6 - Affect of light intensity on photosynthesis

• What is the independent variable?

  • Light intensity - use a lamp at different distances from the pond weed each time
• What is the dependant variable?

  • Rate of photosynthesis
  • Calculate this by counting number of air bubbles in a period of time
• What is the control variable?

  • water volume
  • CO2 concentration
  • Temperature
  • Type of plant - we use pondweed
  • Equipment
• What equipment is used?

  • Lamp - an LED bulb won't heat the water
  • Meter ruler
  • Glass block between the pond weed beaker and the lamp - absorbs the heat energy so the water isn't heated
  • Boiling tube containing water and sodium hydrocarbonate (NaHCO3 )
• Process 


  • Add water and sodium hydrocarbonate to boiling tube
  • Place a piece of pondweed in the water solution - cut off the top bit as it will be easier to see the bubbles produced
  • Either use a led bulb or a glass block to control the water temperature as it can affect the rate and make the test unfair
  • turn off any other lights so the only light source is the lamp
  • Start with the pondweed 10cm away from the lamp and leave it for a few minutes to adjust to the light
  • Then count the bubbles produced in 1 minute - rate
  • Repeat process moving the test tube back 10 cm each time
• Why is sodium hydrocarbonate used instead of pure water

  • Ensures CO2 isnt a limiting factor and it encourages photosynthesis
• What are the limitations of the practical?

  • Might miss count bubbles if they are coming very fast
  • Some bubbles might have a larger volume of oxygen - Instead you could count the oxygen in an upturned measuring cylender
  • May run out of CO2 if you dont use NaHCO3
• What are saftey measures

  • Dont touch the hot lamp you might burn
  • Dont let the glass smash
  • The scalpul could cut you