5.1.7

Limiting Factors

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The Rate of Photosynthesis

The four key factors affecting the rate of photosynthesis are:

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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.
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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.
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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.
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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.

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Light intensity

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

Jump to other topics

1Biological Molecules

2Cells

3Substance Exchange

4Genetic Information & Variation

5Energy Transfers (A2 only)

6Responding to Change (A2 only)

7Genetics & Ecosystems (A2 only)

8The Control of Gene Expression (A2 only)

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