(6)Effect of light intensity

Method

  1. Place a piece of pondweed (cut end up) in a beaker of water.

  2. Add sodium hydrogen carbonate (provides carbon dioxide).

  3. Place a lamp at a measured distance (e.g. 10 cm).

  4. Allow the plant to equilibrate.

  5. Measure the rate of photosynthesis by:

    • Counting oxygen bubbles per minute, OR

    • Measuring the volume of oxygen collected (more accurate).

  6. Repeat at different distances (e.g. 10 cm, 20 cm, 30 cm).

  7. Repeat trials and calculate a mean.

Variables

Independent variable:

  • Light intensity (often changed by altering distance from lamp)

Dependent variable:

  • Rate of photosynthesis (bubbles per minute or volume of oxygen)

Control variables:

  • Temperature

  • Carbon dioxide concentration

  • Type/length of pondweed

  • Volume of water

Evaluation Points

  • Counting bubbles is not very accurate (bubble size varies).

  • Measuring gas volume with a gas syringe is more precise.

  • Heat from the lamp may increase temperature → use LED lamp or water bath.

  • Allow plants to equilibrate→wait a few minutes before timing so the plant adjusts to new light intensity.

  • Repeat and calculate a mean to improve reliability.