Name: ________________________________ Date: _________________ Period: __________
Photosynthesis and Cellular Respiration in Elodea
Based on the CIBT Photosynthesis and Respiration in Elodea Lab – Cornell & HHMI
Background:
Plants can carry out both photosynthesis and cell respiration simultaneously.
During photosynthesis, plants are using the energy of the sun to build molecules,
which effectively store this energy (glucose). Chemically, the photosynthetic reaction
looks like this:
6H2O + 6CO2 + Light energy → C6H12O6 + 6O2
During cellular respiration, plants are using this stored energy (glucose) to fuel their
metabolic processes. Chemically, the cellular respiration reactions look like this:
C6H12O6 + 6O2 → 6H2O + 6CO2 + Energy (ATP)
Remember that plants carry out cell respiration all the time!
Among other things, the converted energy from cell respiration is used to synthesize
molecules, move materials around within the organism, grow (create new cells) and
reproduce. Notice that in photosynthesis, CO2 (carbon dioxide) is being used up as
it is turned into glucose molecules. During cellular respiration, the opposite is
true. As the plant releases energy stored in glucose by breaking it down, CO2 is
being given off into the surrounding water or atmosphere. The relationship between
these two processes is special in that it allows plants to recycle some of their byproducts. (While CO2 is being given off during cell respiration, it can be re-utilized
during photosynthesis).
Purpose:
In this lab, you will try to demonstrate the net change in carbon dioxide when the
common freshwater plant Elodea is placed under different conditions. You will be
using a pH indicator, bromthymol blue (BTB), as a means of determining the
presence of absence of CO2.
The solution of BTB changes color when CO2 is introduced. BTB changes color due
to a change in pH.
• When CO2 is dissolved in water, it forms carbonic acid. This lowers the pH of
the solution and causes BTB to change its appearance. Here is how it works:
H2O + CO2 → H2CO3 → H+ + HCO3carbonic acid
BTB pH color scale:
< pH 6 = yellow
increase in H+  decrease in pH
pH 7 = green
> pH 7.6 = blue
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By the end of this experiment, in your group and as a class, we will be able to
answer the following two research questions:
A. Is environmental CO2 used during photosynthesis in Elodea?
B. Is there a net production of CO2 when Elodea undergoes cellular
respiration in the absence of photosynthesis?
Propose hypotheses for research questions A and B:
A. ____________________________________________________________________________
____________________________________________________________________________
B. ____________________________________________________________________________
____________________________________________________________________________
Materials:
• Elodea plants
• Test tubes
• Test tube rack
• Bromthymol blue (BTB)
• Straws
• 250 mL flask
•
•
•
•
•
•
100 mL graduated cylinder
light source
Aluminum foil
Tape
Rubber stoppers / corks
Safety goggles
Procedure:
1. Your group should obtain a solution of bromthymol blue (BTB). Review the
information on BTB on page 1. You will be adding the CO2 to the BTB
solution. Remember that you are a living generator of CO2. Choose one person
from your group to be the living CO2 generator. Put on your safety goggles
before beginning. Use a straw to gently exhale into a 250 mL flask containing
20 mL of BTB. Continue exhaling into the BTB solution for about one minute.
Do not discard the solution. You will need it later. When CO2 is removed from
the BTB solution, the solution turns back to its original color (blue).
CAUTION: DO NOT SWALLOW ANY BROMTHYMOL BLUE OR SPLASH ANY
ON YOUR FACE! IT IS TOXIC IF SWALLOWED!
What happened when you exhaled into the bromthymol blue solution? Why?
____________________________________________________________________________
____________________________________________________________________________
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2. Your group will be assigned to set up different experimental conditions to
demonstrate photosynthesis and cellular respiration in Elodea, a water plant.
You will use the test tubes, corks, tape, bromthymol blue, aluminum foil, light
source, and Elodea as needed to set up your assigned experimental conditions
according to the table on page 3.
a. 16 test tubes per class, 8 experimental conditions total, repeated 2X
b. If you are assigned a “dark” test tube, you will need to wrap your test
tube in aluminum foil.
c. If you are assigned a “light” test tube, you will need to place your test
tube under the light source overnight with plenty of access to the light.
d. If your test tube has “CO2 added”, you will need to use the yellow BTB
that you made from step 1.
e. If your test tube has Elodea, obtain a small sprig (piece) of the plant
from Ms. Ciovacco.
f. Record the start color of all test tubes in the data table.
Data Table. Elodea Experiment
Test tube
A
B
C
D
E
F
G
H
light
dark
light
dark
Elodea,
CO2
added
Elodea,
CO2
added
Light
treatment
light
dark
light
dark
Test tube
treatment
no
plant,
BTB
only
no
plant,
BTB
only
no
plant,
CO2
added
no
plant,
CO2
added
Elodea, Elodea,
BTB
BTB
Color of
bromthymol
blue at
start
Color of
bromthymol
blue after
24 hours
Explanation
for the color
changes
3. After 24 hours, the class will obtain all 16 test tubes from the light source and
analyze the results. The original CO2 concentration of the test tubes has been
altered by photosynthesis and cell respiration. Today you will make
observations of the test tubes, record changes, and interpret the data you
have collected. Fill in the Data Table with your 24-hour observations and your
explanations for the color changes.
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Please clean out all test tubes and wash the lab equipment thoroughly!
Analysis:
1. Restate research questions A and B in your own words:
a. ______________________________________________________________________
______________________________________________________________________
b. ______________________________________________________________________
______________________________________________________________________
2. Which test tubes provide the evidence for research question A? Why?
3. Which test tubes provide the evidence for research question B? Why?
4. What is the independent variable for this lab (What are you
studying/manipulating)?
5. What is the dependent variable for this lab (What are you measuring)?
6. What are the controlled variables in this lab?
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7. Analyze your hypotheses: were they correct, partially correct, or incorrect?
Explain.
8. Regarding photosynthesis and cellular respiration,
a. Which process(es) occur(s) only in the presence of light?
b. Which process(es) occur(s) both in the light and dark?
9.
What are three “ingredients” that a plant must obtain in order to do
photosynthesis?
a. How was carbon dioxide supplied to the plant?
10. How did you observe that carbon dioxide was present in the solution?
11. From the results of your investigation, which process is occurring more in a
plant that is being supplied with sunlight: photosynthesis or cellular
respiration?
a. What evidence have you used to come to this conclusion?
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12. One of the implications of this investigation is that plants can recycle some
of their “waste” products. CO2 is clearly an example of a material that can be
recycled by plants. What other gas might plants generate as a “waste”
through one metabolic process, but re-use in a subsequent process?
Explain.
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