What Factors Limit the Rate of Photosynthesis?
Learn how light intensity, carbon dioxide, temperature and chlorophyll can limit photosynthesis, with a clear GCSE Biology example and exam tip.
The factors that can limit the rate of photosynthesis are light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll. The factor in shortest supply is called the limiting factor because it prevents the rate from increasing further.
The four limiting factors
- Light intensity: Light provides the energy needed for photosynthesis. Increasing light intensity usually increases the rate until another factor becomes limiting.
- Carbon dioxide concentration: Carbon dioxide is a reactant in photosynthesis. A low concentration means less carbon dioxide is available to make glucose.
- Temperature: Photosynthesis is controlled by enzymes. At low temperatures, reactions happen slowly. As temperature rises, the rate increases up to an optimum. Above the optimum, enzymes may denature, so the rate decreases.
- Amount of chlorophyll: Chlorophyll absorbs light energy. A plant with less chlorophyll absorbs less light, reducing its rate of photosynthesis.
Photosynthesis can be represented by the word equation:
carbon dioxide+water→lightglucose+oxygen\text{carbon dioxide} + \text{water} \xrightarrow{\text{light}} \text{glucose} + \text{oxygen}carbon dioxide+waterlightglucose+oxygenWorked example
Suppose increasing light intensity initially increases the rate of photosynthesis, but the graph eventually levels off. Light is no longer the limiting factor at the plateau. Carbon dioxide concentration, temperature or the amount of chlorophyll is now preventing any further increase.
If carbon dioxide is then added and the rate rises, this shows that carbon dioxide was the limiting factor.
On Higher Tier, you should be able to explain how several factors interact and use graphs to decide which factor is limiting. This answer assumes Higher Tier, although the effects of all four factors are also required at Foundation Tier.
Common exam mistake
Do not say that increasing one factor will always increase the rate. Once that factor is no longer limiting, the rate levels off because a different factor limits photosynthesis. Also remember that chlorophyll amount is one of the specified factors—it is often forgotten.
