Analyzing Photosynthesis and Cellular Respiration
Background: Bromothymol Blue (BTB) is an acidic pH indicator. This means that BTB can be added to water,
to detect the presence of acids in solution. When BTB is added to a neutral or basic solution, it turns the
solution blue. When BTB is added to an acidic solution, it turns the solution yellow.
BTB can be used to detect the presence of carbon dioxide gas in water. When carbon-dioxide is present in
water, it combines with water molecules to form carbonic acid (H2CO3). As the amounts of carbon dioxide
increase, the BTB will change from a blue color to green, and finally yellow.
1. BTB is normally _________________ in color.
2. When BTB changes colors, it indicates the presence of _________________________________.
3. In water, carbon dioxide can change into a(n): acid or base.
4. When there are small amounts of carbon-dioxide present in water, BTB turns the water _________________.
5. When there are large amounts of carbon-dioxide present in water, BTB turns the water _________________.
Emily is learning about photosynthesis and cellular respiration and decided to use BTB to observe the
relationships between the 2 processes. She came up with several hypotheses and then set up experiments
to test those hypotheses. Her experiments involved using pond snails and a pond plant called Elodea.
Her first hypothesis was SNAILS GIVE OFF CARBON DIOXIDE. She put a snail in a small container with pond
water and added BTB which initially colored the water blue. The next day, she observed the water, and
found that it turned yellow.
6. Based on the data, was her hypothesis supported? Yes or No
7. Explain why the water turned yellow.
BTB
Water (starts blue)
8. Describe how she could set up another container that would serve as a control group for the experiment.
9. Snails give off carbon dioxide during the process of _________________________________________________.
Emily’s second hypothesis was that ELODEA TAKES IN CARBON DIOXIDE. She set up another container
filled with pond water and added BTB and the plant to it. This time, she used a straw to exhale into the water.
This caused the water to turn yellow. 10. Why did the water turn yellow???
For a control group, she set up a vial with pond water and BTB, but left out the plant. She used a straw to
exhale into this container also, so the water turned yellow. 11. Why does Emily need a control group???
BTB, water
Container 1 (starts yellow)
BTB, water
Control Group (starts yellow)
12. Plants take in carbon dioxide during the process of _______________________________________.
13. Why did Emily exhale into the containers?
14. The next day, she discovers that the first container is a greenish color now, and the control group
is still yellow. Was her hypothesis correct? EXPLAIN HOW YOU KNOW!!!
15. She leaves the containers for several days, but the one with the plant never fully turns blue.
Why not? (Hint: what other process is the plant going through which involves releasing carbon
dioxide?)
16. She decides to try the experiment all over again, but leaves it in total darkness this time. Using
what you know, predict what will happen to the container with the plant. (What color change if any? Why?)
Emily’s third hypothesis is that ELODEA USE THE CARBON DIOXIDE THAT SNAILS GIVE OFF.
She sets up a control group as shown below:
Water, BTB
CONTROL GROUP
(starts blue)
17. DRAW the experimental set up she should use to test her hypothesis. List the materials involved and the
original color of the water.
18. If her hypothesis is correct, what color should the water be in the experimental container the next day?
Pick one: Blue
Yellow
Green
Clear
19. What color would you expect in the control group container the next day? ________________________
20. The carbon dioxide is being released from the snail during the process of __________________________
and it is being used by the Elodea during the process of ____________________________.
21. Would the Elodea take in carbon dioxide given off by snails if the container was placed in the
dark for several days? Explain.