Respiration in Plants — Class 11 Biology NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 11 Biology chapter "Respiration in Plants" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 11 Biology chapter "Respiration in Plants" — 8 important questions with detailed answers for CBSE board ex…
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NCERT Solutions for Class 11 Biology — Respiration in Plants. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
Respiration in Plants — All 8 Questions Solved
Q1. Define respiration. Why is it essential for plants?
Respiration is a catabolic process in which organic compounds are oxidized to release energy in the form of ATP. In plants, it occurs continuously in all living cells.
Essential for:
- Providing ATP (energy currency) for all life processes
- Biosynthetic reactions (protein, nucleic acid synthesis)
- Active transport and ion uptake
- Maintenance of cell structure and turgor
- Growth and reproduction
Unlike photosynthesis, respiration occurs 24 hours in all parts of the plant.
Q2. What are the stages of aerobic respiration?
Aerobic respiration has three stages:
1. Glycolysis (cytoplasm): Glucose (6C) → 2 Pyruvate (3C) + 2 ATP + 2 NADH
2. Krebs cycle/Citric Acid Cycle (mitochondrial matrix): Acetyl-CoA completely oxidized, releases CO2 + 8 NADH + 2 FADH2 + 2 ATP
3. Electron Transport Chain (inner mitochondrial membrane): NADH and FADH2 oxidized, O2 reduced to H2O, ~32-34 ATP generated
Total yield: ~36-38 ATP per glucose molecule
Q3. Describe glycolysis. Where does it occur and what are its products?
Glycolysis is the breakdown of one molecule of glucose (6C) into two molecules of pyruvate (3C).
Location: Cytoplasm
Main steps:
- Glucose → Glucose-6-phosphate → Fructose-1,6-bisphosphate (uses 2 ATP)
- Fructose-1,6-bisphosphate → 2 G3P (3-carbon molecules)
- G3P → Pyruvate (produces 4 ATP + 2 NADH)
Net products: 2 Pyruvate + 2 ATP (net) + 2 NADH
Glycolysis is anaerobic and does not require oxygen.
Q4. What is anaerobic respiration? Give examples in plants.
Anaerobic respiration is oxidation of organic compounds without oxygen. Two main types in plants:
1. Alcoholic fermentation: Pyruvate → Ethanol + CO2 (occurs in yeast, many plant roots)
2. Lactic acid fermentation: Pyruvate → Lactic acid (occurs in muscle cells and some plants)
Both regenerate NAD+ from NADH so glycolysis continues. Yields only 2 ATP per glucose (much less than aerobic).
Occurs when oxygen is limited, e.g., in waterlogged soils, during flooding, in germinating seeds without sufficient oxygen.
Q5. Distinguish between aerobic and anaerobic respiration.
Aerobic respiration: Requires O2 as final electron acceptor. Complete oxidation of glucose. Occurs in cytoplasm and mitochondria. Produces ~36-38 ATP per glucose. End products: CO2 and H2O.
Anaerobic respiration: No O2 required. Incomplete oxidation. Occurs in cytoplasm only. Produces only 2 ATP per glucose. End products: Ethanol/Lactic acid and CO2 (in fermentation).
Aerobic is much more efficient and is the primary mode in most plant cells under normal conditions.
Q6. What is the Krebs cycle? List its main products.
The Krebs cycle (Citric Acid Cycle/TCA cycle) is a series of enzyme-catalyzed reactions occurring in the mitochondrial matrix. Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C), which is progressively oxidized back to oxaloacetate.
Main products per turn (1 Acetyl-CoA):
- 3 NADH (used in electron transport chain)
- 1 FADH2 (used in electron transport chain)
- 1 GTP/ATP (direct energy)
- 2 CO2 (complete oxidation of acetyl group)
The cycle occurs twice per glucose (2 Acetyl-CoA from pyruvate).
Q7. What is the role of mitochondria in respiration?
Mitochondria is the 'powerhouse' of the cell and houses most of aerobic respiration:
- Outer membrane: Permeable to molecules
- Inner membrane: Contains electron transport chain complexes and ATP synthase; impermeable to many molecules
- Matrix: Contains Krebs cycle enzymes and pyruvate dehydrogenase complex
Krebs cycle and electron transport chain together generate ~34 ATP per glucose. This compartmentalization allows for efficient energy production and regulation of respiration rates.
Q8. What is the respiratory quotient (RQ)? Give its significance.
Respiratory quotient (RQ) = Volume of CO2 released / Volume of O2 consumed
For carbohydrates: RQ = 1 (equal volumes of CO2 and O2)
For fats: RQ = 0.7 (less CO2 released than O2 consumed)
For proteins: RQ = 0.8-0.9
Significance: RQ indicates the type of substrate being respired. RQ > 1 indicates organic acid respiration (CO2 release from decarboxylation in addition to oxidation). RQ is used to study the pattern of respiration and nutritional status of plants.
Respiration in Plants — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. Where does glycolysis occur in a plant cell?
- A. Cytoplasm
- B. Mitochondria
- C. Chloroplast
- D. Peroxisome
Answer: A. Cytoplasm — Glycolysis is the first stage of cellular respiration and takes place in the cytoplasm of the cell, both in aerobic and anaerobic conditions.
MCQ 2. What are the net products formed from one molecule of glucose during glycolysis?
- A. 2 Pyruvic acid, 2 ATP, 2 NADH
- B. 1 Pyruvic acid, 2 ATP, 2 NADH
- C. 2 Pyruvic acid, 4 ATP, 2 NADH
- D. 2 Pyruvic acid, 2 ATP, 4 NADH
Answer: A. 2 Pyruvic acid, 2 ATP, 2 NADH — For one molecule of glucose, glycolysis yields 2 molecules of pyruvic acid, a net of 2 ATP molecules (4 produced, 2 consumed), and 2 NADH molecules.
MCQ 3. Which of the following complexes in the Electron Transport System (ETS) contains both FMN (Flavin Mononucleotide) and Fe-S clusters?
- A. Complex I (NADH dehydrogenase)
- B. Complex II (Succinate dehydrogenase)
- C. Complex III (Cytochrome bc1 complex)
- D. Complex IV (Cytochrome c oxidase)
Answer: A. Complex I (NADH dehydrogenase) — Complex I, also known as NADH dehydrogenase, accepts electrons from NADH and contains flavin mononucleotide (FMN) and iron-sulfur (Fe-S) clusters as prosthetic groups.
MCQ 4. What is the final electron acceptor in the electron transport chain during aerobic respiration?
- A. Water
- B. Oxygen
- C. Carbon dioxide
- D. ATP
Answer: B. Oxygen — Molecular oxygen serves as the final electron acceptor in the electron transport chain, combining with electrons and protons to form water, thereby driving the entire process.
MCQ 5. Where does the Krebs cycle (TCA cycle) take place in eukaryotic plant cells?
- A. Cytoplasm
- B. Inner mitochondrial membrane
- C. Mitochondrial matrix
- D. Outer mitochondrial membrane
Answer: C. Mitochondrial matrix — The Krebs cycle, also known as the tricarboxylic acid (TCA) cycle, occurs in the mitochondrial matrix of eukaryotic cells.
MCQ 6. What are the end products of alcoholic fermentation in plants (e.g., in yeast)?
- A. Lactic acid + CO2
- B. Ethanol + CO2
- C. Pyruvic acid + ATP
- D. Acetic acid + H2O
Answer: B. Ethanol + CO2 — In alcoholic fermentation, pyruvate is converted into ethanol and carbon dioxide, a process commonly observed in yeast and some plant cells under anaerobic conditions.
MCQ 7. What is the typical respiratory quotient (RQ) value for fats?
- A. 1
- B. 0.7
- C. 0.9
- D. More than 1
Answer: B. 0.7 — Fats contain less oxygen relative to carbon and hydrogen, so they require more oxygen for complete oxidation, resulting in an RQ value typically around 0.7.
MCQ 8. How many ATP molecules are generally produced from one molecule of NADH and one molecule of FADH2, respectively, during oxidative phosphorylation in eukaryotes?
- A. 3 ATP from NADH, 2 ATP from FADH2
- B. 2 ATP from NADH, 3 ATP from FADH2
- C. 2.5 ATP from NADH, 1.5 ATP from FADH2
- D. 1.5 ATP from NADH, 2.5 ATP from FADH2
Answer: C. 2.5 ATP from NADH, 1.5 ATP from FADH2 — Modern estimates for ATP yield in eukaryotic cells are 2.5 ATP per NADH and 1.5 ATP per FADH2, reflecting the precise proton motive force generated and utilized by ATP synthase.
MCQ 9. Oxidative phosphorylation refers to:
- A. The formation of ATP by transfer of phosphate group from a substrate to ADP.
- B. The synthesis of ATP by chemiosmosis, using energy from the electron transport chain.
- C. The breakdown of glucose in the absence of oxygen.
- D. The conversion of ADP to ATP in the cytoplasm.
Answer: B. The synthesis of ATP by chemiosmosis, using energy from the electron transport chain. — Oxidative phosphorylation is the process where ATP is synthesized using the energy released from the oxidation of reduced coenzymes (NADH, FADH2) through the electron transport chain and chemiosmosis.
MCQ 10. In aerobic respiration, oxygen is essential because:
- A. It acts as the primary substrate for glycolysis.
- B. It directly participates in the Krebs cycle.
- C. It is the final electron acceptor in the electron transport chain.
- D. It is required for the conversion of pyruvate to acetyl CoA.
Answer: C. It is the final electron acceptor in the electron transport chain. — Oxygen is crucial in aerobic respiration as it acts as the terminal electron acceptor, removing electrons and protons to form water, which is vital for the continuous operation of the electron transport chain.
MCQ 11. According to the chemiosmotic hypothesis, ATP synthase functions due to:
- A. The movement of electrons across the inner mitochondrial membrane.
- B. The creation of a proton gradient across the inner mitochondrial membrane.
- C. The breakdown of NADH and FADH2 directly providing energy.
- D. The accumulation of CO2 in the mitochondrial matrix.
Answer: B. The creation of a proton gradient across the inner mitochondrial membrane. — The chemiosmotic hypothesis states that a proton gradient, established by the electron transport chain across the inner mitochondrial membrane, drives the rotation of ATP synthase, leading to ATP synthesis.
MCQ 12. The intermediate product that links glycolysis with the Krebs cycle is:
- A. Pyruvic acid
- B. Acetyl CoA
- C. Oxaloacetic acid
- D. Citric acid
Answer: B. Acetyl CoA — Pyruvic acid, produced during glycolysis, is converted into acetyl CoA in the mitochondrial matrix through oxidative decarboxylation, which then enters the Krebs cycle.
MCQ 13. Fermentation occurs in plant cells under which condition?
- A. Presence of excess oxygen
- B. Presence of light
- C. Absence of oxygen
- D. Low temperature
Answer: C. Absence of oxygen — Fermentation is an anaerobic process that occurs in the absence of oxygen, allowing for the regeneration of NAD+ for glycolysis to continue.
MCQ 14. How many molecules of CO2 are released during one complete turn of the Krebs cycle?
- A. 1
- B. 2
- C. 3
- D. 6
Answer: B. 2 — For each acetyl CoA that enters the Krebs cycle, two molecules of CO2 are released, specifically during the conversions of isocitrate to α-ketoglutarate and α-ketoglutarate to succinyl CoA.
MCQ 15. How many ATP molecules are consumed during the preparatory phase of glycolysis?
- A. 0
- B. 1
- C. 2
- D. 4
Answer: C. 2 — Two ATP molecules are consumed in the initial steps of glycolysis to phosphorylate glucose to glucose-6-phosphate and fructose-6-phosphate to fructose-1,6-bisphosphate.
MCQ 16. Respiration is essentially a process of:
- A. Anabolism
- B. Catabolism
- C. Photosynthesis
- D. Polymerization
Answer: B. Catabolism — Respiration is a catabolic process where complex organic molecules (like glucose) are broken down into simpler ones, releasing energy.
MCQ 17. Which two enzymes are involved in alcoholic fermentation in plant cells?
- A. Pyruvate dehydrogenase and alcohol dehydrogenase
- B. Pyruvate decarboxylase and alcohol dehydrogenase
- C. Lactic acid dehydrogenase and alcohol dehydrogenase
- D. Pyruvate kinase and alcohol dehydrogenase
Answer: B. Pyruvate decarboxylase and alcohol dehydrogenase — Alcoholic fermentation involves pyruvate decarboxylase, which converts pyruvate to acetaldehyde, and alcohol dehydrogenase, which then reduces acetaldehyde to ethanol.
MCQ 18. Substrate-level phosphorylation occurs during which steps of aerobic respiration?
- A. Glycolysis and Krebs cycle
- B. Krebs cycle and ETS
- C. Glycolysis and ETS
- D. Only during ETS
Answer: A. Glycolysis and Krebs cycle — Substrate-level phosphorylation, the direct transfer of a phosphate group from a substrate to ADP, occurs once in glycolysis (PEP to pyruvate) and once in the Krebs cycle (succinyl CoA to succinate).
MCQ 19. The most common respiratory substrate is:
- A. Protein
- B. Fat
- C. Glucose
- D. Vitamin
Answer: C. Glucose — Glucose is the most common and preferred respiratory substrate, readily broken down to release energy in most living organisms.
MCQ 20. What would be the respiratory quotient (RQ) for respiration utilizing an organic acid like oxalic acid (C2H2O4)?
- A. Less than 1
- B. Equal to 1
- C. Greater than 1
- D. Zero
Answer: C. Greater than 1 — Organic acids are rich in oxygen, so they require less external oxygen for complete oxidation, leading to an RQ value greater than 1 (e.g., for oxalic acid, RQ is 4).
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.