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Effect of Light on Photosynthesis Experiment

Apr 9, 2025

Investigating the Effect of Light Intensity on Photosynthesis

Objective

  • Understand how to investigate the effect of light intensity on the rate of photosynthesis.
  • Higher-tier students should learn how the results are affected by the inverse square law.

Required Practical Setup

  1. Equipment Needed

    • Boiling tube
    • LED light source (to minimize heat, as excessive heat can alter experiment conditions)
    • Beaker of water (if using a normal light bulb to absorb heat)
    • Sodium hydrogen carbonate solution (provides CO2 for photosynthesis)
    • Pondweed cut at one end
    • Stopwatch

  2. Procedure

    • Place the boiling tube 10 cm away from the LED light source.
    • Fill the boiling tube with sodium hydrogen carbonate solution.
    • Insert the pondweed into the boiling tube with the cut end at the top.
    • Allow 5 minutes for the pondweed to acclimatize.
    • Observe and count the bubbles of oxygen gas produced at the cut end for one minute.
    • Repeat the count two more times and calculate the mean number of bubbles per minute.
    • Repeat the entire process at distances of 20, 30, and 40 cm.

Problems and Solutions

  • Problems:
    1. Fast bubble count can be difficult.
    2. Bubbles vary in size (large counts same as small).
  • Solution:
    • Measure the volume of oxygen produced using a funnel and a measuring cylinder to catch and measure the bubbles.

Higher Tier Explanation

  • Graphing Results:
    • Plot mean number of bubbles per minute or volume of oxygen per minute against distance from the light source.
    • Doubling the distance results in the number of bubbles per minute falling by a factor of four.
    • This demonstrates the Inverse Square Law:
      • If distance is doubled, light intensity and consequently the photosynthesis rate (oxygen production rate) falls by four times.

Conclusion

  • You should now be able to describe the method to investigate light intensity effects on photosynthesis.
  • Higher-tier students should understand the inverse square law application.

Additional Resources

  • Revision workbook questions available for practice.

These notes cover the key concepts and steps required to conduct and understand the experiment on photosynthesis and light intensity, including the inverse square law for advanced students.

Full transcript