As we have learnt in Lesson 7, plants need water, carbon dioxide, light and chlorophyll to photosynthesise, but changing the amount of light, carbon dioxide and temperature can affect the rate at which photosynthesis happens.
For most plants, light intensity can affect the rate of photosynthesis. The brighter the light, the faster the rate of photosynthesis will be, up to a point where light will no longer be a limiting factor. A limiting factor is a factor that is limiting the rate of photosynthesis and in this case it could be temperature or carbon dioxide concentration.
Experiment to show change in rate of photosynthesis with change in light intensity
When the light bulb is moved closer to the plant and the light intensity increased, the number of oxygen bubbles produced by photosynthesis also increases. If the number of oxygen bubbles increases, this indicates that the rate of photosynthesis has increased.
Below are the results of this experiment plotted in a graph. You may notice that the graph flattens or plateaus at the end, this means that no matter how much more you increase the light intensity, it will no longer change the photosynthesis rate. This suggests that carbon dioxide concentration or temperature is a limiting factor.
Another factor that can affect photosynthesis rate is carbon dioxide concentration. Air contains only 0.04% of carbon dioxide and increasing this level can increase the rate of photosynthesis. Sometimes, commercial greenhouses artificially increase their carbon dioxide levels in order to grow larger plants and crops.
The graph shows that with increased carbon dioxide concentration, the rate of photosynthesis increases rapidly and then stays the same, or plateaus at the end. This is because the plant has plenty of carbon dioxide for photosynthesis, and increasing the concentration will make no difference to the photosynthesis rate. Temperature or light may now be a limiting factor, and increasing these will increase the rate of photosynthesis.
Reactions and processes in living things are controlled by enzymes and as we learnt in lesson 5, increasing temperature too high can cause an enzyme’s shape to be damaged which we call denaturing. This enzyme damage will stop the enzyme from working and can slow or stop a reaction altogether. The reaction in this case is photosynthesis.
As you can see from the graph, when we increase the temperature, the rate of photosynthesis increases up to an optimum point when photosynthesis rate is at its maximum. The rate of photosynthesis falls rapidly after the optimum as enzymes are damaged by the heat.
You need to learn to interpret graphs like the ones above.
Below is a graph showing how different conditions affect the rate of photosynthesis for tomato plants being grown in a greenhouse.
- Look at the graph carefully and describe the trends it shows.
- Conclude the best results for growing tomato plants in a greenhouse.
- The graph shows that with increasing carbon dioxide concentration, the rate of photosynthesis increases up to a point where the graph flattens. At this point, increasing the carbon dioxide concentration any further makes no difference to the photosynthesis rate. Temperature affects photosynthesis rate. When the temperature is 10°C the photosynthesis rate is less than if the temperature was to be raised to 25°C.
- Tomato plants grow at an optimum carbon dioxide level, indicated at the point where the graph starts to flatten with a temperature of 25°C.