Practice important
Antioxidant Defense
MCQ questions and answers.
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10
MCQ Questions
1. During drought stress, abscisic acid helps limit oxidative damage primarily by promoting which coordinated response?
A.
Suppression of all redox enzymes to prevent oxygen consumption
B.
Activation of antioxidant enzymes that remove reactive oxygen species
C.
Conversion of all cellular oxygen into carbon dioxide
D.
Permanent inhibition of chloroplast electron transport
Correct Answer:
Activation of antioxidant enzymes that remove reactive oxygen species
Explanation:
ABA contributes to antioxidant defense by inducing enzymes such as superoxide dismutase, catalase, and peroxidases, which reduce the accumulation of reactive oxygen species during stress.
2. A plant exposed to salinity shows increased superoxide production. Which enzyme most directly converts superoxide radicals into hydrogen peroxide and oxygen as an initial antioxidant step?
A.
Catalase
B.
Superoxide dismutase
C.
Ascorbate peroxidase
D.
Glutathione reductase
Correct Answer:
Superoxide dismutase
Explanation:
Superoxide dismutase is the first major enzymatic defense against superoxide radicals, converting them into hydrogen peroxide and molecular oxygen. The hydrogen peroxide must then be detoxified by other enzymes.
3. Why must increased superoxide dismutase activity during ABA-associated stress responses be accompanied by efficient hydrogen-peroxide-scavenging systems?
A.
Superoxide dismutase converts hydrogen peroxide into a more toxic radical
B.
Superoxide dismutase produces hydrogen peroxide, which can damage cells if it accumulates
C.
Superoxide dismutase prevents catalase from entering peroxisomes
D.
Superoxide dismutase consumes all cellular ascorbate during the reaction
Correct Answer:
Superoxide dismutase produces hydrogen peroxide, which can damage cells if it accumulates
Explanation:
Although superoxide dismutase removes superoxide, its reaction generates hydrogen peroxide. Catalase, peroxidases, and related antioxidant systems must remove this hydrogen peroxide to prevent oxidative injury.
4. Which pairing correctly matches an antioxidant enzyme with its principal reactive oxygen species substrate in stressed plant cells?
A.
Catalase — hydrogen peroxide
B.
Superoxide dismutase — lipid hydroperoxide
C.
Ascorbate peroxidase — molecular nitrogen
D.
Glutathione reductase — superoxide anion directly
Correct Answer:
Catalase — hydrogen peroxide
Explanation:
Catalase decomposes hydrogen peroxide into water and oxygen, thereby preventing hydrogen peroxide from contributing to oxidative damage or forming highly reactive hydroxyl radicals.
5. A mutant plant cannot increase catalase and peroxidase activity after ABA treatment but retains normal stomatal closure. Which consequence is most likely during prolonged drought?
A.
Reduced hydrogen-peroxide accumulation because stomatal closure eliminates all ROS formation
B.
Greater oxidative damage despite the initial reduction in water loss
C.
Complete prevention of lipid peroxidation by ABA-independent pathways
D.
Increased conversion of hydrogen peroxide into superoxide by catalase
Correct Answer:
Greater oxidative damage despite the initial reduction in water loss
Explanation:
Stomatal closure reduces water loss but can also restrict carbon dioxide entry and disturb photosynthetic electron flow, increasing ROS formation. Without adequate catalase and peroxidase activity, oxidative damage is likely to rise.
6. Which observation would provide the strongest evidence that ABA has enhanced antioxidant defense rather than merely reduced water loss?
A.
Lower transpiration with unchanged antioxidant enzyme activity
B.
Higher activities of superoxide dismutase, catalase, and peroxidase together with lower lipid peroxidation
C.
Reduced leaf area without any change in reactive oxygen species
D.
Increased stomatal closure accompanied by greater hydrogen-peroxide accumulation
Correct Answer:
Higher activities of superoxide dismutase, catalase, and peroxidase together with lower lipid peroxidation
Explanation:
Enhanced antioxidant defense should be reflected by increased ROS-scavenging enzyme activity and reduced oxidative damage, commonly assessed through indicators such as lipid peroxidation.
7. Under drought, hydrogen peroxide can serve both as a signaling molecule and as a damaging oxidant. Which condition best represents effective antioxidant regulation?
A.
Complete elimination of hydrogen peroxide immediately after its formation
B.
Controlled hydrogen-peroxide accumulation followed by efficient detoxification
C.
Unrestricted hydrogen-peroxide accumulation until stress ends
D.
Conversion of hydrogen peroxide into superoxide to strengthen signaling
Correct Answer:
Controlled hydrogen-peroxide accumulation followed by efficient detoxification
Explanation:
Hydrogen peroxide may participate in stress signaling at controlled concentrations, but excessive accumulation damages proteins, membranes, and nucleic acids. Antioxidant systems maintain this balance by removing excess hydrogen peroxide.
8. If a drought-stressed plant has normal superoxide dismutase activity but defective ascorbate peroxidase, which change is most directly expected?
A.
Reduced conversion of superoxide into hydrogen peroxide
B.
Increased accumulation of hydrogen peroxide in chloroplasts
C.
Complete absence of reactive oxygen species formation
D.
Enhanced degradation of ascorbate without any change in hydrogen peroxide
Correct Answer:
Increased accumulation of hydrogen peroxide in chloroplasts
Explanation:
Superoxide dismutase can still convert superoxide into hydrogen peroxide, but defective ascorbate peroxidase reduces hydrogen-peroxide removal, especially in chloroplasts, causing its accumulation.
9. Which result would indicate that an ABA-induced antioxidant response has protected cellular membranes from oxidative stress?
A.
Increased malondialdehyde concentration
B.
Decreased malondialdehyde concentration
C.
Increased superoxide production with unchanged membrane integrity
D.
Decreased catalase activity with increased electrolyte leakage
Explanation:
Malondialdehyde is a product of lipid peroxidation. Its decrease after ABA treatment suggests that antioxidant defenses have reduced ROS-mediated damage to membrane lipids.
10. A plant line overproduces glutathione reductase during ABA-mediated salt stress. What is the most likely antioxidant advantage of this change?
A.
Regeneration of reduced glutathione, strengthening redox-based peroxide detoxification
B.
Direct conversion of oxygen into superoxide for stress signaling
C.
Inhibition of all peroxidase reactions to conserve cellular energy
D.
Replacement of superoxide dismutase by preventing superoxide formation completely
Correct Answer:
Regeneration of reduced glutathione, strengthening redox-based peroxide detoxification
Explanation:
Glutathione reductase helps regenerate reduced glutathione, which supports antioxidant cycles and maintains the reducing conditions required for efficient detoxification of reactive oxygen species.
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