How does light intensity affect the rate of photosynthesis

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

• the rate of oxygen output
• the rate of carbon dioxide uptake
• the rate of carbohydrate (glucose) production

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 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, such as downy mildew in grape vines

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.

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 different factor may become limiting. Beyond this concentration, further increases in the concentration of carbon dioxide will not result in a faster rate of photosynthesis, and would appear on a graph as a horizontal line.

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.

Chlorophyll

Chlorophyll absorbs the light 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.

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. For instance, if a graph is plotted of carbon dioxide uptake against light intensity:

The compensation point is the light intensity at which the rate of photosynthesis is equal to the rate of respiration.

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