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
Method
- Students should cut a fresh 10cm length of pondweed and place this into a beaker of water.
- A couple of blobs of modelling material should be made and they should be attached to the rim of the funnel (this will raise the funnel from the bottom of the beaker and will weigh the funnel down to secure it over the pondweed).
- The funnel is placed over the sprig of pondweed and students must ensure that the trimmed end of the pondweed stem is in the funnel stem facing upwards.
- Students should fill the measuring cylinder with water, place the palm of their hand over the opening and carefully position this onto the end of the funnel stem until the water in the cylinder meets the water level in the beaker. Make a note of where the water level is in the measuring cylinder once the apparatus is ready.
- The beaker of pondweed and water should be positioned 1 metre away from the light source. The beaker should be given a few minutes to acclimatize to the lighting.
- Students then start the stopwatch and begin to count the number of bubbles which are produced by the pondweed. After a fixed time has elapsed, students make another note of the water level in the measuring cylinder so they can see the volume of gas produced along with the number of bubbles.
- Re-position the light source so that it is 80cm away from the beaker, giving a few minutes to acclimatize and repeat the process.
- The same process should be repeated whilst the beaker is 60cm, 40cm and 20cm away from the light source.
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.
