Name: _______________________________________ Date: ______________
Lab # ___________
Is Photosynthesis Occurring?
Background:
The production of food from carbon dioxide and water in the presence of chlorophyll utilizing light energy and
releasing oxygen is called photosynthesis. Plants produce food in the form of simple sugars – mainly glucose (a
carbohydrate). In order for photosynthesis to occur carbon dioxide (CO2), sunlight and chlorophyll must be
present. Chlorophyll is a pigment which is stored in chloroplasts and is used to capture light energy from the
sun. In this activity, we will be using an indicator called Bromothymol Blue to prove that photosynthesis is
occurring.
Problem: How can we tell when photosynthesis is occurring, and what does it need to occur?
Materials:
Bromothymol blue
water
3 Elodea sprigs
aluminum foil
3 test tubes with rubber stoppers
Procedure: In this lab you will compare the rate of photosynthesis in three test tubes: (1) a plant with light; (2)
a plant with no light; and (3) light with no plant.
Which test tube is the control? ________________________________ Why? ___________________________
__________________________________________________________________________________________
Part A: How Does Bromothymol Blue Work?
Bromothymol blue is an indicator used to test for pH changes. It is blue in base, but yellow-green in an acid.
When CO2 mixes with water, it makes carbonic acid.
If carbonic acid is present, what color would we expect the indicator to be? __________________________
Let’s test this hypothesis. When humans exhale, what gas is released? ___________________ So if a student
exhales into a mixture of water and Bromothymol blue, carbonic acid will be produced.
What is the result of a student exhaling in to a solution? _________________________________________
Part B: How can bromothymol blue be used to prove that photosynthesis is occurring?
Now we will use the Bromothymol Yellow/Green from Part A to see if photosynthesis occurs under different
conditions.
Test Tube X – Bromothymol Yellow-Green + Elodea
Test Tube Y (wrapped in foil) – Bromothymol Yellow/Green + Elodea
Test Tube Z – Bromothymol Yellow-Green
Why are we using Bromothymol Yellow-Green rather than Bromothymol Blue? (Hint: what makes
Bromothymol blue turn yellow-green?)
_________________________________________________________________________________________
Predict which test tubes will turn blue: _________________________________________________________
In the table below, fill in the initial colors of each test tube. Then fill in your prediction for what you think will
happen in each test tube after 48 hours.
Test
Tube
Initial Color
Hypothesis Color
Explain Your Hypothesis
X
Y
Z
Fill in the actual results of the experiment below from your observations 48 hours later.
Test
Tube
Initial Color
Final Color
Is CO2 Present?
Did Photosynthesis
Occur?
X
Y
Z
Did photosynthesis occur in test tube X?
Yes
No
How do you know? Explain your answer. _______________________________________________________
Did photosynthesis occur in test tube Y?
Yes
No
How do you know? Explain your answer. _______________________________________________________
Did photosynthesis occur in test tube Z?
Yes
No
How do you know? Explain your answer. _______________________________________________________
One test tube was covered in foil. What was the purpose in doing this? ______________________________
_________________________________________________________________________________________
Now test the solutions in each test tube for the presence of oxygen. Which test tube would you expect to contain
the largest amount of oxygen? X Y Z
Record the results of the oxygen test below:
X
Amount of Oxygen
(O2) Present
Which test tube contained the most O2 gas?
Y
Z
Why is this? _____________________________________________________________________________
________________________________________________________________________________________
Did this agree with your hypothesis?
Yes
No
Conclusion:
1. How can we tell if photosynthesis is occurring? _______________________________________________
________________________________________________________________________________________
2. If Bromothymol Blue is an acid indicator, how is it possible to use it to test for the presence of carbon
dioxide? ________________________________________________________________________________
_______________________________________________________________________________________
3. Can Bromothymol Blue be used to detect the presence of oxygen? _________________ Why or why not?
_______________________________________________________________________________________
4. What 3 things do plants require to perform photosynthesis? ______________________________________
________________________________________________________________________________________
5. What 2 things do plants create as a result of photosynthesis? _____________________________________
________________________________________________________________________________________
6. What other life function can Bromothymol Blue be used to monitor? ______________________________
Why? __________________________________________________________________________________