Respiration in plants MCQs With Answer

Respiration in plants MCQs With Answer is an essential topic for B. Pharm students studying plant physiology and biochemical energetics. This concise introduction explores plant respiration pathways—glycolysis, pyruvate oxidation, TCA cycle, electron transport chain and alternative oxidase—highlighting ATP synthesis, respiratory substrates, inhibitors, and physiological regulation. Emphasis on aerobic vs anaerobic respiration, photorespiration interactions, measurement of respiratory rates, and the pharmacological relevance of respiratory inhibitors equips pharmacy students to connect plant bioenergetics with broader biochemical principles. The following questions are designed to deepen understanding, reinforce key enzymes and mechanisms, and prepare you for exams and applied contexts. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the primary site of ATP synthesis during aerobic respiration in plant cells?

• Mitochondrial matrix enzymes
• Chloroplast stroma
• Inner mitochondrial membrane
• Cell cytosol

Correct Answer: Inner mitochondrial membrane

Q2. Which pathway converts glucose to pyruvate and produces a small net ATP and NADH in plant cells?

• Calvin cycle
• Glycolysis (Embden–Meyerhof pathway)
• Pentose phosphate pathway
• Photorespiration

Correct Answer: Glycolysis (Embden–Meyerhof pathway)

Q3. Pyruvate dehydrogenase complex in plants catalyzes which conversion?

• Pyruvate to lactate
• Pyruvate to acetyl-CoA
• Acetyl-CoA to citrate
• Oxaloacetate to malate

Correct Answer: Pyruvate to acetyl-CoA

Q4. Which enzyme in the TCA cycle directly contributes electrons to the electron transport chain via FADH2?

• Isocitrate dehydrogenase
• Alpha-ketoglutarate dehydrogenase
• Succinate dehydrogenase
• Malate dehydrogenase

Correct Answer: Succinate dehydrogenase

Q5. Which compound is the final electron acceptor in the plant mitochondrial electron transport chain?

• NAD+
• Oxygen
• ATP
• Carbon dioxide

Correct Answer: Oxygen

Q6. Alternative oxidase (AOX) in plant mitochondria provides what function?

• Increases ATP yield per electron transferred
• Bypasses complexes III and IV to reduce oxygen without pumping protons
• Transfers electrons to chloroplasts
• Converts oxygen to carbon dioxide directly

Correct Answer: Bypasses complexes III and IV to reduce oxygen without pumping protons

Q7. Which inhibitor specifically blocks complex IV (cytochrome c oxidase) in the electron transport chain?

• Rotenone
• Antimycin A
• Cyanide (CN-)
• Oligomycin

Correct Answer: Cyanide (CN-)

Q8. The respiratory quotient (RQ) for carbohydrate oxidation is approximately:

• 0.7
• 0.8
• 1.0
• 1.3

Correct Answer: 1.0

Q9. During anaerobic conditions in plants, pyruvate is commonly converted to:

• Acetyl-CoA and CO2
• Ethanol and CO2 via alcoholic fermentation
• Lactate via lactate dehydrogenase
• Oxaloacetate via carboxylation

Correct Answer: Ethanol and CO2 via alcoholic fermentation

Q10. Which enzyme catalyzes the conversion of pyruvate to acetaldehyde in plant anaerobic fermentation?

• Alcohol dehydrogenase
• Pyruvate decarboxylase
• Lactate dehydrogenase
• Pyruvate carboxylase

Correct Answer: Pyruvate decarboxylase

Q11. What is the main role of the TCA cycle in plant respiration besides generating ATP?

• Photosynthetic carbon fixation
• Providing carbon skeletons for biosynthesis and reducing equivalents (NADH, FADH2)
• Storage of starch
• Transport of water

Correct Answer: Providing carbon skeletons for biosynthesis and reducing equivalents (NADH, FADH2)

Q12. Which coenzyme is reduced during glycolysis and later oxidized in the mitochondrial electron transport chain?

• FAD
• NAD+
• Coenzyme A
• ATP

Correct Answer: NAD+

Q13. In plant mitochondria, ATP synthase uses which force to synthesize ATP?

• Chemical gradient of ATP
• Proton-motive force across inner mitochondrial membrane
• Electrical potential across plasma membrane
• Concentration gradient of glucose

Correct Answer: Proton-motive force across inner mitochondrial membrane

Q14. Which respiratory inhibitor is commonly used to block ATP synthase?

• Oligomycin
• Rotenone
• Cyanide
• Antimycin A

Correct Answer: Oligomycin

Q15. Photorespiration is most closely associated with which enzyme activity?

• Rubisco oxygenase activity
• Phosphofructokinase activity
• Pyruvate kinase activity
• ATP synthase activity

Correct Answer: Rubisco oxygenase activity

Q16. Which organelle is primarily involved in photorespiration together with mitochondria and chloroplasts?

• Golgi apparatus
• Peroxisome
• Endoplasmic reticulum
• Vacuole

Correct Answer: Peroxisome

Q17. Which parameter decreases in plant tissues under hypoxic conditions leading to increased anaerobic respiration?

• CO2 concentration
• ATP/ADP ratio
• Glucose concentration
• Pyruvate concentration

Correct Answer: ATP/ADP ratio

Q18. Which of the following is TRUE about the pentose phosphate pathway in relation to respiration?

• It directly produces ATP via substrate-level phosphorylation
• It generates NADPH and pentoses, supporting biosynthesis rather than direct ATP production
• It is the major source of FADH2 for the ETC
• It is localized exclusively in mitochondria

Correct Answer: It generates NADPH and pentoses, supporting biosynthesis rather than direct ATP production

Q19. Which substrate yields the lowest respiratory quotient (RQ) when oxidized?

• Glucose
• Sucrose
• Fatty acids
• Amino acids

Correct Answer: Fatty acids

Q20. Inhibition of complex I (NADH dehydrogenase) is classically caused by which compound?

• Antimycin A
• Rotenone
• Oligomycin
• SHAM (salicylhydroxamic acid)

Correct Answer: Rotenone

Q21. Which process links glycolysis to the TCA cycle and generates CO2 as a byproduct?

• Pentose phosphate pathway
• Pyruvate oxidation by pyruvate dehydrogenase complex
• Fermentation
• Photorespiration

Correct Answer: Pyruvate oxidation by pyruvate dehydrogenase complex

Q22. Which compound acts as an allosteric activator of pyruvate dehydrogenase kinase, thereby inhibiting PDH activity?

• High ADP
• High NAD+
• High ATP
• High pyruvate

Correct Answer: High ATP

Q23. How does temperature generally affect the rate of plant respiration?

• Rate decreases with increasing temperature
• Rate is independent of temperature
• Rate increases with temperature up to an optimum, then declines due to enzyme denaturation
• Rate increases indefinitely with temperature

Correct Answer: Rate increases with temperature up to an optimum, then declines due to enzyme denaturation

Q24. Which molecule is directly responsible for coupling electron transport to ATP synthesis?

• NADH
• ATP synthase rotor
• Proton gradient (H+ electrochemical gradient)
• Oxygen radical

Correct Answer: Proton gradient (H+ electrochemical gradient)

Q25. Which technique is commonly used to measure whole-tissue respiration rate in plants?

• Gas chromatography of lipids
• Respirometry measuring O2 uptake or CO2 evolution
• Spectrophotometry of chlorophyll
• Electrophoresis of proteins

Correct Answer: Respirometry measuring O2 uptake or CO2 evolution

Q26. Which statement best describes cyanide-insensitive respiration in plants?

• It is completely inactive under stress
• It involves AOX that allows electron flow to oxygen even when cytochrome pathway is blocked
• It increases ATP production efficiency
• It occurs only in chloroplasts

Correct Answer: It involves AOX that allows electron flow to oxygen even when cytochrome pathway is blocked

Q27. In plants, which amino acid is commonly produced as an intermediate during stress-induced changes in respiration?

• Glutamate
• Proline
• Tryptophan
• Lysine

Correct Answer: Proline

Q28. The malate-aspartate shuttle functions to:

• Transport ATP out of mitochondria
• Transfer reducing equivalents (NADH) from cytosol into mitochondria
• Export electrons to chloroplasts
• Synthesize malate for storage

Correct Answer: Transfer reducing equivalents (NADH) from cytosol into mitochondria

Q29. What is the effect of oligomycin on plant mitochondrial respiration?

• It uncouples electron transport from ATP synthesis
• It blocks ATP synthase, reducing ATP production and slowing oxygen consumption linked to phosphorylation
• It inhibits complex III
• It stimulates AOX activity

Correct Answer: It blocks ATP synthase, reducing ATP production and slowing oxygen consumption linked to phosphorylation

Q30. Which metabolite accumulates during intense anaerobiosis in plant roots leading to cytosolic acidification?

• Sucrose
• Ethanol
• Lactic acid
• Citrate

Correct Answer: Ethanol

Q31. Which pathway produces the majority of cellular NADH used by the mitochondrial electron transport chain in plants?

• Calvin cycle
• Glycolysis and TCA cycle combined
• Pentose phosphate pathway
• Starch biosynthesis

Correct Answer: Glycolysis and TCA cycle combined

Q32. Which plant respiratory enzyme requires thiamine pyrophosphate (TPP) as a cofactor?

• Succinate dehydrogenase
• Pyruvate dehydrogenase complex
• Cytochrome c oxidase
• ATP synthase

Correct Answer: Pyruvate dehydrogenase complex

Q33. In C4 plants, how is photorespiration affected compared to C3 plants?

• Photorespiration is increased in C4 plants
• Photorespiration is nearly eliminated due to CO2 concentration mechanism in bundle sheath cells
• Photorespiration and photosynthesis are identical to C3 plants
• Photorespiration is replaced by anaerobic respiration

Correct Answer: Photorespiration is nearly eliminated due to CO2 concentration mechanism in bundle sheath cells

Q34. Which ion gradient is primarily responsible for driving ATP synthesis in mitochondria?

• Sodium (Na+) gradient
• Chloride (Cl-) gradient
• Proton (H+) gradient
• Calcium (Ca2+) gradient

Correct Answer: Proton (H+) gradient

Q35. Which enzyme catalyzes the conversion of malate to oxaloacetate in the TCA cycle?

• Malate dehydrogenase
• Fumarase
• Malic enzyme
• Isocitrate lyase

Correct Answer: Malate dehydrogenase

Q36. The glycerol-3-phosphate shuttle in plants primarily transfers reducing equivalents to which cofactor in the mitochondrion?

• Matrix NADH
• FADH2 in the inner mitochondrial membrane
• ATP directly
• NADPH

Correct Answer: FADH2 in the inner mitochondrial membrane

Q37. Which respiratory process is essential during seed germination to mobilize stored reserves?

• Photosynthesis
• Active transport only
• Respiration of stored lipids and carbohydrates to generate ATP and biosynthetic precursors
• Photorespiration

Correct Answer: Respiration of stored lipids and carbohydrates to generate ATP and biosynthetic precursors

Q38. What is the likely effect of an increase in cytosolic NADH/NAD+ ratio on fermentation in plants?

• Fermentation decreases due to substrate limitation
• Fermentation increases to regenerate NAD+ allowing glycolysis to continue
• No change in fermentation
• Shift to increased oxidative phosphorylation

Correct Answer: Fermentation increases to regenerate NAD+ allowing glycolysis to continue

Q39. Which molecule is a common respiratory inhibitor used experimentally to distinguish cyanide-sensitive and cyanide-insensitive pathways?

• Rotenone
• Antimycin A
• SHAM (salicylhydroxamic acid)
• Glutamate

Correct Answer: SHAM (salicylhydroxamic acid)

Q40. In plant cells, which organelle hosts the TCA cycle?

• Plastid
• Mitochondrion
• Peroxisome
• Vacuole

Correct Answer: Mitochondrion

Q41. Which statement best describes the relationship between respiration and biosynthetic pathways in plants?

• Respiration only generates ATP and has no role in biosynthesis
• Respiration supplies ATP and carbon skeletons (intermediates) required for anabolic reactions
• Respiration inhibits biosynthetic pathways by consuming precursors
• Biosynthesis occurs independently of respiration

Correct Answer: Respiration supplies ATP and carbon skeletons (intermediates) required for anabolic reactions

Q42. The classical chemiosmotic hypothesis explains ATP synthesis by proposing that:

• ATP is made by direct substrate-level phosphorylation only
• Energy for ATP synthesis is stored as a transmembrane proton gradient and used by ATP synthase
• ATP is synthesized in the cytosol and imported into mitochondria
• Electrons bind directly to ADP to form ATP

Correct Answer: Energy for ATP synthesis is stored as a transmembrane proton gradient and used by ATP synthase

Q43. Which of the following is a major consequence of prolonged anaerobic respiration in plant tissues?

• Increased ATP yield per glucose
• Depletion of NADH pools
• Accumulation of toxic fermentation products and energy deficit
• Enhanced TCA cycle activity

Correct Answer: Accumulation of toxic fermentation products and energy deficit

Q44. Which enzyme of the TCA cycle is inhibited by high levels of ATP and NADH?

• Citrate synthase
• Succinate dehydrogenase
• Triose phosphate isomerase
• Hexokinase

Correct Answer: Citrate synthase

Q45. Which respiratory process contributes directly to heat production in plants (thermogenesis) via uncoupling?

• ATP synthase hyperactivity
• Uncoupled respiration through alternative oxidase or uncoupling proteins dissipating proton gradient as heat
• Increased glycolytic ATP production
• Photorespiration producing additional ATP

Correct Answer: Uncoupled respiration through alternative oxidase or uncoupling proteins dissipating proton gradient as heat

Q46. Which metabolite serves as a key junction between carbohydrate metabolism and the TCA cycle?

• Fructose-6-phosphate
• Acetyl-CoA
• Glucose-6-phosphate
• Sucrose

Correct Answer: Acetyl-CoA

Q47. Which factor most directly limits the rate of aerobic respiration in waterlogged roots?

• Excess sugar availability
• Oxygen diffusion limitation leading to hypoxia
• High temperature only
• Excess light

Correct Answer: Oxygen diffusion limitation leading to hypoxia

Q48. Which compound is generated by the oxidative decarboxylation steps in the TCA cycle and released as CO2?

• Oxaloacetate
• Succinate
• Alpha-ketoglutarate to succinyl-CoA conversion releases CO2
• Malate to oxaloacetate conversion releases CO2

Correct Answer: Alpha-ketoglutarate to succinyl-CoA conversion releases CO2

Q49. During dark respiration in leaves, which carbon source is often metabolized to support energy needs?

• Atmospheric CO2
• Stored sugars and starch reserves
• Chlorophyll
• Inorganic phosphate

Correct Answer: Stored sugars and starch reserves

Q50. Which of the following best approximates the ATP yield per glucose molecule under ideal plant aerobic respiration conditions?

• 2 ATP
• ~30–32 ATP
• ~100 ATP
• 0 ATP

Correct Answer: ~30–32 ATP

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

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