When increasing light no longer raises the measured rate of photosynthesis, light is no longer limiting the rate over that interval. Carbon dioxide concentration or temperature may be limiting instead. Use a second comparison to find evidence for which factor is responsible.
The 2027 SEC G3 Biology K325 syllabus includes light intensity, carbon dioxide concentration and temperature as limiting factors. This guide shows what a plateau alone can tell you and how to interpret a comparison at two carbon dioxide concentrations.
- Read the section where the rate rises with light.
- Identify the range over which the rate levels off.
- Compare rates at the same light level across treatments.
- Name the factor supported by the comparison.
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Explain the rising and flat sections
At low light intensity, raising the light level can increase the rate. That pattern supports the explanation that light is limiting under those conditions.
A flat section means that raising light across the tested interval produced no measured increase. Several explanations can fit that observation, so a single plateau does not identify the limiting factor by itself.
For a controlled comparison, change carbon dioxide concentration while keeping light, temperature and the amount of plant material comparable at each measurement.
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Worked example | Compare two carbon dioxide conditions
A pupil compares the oxygen collected per minute from similar aquatic shoots at three light intensities. Temperature is kept the same. The readings below are illustrative.
| Light intensity | Lower carbon dioxide | Higher carbon dioxide |
|---|---|---|
| Low | 2 | 2 |
| Medium | 4 | 5 |
| High | 4 | 7 |
At lower carbon dioxide, the reading stays at 4 from medium to high light. At the same high light, the higher-carbon-dioxide treatment gives 7. This supports the conclusion that carbon dioxide availability limited the rate in the lower-carbon-dioxide condition at high light.
At low light the readings are both 2. In this example, raising carbon dioxide alone did not raise the measured rate there.
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Practise explaining an uncertain result
Suppose both treatments instead level off at 4 mL per minute. Those readings give no evidence that the tested increase in carbon dioxide raised the rate. Temperature could be investigated separately by varying it while controlling the other conditions.
If a lamp also warms the water, changing its distance could change light and temperature together. Monitor the water temperature and control it before attributing a difference to light.
Oxygen collected is an indicator of photosynthesis in this setup. When a question gives only bubble counts, explain that bubbles may vary in size and a gas volume per minute provides a more direct comparison of gas output.
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Frequently asked questions
Does a plateau mean photosynthesis stopped?
A plateau above zero means the measured rate stayed roughly constant over that range.
Which two readings should I compare to test carbon dioxide?
Use two readings at the same light intensity and temperature, with different carbon dioxide conditions.
How can I give evidence in one sentence?
Name the controlled comparison and quote the change in the measured rate.
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