Effect of light intensity on photosynthesis rate

• Step 1

  Start by taking a boiling tube and placing it 10cm away from an LED light source
• why is an LED light used

  They don’t produce much heat. Too much heat would affect the results
• Step 2

  Now fill the boiling tube with sodium hydroxide carbonate solution. This releases carbon dioxide whcih is needed for photosynthesis
• Step 3

  Put a piece of pondweed into the boiling tube with the cut end at the top. Leave this for five minutes to acclimatise to the conditions in the boiling tube
• Step 4

  Bubbles of gas should start to appear from the cut end of the pondweed. This gas is oxygen and is produced by photosynthesi.
• Step 5

  Start a stopwatch and count the number of bubbles produced in a minute. Repeat this two more times and calculate a mean
• Step 6

  Restart the experiment but this time but this time at 20 cm and continue repeating until you reach 40cm
• Problems with the practical

  Bubbles can be too fast to count

  Bubbles can be different sizes. A large bubble would count the same as a small one
• solve the problems

  Measure the amount of oxygen produced instead of bubbles
• Step 1 of oxygen method

  Place the pondweed under a funnel and catch the bubbles using a measuring cylinder
• Step 2 of oxygen method

  Use the measuring cylinder to measure the amount of gas produced.
• How are the resultd affected by inverse square law

  If we double the distance then the number of bubbles per minute falls by a factor of four