Measuring the rate of photosynthesis

Factors limiting photosynthesis

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.

Temperature

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.

Graph showing how temperature affects the rate of photosynthesis. The y axis is labelled 'Rate of photosynthesis', the x axis is labelled 'Temperature'.Graph showing rate of photosynthesis and temperature

Light intensity

Investigating light intensity

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.

Graph showing how light intensity affects the rate of photosynthesis. The y axis is labelled 'Rate of photosynthesis', the x axis is labelled 'Light intensity'.Graph showing rate of photosynthesis and light intensity

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.

Diagram of an experiment investigating the role of light in photosynthesis. A lamp is pointed at a beaker containing water and Elodea. A ruler measures the distance between the lamp and the beaker.A light source positioned at a distance from a photosynthesising plant

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.

Calculating light intensity

Light intensity can be calculated using this formula.

{\text{Light intensity}}\propto\frac{1}{{\text{Distance}}^{2}}

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

\frac{1}{{0.2}{\text{ m}}^{2}} = 25 arbitrary units

But when the light is 40 cm from the plant, it will only receive

\frac{1}{{0.4}{\text{ m}}^{2}} = 6.25 arbitrary units

curriculum-key-fact
The SI unit for light intensity is lux. However, different light sources provide different initial values of lux, so the term ‘arbitrary units’ has been used in this example.

Carbon dioxide

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.

Graph showing how the rate of photosynthesis is affected by carbon dioxide concentration. The y axis is labelled 'Rate of photosynthesis', the x axis is labelled 'Carbon dioxide concentration'.Graph showing rate of photosynthesis and carbon dioxide concentration
Question

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.