Name ______

Date ______

Period ______

Assignment: Photosynthesis and Respiration Experiment


  • 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.


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.


  • Water
  • Bromothymol Blue
  • Water plants
  • You creating CO2 through respiration
  • 125 ml Erlenmeyer flask


  • 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 + aluminum foil:
No Plant:


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)

4Compare 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 "greenish-blue" 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 waterplants 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 CO2measured by bromothymol blue color
Before CO2 / 0 minutes / Next day
Plant + aluminum foil
No plant

Trial 2:

Amount of Light / Amount of CO2measured by bromothymol blue color
Before CO2 / 0 minutes / Next day
Plant + aluminum foil
No plant

Trial 3:

Amount of Light / Amount of CO2measured by bromothymol blue color
Before CO2 / 0 minutes / Next day
Plant + aluminum foil
No plant

Conclusion Write/Type your conclusion on a separate piece of paper.

  • 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