Core practical: light intensity and photosynthesisSpec 6.5
In short
The light intensity core practical investigates how light intensity affects the rate of photosynthesis in pondweed. Light intensity is changed by moving a lamp to different distances, and the rate is measured by counting oxygen bubbles per minute or collecting the gas. Temperature is kept constant with a heat shield, and carbon dioxide with sodium hydrogencarbonate.
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 (what this means)
Higher tier: only on the Higher tier papers. Foundation students can skip it. What the labels meanBecause 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.
Safety: keep the mains lamp and its cable away from the water, and do not touch a hot lamp. Controlling temperature matters because a lamp placed close to the plant also warms the water, which would change the rate.
'Light intensity' is the independent variable, not 'the distance'. Say that the distance is how you change the light intensity.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026