Photosynthesis—Tracing Oxygen Atoms from Labeled CO2 If the oxygen atom is labeled in carbon dioxide (CO2) and this labeled CO2 is supplied to a plant, after photosynthesis where would you expect to find that labeled oxygen atom?

Introduction / Context:
This question tests understanding of the fate of oxygen atoms during photosynthesis by using an isotopic tracer. It probes whether the oxygen released as O2 comes from water (H2O) or from carbon dioxide (CO2), and where the oxygen from CO2 ends up in the final products of photosynthesis.

Given Data / Assumptions:

• Labeled oxygen is placed in CO2 (for example, 18O in CO2).
• Plant performs oxygenic photosynthesis with a light-dependent stage and the Calvin cycle.
• We track the labeled oxygen after the process proceeds.

Concept / Approach:
Classical isotope-labeling experiments showed that the O2 evolved in photosynthesis derives from water, not from CO2. In water-splitting at photosystem II, H2O is oxidized to O2, and those oxygen atoms are released as gas. The CO2 carbon is reduced and incorporated into carbohydrates; its oxygen atoms ultimately reside in sugar carbonyl/hydroxyl groups or in downstream metabolites derived from these sugars.

Step-by-Step Solution:

Verification / Alternative check:
Historical tracer studies using 18O-labeled water vs 18O-labeled CO2 demonstrate that labeled O2 appears only when water is labeled, confirming that O2 comes from H2O and CO2 oxygen is retained within organic products.

Why Other Options Are Wrong:

• In the water used: that is the source of evolved O2, not the sink for CO2 oxygen.
• NADPH: carries electrons and a proton; it does not sequester the CO2 oxygen.
• In the oxygen given off: incorrect—released O2 originates from water.
• ATP phosphate groups: ATP synthesis does not incorporate CO2 oxygen into its phosphates.

Common Pitfalls:
Assuming that because O2 is a product, its oxygen must come from CO2; experiments show it comes from H2O. Also confusing electron flow (to NADPH) with oxygen atom tracing.

Final Answer:
In the carbohydrate (sugars) synthesized during the Calvin cycle