Measuring the Rate of Photosynthesis

• Pondweed is used to measure the effect of light intensity on the rate of photosynthesis as it produces oxygen at the same rate
• The faster the rate of oxygen production, the faster the rate of photosynthesis
• 1. a source of white light is placed at a specific distance from the pondweed
  2. the pondweed is left to photosynthesise for a set amount of time, and the oxygen released will collect in a capillary tube
  3. at the end of the experiment, the syringe is used to draw the gas bubble up the tube alongside a ruler and the length of the bubble is measured (volume of oxygen)
• independent variable: distance of light source from pondweed
• dependent variable: length of gas bubble
• control variables:

  • difference in distances of the light source
  • same pondweed
  • same surrounding temperature
  • pH of water
• the test tube of pondweed can be put into a water bath at a set temperature to keep it constant