Role of electron transport: Electron transport systems are essential to which stage(s) of photosynthesis and related plant processes?
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ACalvin cycle (carbon fixation)
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BPhotorespiration
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CLight-dependent reactions (photosynthetic light reactions)
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DAll of these
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ENone of these
Answer
Correct Answer: Light-dependent reactions (photosynthetic light reactions)
Explanation
Introduction / Context:Electron transport chains move electrons through redox carriers to generate a proton gradient and reducing power. In photosynthesis, this machinery is central to the light reactions but not to the stromal CO2-fixing reactions. This question asks where electron transport is vital.
Given Data / Assumptions:
- Photosynthetic electron transport occurs in thylakoid membranes and includes photosystem II, cytochrome b6f, photosystem I, plastoquinone, plastocyanin, and ferredoxin.
- The Calvin cycle takes place in the stroma and uses ATP and NADPH made by the light reactions.
- Photorespiration involves peroxisomes and mitochondria and does not rely on the thylakoid electron transport chain as its core driver.
Concept / Approach:Match each process to its dependence on an electron transport system. Only the light-dependent reactions require the thylakoid electron transport chain to capture light energy, generate ATP via photophosphorylation, and reduce NADP+ to NADPH. The Calvin cycle consumes these products but does not contain an electron transport chain. Photorespiration is a salvage pathway triggered by Rubisco oxygenase activity.
Step-by-Step Solution:
Identify electron transport location: thylakoid membranes.Link to function: ATP and NADPH generation in light reactions.Exclude Calvin cycle and photorespiration as primary sites for electron transport systems.Select light-dependent reactions as the correct answer.Verification / Alternative check:Disruption of cytochrome b6f or photosystem components abolishes light-driven electron flow and ATP/NADPH production but does not directly disable Calvin cycle enzymes in vitro if ATP/NADPH are provided.
Why Other Options Are Wrong:
- Calvin cycle: enzyme-mediated carbon chemistry without membrane electron transport.
- Photorespiration: involves peroxisomal and mitochondrial steps, not the thylakoid electron chain.
- All of these or none: both contradict the specific location of electron transport in photosynthesis.
Common Pitfalls:Confusing ATP/NADPH consumption by the Calvin cycle with their production via electron transport in the light reactions.
Final Answer:Light-dependent reactions (photosynthetic light reactions)