Question

8. Students were asked to design an experiment to determine whether light is necessary for photosynthesis. They planned to use the following in their experiment:
Elodea plants in BTB solution, labeling one test tube “in light”
Elodea plants in BTB solution, labeling one test tube “in dark.”
A control test tube that contained only BTB solution and no plant.

After 15 minutes, only the tube with Elodea exposed to light changed color from yellow to green/blue. The class analyzed the results their experiment to arrive at their conclusions.


Which THREE statements correctly describe what this investigation demonstrates or supports?

Responses

A. Light energy is required for photosynthesis to occur.
A. Light energy is required for photosynthesis to occur.

B. The color change in BTB indicates that carbon dioxide was being used by the plant.
B. The color change in BTB indicates that carbon dioxide was being used by the plant.

C. The control tube helps confirm that BTB changes color even without a plant.
C. The control tube helps confirm that BTB changes color even without a plant.

D. The plant in the dark still performed photosynthesis but at a slower rate.
D. The plant in the dark still performed photosynthesis but at a slower rate.

E. The experiment supports the idea that energy is transferred from light to chemical products.
E. The experiment supports the idea that energy is transferred from light to chemical products.
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Answer 1

Correct answers: A, B, and E.

- A (True): Only the Elodea exposed to light removed CO2 (indicator changed), showing light is required for photosynthesis under these conditions.
- B (True): BTB is a pH indicator; the color shift from yellow (acidic, higher CO2) to green/blue indicates CO2 was being consumed by the plant.
- E (True): Demonstrating that light is required for photosynthesis supports the idea that light energy is converted into chemical energy (products of photosynthesis).

Why C and D are incorrect:
- C is wrong because the control (BTB only) did not change color; it shows BTB does not change on its own without a plant.
- D is wrong because the dark tube showed no color change, so the plant in the dark did not perform detectable photosynthesis.