L'expérience d'Elodea : une conclusion sur deux variables à la fois
A grade-9 class investigates photosynthesis with Elodea. Three beakers contain water and sodium hydrogencarbonate; each is placed 10 cm, 30 cm and 50 cm from the same lamp, and the bubbles released are counted for one minute: 42, 24 and 11. A fourth beaker wrapped in aluminium foil gives 2 bubbles per minute. The pupil's report concludes: «la lumière est le seul facteur qui contrôle la photosynthèse ; à 50 cm la photosynthèse s'arrête». The teacher's notes record what the pupil did not: the 10 cm beaker was prepared with 2 g/L of hydrogencarbonate while the other two had 1 g/L, and the water in the nearest beaker reached 32 °C against 22 °C elsewhere because the lamp was close. The report is worth 20 marks, of which 6 are for the conclusion. The teacher must decide how many of those 6 the conclusion deserves, and what feedback will teach the pupil to separate observation from inference.
1. Identify the variables in this experiment and evaluate the pupil's conclusion. How many of the 6 marks should it receive?
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Variables. Intended independent variable: distance from the lamp (a proxy for light intensity). Dependent variable: bubbles per minute (oxygen released). Variables that should have been controlled: mass of Elodea, volume of water, hydrogencarbonate concentration, temperature, duration of counting.
Two controls failed. The 10 cm beaker had 2 g/L of hydrogencarbonate against 1 g/L, so it had more dissolved CO₂ - a raw material of photosynthesis - as well as more light. And it was at 32 °C against 22 °C, which within this range also raises the rate. The 42 bubbles therefore combine at least three causes; the fall to 24 and 11 is consistent with light but is not attributable to light alone.
Conclusion: 2/6. One mark for reading the trend correctly, with the numbers (42 > 24 > 11) and the direction of the distances. Zero for «le seul facteur» - a single experiment with uncontrolled variables cannot establish exclusivity. Zero for «à 50 cm la photosynthèse s'arrête» - 11 bubbles per minute is not zero, and the pupil contradicts their own data. One mark for a hypothesis stated as a hypothesis (light probably matters) provided the pupil flags that the protocol does not prove it.
2. What is the value of the foil-wrapped beaker giving 2 bubbles per minute rather than 0?
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It is the baseline, and it is the best part of the protocol. A non-zero dark value is normal and informative:
- not all bubbles are necessarily oxygen - air can escape from the cut stem, and dissolved gases come out of solution;
- the plant continues for a short time on products of the light reactions already stored;
- counting itself has a margin of error, especially at low rates.
So the correct quantity is the difference: net photosynthesis ≈ 42 − 2, 24 − 2, 11 − 2, i.e. 40, 22 and 9. The conclusion «photosynthesis stops at 50 cm» is contradicted twice over: 11 bubbles were counted, and 9 of them are above the dark baseline.
The teacher should treat this beaker as a model of good practice in the feedback, and ask the pupil what it would mean if the dark beaker had given 11 bubbles as well: it would mean the measurement detects something other than photosynthesis, and the whole series would have to be reinterpreted.
3. Rewrite the conclusion the pupil is allowed to draw, and state the minimum redesign that would justify a causal claim about light.
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Permitted conclusion: «Dans les conditions de l'expérience, plus la lampe est éloignée, moins l'élodée rejette de bulles en une minute (42, puis 24, puis 11). Cette observation est compatible avec un rôle de la lumière, mais elle ne permet pas de l'isoler : la concentration en hydrogénocarbonate et la température ont varié en même temps que la distance.»
Minimum redesign:
- Place every beaker in a water bath at 22 °C, or insert a transparent heat filter between lamp and beaker.
- Prepare all solutions at the same hydrogencarbonate concentration (1 g/L).
- Use the same mass of Elodea and the same volume in every beaker.
- Keep the foil-wrapped beaker as the dark control.
- Repeat each count three times and average, to reduce counting error.
- If possible, add a dissolved-oxygen sensor: it is more reliable than bubbles and avoids the gas-escaping artefact.
With that protocol, the trend 40 > 22 > 9 can be attributed to light intensity, and the pupil may then write a causal sentence - but still a bounded one («dans ces conditions»), since a single species and a single temperature range do not license a general law.