Photosynthesis vs. Cellular Respiration Organisms interact with many substances in nature. One such substance, CO2, is exchanged between organisms and their environments during the process of photosynthesis and respiration. This investigation uses an indicator, bromothymol blue (BTB), which turns yellow when CO2 is added to a solution. CO2 and water form a weak carbonic acid solution. BTB is light green in a neutral solution, yellow in acidic, and blue in basic (alkaline). Problem statement Do plants and animals need oxygen? Do plants and animals release CO2? Hypothesis ________________________________________________________________________ Results Container Initial Color Final Color Process that led to final color BTB + human breath BTB+ human breath+ Plant BTB+ human breath + Plant (DARK) Conclusion Questions 1. What gas did you exhale? Where did this gas come from (biological process)? 2. Explain why BTB indicator can still be used to detect exhalation even though it does not directly test for the substance that is exhaled. 3. What was the purpose of sealing the test tubes with parafilm? 4. Why do plants have chloroplast AND mitochondria? 5. If plants need oxygen, why do they release it?