practical: photosynthesis

• What should you be able to describe by the end of the video?

  How to investigate the effect of light intensity on the rate of photosynthesis
• What additional concept should higher tier students be able to explain?

  How the results are affected by the inverse square law
• What are the steps to investigate the effect of light intensity on photosynthesis?

  1. Place a boiling tube 10 cm from an LED light source.
  2. Fill the boiling tube with sodium hydrogen carbonate solution.
  3. Add a piece of pond weed and leave for 5 minutes.
  4. Count the number of bubbles produced in one minute.
  5. Repeat at distances of 20 cm, 30 cm, and 40 cm.
• Why is an LED light source preferred in this experiment?

  Because it doesn't release much heat
• What should be done if a normal light bulb is used instead of an LED?

  A beaker of water should be placed between the light and the boiling tube
• What does sodium hydrogen carbonate solution release that is needed for photosynthesis?

  Carbon dioxide
• What is observed from the cut end of the pond weed after acclimatization?

  Bubbles of gas being produced
• What gas is produced by the pond weed during photosynthesis?

  Oxygen
• How many times should the experiment be repeated to calculate the mean number of bubbles produced?

  Two more times
• What are the two main problems with counting bubbles in this practical?

  1. Bubbles can be too fast to count accurately.
  2. Bubbles are not always the same size.
• How can the problems with counting bubbles be solved?

  By measuring the volume of oxygen produced instead of counting bubbles
• What equipment is used to measure the volume of oxygen produced?

  A measuring cylinder
• What happens to the number of bubbles per minute as the distance from the light source increases?

  • The number of bubbles per minute falls by a factor of four when the distance is doubled.
  • Example: From 10 cm to 20 cm, and from 20 cm to 40 cm.
• What is the scientific term for the relationship between distance and light intensity in this experiment?

  The inverse square law
• What happens to light intensity when the distance is doubled?

  It falls by a factor of four
• Why is light necessary for photosynthesis?

  It is required for the process that produces oxygen
• What should students be able to do after watching the video?

  • Describe how to investigate the effect of light intensity on photosynthesis.
  • Explain how results are affected by the inverse square law (for higher tier students).