What Factors Limit the Rate of Photosynthesis?
Learn how light intensity, carbon dioxide concentration and temperature limit photosynthesis, and how to identify the limiting factor on an AQA Biology graph.
Light intensity, carbon dioxide concentration and temperature are the main factors that can limit the rate of photosynthesis. A limiting factor is the factor currently preventing the rate from increasing: changing it raises the rate, until another factor becomes limiting.
How each factor affects the rate
- Light intensity: Chlorophyll absorbs light for the light-dependent reaction. At low light intensity, less ATP and reduced NADP are produced, so the rate of photosynthesis is low. Increasing light intensity raises the rate until another factor limits it.
- Carbon dioxide concentration: Carbon dioxide is used in the Calvin cycle, the light-independent reaction. Rubisco catalyses its combination with RuBP. If carbon dioxide is scarce, less GP is formed, limiting the production of sugars and other organic substances.
- Temperature: Photosynthesis is controlled by enzymes, including rubisco. At low temperatures, enzyme-controlled reactions are slower. As temperature rises, the rate may increase, but above the optimum temperature enzymes can denature, causing the rate to fall.
Reading a limiting-factor graph
Suppose a graph shows photosynthesis increasing as light intensity rises, then levelling off. Light is limiting on the rising part: more light gives a higher rate. At the plateau, more light alone will not increase the rate. Carbon dioxide concentration or temperature may now be limiting. To find out which, change one of those factors while keeping the others constant.
For example, compare two plants under the same light intensity and temperature, but give one a higher carbon dioxide concentration. If its photosynthesis rate increases, carbon dioxide was limiting at the original concentration. If the rate stays the same, another factor is limiting under those conditions.
A common exam mistake is to say that the factor on the graph is always limiting. A flat section does not mean photosynthesis has stopped; it means increasing the factor shown no longer increases the rate under those conditions.
