Photosynthesis

AP Biology· difficulty 1/5

The oxygen released during photosynthesis comes from

A
$Atmospheric O₂ (recycled in the thylakoid)$
B
$Glucose (broken down in stroma releasing O₂)$
C
$H₂O (split during the light reactions at PSII)$
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D
$CO₂ (split during the Calvin cycle)$

Explanation

Isotope-labeling experiments confirmed that the oxygen released is from water, not CO₂. Carbon atoms from CO₂ end up in glucose.

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