Elodea, Light Intensity, and a Sealed Beaker
In a Grade 8 lab, groups place 5 g of Elodea in separate sealed 500 mL beakers filled with pond water and 0.3% sodium bicarbonate. Beaker A sits 30 cm from a 60 W lamp; beaker B is wrapped in foil. Both beakers share a water bath at 22 °C. Students count oxygen bubbles per minute for 10 minutes. For minutes 3–7, A holds steady at 28–32 bubbles/min while B shows 2–4. At minute 8, a student moves A's lamp to 10 cm from the beaker. The count rises to 55/min, then falls to 12/min by minute 10. During the same interval, B's water warms by 1.5 °C. The teacher asks whether the class can conclude that light intensity controls the rate of photosynthesis. Two groups disagree: one says the data prove it; the other says the whole experiment is invalid because the sealed beakers change over time. The class must decide what claim, if any, the evidence supports.
1. What claim about light intensity and photosynthesis is best supported by beaker A's data? Explain how the minute 8–10 pattern shapes your answer.
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Beaker A's data support a limited claim: increasing light intensity can increase the photosynthetic rate in the short term. The jump from about 30 to 55 bubbles/min immediately after the lamp moved closer is consistent with more light driving more light-dependent reactions. However, the fall to 12/min while the lamp stayed at 10 cm shows that light was not the only limiting factor. A sealed beaker has a fixed carbon pool; as photosynthesis proceeds, dissolved CO2 is consumed and O2 accumulates, and the water also warms. So the evidence supports 'light can limit the rate at first,' not 'light intensity controls the rate' in a simple proportional way. A claim of control would require holding other variables constant and measuring steady rates.
2. Identify two unintended changes in the setup and explain how each offers an alternative explanation for the bubble counts.
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One unintended change is temperature: moving the lamp closer and sharing a warm water bath raised the water temperature, and B warmed 1.5 °C, so A likely warmed too. Warmer water can speed enzyme-controlled reactions and reduce gas solubility, changing bubble rate independently of light. A second is the sealed system's chemistry: photosynthesis removes dissolved CO2 and adds O2, so the CO2 supply declines and gas saturation changes over the 10 minutes. The minute-8 rise could be a light effect, but the later fall could be CO2 limitation, O2 buildup, or heating rather than a light effect. Because these variables changed at the same time as light distance, the bubble count cannot be assigned to light alone.
3. Propose one practical revision that would let the class test light intensity more cleanly, and predict the result.
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Use several identical beakers at fixed distances (for example 10, 20, 30, and 40 cm) with the same starting water, bicarbonate concentration, and temperature, and measure each at the same elapsed time after the lamp is turned on. Include a foil-wrapped dark control and at least three replicates per distance; record temperature and replace the water or use a flow-through system to avoid CO2 depletion. Prediction: the initial bubble rate should rise with light intensity and then level off when another factor, such as CO2 or temperature, becomes limiting. If all distances give the same rate, light is not limiting under these conditions.