Required practical activity

The effect of light intensity on the photosynthesis rate using an aquatic organism

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

An image showing the instructions on how to investigate the effect of light intensity on the rate of photosynthesis.
  1. Set up a boiling tube containing 45 cm3 of sodium hydrogencarbonate solution (1%). 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. The pondweed should be 8 cm long.
  3. Use forcepts to place the pondweed in the boiling tube carefully. 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 average 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/light intensity.
  • Dependent variable – the number of bubbles produced per minute.
  • Control variables – concentration of sodium hydrogencarbonate 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.

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.

The effect of chlorophyll

The effect of the presence or absence of chlorophyll on photosynthesis can be investigated using a variegated plant. Variegated plants have regions of their leaves with, and without, chlorophyll.

Only those areas of the leaf with chlorophyll photosynthesise. They will test positive for starch, which is built up from the glucose produced.

Geranium flowers
  1. The leaf of a variegated Pelargonium is dropped in boiling water to kill and preserve it
  2. The leaf is left for 10 minutes in hot ethanol in a boiling tube. This removes the chlorophyll
  3. The leaf is dipped in boiling water to soften it
  4. The leaf is spread out in a Petri dish and covered with iodine solution
  5. The areas that had the chlorophyll stain blue-black. The areas that had no chlorophyll remain pale
An image showing the stages of leaf when dropped in boiling water.

Risks

Care must be taken when using boiling ethanol. Make sure that no Bunsen burners are turned on as the ethanol is highly flammable.