Required Practical 6: Photosynthesis

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

  1. Take a boiling tube

  2. Place 10 cm away from an LED light source

  3. Fill boiling tube with sodium hydrogen carbonate solution

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

  5. Leave for 5 minutes to acclimatize

  6. Start stopwatch and count bubbles produced in 1 minute

  7. Repeat 2 more times and calculate mean

  8. Repeat experiment at 20 cm, 30 cm, and 40 cm
• 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 equipment to catch the bubbles in a measuring cylinder filled with water

  3. Use the measuring cylinder to measure the volume of oxygen produced
• Doubling the distance from the light to the pondweed

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


  If we double the distance, the light intensity falls by 4 times, which causes the number of oxygen bubbles to fall by 4 times
• Photosynthesis


  The reaction that plants use to trap light energy
• Plants use light for their source of energy
• Photosynthesis


  • It is an endothermic reaction
  • It takes place in the leaves of a plant
  • Leaves contain the green chemical chlorophyll
• Photosynthesis


  1. Plant takes in carbon dioxide and water

  2. Light energy is absorbed by chlorophyll

  3. Light energy is used to convert carbon dioxide and water into glucose

  4. Oxygen is also produced
• Chemical formulas


  • Carbon dioxide (CO2)
  • Water (H2O)
  • Glucose (C6H12O6)
  • Oxygen (O2)
• Light intensity increases


  Rate of photosynthesis increases
• Limiting factor


  The factor that is in short supply and is limiting the rate of photosynthesis
• Carbon dioxide level increases

  Rate of photosynthesis increases
• Factors affecting rate of photosynthesis

  • Light intensity
  • Carbon dioxide level
  • Amount of chlorophyll in leaf
  • Temperature
• As temperature increases, enzymes involved in photosynthesis work faster, increasing the rate, but if temperature keeps increasing, enzymes will denature and the rate will fall