Core practical: light intensity and photosynthesisSpec 6.5
You can investigate how light intensity affects the rate of photosynthesis using an aquatic plant such as pondweed. The light intensity is changed by moving a lamp to different distances from the plant.
- Cut a piece of pondweed and place it in a boiling tube or beaker of water with a little sodium hydrogencarbonate, which supplies carbon dioxide. Place a heat shield (a clear container of water) between the lamp and the plant, or use an LED lamp, to keep the temperature constant.
- Place a lamp a measured distance from the pondweed, for example 10 cm, using a ruler. Switch off or block other light sources.
- Leave the pondweed for a couple of minutes to adjust to the new light level.
- Count the number of bubbles of oxygen released from the cut end of the stem in one minute. (You can instead collect the gas and measure its volume.)
- Repeat at different distances, for example 20, 30, 40 and 50 cm.
- Repeat each distance at least three times and calculate a mean rate.
| Type | Variable | How it is controlled or measured |
|---|---|---|
| Independent | Light intensity | Changed by changing the distance between the lamp and the plant |
| Dependent | Rate of photosynthesis | Measured as the number of bubbles per minute, or the volume of gas collected per minute |
| Control | Temperature | Heat shield or water bath, and a lamp that gives out little heat |
| Control | Carbon dioxide concentration | Same amount of sodium hydrogencarbonate in the water each time |
| Control | The plant | Same piece of pondweed, with the same length of stem, for every distance |
Typical results: the further the lamp is from the plant, the lower the light intensity, so fewer bubbles are released per minute. Plot the mean rate (y-axis) against the distance or light intensity (x-axis).
Higher tier
Because light intensity is inversely proportional to the square of the distance, plotting the rate against 1 ÷ distance² should give a straight line while light is the limiting factor.
Counting bubbles is not very accurate because bubbles can be different sizes. Collecting the gas and measuring its volume gives better data.