Leaf Disk Photosynthesis

In this investigation, leaf disks are vacuum-infiltrated with a baking soda solution so they sink. When exposed to light, oxygen from photosynthesis causes the disks to float, and the time until half of them float (ET50) indicates the photosynthesis rate.

1. Prepare three clear cups with about 300 mL water each; dissolve a small pinch of baking soda and 1 drop of dish soap in each and stir gently.
2. Punch out 30 uniform disks from fresh spinach or ivy leaves, avoiding major veins.
3. Load 10 disks into a syringe (no needle), draw in a little of the baking soda solution, and hold the syringe upright.
4. Expel excess air, seal the tip with a finger, pull back the plunger to create a vacuum for about 10–15 seconds, then release; repeat until the disks sink.
5. Pour the sunken disks and solution into Cup 1 so the disks are covered by a few centimeters of solution; label the cup.
6. Repeat the infiltration for two more batches of 10 disks to prepare Cup 2 and Cup 3.
7. Place all cups under the same bright light from above; start a timer.
8. Each minute, note how many disks are floating in each cup; gently swirl to free any stuck disks.
9. Continue until all disks float; determine the time when 5 of 10 disks float (ET50) for each cup.
10. Average the three ET50 values, then compute 1/ET50 to compare photosynthesis rates between conditions.

Measuring Photosynthesis: Leaf Disk Assay - LabXchange:

Leaf Disc Photosynthesis - Casey Tech School:

📄 Use Floating Leaf Disks to Study Photosynthesis - Science Buddies: https://www.sciencebuddies.org/science-fair-projects/project-ideas/PlantBio_p053/plant-biology/photosynthesis-leaf-disk-assay

• Change light intensity by moving the lamp closer or farther.
• Test different light colors using colored film (red, blue, green, etc.).
• Compare light sources (LED, CFL, incandescent, halogen, sunlight).
• Vary water temperature (for example, ~10°C, 20°C, 30°C, 40°C).
• Adjust baking soda concentration (for example, 0 to 2 g per 100 mL).
• Use different plant species or different leaf colors from the same plant.
• After disks float, cover a cup with foil to observe sinking in the dark (cellular respiration).

• Use a syringe without a needle; do not point the tip at eyes or face when releasing vacuum.
• Do not drink the solutions; wash hands after the activity and clean spills promptly.
• If using hot lamps or warm water, prevent burns and keep lamps a safe distance from cups.
• Check for plant or soap sensitivities; avoid contact with eyes.
• Work on a stable surface to prevent glass or cup tipping.

• Why do the disks start to float in the light? (Oxygen from photosynthesis forms bubbles on the disks, increasing buoyancy.)
• What role does baking soda play? (It provides dissolved carbon dioxide needed for photosynthesis.)
• Why add a tiny drop of dish soap? (It reduces surface tension so bubbles form and release more consistently.)
• How does temperature affect ET50? (Moderate warming speeds enzyme activity and lowers ET50; too hot can damage enzymes and increase ET50.)
• Why use 1/ET50 as the “rate”? (Shorter ET50 means faster oxygen production; taking the reciprocal converts time-to-50% into a value proportional to rate.)
• What would you expect in green vs. red leaves? (Leaves with more chlorophyll generally photosynthesize faster and reach 50% floating sooner.)