Elodea and Light: When More Light Stops Helping
Students investigate photosynthesis in Elodea using a CO2 indicator solution that shifts from red to yellow as CO2 is consumed. They place tubes at 10, 20, 30, and 40 cm from a 100 W lamp and record the time to color change: 40 minutes at 40 cm, 22 minutes at 30 cm, 12 minutes at 20 cm, and 11 minutes at 10 cm. A student concludes that photosynthesis increases with light intensity, so moving the lamp closer always increases the rate. The teacher notes that the lamp heats the water at 10 cm, that the indicator was not degassed between trials, and that the 30 cm and 40 cm trials used a different batch of indicator made with tap water rather than distilled water. Another group using the same lamp at 20 cm got 35 minutes instead of 12 minutes. The class must decide whether to revise the conclusion or repeat the experiment.
1. Evaluate the student's conclusion using the data. Explain why the pattern from 20 cm to 10 cm is especially important, and connect it to a biological limiting factor.
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The claim that closer always means faster is not supported. Time to color change falls from 40 min at 40 cm to 22 min at 30 cm and 12 min at 20 cm, but only from 12 to 11 min between 20 and 10 cm. If distance were the only factor, the inverse-square law predicts a much larger change from 20 to 10 cm because light intensity should roughly quadruple. The plateau suggests light saturation: at high intensity, another factor such as CO2 concentration, temperature, or the capacity of the light reactions and Calvin cycle limits the rate. Alternatively, the lamp heats the water at 10 cm and may inhibit photosynthesis or change the indicator. The 20 cm result from another group, 35 min versus 12 min, also shows poor reproducibility. The defensible conclusion is that the rate increases with light up to a point and then levels off; the data do not show a continuing increase at the closest distance.
2. Identify the confounds in the experiment and explain how each could produce a misleading result.
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The 30 cm and 40 cm trials used a different indicator batch made with tap water, so the color-change endpoint may differ because of buffering capacity, initial pH, or dissolved ions; the comparison across distances is therefore not clean. The indicator was not degassed between trials, so residual CO2 or a different starting color could change the time to the endpoint. The lamp heats the water at 10 cm, adding temperature as a variable; temperature affects enzyme activity and CO2 solubility, so the 10 cm result may reflect heat rather than light alone. The time-to-color endpoint is subjective and depends on the observer. The discrepancy between the two groups at 20 cm, 12 min versus 35 min, indicates that uncontrolled variables are large. Any of these could create either a false plateau or a false steep increase.
3. Design a controlled experiment to test the effect of light intensity on the rate of photosynthesis. Include how you would measure the rate, control temperature, and analyze the data.
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Use a single batch of degassed indicator, identical Elodea sprigs of similar mass, and identical tubes. Place tubes at distances such as 10, 20, 30, 40, and 50 cm from the same lamp, with at least three replicates per distance. Put all tubes in a water bath at 25 °C and shield them from stray light; measure the lamp output with a light meter if available. Measure the rate as the inverse of time to a defined color endpoint, or better, use an oxygen sensor to record the initial linear increase in dissolved oxygen. Include a dark control and a boiled-plant control. Convert distance to relative light intensity using the inverse-square relationship, then plot rate against intensity. If the plot rises and then plateaus, light saturation is supported; if the 10 cm rate falls, heat or photoinhibition is likely. Report means with error bars and test whether the plateau is statistically meaningful.