Photosynthesis source of oxygen: Identify the molecule that provides the O₂ released. Full question: During photosynthesis, the oxygen gas (O₂) evolved comes from which reactant molecule? Choose the correct option.

Difficulty: Easy

Water (H₂O)
Carbon dioxide (CO₂)
Glucose (C₆H₁₂O₆)
Chlorophyll
Oxygen dissolved in cytoplasm

Correct Answer: Water (H₂O)

Explanation:

Given data

Target: Identify the source of released O₂ in photosynthesis.

Concept / Approach
In the light reactions, water undergoes photolysis at photosystem II, producing electrons, protons, and O₂. Isotopic tracing (using H¹⁸₂O) shows that the oxygen comes from water, not CO₂.

Key reaction fragment
2 H₂O → O₂ + 4H⁺ + 4e⁻ (driven by light energy)

Final Answer
Water (H₂O).

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