Edexcel IGCSE Biology · Spec 2.20
Limiting Factors
How light intensity, carbon dioxide and temperature limit the rate of photosynthesis, and the shape of each graph.
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Limiting Factors
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Explained
Limiting factors, and reading the shape of the graph
A limiting factor is whatever is in shortest supply and therefore stops photosynthesis going any faster. Three are examinable: light intensity, carbon dioxide concentration and temperature. At any moment only one of them is doing the limiting, and the skill is working out which.
Light intensity and carbon dioxide give the same shape
Increase light intensity from low and the rate rises steeply, because light is the factor in shortest supply. Keep increasing it and the line flattens. The rate has stopped responding to light, which means something else has become the limiting factor, usually carbon dioxide or temperature.
Carbon dioxide behaves the same way and for the same reason: a steep rise, then a plateau once a different factor takes over.
The plateau is where the marks are. When a graph levels off, the answer is not "the plant has had enough light". It is that light is no longer limiting, and some other factor now is.
Temperature is the one that behaves differently
Temperature rises to a peak and then falls, and the fall is what sets it apart. Up to the optimum, warmer conditions mean faster moving particles and more frequent collisions with enzymes, so the rate rises.
Past the optimum the rate drops sharply, because the enzymes controlling photosynthesis are denatured. Their active sites change shape, so the substrate no longer fits. Light and carbon dioxide have no equivalent to this, which is why only the temperature graph comes back down.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Biology show that most candidates pick up something on these questions, and that the best responses go further by linking conditions to consequences. One report describes strong answers explaining the effect of lower temperatures and fewer hours of sunlight on both the rate of photosynthesis and the number of producers.
That is the move worth copying. Do not stop at the rate. If the question is about a habitat or a season, carry the reasoning through to what it means for the plants and for everything that feeds on them.
Identifying which factor is limiting
Look at what happens when one factor is changed. If the rate increases, that factor was limiting. If nothing happens, it was not, and something else is holding the rate down.
Questions often show two curves at different carbon dioxide concentrations. Where the curves overlap, light is limiting for both. Where they separate, carbon dioxide has become the limiting factor for the lower curve, and saying exactly where that happens on the graph is usually worth a mark.
Why growers care
Commercial greenhouses raise all three deliberately: lamps for light, heaters for temperature, and sometimes added carbon dioxide. The cost has to be weighed against the extra yield, and questions frequently ask you to make that judgement rather than simply describe the science.
Spec 2.20
What you need to know
- Explain what a limiting factor is
- Describe how light and carbon dioxide affect the rate
- Describe how temperature affects the rate
Active recall
Quick check
Answer each question before opening the answer.
Name the three limiting factors of photosynthesis.
Light intensity, carbon dioxide concentration and temperature.
What is meant by a limiting factor?
The factor in shortest supply that limits (caps) the rate of photosynthesis.
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