Study
Heavy oxygen (O18) as a tracer in the study of photosynthesis
Ruben, Samuel; Randall, Merle; Kamen, Martin; Hyde, James L.
Journal of the American Chemical Society, 1941
Algae were supplied with water enriched in the heavy oxygen isotope O-18 while carbon dioxide carried ordinary oxygen, and then the reverse. The oxygen gas released was collected and its isotopic composition measured.
- Studied in
- Chlorella
- Sample size
- algal cultures supplied with isotopically labelled water or carbon dioxide
The oxygen released carried the isotopic signature of the water, not of the carbon dioxide, in both conditions.
What it means
The authors' reading, and ours. Where they differ, that difference is the point.
How the authors put it
The oxygen produced in photosynthesis originates from water, not from carbon dioxide.
How NatureHQ reads it
An elegant use of a label to answer a question chemistry alone could not. Both candidate molecules contain oxygen, so no amount of measuring how much gas comes off will say which one it came from — you have to mark the atoms and see which mark appears. The answer rewrites the summary equation people memorise: plants are not taking carbon dioxide apart to release its oxygen, they are splitting water, and the oxygen in the atmosphere is water that has been taken apart by light.
- Performed on algae; the conclusion generalises on the strength of shared machinery rather than direct testing across plants.
- Early isotope-ratio measurement carried substantial uncertainty.
- Says nothing about how the splitting is accomplished, which took decades more.
Labelling the atoms to find where oxygen comes from
Photosynthesis consumes water and carbon dioxide and releases oxygen. Both inputs contain oxygen — so which one does the released gas come from?
What this study is used for on NatureHQ
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Published by: Academic book publishersPublished as an academic monograph.
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