Factors affecting the rate of photosynthesis

The rate of photosynthesis is how quickly a plant carries out photosynthesis. It depends on conditions including light intensity, carbon dioxide concentration and temperature. A factor that restricts the rate under the current conditions is called a limiting factor.

Light intensity and carbon dioxide concentration

Photosynthesis requires light and carbon dioxide. If light intensity is limiting, increasing it raises the rate of photosynthesis. If carbon dioxide concentration is limiting, increasing it raises the rate.

The rate does not keep rising indefinitely as either factor increases. Eventually, another factor becomes limiting, so increasing the first factor further has little or no effect. For example, increasing light intensity may initially increase the rate, but the rate may then level off because carbon dioxide is limiting. Increasing the carbon dioxide concentration could allow the rate to rise again.

Temperature and photosynthesis

Photosynthesis involves enzyme-controlled reactions. As temperature rises, these reactions generally become faster, so the rate of photosynthesis increases towards an optimum temperature.

Above the optimum, the rate falls. High temperatures can cause enzymes to lose their working shape, so they can no longer carry out their reactions effectively.

Investigating a factor affecting photosynthesis

To investigate one factor, change that factor and measure the rate in the same way at each value. Keep other relevant conditions controlled so that they do not cause the observed change.

For example, to investigate light intensity, a water plant can be exposed to different light intensities while its temperature and carbon dioxide supply are kept constant. The volume of oxygen produced in a fixed time can be used as an indicator of the photosynthetic rate. Comparing the measurements shows how the rate changes as light intensity changes.