Factors affecting the rate of photosynthesis

In a nutshell

There are many factors that impact the rate of photosynthesis. These are called limiting factors and the main three are: light intensity, carbon dioxide concentration and temperature. If any of these factors is not at their optimum level, the rate of photosynthesis will be impacted, regardless of the level of the other factors.

Law of limiting factors

Definition

At any given moment, the rate of a photosynthesis is limited by the factor that is nearest to its minimum value.

Light intensity

Light is needed for the light-dependent reaction of photosynthesis. Therefore, the greater the light intensity, the more energy the plant receives and the quicker the light-dependent stage will occur. This will produce ATP at a faster rate which will be used in the light-independent stage of photosynthesis. This means that the rate of the light-independent stage will still be impacted by the light intensity.

Note: Chlorophyll a, chlorophyll b and carotene only absorb light in the red and blue wavelengths of the visible light spectrum.

As the light intensity increases, so does the rate of photosynthesis (1.). However, there will be a point where there is another factor limiting the rate of photosynthesis (2.) and the rate will plateau (3.). This factor could be the temperature or the carbon dioxide concentration.

Biology; Energy for biological processes; KS5 Year 12; Factors affecting the rate of photosynthesis

Carbon dioxide concentration

Carbon dioxide is a reactant of photosynthesis that is required for the Calvin cycle as it combines with ribulose bisphosphate (RuBP). Higher $CO_2$ concentrations will mean this carbon fixation step will occur quicker, which will increase the rate of the rest of the Calvin cycle.

Therefore, as the carbon dioxide concentration increases, so does the rate of photosynthesis (1.). However, there will be a point where there is another factor limiting the rate of photosynthesis and the rate will plateau (2.). This factor could be the temperature or the light intensity.

Biology; Energy for biological processes; KS5 Year 12; Factors affecting the rate of photosynthesis

Temperature

Photosynthesis is controlled by enzymes therefore increasing the temperature will increase the rate of photosynthesis. However, enzymes denature at temperatures higher than their optimum so increasing the temperature too much will cause a decrease in the rate of photosynthesis (1.). The temperature will impact the light-independent reaction more than the light-dependent reaction as the latter is largely controlled by light energy as opposed to enzymes.

The Calvin cycle is regulated by enzymes.

Example

Carbon fixation is catalysed by ribulose bisphosphate carboxylase (rubisco).

Biology; Energy for biological processes; KS5 Year 12; Factors affecting the rate of photosynthesis

Another way that temperature can impact the rate of photosynthesis is through the impact on the stomata. Increased temperatures lead to increased transpiration, therefore, plants may close their stomata to reduce the potential water loss. However, this will stop carbon dioxide entering the leaves and therefore the rate of photosynthesis will decrease.

Overcoming the limiting factors

Glasshouses (or greenhouses) can be used to create the optimal environment for plants to grow in. By controlling the limiting factors, plant growth and crop yield can be maximised. Some features of glasshouses are explained below.

Feature	Explanation
Heaters	Glasshouses trap heat which ensures that temperature is not a limiting factor. At colder times, a heater could be put in the glasshouse and the temperature can be regulated easily. *Note:* Airflow systems could be used in conjunction with heaters to ensure even heat distribution around the glasshouse.
Lights	Glasshouses can also have lights which can maximise the amount of light that plants receive for photosynthesis. During the winter months when the days are shorter, lights can be turned on to artificially increase the day length.
Paraffin heaters	Paraffin heaters can be used in glasshouses to both increase the temperature and carbon dioxide levels. These heaters burn paraffin to generate heat. Carbon dioxide is a by-product.