photosynthesis - required practical

• 1. start by taking a boiling tube and placing it 10cm away from an LED light source ; why is the LED light source used?


  as these don't release too much heat
• 2) fill the test tube with sodium hydrogen carbonate solution : why do we use sodium hydrogen carbonate solution?


  it releases carbon dioxide which is needed for photosynthesis
• 3) place piece of pondweed in the test tube with the cut end at the top
• 4) leave pondweed for five minutes to get used to the conditions within the boiling tube.
• 5) bubbles of oxygen produced by photosynthesis should start appearing at the cut end of the pondweed.
• 6) start a stop-watch and count number of bubbles produced in one minute. reap twice more to calculate mean number of bubbles produced in one minute.
• 7) repeat the whole experiment again and move the test tube away from the light 10cm each time.
• problems in experiment;

  bubbles are too fast to count accurately and bubbles are not always the same size.


  solve by measuring the volume of oxygen produced.
• measure volume;

  1. place pond-weed under a funnel and catch the bubbles in a measuring cylinder filled with water.
  2. use the measuring cylinder to measure the volume of oxygen produced.
• if we double the distance, the number of bubbles per minute drops - inverse-square law.