Effects of light intensity and chlorophyll on photosynthesis

Factors affecting photosynthesis

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

  • the rate of oxygen output
  • the rate of carbon dioxide uptake
  • the rate of carbohydrate 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

Light intensity

Without enough light, a plant cannot photosynthesise very quickly - even if there is plenty of water, 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

Carbon dioxide concentration

Carbon dioxide 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 low number of molecular collisions between enzymes and substrates. At high temperatures, enzymes are denatured and so the reaction slows and stops.

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.