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

First, let's restate what the question is asking: if water labeled with the radioactive oxygen-18 is split during photosynthesis, the oxygen gas released should carry the same label, and we need to identify which description best explains this.
Option 1: 'During the light reactions of photosynthesis, water is split, removing electrons and protons, and oxygen gas is released.' This matches the known light reactions process in which water is split (photolysis) to replace electrons in photosystem II, releasing O2 as a byproduct. The splitting of water provides electrons, protons, and molecular oxygen, so labeling O from water ends up in O2 gas.
Option 2: 'During the light reactions of photosynthesis, water is split, the hydrogen atoms combine with the CO2, and oxygen gas is released.' While the light reactions generate ATP and NADPH and release O2, the statement misstates that hydrogen from water directly combines with CO2 during the light reactions to form sugar; CO2 fixation and sugar synthesis occur later, in the Calvin cycle. This conflates stages and misattributes hydrogen usage.
Option 3: 'During the Calvin cycle, water is split, regenerating NADPH from NADP+, and oxygen gas is released.' The Calvin cycle does not split water and does not release O2; it uses the products (ATP, NADPH) from the light reactions to fix CO2 into sugars. Water splitting and O2 evolution occur during the light reactions, not in the Calvin cycle, making this option incorrect.
Option 4: 'During the Calvin cycle, water is split, the hydrogen atoms are added to intermediates of sugar synthesis, and oxygen gas is released.' This statement again places water splitting and O2 production in the Calvin cycle, which is inaccurate. In the Calvin cycle, water is not split and no oxygen gas is released; the cycle relies on CO2 reduction and carbohydrate formation using ATP and NADPH from the light reactions.
In summary, the only choice that correctly aligns with the actual sequence—water splitting during the light reactions providing electrons and releasing O2—while also accounting for the fate of the labeled oxygen, is the first option. The other statements either place the process in the wrong stage (Calvin cycle) or misstate the role of water splitting and its connection to CO2 fixation and sugar synthesis.

APBIO141-031 with labs Unit 3 Test

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