Interpreting CO2 Changes in Sealed Elodea–Snail Flasks
Students place equal masses of Elodea and equal numbers of snails in four sealed 250-mL flasks with CO2 sensors. Flask 1 contains Elodea plus a snail; Flask 2 a snail only; Flask 3 Elodea only; Flask 4 water only. All flasks are held at 22°C for 12 hours at 1000 lux, then 12 hours in darkness. Initial CO2 is 500 ppm in every flask. After the light period, readings are Flask 1: 320 ppm; Flask 2: 610 ppm; Flask 3: 240 ppm; Flask 4: 500 ppm. After the dark period, readings are Flask 1: 540 ppm; Flask 2: 680 ppm; Flask 3: 430 ppm; Flask 4: 505 ppm. Students are asked to explain why Flask 1 is lower than Flask 3 in the light, why Flask 1 rises in the dark, and whether the data show that Elodea photosynthesizes. They have no sensor for oxygen.
1. Calculate the net CO2 change for each flask in the light and dark periods. Use the values to explain the roles of Elodea and the snail, and explain why Flask 1 is lower than Flask 3 in the light.
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Light-period changes: Flask 1: 320 - 500 = -180 ppm; Flask 2: 610 - 500 = +110 ppm; Flask 3: 240 - 500 = -260 ppm; Flask 4: 500 - 500 = 0 ppm. Dark-period changes from the light endpoint: Flask 1: 540 - 320 = +220 ppm; Flask 2: 680 - 610 = +70 ppm; Flask 3: 430 - 240 = +190 ppm; Flask 4: 505 - 500 = +5 ppm. The snail-only flask rises, showing respiration releases CO2. The Elodea-only flask falls in light and rises in dark, showing photosynthesis exceeds respiration in light but stops or falls below respiration in darkness. Flask 1 is lower than Flask 3 in light because Elodea photosynthesizes in both, but the snail in Flask 1 adds CO2 through respiration, partially offsetting the plant's uptake. The +80 ppm difference between Flask 1 and Flask 3 in light is consistent with the snail's respiratory contribution.
2. Predict how the CO2 readings would change if the light period were run at 2000 lux instead of 1000 lux. Justify your prediction using the data and identify one limiting factor that could prevent a proportional response.
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If light was limiting photosynthesis at 1000 lux, Flask 3 would likely fall below 240 ppm during the brighter light period, and Flask 1 would likely fall below 320 ppm but remain above Flask 3 because the snail still respires. The dark-period starting values would then be lower, so final dark readings could also be lower, though respiration rates would be similar. The response may not double because CO2 concentration in the sealed flask can become limiting as it drops, and temperature, nutrients, or plant surface area may also limit photosynthesis. At very high light, photoinhibition or heat could reduce the rate. The prediction is therefore directional, not proportional: brighter light should increase net CO2 uptake if light is the limiting factor, but the sealed system sets a limit.
3. Identify one controlled variable and one variable that was not adequately controlled in this experiment. Explain how the uncontrolled variable could affect the conclusion, and propose one improvement.
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A controlled variable is the initial CO2 concentration, which was 500 ppm in all flasks, and the temperature was held at 22°C. A poorly controlled variable is the amount of active photosynthetic tissue: equal masses of Elodea are not necessarily equal leaf surface area, and equal numbers of snails are not necessarily equal respiration rates if the snails differ in size or activity. If Flask 3 had more leaf surface, its larger CO2 drop could reflect tissue amount rather than a treatment difference. Light distribution and microbial respiration in the water are also possible confounds. An improvement is to use replicated flasks per treatment, select similar-sized snails, and standardize Elodea by surface area or shoot length rather than mass alone. Acclimating organisms to the flasks before measurement would also reduce stress effects.