Passage IV

Carbon dioxide (CO₂) is consumed during photosynthesis and produced, during cellular respiration. Photosynthesis and cellular respiration can be summarized by equations 1 and 2, respectively:

1) 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O2

2) C₆H₁₂O₆ + 6O2 → 6CO₂ + 6H₂O + energy

When CO₂ dissolves 1in an aqueous solution, an acid is formed. Thus, the pH of an aqueous solution is affected by the concentration of CO₂ dissolved in the solution. A student performed an experiment to determine how the pH of an aqueous solution is affected by the addition to the solution of an organism that is undergoing both photosynthesis and cellular respiration, an organism that is undergoing cellular respiration but not photosynthesis, or both such organisms.

Experiment

The student prepared a sterile aqueous solution containing bromothymol blue. A bromothymol blue solution can be yellow, green, or blue in color, depending on its pH (see Table 1). The student's solution was green.

The student placed 10 mL of the solution into each of 8 sterile tubes (Tubes 1-8). Then she added a piece of elodea (a water plant), a snail, or both to each of Tubes 3-8, placed a cap on each of Tubes 1-8, and incubated all 8 tubes at 25°C for 24 hr. During the incubation, each rube received either no light for 24 hr (dark treatment) or constant light for 24 hr (light treatment). Table 2 lists, for each tube, the organism(s) present (if either), whether the dark or the light treatment was received, and the color of the solution at the end of the 24 hr incubation.