The chemical reaction for photosynthesis is

6 CO2 + 12 H2O + light energy → C6H12O6 + 6 O2 + 6 H2O

If the input water is labeled with a radioactive isotope of oxygen, 18O, then the oxygen gas released as the reaction proceeds is also labeled with 18O.

Which of the following is the most likely explanation?

Question

The chemical reaction for photosynthesis is

6 CO2 + 12 H2O + light energy → C6H12O6 + 6 O2 + 6 H2O

If the input water is labeled with a radioactive isotope of oxygen, 18O, then the oxygen gas released as the reaction proceeds is also labeled with 18O.

Which of the following is the most likely explanation?

BlackTom AI · Accepted Answer

Photolysis in Photosystem II splits water molecules, producing O2, protons, and electrons. The oxygen-evolving complex (OEC) in the light reactions oxidizes water: 2 H2O → O2 + 4 H+ + 4 e−. This is the key biochemical step that generates molecular oxygen in photosynthesis.

Relevant knowledge points and concepts:
- Photosynthesis has light-dependent (light reactions) and light-independent (Calvin cycle) stages.
- The light reactions occur in the thylakoid membranes and include photolysis (water splitting) at Photosystem II.
- The oxygen produced by photosynthesis originates from water, not from CO2.
- Isotopic labeling (18O) traces the origin of atoms in reaction products.

Step-by-step reasoning and intermediate steps:
1. Label water with 18O. If O2 produced contains 18O, the oxygen atoms in O2 must come from water.
2. The OEC reaction (balanced per O2) is: 2 H2(18)O → 18O2 + 4 H+ + 4 e−. Thus each O2 molecule contains oxygen atoms derived directly from two water molecules.
3. In the overall balanced photosynthesis shown (6 CO2 + 12 H2O + light → C6H12O6 + 6 O2 + 6 H2O), twelve waters are split during the light reactions to yield six O2 (12 H2O → 6 O2 + 24 H+ + 24 e−). The released electrons and protons are used to reduce CO2 to carbohydrate in the Calvin cycle.
4. Observing 18O in evolved O2 empirically demonstrates that photolysis of water is the source of atmospheric O2.

Why the correct answer is correct:
The experimental result (18O-labeled O2 when water is labeled) directly supports water-splitting during the light reactions; the mechanism and stoichiometry above show how water yields O2 and provides electrons for CO2 reduction.

Why other common ideas are incorrect:
- Oxygen does not come from CO2; if CO2 were the source, label in water would not appear in O2.
- Oxygen is not formed by splitting carbon compounds or chlorophyll; experimental isotope tracing rules these out.

Conclusion: The labeled-O18 experiment confirms that during the light reactions of photosynthesis water is split (photolysis), releasing O2—matching the provided correct answer.

Similar Questions

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