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In this lab you will investigate the impact of cellular respiration and photosynthesis on the environment. Specifically, you will be monitoring the changes in the carbon dioxide level as an indicator of the activity of these two processes. Photosynthesis uses CO₂ as a starting material and cellular respiration produces it as a waste product.

All cells do cellular respiration all the time. The cells of green plants can also do photosynthesis when light is available. When photosynthesis is running rapidly the level of CO₂ in the immediate environment would be expected to decrease; but, when light is not available photosynthesis does not occur to utilize the waste CO₂ that cellular respiration produces and the CO₂ level would increase.

Monitoring CO₂
When CO₂ enters an aqueous solution it reacts with the water to produce a weak acid -- the more CO₂, the more acidic the solution becomes. Therefore, measuring the change in acidity would be a way to monitor relative carbon dioxide levels.

pH refers to acidity and basicity. The lower the pH number, the greater the acidity. Brom thymol blue (BTB) is a pH indicator: it changes color as the pH varies. BTB turns blue at pH 7.6 (more basic), it becomes yellow at pH 6 (more acidic). Mix yellow and blue and you get green -- BTB is green midway between these pH extremes.

         acidic                   approximately neutral                   basic

The Experiment

This is how the setup looks at the beginning of exercise 6-1.

The two tubes at the right each contain aquatic plant material (elodea). One tube is in the light and the other is covered with aluminum foil to keep its contents in the dark. Both tubes contain BTB solution, adjusted to be about in the middle of its pH range.

The two tubes on the left do not contain plants but are like the other pair of tubes in every other way. They will show how (and if) the BTB solution changes color without plant activity.