(P) Oxygen production shows the rate of photosynthesis

• Canadian pondweed can be used to measure the effect of light intensity on the rate of photosynthesis
• The rate at which the pondweed produces oxygen corresponds to the rate at which it's photosynthesising - 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. As it photosynthesises, the oxygen released will collect in the capillary tube

  
• 3) At the end of the experiment, the syringe is used to draw the gas bubble in the tube up alongside a ruler and the length of the gas bubble is measured. This is proportional to the volume of O2 produced

  
• 4) For this experiment, any variables that could affect the results should be controlled, e.g. the temperature and time the pondweed is left to photosynthesise

  
• 5)The experiment is repeated twice with the light source at the same distance and the mean volume of O2 produced is calculated

  
• 6) Then the whole experiment is repeated with the light source at different distances from the pondweed

  
• The apparatus below can be altered to measure the effect of temperature or CO2 on photosynthesis. E.g. the test tube of pondweed can be put into a water bath at a set temperature, or a measured amount of sodium hydrogencarbonate can be dissolved in the water (which gives off CO2)

  
• The experiment can then be repeated with different temperatures of water / concentrations of sodium hydrogencarbonate

  
• You can compare the results at different light intensities by giving the rate as the length of the bubble per unit time, e.g. cm/min