RP5

• Investigating the effect of light intensity on the rate of photosynthesis

  1. Take a boiling tube and place it 10 cm away from an LED light source

  2. Fill the boiling tube with sodium hydrogen carbonate solution

  3. Put a piece of pond weed into the boiling tube

  4. Leave for 5 minutes to acclimatize

  5. Start a stopwatch and count the number of bubbles produced in 1 minute

  6. Repeat the experiment at 20 cm, 30 cm, and 40 cm from the light source
• Problems with the practical

  • Number of bubbles can be too fast to count accurately
  • Bubbles are not always the same size
• Solving the problems

  1. Measure the volume of oxygen produced instead of counting bubbles

  2. Use a measuring cylinder to catch the bubbles and measure the volume of oxygen
• Doubling the distance from the light source

  Number of bubbles per minute falls by a factor of 4
• Inverse square law

  If the distance is doubled, the light intensity falls by a factor of 4, which causes the number of oxygen bubbles to fall by 4 times
• Photosynthesis


  When green plants use carbon dioxide and water in the presence of light energy to produce glucose and oxygen
• Required practical to investigate factors affecting rate of photosynthesis


  1. Use an aquatic plant (pond weed)

  2. Add sodium hydrogen carbonate solution

  3. Cut stem under water

  4. Measure oxygen bubbles produced in 1 minute

  5. Vary light intensity by changing distance from light source
• Independent variable

  • Light intensity
• As distance between light source and pond weed increases

  Number of oxygen bubbles produced in 1 minute decreases
• Alternative methods

  • Collect volume of oxygen produced
  • Test collected gas with glowing splint to prove it is oxygen
• Assuming each bubble is the same volume is an error that may be introduced in the first method
• Collecting the oxygen in an inverted boiling tube displaces the water and allows measurement of the volume or testing with a glowing splint