Honors Biology
Photosynthesis and Respiration in Plants
Name
Date
Introduction
In this lab you will explore how Elodea plants affect bromothymol blue in a variety of situations.
This experiment will allow you to make some generalizations about photosynthesis AND
respiration in plants. Note that bromothymol blue turns yellow-green (or clear) in the presence of
carbon dioxide. Bromothymol blue turns from yellow-green to blue after oxygen replaces the
carbon dioxide in the solution.
Pre-lab assignment
1) Read the procedure carefully.
2) Complete the table below with predictions for the test tubes listed (step #13). Each prediction
must have an explanation supporting it.
a. Students without predictions and clear, reasonable explanations will not be able to begin
the lab in class and may be required to complete the lab after school.
Record YOUR predictions as a pre-lab assignment in this table after reading the procedure.
Tube
A
B
C
D
E
F
G
H
Predicted
Color
Reason for Prediction. Explain Fully!
Procedure – Day 1
1. Working in your full lab groups, determine which student will be in charge of reading directions.
2. Sign in. Read the sign in instructions carefully.
3. Obtain eight test tubes and two test tube racks. Make sure all of your test tubes are the same
size. Place a small piece of blue tape on each of the test tubes.
4. Label the eight test tubes A – H.
A = Bromothymol blue & Elodea, in the DARK
B = Bromothymol yellow-green & Elodea, in the DARK
C = Bromothymol blue & no plant, in the DARK
D = Bromothymol yellow-green & no plant, in the DARK
E = Bromothymol blue & Elodea, in the LIGHT
F = Bromothymol yellow-green & Elodea, in the LIGHT
G = Bromothymol blue & no plant, in the LIGHT
H = Bromothymol yellow-green & no plant, in the LIGHT
5. Fill the “bromothymol blue” tubes (A, C, E, G) about 2/3 full with bromothymol blue solution
6. Fill a small flask with about 30 mL of bromothymol blue solution. Blow gently into it until it turns
yellow, and then continue blowing for 15 seconds. You want the solution to be really yellow
and full of carbon dioxide. Pour the contents into a “bromothymol yellow-green” tube (B, D, F, or
H) until it is about 2/3 full of the bromothymol solution. Repeat this with the remaining
“bromothymol yellow-green” tubes until all tubes are about 2/3 full.
7. Get an 8 inch piece of healthy Elodea (if it’s brown and looks unhealthy, don’t use it). Cut off,
gently rinse, and place an approximately 2 inch piece of Elodea in each of the four Elodea test
tubes (A, B, E, F). Make sure each piece is about the same size and types as the others.
8. Double check your tubes: four tubes should contain a blue solution (A, C, E, G). The other four
should contain a yellow-green solution (B, D, F, H). Correct any mistakes. Double check that
Elodea are in the correct tubes, too (A, B, E, F).
9. Tightly cover the tops of all test tubes with aluminum foil.
10. Using blue tape, label two test tube RACKS with your period, lab bench numbers, and the
name of someone in your lab group.
11. Place the four “dark” test tubes the cabinet identified by your teacher.
12. Place the four “light” test tubes in a rack and place under the lights (24 hours light).
13. As you learned in the previous chapter, plant cells contain both chloroplasts (photosynthesis)
and mitochondria (respiration). Given this information, make a prediction about what color you
think each test tube will be at the conclusion of this experiment tomorrow and briefly explain
your reasoning (complete the “Predicted Color and Reason for Prediction” columns). Will the
tube be yellow or blue tomorrow? Why? Think about what the plant will be producing (carbon
dioxide, oxygen, both, neither?) and what that will do to the bromothymol indicator. Pre-lab
assignment: complete this individually for your pre-lab; during the lab setup your group will
discuss and make a group prediction.
14. Sign out, and get a stamp here. Your table must be completed to get credit. __________________
LAB GROUP PREDICTIONS
Tube
Predicted
Color
(complete this
as a group
before the lab)
Reason for Prediction. Explain Fully!
(complete this as a group before the lab)
Actual Color
(complete this
as a group
after the lab)
A
B
C
D
E
F
G
H
Procedure – Day 2
1. Locate all eight of your test tubes and place them in a single rack for viewing.
2. Record the color and appearance of each test tube in the “Actual Color (Day 2)” column.
3. Pour contents of test tubes into the main “waste container” on the lab bench. Rinse all test
tubes and return them to the main lab bench. Take off all tape, and discard all aluminum foil.
Check with Mr. Welman before returning to your seat.
Conclusion Questions
Using your book, your organelle presentation notes, and your experience in this activity, answer the following
questions and be prepared to discuss them in class.
1. Photosynthesis occurs in which organelle? ___________________________
2. What two chemicals are used up during photosynthesis (reactants)?
3. What two chemicals are produced during photosynthesis (products)?
4. What is the balanced chemical equation used to represent photosynthesis? Write it with symbols (e.g. O2) and
words (e.g. oxygen).
5. Record below the final color of each of the tubes (copy from the table on the previous page). Then, given what you
know about how bromothymol blue reacts in the presence of oxygen or carbon dioxide, explain what you observed
in each test tube. Why did the solution change color (or not change color)? For example, if a tube containing
Elodea turned from yellow to blue, explain why.
A = Bromothymol blue & Elodea, in the DARK
Final Color: _________________
B = Bromothymol yellow-green & Elodea, in the DARK
Final Color: _________________
E = Bromothymol blue & Elodea, in the LIGHT
Final Color: _________________
F = Bromothymol yellow-green & Elodea, in the LIGHT
Final Color: _________________
6. Do plants complete cellular respiration, photosynthesis, or both? Explain, focusing on the timing of when and the
amount each process occurs.