Required practical

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

The effect of light intensity on 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

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 forceps to place the pondweed in the boiling tube carefully, with the 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.
5. Allow the boiling tube to stand for five minutes.
6. Count the number of bubbles emerging from the cut end of the stems in one minute.
7. Repeat the count five times and record your results.
8. Calculate the average number of bubbles produced per minute.
9. 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.

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

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

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

Results

The table shows a set of results obtained by students investigating the effect of light intensity on photosynthesis. They varied the distance of the plant from the light source.

In the table below, the rate of photosynthesis = number of bubbles produced per minute.

DistanceRate
10 cm120
15 cm54
20 cm30
25 cm17
30 cm13

The data plotted on a graph:

Question

From the graph, suggest the rate of oxygen production when the light source is 18 cm away.

40 bubbles per minute.

Using the graph: