photosynthesis - light

• Why is the stem of the pond weed cut at an angle?

  To ensure a steady stream of oxygen bubbles is produced.
• What is the independent variable in the photosynthesis experiment?

  The light intensity, which is changed by varying the distance from the light source.
• How is light intensity changed in the experiment?

  By changing the distance between the pond weed and the light source.
• What happens to the number of bubbles produced as the distance from the light source increases?


  The number of bubbles produced decreases.
• What is an alternative method to measure the rate of photosynthesis more accurately?

  • Collect the volume of oxygen produced.
  • Use a glass funnel and boiling tube to capture gas.
  • Measure the volume or test with a glowing splint.
• What is the purpose of using a glowing splint in the experiment?

  To test if the collected gas is oxygen.
• What is need for the Practical investigating photosynthesis for light intensity?

  Use water plants, such as Elodea which release bubbles of oxygen when photosynthesising

  A lamp with an LED bulb is set up beside the beaker of water containing the water plant. An LED is best as it will not raise the temperature of the water.

  Sodium hydrogen carbonate (NaHCO,) is added to the water to supply carbon dioxide

  This set up can be used to investigate the evolution of oxygen from a water plant.
• How to investigate the effect of light intensity?


  1)Place pondweed in water and set up a desk lamp next to a ruler so that you can measure the distance between the light and the beaker. Add a fixed volume of 0.2% of sodium hydrogen carbonate solution to the water.

  2) Move the lamp away by 10cm

  3) Leave for 5 minutes to allow for the pondweed to adapt

  4)Count the number of bubbles given off in 1 minute and record

  5) Repeat steps 2-4 but with more distances (20,30,40cm)
• What should you do after doing the investigation?

  Plot a graph of the rate of photosynthesis (given by the no. of bubbles) against light intensity (using the inverse square law, light intensity∝1/distance^2 between pondweed and light source).
• What are the controlled variables?


  Species and size of pondweed

  Temperature

  Volume and concentration of hydrogen carbonate solution

  Time for counting bubbles. volume of water