Question

All organisms are dependent on a healthy carbon dioxide-oxygen balance. Photosynthesis and cellular respiration are key processes in maintaining this balance. Your class is conducting a lab experiment to investigate the relationship between photosynthesis and cellular respiration by observing how carbon dioxide cycles through a biological system. To conduct the experiment, each group set up four test tubes containing 30 ml distilled water and 3 ml of bromothymol blue. Bromothymol blue is a pH indicator that changes from blue (alkaline) to yellow (acid). When carbon dioxide dissolves in water, it produces carbonic acid, turning the indicator from blue to yellow. The level of CO2 was inferred from a change in the indicator color.

A snail, elodea (aquatic plant), or both were added to the tubes and they were stoppered. Tubes were placed under a grow light for 24 hours and the results recorded. The test tubes were then covered for 24 hours to produce a dark environment and the results recorded. Which statement best explains the change in Test Tube D from the light setup to the dark setup.
Responses
A The rate of photosynthesis decreased in the dark, thus the elodea used less CO2. The elodea and snails produced less CO2 through cellular respiration, causing the CO2 to be in equilibrium, making the solution more neutral (yellow).The rate of photosynthesis decreased in the dark, thus the elodea used less CO 2 . The elodea and snails produced less CO 2 through cellular respiration, causing the CO 2 to be in equilibrium, making the solution more neutral (yellow).
B The rate of photosynthesis decreased in the dark, thus the elodea used less CO2. The elodea and snails continued to produce CO2 through cellular respiration, raising the level of CO2 in the tube, causing the solution to become more acidic (yellow).The rate of photosynthesis decreased in the dark, thus the elodea used less CO 2 . The elodea and snails continued to produce CO 2 through cellular respiration, raising the level of CO 2 in the tube, causing the solution to become more acidic (yellow).
C The rate of photosynthesis decreased in the dark, thus the elodea used more CO2. The elodea and snails continued to consume CO2 through cellular respiration, lowering the level of CO2 in the tube, causing the solution to become more basic (yellow).The rate of photosynthesis decreased in the dark, thus the elodea used more CO 2 . The elodea and snails continued to consume CO 2 through cellular respiration, lowering the level of CO 2 in the tube, causing the solution to become more basic (yellow).
D The rate of photosynthesis decreased in the dark, thus the elodea used less CO2. The elodea and snails continued to consume CO2 through cellular respiration, raising the level of CO2 in the tube, causing the solution to become more acidic (yellow).

Answer 1

B

In the light the elodea was photosynthesizing and removing CO2 (making the solution more basic/blue). In the dark photosynthesis stopped but the elodea and snail kept respiring, producing CO2; the increased CO2 forms carbonic acid and turns the bromothymol blue yellow.