenance.

Q43. What is the total ATP yield per glucose in plants?

Ans:
Approximately 36 ATP molecules are produced per glucose molecule in plants.

Q44. How is respiration different from photosynthesis?

Ans:
Respiration releases energy by breaking down food, while photosynthesis stores energy by synthesising food.

Q45. Why is respiration a stepwise process?

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

Q46. How are respiration and photosynthesis interrelated?

Ans:
Photosynthesis produces glucose and oxygen used in respiration, while respiration releases CO₂ used in photosynthesis.

Q47. Why is respiration called an amphibolic pathway?

Ans:
It performs both energy-releasing and biosynthetic roles.

Q48. Name two factors affecting rate of respiration.

Ans:
Temperature and availability of oxygen affect respiration rate.

Q49. What happens to respiration rate at high temperature?

Ans:
It increases up to optimum temperature and decreases beyond it due to enzyme denaturation.

Q50. Why is respiration essential even in dormant seeds?

Ans:
Respiration supplies energy required for maintenance and future germination.

Best Suited For

• CBSE Class 11 Annual Examinations
• NCERT-based 3–4 mark questions
• Conceptual clarity and structured answers