Source of oxygen for respiration: The O2 that serves as the terminal electron acceptor in oxidative phosphorylation ultimately comes from which biological process on Earth?
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AFermentation in microbes and tissues
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BThe light reactions of oxygenic photosynthesis (water splitting)
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CThe dark reactions (Calvin cycle) of photosynthesis
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DCarbon fixation pathways in plants and algae
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EAbiotic production in the Golgi apparatus
Answer
Correct Answer: The light reactions of oxygenic photosynthesis (water splitting)
Explanation
Introduction / Context:Aerobic respiration depends on molecular oxygen as the terminal electron acceptor. At a planetary scale, Earth's atmospheric O2 reservoir originates from biological oxygen evolution. Understanding its source clarifies the interdependence between photosynthesis and respiration.
Given Data / Assumptions:
- Oxygenic photosynthesis splits water at photosystem II, releasing O2.
- Respiration consumes O2 and produces CO2 and H2O.
- Fermentation pathways operate without O2 and do not generate it.
Concept / Approach:During the light reactions, the oxygen-evolving complex of photosystem II oxidizes water: 2 H2O → O2 + 4 H+ + 4 e−. This O2 diffuses and accumulates in the atmosphere. Oxidative phosphorylation in mitochondria depends on atmospheric O2, linking respiratory metabolism to photosynthetic O2 production across ecosystems.
Step-by-Step Solution:
Identify the process that generates O2: water oxidation in light reactions.Recognize that the Calvin cycle fixes CO2 but does not produce O2.Exclude fermentation: it neither uses nor produces O2.Conclude that the light reactions are the ultimate source of atmospheric O2 used in respiration.Verification / Alternative check:Isotope labeling with H2^18O demonstrates that evolved O2 derives from water, not CO2, confirming photosystem II as the source.
Why Other Options Are Wrong:
- Fermentation: anaerobic; does not yield O2.
- Dark reactions and carbon fixation consume ATP/NADPH; they do not produce O2.
- Golgi apparatus: an organelle with no role in O2 generation.
Common Pitfalls:Assuming O2 arises from CO2 reduction; in oxygenic photosynthesis, O2 comes from water splitting.
Final Answer:The light reactions of oxygenic photosynthesis (water splitting)