IGCSE Biology / Plants and food
Light, carbon dioxide and temperature
Edexcel International GCSE Biology (4BI1), chapter 10: Plants and food, specification point 2.20. Free lesson.
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A limiting factor is the factor in shortest supply, so it controls the rate of photosynthesis. Light and carbon dioxide graphs level off when another factor takes over, while high temperatures make the rate fall because enzymes denature.
What a limiting factor means
Photosynthesis depends on several conditions, including light intensity, carbon dioxide concentration and temperature. The limiting factor is the one in shortest supply. It controls the rate, even if the other conditions are suitable. Adding more of the limiting factor can increase the rate. Once another factor becomes limiting, adding more of the first factor no longer increases the rate.
Why the light graph levels off
The graph of light intensity against rate rises steeply at first. More light directly increases the rate because light is limiting. The graph then levels off: extra light makes no difference because another factor, usually carbon dioxide concentration or temperature, is now limiting. On the rising part, light is limiting. On the flat part, name the factor that has become limiting instead.
Why the carbon dioxide graph levels off
The carbon dioxide concentration graph has the same shape. The rate rises while carbon dioxide is limiting, then levels off when another factor controls the rate. In a glasshouse on a bright day, carbon dioxide is often limiting because air contains only about 0.04% carbon dioxide. Commercial growers burn fuel in glasshouses to add carbon dioxide and heat. On the rising part, carbon dioxide is limiting. On the flat part, another factor is limiting.
Why the temperature graph falls
As temperature rises, molecules collide more often, so the enzyme-controlled reactions of photosynthesis happen faster. The rate reaches an optimum, around 25°C to 30°C for most plants, then falls sharply. Above the optimum, enzymes denature: their active sites change shape, so they cannot work as well. This fall makes the temperature graph different from the light and carbon dioxide graphs. Temperature graph: rises, reaches an optimum, then falls. Light and carbon dioxide graphs rise, then level off.
How to answer a limiting-factor question
A flat graph tells you that the factor on the horizontal axis is no longer limiting. Do not just describe the flat line or say the plant has enough of that factor. State that another factor now controls the rate. For example: light is no longer limiting, so increasing it has no further effect. Carbon dioxide concentration or temperature is now limiting the rate. Naming the new limiting factor is where the mark is.