Elodea, Bromothymol Blue and the Light–Dark Balance
Three transparent test tubes are filled with equal volumes of dilute bromothymol blue solution. Tube A is sealed with a sprig of the water plant Elodea and kept in bright light. Tube B is sealed with an equal sprig and wrapped in black paper. Tube C is sealed with no plant and kept in bright light. After four hours the colour in A is blue-green, B is yellow, and C remains green. The teacher explains that bromothymol blue turns yellow as carbon dioxide dissolves in water and becomes more acidic, while it stays blue-green when carbon dioxide is removed. A student concludes that Elodea only performs photosynthesis and does not respire, because tube A did not turn yellow. Another student says the experiment shows both processes but does not prove which one is faster. The plant sprigs are healthy and of similar mass. The tubes are kept at 25 °C.
1. Account for the final colour in each tube A, B and C. Which tube is the control, and what does it control for?
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In tube A, light allows photosynthesis, so the plant uses dissolved CO2 faster than respiration releases it. The net removal of CO2 makes the solution less acidic, and the indicator becomes blue-green. In tube B, light is excluded, so photosynthesis cannot occur; respiration continues and releases CO2, making the water more acidic and turning the indicator yellow. Tube C has no plant and stays green, showing that the indicator does not change by itself under the same light and room-air conditions. Tube C is therefore the control for the indicator solution and for any CO2 exchange with the air. Comparing A and B with C shows that the colour changes are caused by the plant, not by the black paper or the lighting alone.
2. Evaluate the first student's conclusion that Elodea does not respire. Use the evidence from tube B and explain why tube A can still be consistent with respiration.
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The conclusion is wrong. Tube B is direct evidence that the plant respires: in the dark, photosynthesis stops, but CO2 still appears and turns the indicator yellow. Respiration is a continuous process in living cells, so it must also occur in tube A. In the light, photosynthesis is simply faster than respiration, so the net change is removal of CO2 rather than addition. The blue-green colour in A does not mean that respiration is absent; it means that CO2 uptake by photosynthesis exceeds CO2 release by respiration. The experiment therefore shows both processes, and it also shows that their relative rates depend on light.
3. The experiment is repeated with the light moved farther away so that tube A turns green rather than blue-green after four hours. What does this suggest about the balance between photosynthesis and respiration?
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Moving the light farther away reduces the light intensity reaching the plant, so the rate of photosynthesis falls. Respiration continues at a similar rate, so the net removal of CO2 is smaller. The indicator therefore ends closer to its original green, between the blue-green of strong net CO2 removal and the yellow of net CO2 release. This does not mean that photosynthesis has stopped or that respiration has stopped. It suggests that the two processes are closer to balancing at that lower light level: the amount of CO2 used in photosynthesis is only slightly greater than the amount released by respiration.