Factors affecting photosynthesis

There are several ways of measuring the rate of photosynthesis in the lab. These include:

These are not perfect methods as the plant will also be respiring, which will use up some oxygen and carbohydrate and increase carbon dioxide output.

Several factors can affect the rate of photosynthesis:

  • light intensity
  • carbon dioxide concentration
  • temperature

The amount of chlorophyll also affects the rate of photosynthesis:

  • plants in lighting conditions unfavourable for photosynthesis may synthesise more chlorophyll, to absorb the light required
  • the effects of some plant diseases affect the amount of chlorophyll, and therefore the ability of a plant to photosynthesise

Light intensity

Without enough light, a plant cannot photosynthesise very quickly - even if there is plenty of water and carbon dioxide and a suitable temperature.

Increasing the light intensity increases the rate of photosynthesis , until some other factor - a limiting factor - becomes in short supply.

Graph showing how increasing the light intensity increases the rate of photosynthesis

At low light intensities the increase in the rate of photosynthesis is linear

At very high light intensities, photosynthesis is slowed and then inhibited, but these light intensities do not occur in nature.

Carbon dioxide concentration

Carbon dioxide - with water - is one of the reactants in photosynthesis.

If the concentration of carbon dioxide is increased, the rate of photosynthesis will therefore increase.

Again, at some point, a factor may become limiting.

Graph showing that if the concentration of carbon dioxide is increased, the rate of photosynthesis will therefore increase

Temperature

The chemical reactions that combine carbon dioxide and water to produce glucose are controlled by enzymes. As with any other enzyme-controlled reaction, the rate of photosynthesis is affected by temperature.

At low temperatures, the rate of photosynthesis is limited by the number of molecular collisions between enzymes and substrates. At high temperatures, enzymes are denatured.

A graph with rate of photosynthesis on the y axis and carbon dioxide concentration on the x axis.  The plotted line rise steeply and then levels off to horizonal.  During the steep part carobn dioxide is the limiting factor.  During the horizontal part another factor has become limiting.

Chlorophyll

Chlorophyll absorbs the light energy required to convert carbon dioxide and water into glucose.

Chlorophyll is green - so absorbs the red and blue parts of the electromagnetic spectrum and reflects the green part of the spectrum.

A diagram showing the cycle of Chlorophyll.

Leaves with more chlorophyll are better able to absorb the light required for photosynthesis.

The compensation point

These graphs have been plotted with rate of photosynthesis against the factor under investigation.

If oxygen production or carbon dioxide uptake is used as a measure of photosynthetic rate, the graphs are slightly different. The line does not go through the origin. This is because oxygen production and carbon dioxide uptake are affected by respiration as well as photosynthesis. Respiration uses oxygen and produces carbon dioxide. Below is a graph measuring the rate of photosynthesis plotted on carbon dioxide against light intensity:

A graph showing the carbon dioxide uptake.

This graph shows that with no light, the carbon dioxide uptake is below zero (point A on the graph). With no light, there is no photosynthesis. So the rate of respiration is greater than the rate of photosynthesis. So this means there is an overall excess of carbon dioxide produced during respiration. An excess is the same as a negative uptake.

As the light intensity increases the rate of photosynthesis follows (point B on the graph). This means more carbon dioxide is being absorbed by the plant for photosynthesis. So this means more carbon dioxide uptake occurs. The graph shows this increase in carbon dioxide uptake.

There will come a point when the rate of photosynthesis has increased to match the rate of respiration. This is called the compensation point (point C on the graph). This is shown on the graph as the point the line crosses zero on the x-axis.

Beyond the compensation point, further increases in light intensity cause an increase in photosynthesis. This means the rate of photosynthesis increases beyond the rate of respiration (point D). Now more carbon dioxide is being taken in by the plant for photosynthesis than is produced by the plant during respiration. So the carbon dioxide uptake increases.

After a specific point, further increases in light intensity give ever smaller increases in photosynthesis. This results in the rate of carbon dioxide uptake slowing (point E on the graph).

A similar graph will be obtained if oxygen production is plotted against light intensity.