Required practicals (photosynthesis)

• Required Practical


  Investigate the effect of light intensity on the rate of photosynthesis in water plants (pondweeds)
• Photosynthesis investigation


  1. Set up plant in beaker with LED bulb at different distances

  2. Measure number of oxygen bubbles released per minute

  3. Calculate average number of bubbles produced
• Pondweed (Cabomba/Elodea)


  • Used as the plant for the investigation
  • Releases oxygen bubbles during photosynthesis
• LED bulb


  • Used as the light source
  • Won't heat up the water
• Sodium hydrogen carbonate (NaHCO3)

  Provides the carbon dioxide reactant for photosynthesis
• Light intensity


  Decreases as distance from the plant increases
• The number of bubbles produced per minute is used to measure the rate of photosynthesis
• Method


  1. Fill boiling tube with sodium hydrogen carbonate solution

  2. Cut 8cm piece of pondweed and place in tube

  3. Position tube 100mm from light source

  4. Allow 5 minutes for plant to acclimatise

  5. Count number of bubbles produced in 1 minute

  6. Repeat 5 times and calculate average
• Variables


  • Independent: distance from light source/light intensity
  • Dependent: number of bubbles produced per minute
  • Controlled: concentration of solution, temperature, volume
• Extension: measure volume of gas produced using a syringe, or measure changes in oxygen concentration using data logging equipment
• Extension: investigate effect of different wavelengths of light using coloured acetate filters
• Distance from light source

  Light intensity is inversely proportional to the square of the distance