Elodea Lab

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?
Process that led to
final color
BTB + human breath
BTB+ human breath+ Plant
BTB+ human breath + Plant
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?
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