Required practical photosynthesis

• How do you investigate the effect of light intensity on the rate of photosynthesis?

  • take a boiling tube and place it 10cm away from an LED light source
  • fill the tube with sodium hydrogen carbohydrate solution
  • put a piece of pondweed into the boiling tube and cut the end top
  • leave for 5 minutes
  • bubbles of oxygen will appear
  • start a stopwatch and count the bubbles produced in one minute
  • repeat 2x and calculate the mean of the bubbles produced
  • repeat the whole practical at 20cm then 30cm and 40cm
• Two reasons why we use a LED light source ?

  they do not release a lot of heat

  too much heat would change the temperature of the experiment
• What would you have to do if you used a light bulb ?Why?

  place a beaker of water in between the light and the tube because it absorbs the heat produce by the light bulb
• Why do we use sodium hydrogen carbonate solution ?

  it produces carbon dioxide which is needed for photosynthesis
• Why do we leave the pondweed for 5 minutes ?

  so it can acclimatise (getting use to)to the conditions of the boiling tube
• What should you see after 5 minutes ?

  see bubbles of oxygen being produced at the cut end
• What are the problems with this practical ?

  • the number of bubbles can be too fast to count accurately ?
  • bubbles are not always the same size e.g. a large bubble would count the same as a small one
• How do we solve the problems with the practical ?

  measuring the volume of oxygen produced
• How do you measure the volume of oxygen being produced ?

  place a funnel above the pondweed and catch the bubbles in a measuring cylinder filled with water,then use the measuring cylinder to measure the volume of the gas produced
• What is the inverse square law? 


  The intensity of a light source is inversely proportional to the square of the distance from the source