Limiting Factors

There are a number of factors that can limit how much photosynthesis can take place in a cell. The rate of photosynthesis is affected by the

  • Intensity of light
  • Concentration of carbon dioxide
  • Temperature

If any one of these three factors is in short supply, the rate of photosynthesis will decrease or plateau. Photosynthesis will not be able to occur any faster. At any point, any one of these factors can limit the rate of photosynthesis. At night, light is the limiting factor, in winter, temperature will slow down the rate of photosynthesis, during the summer carbon dioxide levels are likely the cause of slowed photosynthesis.

Chlorophyll levels can also limit the rate of photosynthesis if there is not enough magnesium in the soil to allow for the creation of chlorophyll. Also some plant diseases can affect the rate of photosynthesis by causing chloroplasts to become damaged.

You are expected to be able to interpret data when presented to you on a graph.

We can see that as light intensity increases the rate of photosynthesis also increases. However midway through at the point highlighted by the red arrow, the rate of photosynthesis plateaus, it levels off. We know that light is only one of the limiting factors for rate of photosynthesis so there could be too little carbon dioxide to supply a high rate of photosynthesis or the temperature could be too cold.

On higher tier papers, they may ask you to Interpret a graph showing two or three factors and determine from these what the limiting factor is. For example, this graph shows rate of photosynthesis against light intensity at different carbon dioxide concentrations.

Both lines level off when light is no longer a limiting factor. The line at the higher carbon dioxide concentration of 0.5% levels off at a higher point then the one for 0.05%. This means that carbon dioxide concentration must have been the limiting factor since the temperature and light intensity was the same for both.

In this graph we will be looking at how rate of photosynthesis is affected by light intensity and temperature.

Here we can see both lines increase as light intensity increases but both lines level off when light is no longer a limiting factor. The line at 25 degrees levels off at a higher point than the line for 15 degrees Celsius.

This tells us that the limiting factor here was temperature because when the temperature increased, so too did the rate of photosynthesis. Increasing temperature too much however will not increase the rate of photosynthesis. If the temperature is too high the enzymes responsible for photosynthesis will denature and photosynthesis will stop.