Science at Home - Photosynthesis

Floating Leaf Disk Assay for Investigating Photosynthesis

Finding a good, quantitative procedure for students to use for exploring photosynthesis is a challenge. The standard procedures such as counting oxygen bubbles generated by an elodea stem tend to be prone to error and so are unreliable. The floating leaf disk assay technique is both reliable and understandable. Once familiar with the technique, students can design experiments to answer their own questions about photosynthesis.


  • Sodium bicarbonate (baking soda)
  • Liquid soap
  • Plastic syringe (10cc or larger) – remove any needle!
  • Leaf material (spinach leaves work great; arugula is fine)
  • Hole punch
  • Plastic cups
  • Timer
  • Light source (the sun!)


  1. Prepare a 0.2% bicarbonate solution by adding 1/8 teaspoon of baking soda in 1 ¼ cup water. Add 1 drop of liquid soap into the solution, mix by stirring. If your solution contains a lot of  suds, then dilute it with more bicarbonate solution.
  2. Use the hole punch to cut 10 or more leaf disks for each trial. Don’t let these dry out! punching them onto a wet paper towel can help.
  3. Remove the piston or plunger from the syringe and place the leaf disks into the syringe barrel. Replace plunger and suck in sodium bicarbonate solution until the syringe is 1/2 full. Point the syringe upward and push out the air. Tap the syringe to suspend the leaf disks in the solution.
  4. Holding a finger over the syringe opening, draw back on the plunger to create a vacuum. This will deplete your leaf disks of air (you’ll see it come out!), causing your leaf disks to sink. Repeat this step 2-3 times to get the majority of the disks to sink. Note - do not repeat more than 5 times, or you could kill your leaf disks.
  5. Pour the disks and solution into a cup and add more solution to a depth of about 2-3 inches.
  6. Place under a light source and time each leaf disk as it photosynthesizes and floats to the top of the solution!

Twists and Controls (checks to see the importance of different variables)

  • For a control, infiltrate leaf disks with a solution of only water with a drop of soap (no bicarbonate).
  • For another control, complete steps 1-5 as directed above, but place cup in the dark.
  • Cover cups with colored cellophane or filters OR use food coloring to color the disk floating solution itself. This provides your leaves with differing wavelengths of light. Which color light works best for photosynthesis?
  • Use different leaf material and have a race! You can try older versus younger spinach leaves, or go in your backyard and find different types of leaves – just remember that the leaf surface you choose should be smooth and not too much thicker or thinner than the spinach leaves.

Making colored solution

  • BLUE: Add 1 drop blue food coloring to 2/3 cup buffer solution, then dilute by half by adding an equal volume of your bicarbonate + soap solution.
  • ORANGE: add 1 drop orange food coloring to 2/3 cup buffer solution.
  • GREEN: Add 1 drop orange food coloring and 1 drop blue food coloring to 2/3 cup buffer solution, then dilute by half.