Purpose
Determine how light intensity affects the rate of photosynthesis, using floating leaf discs.
The trick that makes this work
Leaf discs sink when the air is drawn out of them with a syringe. As photosynthesis produces oxygen, the discs become buoyant and rise. Time to float is a measure of photosynthetic rate.
Materials
- Spinach leaves, hole punch, 10 mL syringe (no needle)
- Sodium bicarbonate solution (the source), drop of dish soap
- Clear cups, lamp, ruler, timer
Procedure
- Punch 10 discs per trial. Avoid large veins.
- Draw bicarbonate solution and discs into the syringe, seal the tip, pull the plunger to remove gas from the leaves, and repeat until all discs sink.
- Place discs in a cup of bicarbonate solution at a measured distance from the lamp.
- Record the time for each disc to reach the surface.
- Repeat at three distances: 10 cm, 20 cm, 40 cm.
Control your variables
Only distance from the lamp changes. Same solution, same leaf, same disc count, same water temperature — and watch the last one, because lamps heat water.
Data
| Distance (cm) | Discs floating at 5 min | Median time to float (s) |
|---|---|---|
| 10 | ||
| 20 | ||
| 40 |
Analysis
- Graph median float time against distance. Describe the shape.
- Light intensity falls off with the square of distance: . Does your data support that? Explain what would convince you either way.
- What would happen with no bicarbonate in the water? Predict, then explain using the equation on Photosynthesis.
- Leave a floating disc in the dark and it sinks again. Using Cellular Respiration, say what happened to the oxygen — and then compare the two processes: what does each one take in, what does each give out, and why does a pond need both of them running at once?
Hand in
- Data table and graph
- Analysis answers
- One limitation of this method, honestly stated
Curriculum connection
A1.2
apply a scientific experimentation process and associated skills to conduct investigations, making connections between their observations and findings and the scientific concepts they are learning
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B2.3
compare and contrast the processes of cellular respiration and photosynthesis, and explain how their complementary relationship contributes to the dynamic equilibrium of ecosystems
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