Many factors can affect the rate of photosynthesis. It is a very complex reaction with lots of different processes involved. Therefore a slight change in one factor can have an adverse effect on the whole process. Limiting factors limit the rate at which photosynthesis can take place. Even if there is a large supply of all the other factors, if there is not enough of one of them, then the whole reaction is limited.
Light is a limiting factor. It is needed directly for the light dependent stage of photosynthesis. When there is no light photosynthesis cannot take place because the light dependent reaction requires it. When there is no light, photosynthesis cannot take place, even if the carbon dioxide concentration and the temperature are at their optimum temperature. The light limits the whole reaction. No matter how much you increase the carbon dioxide concentration and the temperature, photosynthesis will still not occur.
As more light is provided, with a good supply of the other factors, then the rate of photosynthesis increases. Now when the level of light is low, then the only factor limiting photosynthesis is still the light. This means that any increase in the light intensity will cause an increase in the rate of photosynthesis. If this was depicted on a graph, then you would see a slope with a positive gradient.
Now the light could continue to be increased, and this would continue the graph with a positive slope. However at some point another factor, such as the temperature or the light would begin to limit the reaction. Therefore the graph would begin to slope off, as even with a further increase in light, the rate of photosynthesis cannot be increase.
There are two ways of measuring the rate of photosynthesis. One is to measure the concentration of oxygen released and the other is to measure the volume of carbon dioxide taken up. When photosynthesis occurs, the oxygen is taken up and the carbon dioxide is released. During aerobic respiration the oxygen is taken up and the carbon dioxide is released.
As the light intensity is increased the carbon dioxide taken up/released and the oxygen taken up/released for each different process will balance out. When this happens it is called the light compensation point. After this point the increasing light intensity continues to increase the rate of photosynthesis. It will then increase greatly until other factors begin to limit the rate – usually the lack of carbon dioxide.
Now it is not just light which can limit the rate of photosynthesis. The concentration of carbon dioxide also has a great effect, which you may have already worked out. There is only a very low level of carbon dioxide in the atmosphere. It has been increasing in recent years due to the overuse of greenhouse gases, but the level is still less than 0.1%. Plants actually need a concentration of about 0.1% carbon dioxide in order to efficiently and effectively photosynthesise at a good rate.
This is why farmers often increase the level of carbon dioxide in greenhouses, to supply an artificially high concentration of carbon dioxide. This helps to prevent the carbon dioxide concentration from becoming a limiting factor. Hence more crops can be grown, and so more profit can be made.
The reason why carbon dioxide is a limiting factor, is because if there is not enough, then the light independent reaction of photosynthesis will not be able to take place as quickly. This will mean that less ribulose biphosphate will be made, which means that the whole reaction will be slowed down. The lack of carbon dioxide will cause a lack of products being made from the light independent stage to be reduced.
The final limiting factor is temperature. As the temperature increases from very low, the rate of photosynthesis increases proportionally. This means that there is a linear relationship at the start, just like all of the other limiting factors.
As the temperature creeps above a certain temperature, it begins to level off, and then the rate of photosynthesis suddenly begins to drop. Now the temperature at which the rate of photosynthesis is the highest is called the optimum temperature. Above this optimum temperature the enzymes and proteins begin to denature. This is a particular issue in photosynthesis, considering the reduction of carbon dioxide, at the very start of the light independent stage is enzyme controlled. When the enzymes are denatured, it means that they can no longer carry out their function. This happens because the structure changes when the temperature gets too high, and this permanently damages the enzymes.
At first only some of the enzymes will be damaged, and the damage may not be severe enough as to stop them from working. As the temperature is increased further the enzymes become more damaged, and so they begin to stop working altogether. If these high temperatures are maintained or increased further, then eventually no photosynthesis will take place at all.
Toole Glenn, Toole Susan “Factors affecting photosynthesis” <em>AQA A2 Biology</em> Chapter 3.4 Pages 41-43 ISBN 9780748798131