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Hands-On with Photosynthesis and Cellular Respiration


Photosynthesis is a topic that many students need help understanding. Using a hands-on experiment to demonstrate the process helps reinforce the concepts with students, especially your kinesthetic learns. Combining the lab with images and video found on the Internet targets your audio and visual learners as well. Below are simple activities that use Elodea to help teach this topic.

Observing photosynthesis

Before describing what we are doing, we will briefly discuss what we are not doing—as in a much-repeated experiment that uses Elodea to produce gaseous O2 in the presence of light. The usual procedure requires gas collection over a relatively long time period. Approaching this experiment differently provides a more rapid response in volume change of gas produced, relative to light intensity. Using this simple system, students observe the immediate effects in variations of physical factors, such as temperature and light intensity, on Elodea producing gaseous O2.


  • 2 Elodea Sprigs
  • Fine Needle
  • Sodium Bicarbonate (0.5% solution)
  • 500-cc Bell Jar or Beaker
  • 200-W Light Bulb (or equivalent fluorescent or LED) and Power Source


  1. Using a fine needle, pierce the Elodea sprig stems several times.
  2. Place the sprigs and sodium bicarbonate solution in the bell jar or beaker.
  3. Expose the sprigs to the lighted 200-W bulb. Note the nearly immediate response as bubbles stream from the holes in the stems.

Cellular respiration and photosynthesis

Create another twist to this activity by comparing Elodea exposed to light with Elodea that is not. Using bromthymol blue as an indicator, students expose a sprig of Elodea to light while covering another sprig to keep out light. The plant exposed to light produces oxygen, so the water remains basic and its color turns blue. The covered sprig respires, releasing carbon dioxide. The carbon dioxide reacting with the water forms carbonic acids and lowers the pH, causing the indicator to remain yellow.


  • 2 Elodea Sprigs
  • Bromthymol Blue
  • Carbonated Water (as an alternative, have students use a straw to bubble their breath through the bromthymol blue, changing the color to yellow)
  • 2 Test Tubes, 18 × 100 mm
  • Grow Light with Power Source
  • Aluminum Foil
  • Test Tube Rack or Large Beaker
  • Distilled Water


  1. Cut Elodea sprigs into 8- to10-cm lengths.
  2. Fill each test tube 2/3 full of water.
  3. Place a sprig in each test tube.
  4. Add several drops of carbonated water to each tube.
  5. Add 15 to 20 drops of bromthymol blue to each test tube. (The water is now slightly acidic, so it is yellow.)
  6. Completely cover one sprig (and its test tube) with aluminum foil.
  7. Place both test tubes in a rack or large beaker under the plant light. Leave undisturbed for 24 hr.
  8. Prior to examining the results, assign students to predict what will happen and why.
  9. Examine the results.

After 24 hours, water in the covered tube remains yellow, while water in the uncovered tube turns blue. The Elodea exposed to light continues to photosynthesize, using carbon dioxide in the water. With less carbon dioxide in the water, less carbonic acid forms, causing the water to turn slightly basic and, as a result, blue. The Elodea in the covered tube respires, breaking down stored carbohydrates and releasing carbon dioxide into the water. The extra carbon dioxide and water react, creating carbonic acid, keeping the water acidic, and remaining yellow.

Get more out of this “green” activity

Having students create diagrams to illustrate the processes of photosynthesis and cellular respiration assists in their understanding. If a student team is struggling, consider combining it with a team that is grasping the concepts more quickly. Having students work together allows them to help each other and emphasizes collaborative learning. Allowing students to find answers collectively also causes them to explain results to each other, which can greatly improve comprehension while engaging them in learning.

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