Question

6. Scientists conducted two experiments using radioactive isotope labeling to show the paths of atoms in photosynthesis. The two experiments we studied the scientists conducted to show the paths of the atoms found in CO₂ and H₂O in photosynthesis using radioactive isotope labeling. In Experiment 1, they used radioactive carbon dioxide (CO₂), and in experiment 2, they used radioactive water (H₂O).

After Experiment 1, the radioactive label was detected in the glucose molecules produced by the plant.
After Experiment 2, the radioactive label was detected in the oxygen gas released into the air.
​​​​​​​Which conclusion is best supported by the experimental evidence?

Responses

A. The oxygen atoms in carbon dioxide become the oxygen molecules released by plants.
A. The oxygen atoms in carbon dioxide become the oxygen molecules released by plants.

B. The carbon atoms in carbon dioxide become part of the glucose molecules produced during photosynthesis.
B. The carbon atoms in carbon dioxide become part of the glucose molecules produced during photosynthesis.

C. Sunlight energy is stored directly inside the oxygen molecules.
C. Sunlight energy is stored directly inside the oxygen molecules.

D. Water molecules are unchanged during photosynthesis and serve only to hydrate the plant.
D. Water molecules are unchanged during photosynthesis and serve only to hydrate the plant.

E. Carbon dioxide and sunlight combine to form glucose without involving water molecules.
E. Carbon dioxide and sunlight combine to form glucose without involving water molecules.
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Answer 1

B. The carbon atoms in carbon dioxide become part of the glucose molecules produced during photosynthesis.

Reason: Experiment 1 shows radioactive carbon from CO₂ ends up in glucose, so CO₂ provides the carbon for sugar. (Experiment 2 separately shows the O₂ released comes from water, not CO₂.)