Limiting Factors of Photosynthesis: Read and Explain the Graph
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Core students investigate how light, carbon dioxide and temperature affect photosynthesis rate. Extended students identify which factor is in shortest supply and explain why a curve rises, plateaus or falls.
The highest-value mark in a limiting-factor question is rarely “the graph levels off”. It is the explanation that the x-axis factor is no longer limiting and a different factor now restricts the rate.
What a limiting factor means
A limiting factor is the factor in shortest supply that restricts the rate of a process. For photosynthesis, the main factors are light intensity, carbon dioxide concentration and temperature.
Increasing a non-limiting factor produces little or no further increase. The limiting factor can change along the same graph.
Explain the rising section and plateau
On a rate-against-light graph, the initial rise means light is limiting: more light transfers more energy for photosynthesis. At the plateau, light is no longer limiting. Carbon dioxide concentration or temperature now restricts the rate.
If a second curve at higher carbon dioxide reaches a higher plateau, explain that added carbon dioxide removes or reduces that limitation.
Temperature behaves differently
Photosynthesis includes enzyme-controlled reactions. Increasing temperature raises collision frequency up to an optimum. Above the optimum, enzymes lose functional active-site shape and the rate falls. Therefore a temperature graph may fall after its peak rather than simply plateau.
Apply the idea to greenhouses
Growers can provide lighting, add carbon dioxide and control temperature. The economic answer is not “maximise everything”: the benefit of removing a limiting factor must exceed the cost. Once a different factor limits the process, more spending on the first factor gives little extra yield.
Try a focused question (4 marks)
A photosynthesis rate rises with light intensity and then becomes constant. At higher carbon dioxide concentration, the constant rate is greater. Explain both observations.
Show the answer and marking guidance
At low light intensity, light is the limiting factor.
Increasing light provides more energy and raises the rate.
At the first plateau, another factor such as carbon dioxide or temperature limits the rate.
Higher carbon dioxide reduces that limitation, allowing a higher rate before another factor limits it.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Writing only that the line becomes flat.
Naming light as limiting on the plateau after light has become abundant.
Ignoring enzyme denaturation on a high-temperature graph.
Assuming the same factor limits every part of a graph.