There are a number of factors which contribute towards the process of photosynthesis. The factor which is working at the lowest level will usually be the limiting factor of the process. There are several limiting factors which can reduce the rate of photosynthesis, eg temperature, light intensity and carbon dioxide concentration.
As with other chemical reactions, the rate of photosynthesis increases as the temperature rises. Photosynthesis is controlled by enzymes, which become denatured if the temperature is too high. As a result, the rate of photosynthesis reaches its maximum at an optimum temperature, then decreases. The maximum rate is determined by the other factors involved.
Light provides the energy needed for photosynthesis. Increasing the light intensity increases the rate of photosynthesis, provided plenty of carbon dioxide and water are available. The maximum rate is determined by the other factors involved.
The role of light in photosynthesis can be investigated in several ways.
In this experiment, a light is moved further from a photosynthesising plant to vary the light intensity the plant receives. The further the light moves from the plant, the fewer oxygen bubbles are produced by photosynthesis.
The intensity of light at different distances from a light source can be described by the inverse square law. This states that the intensity of light is inversely proportional to the square of the distance from the source.
In practical terms, this means that when the light is moved twice as far from the plant it will receive a quarter of the energy.
Light intensity can be calculated using this formula.
The symbol ∝ means ‘is proportional to’, and distance is measured in metres.
Therefore, when the light is 20 cm from the plant, it will receive
= 25 arbitrary units
But when the light is 40 cm from the plant, it will only receive
= 6.25 arbitrary units
Carbon dioxide is needed for photosynthesis. Increasing the carbon dioxide concentration increases the rate of photosynthesis, provided the plant is warm enough, and has plenty of light and water. The maximum rate is determined by the other factors involved.
What is limiting the rate of photosynthesis at points A and B on the graph? What information from the graph can be used as evidence for this?
The rate of photosynthesis at point A is being limited by the concentration of carbon dioxide. The evidence for this is that as the concentration of carbon dioxide is increased, the rate of photosynthesis also increases.
The limiting factor at point B could be temperature or light intensity. We know it is not carbon dioxide because any further increase in carbon dioxide does not increase the rate of photosynthesis. Therefore something else must be preventing an increase in rate.