Required practical activity 6

Investigate the effect of light intensity on the rate of photosynthesis

Greg Foot explains the effect of temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis

Investigating photosynthesis

The effect of light intensity on photosynthesis can be investigated in water plants such as Cabomba or Elodea, types of pond weed, which are sold in aquarium shops.

The plants will release bubbles of oxygen, a product of photosynthesis, which can be counted.

Aim

To investigate the effect of light intensity on the rate of photosynthesis.

Method

A lamp with an LED bulb is set up at different distances from the plant in a beaker of water:

  • an LED bulb is best as this will not raise the temperature of the water
  • sodium hydrogen carbonate (NaHCO3) is added to the water to supply the reactant carbon dioxide to the plant
  • the light intensity is proportional to distance - it will decrease as the distance away from the bulb increases.
An image showing the instructions on how to investigate the effect of light intensity on the rate of photosynthesis.

The bubbles produced over one minute periods are recorded. The rate of photosynthesis is the number of bubbles produced per minute.

Variables

  • independent variable - distance from the light source/light intensity
  • dependent variable - the number of bubbles produced per minute
  • control variables - concentration of sodium hydrogen carbonate solution, temperature, using the same piece of pondweed each time

Risks

Care must be taken when using water near electrical equipment. Ensure that your hands are dry when handling the lamp.

Extension activities

The volume of oxygen produced could be measured by collecting the gas produced in a gas syringe.

A diagram showing and experiment of the volume of oxygen.

The changes in the oxygen concentration in the water could be measured using data logging equipment.

An image showing the changes in oxygen.

You could investigate the effect of different wavelengths of light on photosynthesis.

Use coloured acetate filters to investigate the effects of the blue, green and red parts of the spectrum on photosynthesis.

An image showing the effect of different wavelengths of light on photosynthesis.