How Do You Measure the Rate of Photosynthesis?

To measure the rate of photosynthesis, place a sprig of elodea in a test tube next to a light source and calculate the number of air bubbles coming out of the plant. Change lighting and availability of carbon dioxide to alter the rate of photosynthesis.

Photosynthesis is the process by which plants absorb light to produce glucose for themselves. In the process, plants consume carbon dioxide and produce oxygen.

  1. Prepare equipment and materials
  2. Cut a stem off an elodea plant that is small enough to fit in a test tube. Fill the test tube with water and put the sprig of elodea inside, having sliced the end off it at an angle. Position the test tube in a test tube rack or tape it to a metal stand. Put an electric light next to the tube.
  3. Carry out measurements
  4. This experiment assumes that the number of air bubbles released by the plant is directly related to the rate of photosynthesis, and that the air is composed mainly of oxygen. Count the number of bubbles that rise from the plant to the surface of the water during 5 minutes. After a break, repeat for another 5 minutes. Calculate the average number of bubbles per minute.
  5. Alter conditions
  6. Run the same two 5-minute counting bubble sessions in different conditions. These can include moving the light away to a certain distance, thus reducing the amount of light the plant receives. Another change can be adding sodium bicarbonate to the water, thus increasing the amount of carbon dioxide available to the plant. Less light should result in a slower rate of photosynthesis and more carbon dioxide in a faster rate. Other conditions to change are: temperature of the water and color of light.