Single answerDifficulty 2/5 · StandardRepresenting dataArgumentation
Heavy oxygen in algae
Topic: Photosynthesis
Green algae (Chlorella) are kept in bright light in two flasks. In one, the water is enriched with the heavy isotope ¹⁸O; in the other, the dissolved CO₂ (supplied as bicarbonate) is enriched instead. Samples of the O₂ the algae give off are analysed in a mass spectrometer. Natural oxygen contains about 0.20% ¹⁸O.
Experimental setup
- Question
- Does the O₂ released in photosynthesis come from water or from CO₂?
- Independent variable
- Which reactant carries the extra ¹⁸O
- Dependent variable
- Percentage of ¹⁸O in the O₂ released
- Kept constant
- Same algal culture · Light intensity · Temperature · Duration
- Groups
- Flask A: ¹⁸O-enriched water, ordinary CO₂
- Flask B: Ordinary water, ¹⁸O-enriched CO₂
Data
| Flask | Water (% ¹⁸O) | CO₂ (% ¹⁸O) | O₂ released (% ¹⁸O) |
|---|---|---|---|
| A | 0.85 | 0.20 | 0.85 |
| B | 0.20 | 0.68 | 0.20 |
What do these results show?
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