The Greenhouse Gas Experiment
A student sets up two identical clear plastic bottles, each containing 200 cm³ of pond water and 10 water snails. Bottle A is sealed with a bung and placed in bright light. Bottle B is sealed and placed in the dark. After 24 hours, the student measures the oxygen concentration in each bottle using an oxygen sensor. Bottle A has 8 mg/dm³ oxygen; Bottle B has 2 mg/dm³. The student also measures the oxygen concentration in a control bottle C containing pond water but no snails, kept in bright light; it has 10 mg/dm³. The student concludes that snails only respire and do not photosynthesise. The teacher asks the student to consider the pond water itself.
1. Explain the results in bottles A, B and C. Include the processes occurring in the snails and in the pond water.
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In bottle C, the pond water contains microscopic algae that photosynthesise in bright light, producing oxygen. The oxygen concentration is highest (10 mg/dm³) because there are no snails to use oxygen. In bottle A, the snails respire and use oxygen, while the algae still photosynthesise and produce oxygen. The net oxygen is lower than C (8 mg/dm³) because respiration by snails uses some oxygen. In bottle B, kept in the dark, photosynthesis stops, but snails and algae continue to respire, using oxygen. The oxygen concentration is lowest (2 mg/dm³). The student's conclusion that snails only respire is correct for the snails, but the experiment shows that the pond water also affects oxygen levels. The difference between A and C is due to snail respiration. The difference between A and B is due to lack of photosynthesis in the dark.
2. Suggest one improvement to the experiment to make the conclusion more reliable, and explain why it would help.
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One improvement is to use more replicates of each bottle (for example, three bottles per condition) and calculate an average oxygen concentration. This would help because individual bottles may vary due to differences in the amount of algae or snail activity. Replicates reduce the effect of random variation and make the results more reliable. Another improvement is to measure the oxygen concentration at the start of the experiment, so the change in oxygen can be calculated rather than just the final value. This would show how much oxygen was produced or used. Also, using a control with dead snails or no snails in the dark would help separate snail respiration from algal respiration. Any one of these would strengthen the conclusion.