Biology
Name ____________________________________
Photosynthesis Lab - Elodea & Bromothymol Blue Period ___________ Date ___________________
Background:
Photosynthesis is the process by which plants take carbon dioxide from the atmosphere, add water, and
use the energy of sunlight to produce sugar. Write the equation for photosynthesis:
Photosynthesis occurs in the chloroplast, an organelle in plant cells that contains the molecule
chlorophyll. Chlorophyll absorbs the energy of sunlight. That light energy is converted to chemical
energy through the steps of photosynthesis.
The reactions of photosynthesis can be divided into two major types: light-dependent reactions and
light-independent reactions. The light-dependent reactions convert energy from the sun into a form that
the chloroplast can then use to make sugar from carbon dioxide; in the process producing oxygen as a
waste product. The light-independent reactions use that energy to make glucose from carbon dioxide and
water.
In this lab, you will investigate the process of photosynthesis. More specifically, you will learn how the
amount of light affects photosynthesis. In order to do this, you will use an Elodea plant, the chemical
Bromothymol Blue (which acts as an indicator to show if photosynthesis is occurring). Bromothymol
blue works because it is able to detect the presence of CO2 (needed for photosynthesis) and O2 (released
during photosynthesis) in solution.
In a test tube, you will see that:

bromothymol blue + CO2 = green color

bromothymol blue + O2 = blue color
Equation:
Write the equation for photosynthesis. ____________________________________________________
Good Title for the Lab: _______________________________________________________________
___________________________________________________________________________________
Hypothesis: (What do you predict will happen to the color in the test tube with plant/light and
with plant/no light?)
____________________________________________________________________________________
____________________________________________________________________________________
Procedure:
1. Pour 75 mL of water into a 250 mL beaker.
2. Add 2 mL of bromothymol blue to the water. It should be a blue solution.
3. Using a straw, GENTLY blow into the solution causing it to bubble for approximately 1 minute.
4. Label 3 large test tubes: 1, 2, & 3
 Tube 1 will be have no Elodea
 Tube 2 will be the Elodea in the dark
 Tube 3 will be the Elodea in the light
5. Pour 25 mL of the bromothymol blue + water solution into each of the three test tubes.
6. Cover Tube 1 with tin foil (there is no Elodea in Tube 1) and place it in the test tube rack
7. Add a 7 cm piece of Elodea to Tube 2. Using your straw, GENTLY push the Elodea to the bottom
of the test tube. Then cover Tube 2 with tinfoil (entirely) and place it in the test tube rack.
8. Add a 7 cm piece of Elodea to Tube 3. Using your straw, GENTLY push the Elodea to the bottom
of the test tube. Cover the test tube with plastic wrap. Then place it in the test tube rack.
9. Place the entire test tube rack under a light source and allow to sit overnight.
Tube 1
No Elodea
+
Covered in tinfoil
Tube 2
Tube 3
Elodea
+
Covered in tinfoil
Elodea
+
In the light
Pre-Lab Questions:
1. Predict what will happen to the color of the solution when you blow into the straw (causing the
bromothymol blue + water solution to bubble). Explain your prediction. ______________________
_________________________________________________________________________________
_________________________________________________________________________________
_________________________________________________________________________________
2. Predict what color Tube 1 will be after the experiment. __________________
3. Predict what color Tube 2 will be after the experiment. __________________
4. Predict what color Tube 3 will be after the experiment. __________________
Results: Record your results in the table below.
Sample
No Elodea +
Tinfoil
Elodea +
Tinfoil
Elodea +
Light
Color Before
Experiment
Color After
Experiment
Post-Lab Analysis:
5. What color does the bromothymol blue solution turn after you exhale into it? __________________
Explain your answer. ______________________________________________________________
_________________________________________________________________________________
6. What happened to the color in Test Tube 1? Explain why you obtained the results that you did.
_________________________________________________________________________________
_______________________________________________________________________________
7. What happened to the color in Test Tube 2? Explain why you obtained the results that you did.
_________________________________________________________________________________
_________________________________________________________________________________
_______________________________________________________________________________
8. What happened to the color in Test Tube 3? Explain why you obtained the results that you did.
_________________________________________________________________________________
_________________________________________________________________________________
_______________________________________________________________________________
9. Compare the Elodea in the dark to the Elodea in the light. Why is there a color difference?
_________________________________________________________________________________
_________________________________________________________________________________
_________________________________________________________________________________
_________________________________________________________________________________
10. What gas (or gases) can bromothymol blue serve as an indicator for? _________________________
_________________________________________________________________________________
11. What gas do you exhale?
_________________________
12. What gas do plants give off?
_________________________
13. List three things that a plant needs to undergo photosynthesis.
a. ________________________
b. ________________________
c. ________________________
14. Write the overall equation for photosynthesis.
_________________________________________________________________________________
15.
What was the control group? _______________________________________________
16.
Why is it important to have a control group?
17.
What was the experimental group? ___________________________________________
18.
What was the independent variable? ______________________________________________
Why was this the independent variable?
19. What was the dependent variable? __________________________________________________
Why was this the dependent variable?
20.
Identify 3 constants (or controlled variables).
21.
Look carefully at the results. Write a conclusion PARAGRAPH analyzing your results.
Include the following:

Summarize the problem/question being investigated in the scenario.

Discuss if you accept or reject the hypothesis?

Provide proof/evidence that supports your answer.

Provide one NEW QUESTION for further investigation.