Biology Lab

Today in biology we set up a classic photosynthesis experiment. Bromothyl blue (BTB) is an indicator chemical that turns yellow in the presence of carbon dioxide. So we started with a beaker of (blue) BTB. Using a straw, each group blew into their beakers of BTB until it turned yellow. We figured out that the color change was due to the carbon dioxide in exhaled air reacting with the BTB. Each group then poured the (now yellow) solution into two separate test tubes, and added an aquatic plant to each.

Question: What will happen to the BTB? After some discussion we decided that the plants, through photosynthesis, will use up the carbon dioxide in the water and produce oxygen. This should result in the BTB color changing back to blue.

Question: How do we know that any color change results from the plant and not from something else? Through discussion, we figured out that oxygen in the air might affect our results, so we decided to seal up the test tubes with parafilm so that they'd be airtight. Now if the color changes, we know that it's from the plants sealed inside and not from the gases in the atmosphere.

Question: How can we make sure that photosynthesis is what produces oxygen, and not some other process in the plant? We thought about this for awhile, and decided that, by keeping one of the test tubes in the dark, we would prevent the plant inside from doing photosynthesis. So each group enclosed one of their two test tubes with aluminum foil to block out all light.

Our working hypothesis, then, is that if a plant is exposed to light, then it will absorb carbon dioxide and produce oxygen which will turn its BTB solution from yellow to blue.