Factors Affecting the Rate of Photosynthesis — AQA GCSE Biology
Several environmental factors control how fast photosynthesis happens. The slowest one acts as the limiting factor.
The three main factors
- Light intensity — more light gives a faster rate (until another factor limits it).
- Carbon dioxide concentration — more CO₂ gives a faster rate.
- Temperature — increases the rate up to an optimum; too hot and the enzymes denature, so the rate falls.
(The amount of chlorophyll also affects the rate and can be reduced by disease or lack of minerals.)
Limiting factors
A limiting factor is the factor in shortest supply that holds back the rate. On a graph, the rate rises as a factor increases, then levels off when a different factor becomes limiting.
Light intensity and the inverse square law
Light intensity is inversely proportional to the square of the distance from the source: $$\text{light intensity} \propto \frac{1}{\text{distance}^2}$$ So doubling the distance reduces the light intensity to one quarter.
Commercial use
Greenhouse growers control light, temperature and CO₂ to maximise the rate and increase yield, balancing the cost of doing so against the extra profit.
Exam tips
- Learn the three main limiting factors.
- On a graph, explain which factor is limiting at each part of the curve.
- Apply the inverse square law to light intensity and distance.
- Link greenhouse conditions to increasing yield as an economic decision.