Required Practical: Effect of Light Intensity on Photosynthesis — AQA GCSE Biology
This required practical investigates how light intensity affects the rate of photosynthesis using pondweed.
Method
- Place a piece of pondweed (e.g. Elodea or Cabomba) in a beaker of water containing a little sodium hydrogencarbonate (a CO₂ source).
- Place a lamp a measured distance from the pondweed.
- Count the number of oxygen bubbles released per minute (or collect and measure the volume of gas for more accuracy).
- Repeat at different distances from the lamp (e.g. 10, 20, 30, 40 cm).
Using the inverse square law
Light intensity is proportional to 1 ÷ distance², so calculate this for each distance and plot the rate of photosynthesis against light intensity.
Results
As light intensity increases (lamp closer), the rate of photosynthesis increases, until another factor (CO₂ or temperature) becomes limiting and the rate levels off.
Control variables
Keep the temperature constant (use a heat shield or water bath, as the lamp produces heat), and keep the same piece of pondweed and CO₂ concentration.
Exam tips
- The independent variable is light intensity (changed via distance); measure the rate by bubbles or gas volume.
- Use 1 ÷ distance² for light intensity.
- Control temperature — the lamp's heat is a common source of error.
- Explain the levelling-off using another limiting factor.