Light Intensity GCSE Biology Practical
Investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed.
Students are required to investigate the rate of photosynthesis by measuring the amount of oxygen produced by an aquatic pondweed within a given time. They will carry this out by counting the number of bubbles generated by a sample of pondweed under illumination from different distances.
Equipment (per pair of participants)
• Pondweed – Egeria najas (10cm piece)
• 250ml beaker
• Filter funnel
• Modelling material
• 10ml measuring cylinder
• Light source
• Metre ruler
• Stopwatch
Investigating the Effect of Light Distance on Photosynthesis
Method
- Preparing the Experiment
- Cut Pondweed
- Take a fresh 10cm sprig of pondweed and place it into a beaker of water.
- Prepare the Funnel
- Use modelling material to create two small blobs.
- Attach the blobs to the rim of the funnel to:
- Raise the funnel above the beaker's bottom.
- Secure the funnel in place over the pondweed.
- Position the Pondweed
- Place the funnel over the pondweed, ensuring the trimmed end of the pondweed stem is inserted into the funnel stem and facing upwards.
- Set Up the Measuring Cylinder
- Fill a measuring cylinder with water.
- Cover the opening with your palm, invert it, and carefully position it over the funnel stem, ensuring no air enters.
- Adjust until the water levels in the cylinder and beaker match.
- Record the initial water level in the measuring cylinder.
- Conducting the Experiment
- Initial Setup
- Place the beaker containing the apparatus 1 metre away from the light source.
- Allow the pondweed to acclimatize to the lighting for a few minutes.
- Measure Photosynthesis
- Start a stopwatch and count the number of bubbles produced by the pondweed within a fixed time period.
- After the set time, note the new water level in the measuring cylinder to determine the volume of gas produced.
- Adjust Distance
- Reposition the light source at:
- 80cm, 60cm, 40cm, and 20cm from the beaker.
- For each new distance:
- Allow the pondweed to acclimatise to the light.
- Repeat the bubble count and record the water level in the measuring cylinder.
Notes
- Ensure the pondweed stem remains trimmed and properly positioned for optimal gas collection.
- Perform the experiment in a controlled environment to minimize external light interference or temperature fluctuations.
Technician tips
Advance preparation
- Egeria najas is the best option for choice of pondweed. Some pondweeds are classed as an invasive species so are no longer suitable for schools to use. Egeria can be purchased from some science suppliers, or from an aquatic pet supplies shop.
- See the CLEAPSS website for more information regarding other suitable types of pondweed: GL333 Pondweed In order to help the experiment along, the pondweed can be placed in a large glass beaker of water (the water should be left to stand for 24 hours before introducing the pondweed) and positioned under a desk lamp for a couple of hours before the plant is used in the classroom, which will initiate the production of oxygen bubbles.
To do this practical you will need
Need some Biology inspiration and help?
The experiment requires students to investigate the rate of photosynthesis by measuring the amount of oxygen produced by an aquatic pondweed within a given time. To do this practical students will need beaker, desk lamp, measuring cylinder, metre ruler, filter funnel, bulbs, stopwatches and a modelling material. At Philip Harris shop our full range of equipment needed for student's biology lessons in light intensity and photosynthesis.
