Role of electron transport: Electron transport systems are essential to which stage(s) of photosynthesis and related plant processes?

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:

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)