AQA GCSE Biology · Bioenergetics
Photosynthesis required practical
Investigate the effect of light intensity on the rate of photosynthesis using pondweed.
Pondweed gives off bubbles of oxygen as it photosynthesises. Moving a lamp closer increases the light intensity, and the bubbles come faster, until another factor limits the rate.
Equipment
- Pondweed (e.g. Cabomba or Elodea) and a paper clip
- Boiling tube in a clamp stand
- Sodium hydrogencarbonate solution
- LED lamp
- Metre ruler
- Stopwatch
Variables
- Independent you change
- Light intensity (distance from the lamp)
- Dependent you measure
- Number of oxygen bubbles per minute
- Control you keep the same
- Temperature
- Carbon dioxide concentration (same sodium hydrogencarbonate)
- Same piece of pondweed
- Same length of time counting
Method
- 1
Fill a boiling tube with sodium hydrogencarbonate solution, which gives the pondweed carbon dioxide.
- 2
Put a fresh cut piece of pondweed in the tube, cut end up, with a paper clip to weigh it down.
- 3
Place an LED lamp 10 cm from the pondweed, measured with a metre ruler.
- 4
Leave it for 5 minutes so the pondweed adjusts to the light.
- 5
Count the number of bubbles given off in one minute.
- 6
Repeat at 20, 30, 40 and 50 cm, and do each distance three times.
Worked example
Mean bubbles per minute at each distance (three repeats).
| Distance (cm) | Bubbles per minute | Relative light intensity (1 ÷ d²) |
|---|---|---|
| 10 | 62 | 0.0100 |
| 20 | 34 | 0.0025 |
| 30 | 17 | 0.0011 |
| 40 | 10 | 0.0006 |
| 50 | 6 | 0.0004 |
Working
Light intensity ∝ 1 ÷ distance² (the inverse square law)
Doubling the distance from 10 cm to 20 cm: intensity × (1 ÷ 2²) = ¼ as bright
Conclusion
The closer the lamp, the faster oxygen is given off, so the rate of photosynthesis increases with light intensity. Because intensity falls with the square of the distance, the rate drops quickly at first and then levels off.
Risks
- Lamps get hotUse an LED lamp and don’t touch the bulb.
- Water near electrical equipmentKeep the lamp and plug dry and away from spills.
Common mistakes
- Using a filament lamp close up without controlling temperature. The heat warms the water and changes the rate too. Use an LED or put a beaker of water in between.
- Not leaving time for the pondweed to adjust after moving the lamp.
- Saying light intensity is proportional to distance. It is inversely proportional to the distance squared.
Better results
- Collect the gas in an upturned measuring cylinder or gas syringe and measure its volume. Counting bubbles is inaccurate because bubbles vary in size.
- Do the practical in a darkened room so the lamp is the only light source.
- Repeat each distance three times and calculate a mean.
Launching November 2026
7 exam questions on this practical.
Practise the method, the variables, the maths and what the results mean, then revise the Bioenergetics flashcards. It opens to students soon: join the waitlist and we’ll email you once, when it does.