Seneca
Sign up

Limiting Factors

Biology: AQA A Level, topic 5.1.7

The Rate of Photosynthesis

The four key factors affecting the rate of photosynthesis are:

Temperature

Temperature

  • Initially, the rate of photosynthesis increases as the temperature increase. Above about 45°C, the rate starts to fall.
  • Enzymes such as rubisco play an important role in photosynthesis.
  • At low temperatures, enzymes have less kinetic energy and are less likely to successfully collide with the substrate. This means fewer enzyme-substrate complexes are formed.
  • Above the optimum temperature, enzymes will denature and the rate of photosynthesis will fall.
Light intensity

Light intensity

  • Increasing light intensity increases the rate of photosynthesis because more energy is provided.
  • In low light intensities the light-dependent reaction cannot take place. This causes levels of reduced NADP and ATP to fall.
  • Reduced NADP and ATP are necessary to convert GP to TP and to regenerate RuBP.
  • Without reduced NADP and ATP the Calvin cycle cannot take place and rate of photosynthesis declines.
Carbon dioxide (CO2) concentration

Carbon dioxide (CO2) concentration

  • Increasing the CO2 concentration increases the rate of photosynthesis.
  • At high CO2 concentrations, there is more carbon available for fixation in the Calvin cycle.
  • If the rate of the Calvin cycle increases, more GP and TP is produced and more RuBP is regenerated.
  • Very high CO2 concentrations can cause stomata to open wider causing more water to be lost.
  • Too much water loss will then cause the stomata to close and the rate of photosynthesis to slow again.
Chlorophyll concentration

Chlorophyll concentration

  • High chlorophyll concentration gives a high rate of photosynthesis.
  • If there is more chlorophyll, light can be absorbed at a faster rate. This will cause the light-dependent reaction rate to increase.

Agricultural Practices

Commercial growers need to understand limiting factors of photosynthesis to try and maximise yields. This is done by preventing low light intensity, low temperatures and low CO2 levels.

Light intensity

Light intensity

  • A high light intensity is optimum for maximising yield.
  • This is achieved by providing lamps and a lit environment at night time.
Temperature

Temperature

  • The optimum temperature for plant enzymes is approximately 25oC.
  • A temperature below 10oC or above 40oC would decrease the rate of photosynthesis and reduce yields.
  • In order to achieve an optimum temperature, glasshouses are used to trap heat energy from sunlight.
  • Heating and cooling systems can also be used.
Carbon dioxide

Carbon dioxide

  • The atmospheric concentration of CO2 is 0.04% but increasing this to 0.4% increases rate of photosynthesis.
  • This can be done by adding CO2 into the air.

Test yourself on Limiting Factors

  • What are the key factors affecting the rate of photosynthesis?

  • What effect does increasing light intensity have on rate of photosynthesis?

  • True or false - increasing CO2 levels increases rate of photosynthesis.

  • What three factors can farmers control to maximise yields?

  • What is the approximate optimum temperature range to maximise plant yields?

Practise this topic for free