Required Practical: Photosynthesis Rate (AQA GCSE Biology: Combined Science)

• Aim: Investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed
• You will:
  • measure the volume of oxygen produced by the pondweed as the light intensity changes as the light source is moved
  • measure and calculate rates of photosynthesis
  • extract and interpret graphs of photosynthesis rate involving one limiting factor

• The most commonly used method to measure the rate of photosynthesis is to measure or observe the oxygen released from aquatic plants

Diagram showing the setup for the investigation using aquatic plants

Method

• Place a piece of pondweed (Elodea or Cabomba are often used), into a beaker of water
• Use a light a set distance from the plant
• Record the number of bubbles observed in three minutes
• Repeat steps for different distances

Improvements

• Use a gas syringe to collect the volume of gas produced
• Repeat the experiment at least twice for each distance and calculate the mean number of bubbles
• Use of a glass tank between lamp and plant to prevent heating of the plant, or using an LED bulb that releases very little heat energy

Variables

• Different variables can be investigated using this method. The three diagrams show the setup for 3 independent variables: light, carbon dioxide and temperature

Results

• A graph of the independent variable used against the number of bubbles produced per minute can be drawn to see the pattern or trend

A graph of distance from the lamp against number of bubbles per minute