30 MCQs on ROS, Abiotic Stress & Antioxidant Pathways, Part B Style
Memory-based MCQs (like CSIR NET Part B style) from the whole topic you gave (ROS as abiotic stress + antioxidants & scavenging).
I will make 30 MCQs that together touch every major aspect → ROS generation, scavenging, enzymes, antioxidants, signaling (Ca²⁺, MAPK), organelles, and redox cycle.
This way, just practicing these questions again and again will work as active revision — no need to reread long paragraphs.
📝 30 MCQs on ROS, Abiotic Stress & Antioxidant Pathways
Q1. Which cellular process is not a natural source of ROS in plants?
a) Photosynthesis
b) Respiration
c) Photorespiration
d) Protein translation
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Q2. Which of the following enzymes is directly involved in ROS generation?
a) NADPH oxidase
b) Glutathione reductase
c) Ascorbate peroxidase
d) Catalase
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Q3. The steady-state level of ROS in a cell depends on:
a) Photosynthetic rate only
b) Balance between ROS generation and scavenging
c) Level of antioxidants only
d) Cell division rate
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Q4. Which small antioxidant is lipid-soluble?
a) Ascorbate
b) Glutathione
c) α-tocopherol
d) NADPH
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Q5. Glutathione exists in two forms:
a) GSH (oxidized) and GSSG (reduced)
b) GSH (reduced) and GSSG (oxidized)
c) GSH (neutral) and GSSG (ionic)
d) Only one form exists
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Q6. Which enzyme maintains glutathione in its reduced form?
a) Glutathione reductase
b) Catalase
c) Ascorbate peroxidase
d) Superoxide dismutase
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Q7. The reaction 2O₂•⁻ + 2H⁺ → H₂O₂ + O₂ is catalyzed by:
a) Catalase
b) Ascorbate peroxidase
c) Superoxide dismutase
d) Glutathione peroxidase
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Q8. Catalase is primarily located in which organelle?
a) Mitochondria
b) Chloroplast
c) Peroxisome
d) Cytosol
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Q9. Which antioxidant enzyme uses ascorbate as the reducing agent?
a) Superoxide dismutase
b) Ascorbate peroxidase
c) Glutathione reductase
d) Catalase
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Q10. Which enzyme detoxifies hydrogen peroxide using reduced glutathione (GSH)?
a) Catalase
b) Glutathione peroxidase
c) Superoxide dismutase
d) Peroxidase
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Q11. The Ascorbate–Glutathione cycle includes all enzymes except:
a) APX
b) MDHAR
c) DHAR
d) Catalase
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Q12. Which antioxidant is also known as vitamin C?
a) Ascorbate
b) α-tocopherol
c) β-carotene
d) Glutathione
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Q13. ROS can activate plasma membrane calcium channels, leading to:
a) DNA replication
b) Increased cytosolic Ca²⁺
c) NADH synthesis
d) Protein degradation
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Q14. Calcium-dependent protein kinases (CDPKs) phosphorylate which protein to enhance ROS production?
a) Catalase
b) RBOH (NADPH oxidase)
c) Superoxide dismutase
d) Glutathione reductase
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Q15. The amino-terminal domain of RBOH proteins contains:
a) Iron-binding motifs
b) Phosphorylatable serine residues & EF-hand Ca²⁺-binding sites
c) ATP synthase subunits
d) DNA-binding domains
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Q16. Which of the following has the longest half-life and can diffuse widely in cells?
a) Hydroxyl radical (•OH)
b) Singlet oxygen (¹O₂)
c) Hydrogen peroxide (H₂O₂)
d) Superoxide anion (O₂•⁻)
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Q17. Which vitamin acts as a lipid-soluble antioxidant in membranes?
a) Vitamin A (β-carotene)
b) Vitamin C (ascorbate)
c) Vitamin E (α-tocopherol)
d) Vitamin D
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Q18. MAPK cascades in plants are composed of:
a) MAP3K → MAP2K → MAPK
b) MAPK → MAP2K → MAP3K
c) CDPK → RBOH → MAPK
d) NADPH oxidase → MAPK → APX
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Q19. Which abiotic stresses share ROS, Ca²⁺, and phosphatidic acid as upstream intermediates?
a) Drought, temperature, salinity, oxidative stress
b) Pathogen infection, UV light, wounding, flooding
c) Senescence, germination, flowering, pollination
d) Nitrogen fixation, nodulation, symbiosis, mycorrhiza
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Q20. Which of the following is not a ROS scavenging enzyme?
a) Catalase
b) Ascorbate peroxidase
c) Glutathione reductase
d) NADPH oxidase
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Q21. H₂O₂ detoxification in peroxisomes mainly involves:
a) APX
b) Catalase
c) Superoxide dismutase
d) DHAR
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Q22. Which enzyme couples oxidation and reduction of superoxide simultaneously?
a) Superoxide dismutase
b) Catalase
c) APX
d) Peroxidase
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Q23. Which cofactor provides reducing power for reductases in the antioxidant cycle?
a) ATP
b) NADH or NADPH
c) FADH₂
d) Coenzyme Q
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Q24. Which of the following is not an enzymatic antioxidant defense?
a) SOD
b) Catalase
c) APX
d) β-carotene
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Q25. Which statement about ROS is correct?
a) They are only harmful and have no signaling role
b) They act as both toxic byproducts and signaling molecules
c) They are stable molecules with long half-life
d) They never interact with Ca²⁺ signaling
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Q26. Which ROS species is most reactive and damaging?
a) O₂•⁻
b) •OH (hydroxyl radical)
c) H₂O₂
d) ¹O₂
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Q27. Cross-talk between ROS and hormone signaling during abiotic stress involves:
a) Only auxin
b) MAPKs and multiple hormones
c) Only abscisic acid
d) No hormonal interaction
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Q28. In the Ascorbate–Glutathione cycle, which enzyme regenerates ascorbate from monodehydroascorbate?
a) MDHAR
b) DHAR
c) GR
d) APX
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Q29. During ROS stress signaling, transcription factors are regulated by:
a) Only phosphorylation
b) Redox status changes and phosphorylation/dephosphorylation
c) ATP hydrolysis
d) Lipid peroxidation
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Q30. Which ROS is directly produced by NADPH oxidase?
a) O₂•⁻ (superoxide)
b) H₂O₂
c) •OH (hydroxyl radical)
d) ¹O₂
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✅ Answer Key
1 → d
2 → a
3 → b
4 → c
5 → b
6 → a
7 → c
8 → c
9 → b
10 → b
11 → d
12 → a
13 → b
14 → b
15 → b
16 → c
17 → c
18 → a
19 → a
20 → d
21 → b
22 → a
23 → b
24 → d
25 → b
26 → b
27 → b
28 → a
29 → b
30 → a
✅ This set ensures coverage of every aspect:
ROS sources (Q1–3, 30)
Antioxidants (Q4–12, 20–24, 28)
ROS–Ca²⁺ cross talk & signaling (Q13–15, 29)
Stability & types of ROS (Q16, 25–26)
MAPK and stress responses (Q18–19, 27)
👉 If you practice these daily, your active recall will replace passive paragraph reading.
30 medium-to-high level MCQs on Oxidation–Reduction (Redox reactions, oxidases, reductases, electron transfer, ROS, antioxidants, etc.), crafted in a style that matches CSIR NET Part B (so slightly conceptual, analytical, experimental reasoning based — not just memory-based).
Here’s a fresh set of 30 MCQs for you
MCQs on Oxidation–Reduction (Part B level, Medium–High Standard)
1. In a redox reaction, the substance that gets reduced always:
a) Loses electrons
b) Gains electrons
c) Acts as oxidizing agent
d) Both b and c
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2. During the action of glucose oxidase, glucose is oxidized to gluconolactone. What happens to oxygen in the process?
a) It is reduced to H2O2
b) It is oxidized to CO2
c) It remains unchanged
d) It is converted to O2–
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3. In terms of oxidation states, which process best describes the conversion of Fe²⁺ → Fe³⁺?
a) Oxidation, loss of electron
b) Oxidation, gain of electron
c) Reduction, loss of electron
d) Reduction, gain of electron
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4. The function of superoxide dismutase (SOD) is to:
a) Oxidize hydrogen peroxide
b) Reduce O2•– into water
c) Convert O2•– into O2 and H2O2
d) Use NADPH to recycle antioxidants
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5. Glutathione reductase maintains the reduced state of glutathione (GSH) using:
a) NAD+
b) NADH
c) NADPH
d) FADH2
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6. Catalase detoxifies hydrogen peroxide by:
a) Oxidizing H2O2 into O3
b) Converting H2O2 into H2O and O2
c) Reducing H2O2 into H2 and O2
d) Transporting H2O2 to mitochondria
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7. Which of the following pairs represents an oxidase and a reductase?
a) Glucose oxidase – Glutathione reductase
b) Catalase – Peroxidase
c) Ascorbate peroxidase – Superoxide dismutase
d) NADH dehydrogenase – Cytochrome oxidase
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8. Oxidation is best defined as:
a) Gain of hydrogen / loss of oxygen
b) Loss of electrons / gain of oxygen
c) Gain of electrons / gain of oxygen
d) Loss of protons / gain of hydrogen
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9. In mitochondria, which component acts as the terminal electron acceptor?
a) Cytochrome c
b) Oxygen
c) NAD+
d) FAD
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10. Which antioxidant enzyme uses ascorbate as a reducing agent?
a) Catalase
b) Superoxide dismutase
c) Ascorbate peroxidase
d) Glutathione peroxidase
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11. The reduced form of glutathione (GSH) becomes oxidized into:
a) Glutathione sulfonate
b) Glutathione disulfide (GSSG)
c) NADH
d) H2O2
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12. Which statement about ROS is correct?
a) ROS are only harmful and never useful
b) ROS can act as signaling molecules at low concentrations
c) ROS are stable molecules with long half-life
d) ROS are only generated during stress, not under normal metabolism
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13. Dehydroascorbate reductase catalyzes:
a) Reduction of oxidized ascorbate back to ascorbate
b) Oxidation of ascorbate to oxalic acid
c) Conversion of NADH to NAD+
d) Decomposition of H2O2
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14. Which process represents a reduction?
a) NADPH → NADP+
b) FAD → FADH2
c) Fe²⁺ → Fe³⁺
d) H2O2 → O2
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15. Which enzyme pair is localized in peroxisomes?
a) Catalase and superoxide dismutase
b) Catalase and glutathione reductase
c) Ascorbate peroxidase and catalase
d) Monodehydroascorbate reductase and superoxide dismutase
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16. In the ascorbate–glutathione cycle, which molecule acts as the electron donor to regenerate ascorbate?
a) NADH
b) Glutathione (GSH)
c) Catalase
d) Cytochrome oxidase
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17. An increase in oxidation state of an element indicates:
a) It has been oxidized
b) It has been reduced
c) It has acted as reducing agent
d) Both a and c
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18. Which antioxidant is lipid-soluble?
a) Ascorbate
b) Glutathione
c) α-Tocopherol (Vitamin E)
d) NADPH
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19. The enzyme that converts O2•– to H2O2 and O2 requires which cofactors?
a) Fe and Mg
b) Zn and Cu / Mn
c) Ca and Mg
d) Ni and Co
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20. When hydrogen peroxide is converted to water and oxygen by catalase, the reaction type is:
a) Oxidation
b) Reduction
c) Disproportionation
d) Hydrolysis
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21. Which antioxidant directly reacts with singlet oxygen (¹O2)?
a) Glutathione
b) β-carotene
c) NADPH
d) Cytochrome oxidase
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22. The redox couple NADP+/NADPH is important because:
a) NADP+ accepts electrons and NADPH donates electrons
b) NADPH accepts electrons and NADP+ donates electrons
c) Both act as oxidizing agents
d) Both act as reducing agents
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23. Which plant organelle generates the highest ROS under high light conditions?
a) Peroxisomes
b) Chloroplasts
c) Nucleus
d) Golgi apparatus
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24. Which enzyme is NOT part of the ascorbate–glutathione cycle?
a) Ascorbate peroxidase
b) Monodehydroascorbate reductase
c) Glutathione reductase
d) Catalase
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25. Which antioxidant enzyme directly uses reduced glutathione (GSH) as substrate?
a) Catalase
b) Ascorbate peroxidase
c) Glutathione peroxidase
d) Superoxide dismutase
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26. Oxidases differ from dehydrogenases in that oxidases:
a) Use NAD+ as electron acceptor
b) Use oxygen as the terminal electron acceptor
c) Produce ATP directly
d) Work only in anaerobic conditions
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27. In redox potential (E°’), a more negative value indicates:
a) Strong oxidizing agent
b) Strong reducing agent
c) Weak reducing agent
d) Electrically neutral state
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28. Which cofactor is common in oxidoreductase enzymes?
a) Biotin
b) Heme / FAD / NAD(P)+
c) Pyridoxal phosphate
d) Thiamine pyrophosphate
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29. During oxidative stress, NADPH is mainly regenerated by:
a) Glycolysis
b) Pentose phosphate pathway
c) TCA cycle
d) Photorespiration
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30. Which statement is TRUE about oxidation–reduction in biology?
a) Oxidation always means addition of oxygen atom only
b) Reduction always means addition of hydrogen atom only
c) Redox reactions always involve electron transfer, sometimes with H/O involvement
d) Oxidation and reduction can occur independently without relation
✅ Answers (check after attempting)
1-b,c
2-a
3-a
4-c
5-c
6-b
7-a
8-b
9-b
10-c
11-b
12-b
13-a
14-b
15-a
16-b
17-a,c
18-c
19-b
20-c
21-b
22-a
23-b
24-d
25-c
26-b
27-b
28-b
29-b
30-c
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