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?

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:

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)