Investigating photosynthesis

The effect of light intensity on photosynthesis can be investigated in water plants. Use Cabomba or Elodea, which are sold in aquarium shops.

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

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

• an LED bulb is best as this will not raise the temperature of the water
• sodium hydrogencarbonate – formula NaHCO₃ – is added to the water to supply carbon dioxide – a reactant in photosynthesis – to the plant
• the light intensity is inversely proportional to the square of the distance – it will decrease as the distance away from the bulb increases – so light intensity for the investigation can be varied by changing the distance from the lamp to the plant

The bubbles produced over one minute periods are recorded.

Aim

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

Method

1. Set up a boiling tube containing 45 cm³ of sodium hydrogen carbonate solution. Allow the tube to stand for a few minutes and shake to disperse any air bubbles that might form.
2. Cut a piece of the pondweed, Cabomba.
3. Use forceps to place the pondweed in the boiling tube carefully. The pondweed should be cut end uppermost. Make sure that you don't damage the pondweed, or cause the liquid to overflow.
4. Position the boiling tube so that the pondweed is 10 cm away from the light source. Allow the boiling tube to stand for five minutes. Count the number of bubbles emerging from the cut end of the stems in one minute. Repeat the count five times and record your results.
5. Calculate the mean number of bubbles produced per minute. Repeat the experiment at different distances away from the light source.

Variables

• Independent variable – distance from the light source.
• Dependent variable – the number of bubbles produced per minute.
• Control variables – concentration of sodium hydrogen carbonate solution, temperature, using the same piece of Cabomba pondweed each time.

Risks

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

Cabomba must be destroyed after use, and not released into natural water courses.

Results

More photosynthesis occurs at higher light intensities. More bubbles of oxygen should be produced when the light is closer to the pondweed. At lower light intensities, when the light is further away, the rate of photosynthesis reduces and so fewer oxygen bubbles should be observed.

Extension activities

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

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

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.