We now know that plants photosynthesise using carbon dioxide and water as the raw materials in the presence of light and chlorophyll, to produce glucose and oxygen, but photosynthesis is a dependent process and many factors can influence the rate at which it takes place. 


If you look at the equation below, which you have seen previously, you may be able to work out for yourself what "factors" can influence the rate at which photosynthesis takes place:


Two of the factors which can limit the rate of photosynthesis is the availability of carbon dioxide and the availability of light.


Additionally, the availability of chlorophyll can affect the rate. Some plants which have become diseased may have damage to the chloroplasts which will restrict the amount of chlorophyll that is actually produced. 


The fourth which is not shown is the temperature.


So how do these factors actually affect things? Well, let's take for example light. 


  • Light - Plants do not photosynthesise in the dark because light is necessary for the reaction to take place. You may find in woodland that certain plants (for example nettles) that live in the shadow of other plants or trees may look quite pale and unwell, the leaves may not appear to be a green as they should be and they just generally don't look healthy. The larger plants are in fact shielding the smaller plants from sunlight which is to their detriment.

  • Carbon dioxide levels - as this is one of the raw materials, if it suddenly becomes scarce then this will of course affect the rate at which photosynthesis can take place. In an experiment to observe the rate of photosynthesis you will find that your sample will be placed into a solution of sodium hydrogen carbonate, this is done so that carbon dioxide availability is quite high and therefore does not become a "limiting factor" in the experiment.

  • Temperature - photosynthesis in plants, and chemical reactions in living things in general require the involvement of enzymes. Enzymes increase the rate of photosynthesis in plants but they are very temperature critical. If the temperature is too low, the enzyme will not function particularly well, and to high a temperature can destroy it (the enzymes become "denatured", we will come to this in another section).