Science project

Elodea Photosynthesis Lab

Research Questions:

  • How do plants perform photosynthesis?
  • What gases are used by plants during photosynthesis?
  • What gases are produced by plants during photosynthesis?
  • What is the role of chlorophyll in photosynthesis?
  • Which wavelengths of light are used by chlorophyll in plants during photosynthesis?

Plants perform photosynthesis to create sugars and other organic compounds that they need to survive. Chlorophyll is a pigment in the chloroplast of the plant cell that absorbs energy from light and uses it to create compounds needed in photosynthesis. Visible light is made up of different colors of different wavelengths and energy. Certain wavelengths of light are used by plants more than others. Plants use carbon dioxide and produce oxygen gas during photosynthesis. They produce carbon dioxide during cellular respiration. In this experiment, the student will place aquatic plants under different colors of light in a solution of bromothymol blue.   Bromothymol blue is an acid-base indicator that turns yellow in acidic solutions and blue in alkaline solutions. Carbon dioxide bubbled into water forms carbonic acid. Thus, the more photosynthetic activity, the less acidic and more blue the bromothymol blue solution becomes as the plant uses carbon dioxide during photosynthesis. The less photosynthetic activity, the more acidic and yellow the bromothymol blue solution becomes as the plant produces carbon dioxide during cellular respiration.


  • Elodea, an aquatic plant available at many pet or gardening stores
  • Bromothymol blue solution (acid-base indicator available for purchase online) 0.04% (Aqueous), 1 L Bottle (1-2 bottles)
  • Transparent plastic films of different colors (heat resistant)
  • Masking tape
  • Clamp lights (one for each color and one additional for the control)
  • Test tubes or small glasses (one for each color and two additional as controls)
  • Plastic wrap
  • Aluminum foil

Experimental Procedure:

  1. Cover each clamp lamp with a colored plastic film. Secure the film with tape. Be sure that the film is heat resistant to avoid fire hazard. Leave one lamp without a colored film as a control. Set up the lamps at least several feet apart and away from windows. Write a hypothesis to explain which color light will be best for photosynthesis and why.
  2. Pour equal amounts of bromothymol blue into the test tubes or glasses (about 2/3 of the test tube or ½ cup in a small glass). If you do not have enough solution, you may dilute it with a little water. Just be sure to use distilled water and dilute the entire supply only slightly. Write the name of the colored film to be tested on each cup (you can also use masking tape to label cups). 
  3. Cut equal-sized pieces of elodea for each test tube or glass (about 3 inches in length). Make sure that each piece looks healthy and has plenty of green leaves. Place a piece of elodea in each test tube or glass. Record the initial color of the bromothymol blue solution. Cover each test tube or glass with plastic wrap. Be sure to completely seal the vessel to keep gas from entering or leaving. For one of the control plants, cover the test tube or glass completely with aluminum foil to block out any light. 
  4. Place one uncovered plant and the covered plant under the light without any colored film. Place the remaining plants each under a colored lamp. The plants should all be 12 inches (30 cm) away from their lamp. Allow the plants to remain under the lamps until the uncovered control has turned dark blue (about 24-48 hours). Record the final color of the bromothymol blue solution for each color of light, as well as both controls. Create a bar graph to illustrate your results. Compare your results to your hypothesis.

Terms/Concepts: Photosynthesis; Cellular respiration; Visible light spectrum; Chlorophyll; Carbon dioxide; Bromothymol blue


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