Name _________________________________________________
Date ____________________
Period ______
Assignment: Photosynthesis and Respiration Experiment
Purpose:
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Student will write a prediction, data table and conclusion for experiment
Students will use water plants to observe photosynthesis and respiration in bromothymol
blue
Students will measure and observe the changes in the color of the bromothymol blue
solution.
Introduction:
You are by now quite familiar with photosynthesis. It is the process by which plants and
some other organisms convert solar energy to chemical energy in the form of sugars. In this
reaction, the energy from sunlight is harnessed and utilized to form sugar molecules from
carbon dioxide and water, forming oxygen gas as a byproduct.
Photosynthesis: 6CO2 + 6H2O + sunlight ---> C6H12O6 + 6O2
While photosynthesis creates sugar with the sun's energy, cellular respiration utilizes the
chemical energy in sugar to meet the energetic needs of organisms. During respiration, sugar is
combined with oxygen, resulting in energy and carbon dioxide. Note that the equation for
respiration is the opposite of photosynthesis, indicating the products of photosynthesis become
reactants in respiration, and vice-versa.
Respiration: 6O2 + C6H12O6 -----> energy + 6CO2 + 6H2O
Bromothymol blue is a pH indicator solution that is blue when its pH is basic (and carbon
dioxide concentrations are low) and yellow when its pH is acidic (and carbon dioxide
concentrations are high). In this experiment, you will be measuring the production of O2 by
plants with bromothymol blue.
Materials:
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Water
Bromothymol Blue
Water plants
You creating CO2 through respiration
125 ml Erlenmeyer flask
Question:
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How do different amounts of light affect the amount of CO2 produced by plants as
measured by bromothymol blue?
Manipulated variable: ________________________________
Responding Variable: _________________________________
Prediction: Predict what you think will happen in each test tube.
Include why you think specific
changes will take place.
Plant:
Plant + aluminum foil:
No Plant:
Procedure:
1. Place about 75 ml of bromothymol blue into a 125 ml Erlenmeyer flask (triangular glass
containers).
2. Observe the color of the solution. Record your observations. Hold the solution in front of a
white piece of paper to observe the color clearly.
3. The next step is to introduce carbon dioxide into the solution. We could do this with a carbon
dioxide tank like those used for soda dispensers, but that would be overkill. Luckily, there is a
ready source of cost-free carbon dioxide within you right now - your lungs. Use a straw to slowly
blow CO2 into the solution until it just turns yellow. (Do not suck in the Bromothymol Blue)
4 Compare the color of the solution to the printout showing bromothymol blue at various pHs.
Find the color on the printout that best matches the color of your solution. If your color is
between those shown, use the pH value that is intermediate between them. Record the color of
the solution after you've introduced CO2 (Initial color) and estimated pH (Initial pH) for each of
the tubes. Be as detailed as possible when describing the color (e.g., use terms like "greenishblue" or "yellow-green").
5. Pour the solution into three screw cap tubes, dividing the solution evenly between them.
6. Obtain two 2-inch pieces of water plants. Place a water plant in one of the tubes, and cap the
tube. Place a water plant in a second tube and completely cover with aluminum foil (to prevent
the entry of light), and cap the tube. In both tubes, make sure the plant is completely submerged
in the solution. Cap the remaining tube – this tube will serve as your control.
7. Place the tubes upright on the lab bench approximately 8-10 inches from the lamp. Ensure
the light hits all three tubes equally and from the side, not from the top. Turn on the lamp. Allow
the plants to sit undisturbed for 24 hours under the light.
8. Carefully remove the water plants from the tubes.
9. Determine the color of each of the solutions against a white background, record the results in
Table 1, and compare the colors to the bromothymol blue printout to determine pH.
10. After all measurements have been completed, throw out your used water plants. Rinse out
your glassware and throw away your used straw.
Data:
Amounts of Light vs. amount of CO2 produced
Trial 1:
Amount of Light
Amount of CO2 measured by bromothymol blue color
Before CO2
0 minutes
Next day
Plant
Plant + aluminum foil
No plant
Trial 2:
Amount of Light
Amount of CO2 measured by bromothymol blue color
Before CO2
0 minutes
Next day
Plant
Plant + aluminum foil
No plant
Trial 3:
Amount of Light
Amount of CO2 measured by bromothymol blue color
Before CO2
0 minutes
Next day
Plant
Plant + aluminum foil
No plant
Conclusion Write/Type your conclusion on a separate piece of paper.
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Answer your investigation question.
What happened in each test tube?
Include supporting data from your Amounts of Light vs. amount of CO2 produced tables
Explain how these data support your conclusion.
Draw a picture below of what is happening in each test tube:
 Draw the O2 and CO2 movement with arrows.
 Draw where the energy from the light is going.
Test tube without plant
Test tube with plant and
aluminum foil
Test tube with plant