30 CSIR NET Life Science – Part C Level Questions (Oxidation–Reduction, ROS, Antioxidants)

30 high-quality CSIR NET Part C-level questions बना रहा हूँ। हर सवाल में multiple statements होंगे (true/false चुनने होंगे, या सही combination चुनना होगा)। इस तरह आप एक ही सवाल से पूरे pathway/enzymes revise कर लोगे।

🔹 30 CSIR NET Life Science – Part C Level Questions (Oxidation–Reduction, ROS, Antioxidants)

Q1. Consider the enzyme Ascorbate peroxidase (APX) in plants.

Which of the following statements are correct?

A. APX reduces H₂O₂ to H₂O using ascorbate as electron donor.

B. The product of ascorbate oxidation by APX is monodehydroascorbate (MDHA).

C. APX is present both in chloroplasts and cytosol.

D. APX activity can be bypassed by catalase for detoxification of H₂O₂ in the same organelle.

Options:

1. A, B, C

2. B, C, D

3. A and D only

4. A, B, D

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Q2. Superoxide dismutase (SOD) catalyzes dismutation of O₂•–.

Which of the following statements are TRUE?

A. Requires metal cofactors (Cu/Zn in cytosol, Mn in mitochondria, Fe/Mn in chloroplasts).

B. Converts two superoxide radicals into H₂O₂ and O₂.

C. SOD functions only in mitochondria.

D. Overexpression of SOD in transgenic plants increases oxidative stress tolerance.

Options:

1. A, B, D

2. B, C

3. A, C, D

4. A only

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Q3. Regarding Glutathione reductase (GR):

A. Uses NADPH to reduce oxidized glutathione (GSSG) back to GSH.

B. Maintains high GSH/GSSG ratio, critical for redox homeostasis.

C. Acts in the ascorbate–glutathione cycle.

D. Requires FAD as prosthetic group.

Options:

1. A, B, C

2. B, C, D

3. A, B, D

4. A, B, C, D

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Q4. ROS scavenging in plants:

A. H₂O₂ is relatively stable and can diffuse through membranes.

B. OH• (hydroxyl radical) is highly reactive and non-enzymatically formed via Fenton reaction.

C. O₂•– can cross membranes freely without enzymatic conversion.

D. ROS have dual roles: damaging at high concentration, signaling at low concentration.

Options:

1. A, B, D

2. A, C

3. B, C, D

4. A, B, C, D

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Q5. Match the following enzymes with their reactions:

a. Catalase

b. Glutathione peroxidase

c. Ascorbate peroxidase

d. SOD

i. 2 H₂O₂ → O₂ + 2 H₂O

ii. 2 GSH + R-OOH → GSSG + ROH + H₂O

iii. Ascorbate + H₂O₂ → DHA + H₂O

iv. 2 O₂•– + 2 H⁺ → O₂ + H₂O₂

Options:

1. a-i, b-ii, c-iii, d-iv

2. a-ii, b-i, c-iii, d-iv

3. a-i, b-iii, c-iv, d-ii

4. a-iv, b-ii, c-i, d-iii

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Q6. Which of the following correctly describe Monodehydroascorbate reductase (MDHAR)?

A. Uses NAD(P)H to reduce MDHA → ascorbate.

B. Prevents non-enzymatic disproportionation of MDHA.

C. Works exclusively in mitochondria.

D. Is a flavoprotein enzyme.

Options:

1. A, B, D

2. A, C

3. B, C, D

4. A only

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Q7. In plants, ROS scavenging involves both enzymatic and non-enzymatic antioxidants. Which are correctly classified?

A. Enzymatic: SOD, CAT, GR

B. Non-enzymatic: Ascorbate, Glutathione, Carotenoids, α-Tocopherol

C. Enzymatic: Peroxidases, APX

D. Non-enzymatic: NADPH

Options:

1. A, B, C

2. B, D

3. A, B

4. A, C, D

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Q8. Hydrogen peroxide scavenging is achieved through:

A. Catalase (CAT) → O₂ + H₂O

B. Ascorbate peroxidase (APX) → Ascorbate + H₂O₂ → DHA + H₂O

C. Peroxiredoxins (PRX) → Thioredoxin-dependent reduction of H₂O₂

D. Glutathione peroxidase (GPX) → GSH-dependent detoxification

Options:

1. A, B, C, D

2. A, C only

3. B, D only

4. A, B, D

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Q9. Regarding the Ascorbate–Glutathione cycle:

A. Involves APX, MDHAR, DHAR, GR.

B. Requires continuous supply of NADPH.

C. Links ascorbate, glutathione, and NADPH pools.

D. Operates only in mitochondria.

Options:

1. A, B, C

2. B, D

3. A, D

4. A, C, D

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Q10. Which statements about glutathione are correct?

A. Exists in reduced (GSH) and oxidized (GSSG) form.

B. Ratio of GSH/GSSG indicates redox status of cell.

C. GSH participates in detoxification of xenobiotics.

D. GSH is only produced in mitochondria.

Options:

1. A, B, C

2. B, C, D

3. A, C, D

4. A and B only

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Q11. Match cofactors with enzymes:

a. SOD (Cu/Zn)

b. Catalase (heme)

c. Glutathione reductase (FAD)

d. Peroxidase (heme)

Options:

1. All correct matches

2. Only a and b correct

3. a, b, d correct

4. a, c correct

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Q12. Fenton reaction generates hydroxyl radicals. Which are correct?

A. Involves Fe²⁺ and H₂O₂.

B. OH• is the most damaging ROS to DNA.

C. Plants avoid free Fe²⁺ accumulation to prevent Fenton reaction.

D. Superoxide dismutase directly prevents Fenton reaction.

Options:

1. A, B, C

2. A and D

3. A, B, D

4. B, C, D

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Q13. The role of NADPH in antioxidant defense:

A. Provides reducing power to GR for GSSG → GSH.

B. Provides electrons to MDHAR.

C. Is regenerated mainly by oxidative pentose phosphate pathway.

D. Is regenerated by mitochondrial electron transport chain.

Options:

1. A, B, C

2. B, C, D

3. A, C, D

4. A, B, C, D

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Q14. Which antioxidant enzyme works directly in lipid membrane protection?

A. Catalase

B. Phospholipid hydroperoxide glutathione peroxidase (PHGPX)

C. SOD

D. Glutathione reductase

Options:

1. B only

2. A and B

3. B and D

4. A, B, C

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Q15. ROS as signaling molecules:

A. H₂O₂ can activate MAPK cascades.

B. ROS regulate stomatal closure via ABA signaling.

C. ROS regulate programmed cell death (PCD).

D. ROS act only as toxic byproducts with no signaling role.

Options:

1. A, B, C

2. B, D

3. A, C, D

4. A and D

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Q16. Match ROS with property:

a. O₂•– (superoxide)

b. H₂O₂ (hydrogen peroxide)

c. OH• (hydroxyl radical)

d. ¹O₂ (singlet oxygen)

i. Highly reactive, no enzymatic scavenger

ii. Stable, membrane-permeable

iii. Produced in chloroplast PSI/PSII

iv. Requires dismutation by SOD

Options:

1. a-iv, b-ii, c-i, d-iii

2. a-iii, b-i, c-ii, d-iv

3. a-ii, b-iii, c-iv, d-i

4. a-iv, b-iii, c-ii, d-i

Q17. Which of the following cofactors usually act as electron carriers in biological redox reactions?

a) NAD⁺/NADH

b) FAD/FADH₂

c) ATP/ADP

d) Both (a) and (b)

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Q18. In the electron transport chain, which complex directly transfers electrons to oxygen?

a) Complex I

b) Complex II

c) Complex III

d) Complex IV

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Q19. The standard reduction potential (E°) of a redox couple determines:

a) The speed of the reaction

b) The direction of electron flow

c) The concentration of substrate

d) The enzyme kinetics

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Q20. During glycolysis, glyceraldehyde-3-phosphate dehydrogenase catalyzes a reaction where:

a) NAD⁺ is oxidized

b) NAD⁺ is reduced

c) FAD is reduced

d) ATP is hydrolyzed

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Q21. Which molecule is the strongest oxidizing agent among the following?

a) O₂

b) H₂O

c) CO₂

d) NADH

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Q22. Which enzyme is most likely to catalyze a reaction where oxygen is reduced to water?

a) Oxidoreductase

b) Oxygenase

c) Oxidase

d) Hydrolase

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Q23. Which of the following is a hallmark of reduction in organic molecules?

a) Increase in number of C–O bonds

b) Decrease in number of C–H bonds

c) Increase in number of C–H bonds

d) Increase in positive charge

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Q24. Which cofactor is commonly used by oxidases?

a) Heme (Fe²⁺/Fe³⁺)

b) NAD⁺

c) Biotin

d) Coenzyme A

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Q25. Why does FAD often participate in redox reactions where NAD⁺ cannot?

a) FAD has a higher reduction potential

b) FAD can accept single electrons

c) FAD is more abundant in cytoplasm

d) FAD is a stronger oxidizing agent

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Q26. In oxidative phosphorylation, reduction of O₂ to H₂O is coupled with:

a) CO₂ release

b) ATP synthesis

c) NAD⁺ breakdown

d) Pyruvate formation

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Q27. Which redox reaction is an example of disproportionation?

a) 2H₂O₂ → 2H₂O + O₂

b) NAD⁺ → NADH

c) O₂ → H₂O

d) Fe²⁺ → Fe³⁺

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Q28. In photosynthesis, water acts as:

a) Oxidizing agent

b) Reducing agent

c) Catalyst

d) Electron carrier

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Q29. The enzyme superoxide dismutase (SOD) catalyzes:

a) Oxidation of O₂ to H₂O₂

b) Reduction of O₂ to H₂O

c) Dismutation of O₂⁻ radicals

d) Reduction of NADP⁺

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Q30. Which of the following best describes the relationship between oxidation potential and tendency to gain electrons?

a) Higher oxidation potential = stronger reducing agent

b) Higher reduction potential = stronger oxidizing agent

c) Lower reduction potential = stronger oxidizing agent

d) Oxidation potential is unrelated to electron affinity

Answer Key (Short Format):

Q1 – A

Q2 – A

Q3 – D

Q4 – A

Q5 – A

Q6 – A

Q7 – A

Q8 – A

Q9 – A

Q10 – A

Q11 – A

Q12 – A

Q13 – A

Q14 – A

Q15 – A

Q16 – A

Q17 – D

Q18 – D

Q19 – B

Q20 – B

Q21 – A

Q22 – C

Q23 – C

Q24 – A

Q25 – B

Q26 – B

Q27 – A

Q28 – B

Q29 – C

Q30 – B