Limiting Factors of Photosynthesis (AQA A Level Biology): Revision Note

Factors that limit the rate of photosynthesis

• In order to carry out photosynthesis at the highest possible rate, plants need a constant supply of reactants, as well as suitable environmental conditions

• If conditions are not ideal then the rate of photosynthesis will be limited

• The main environmental factors that can limit the rate of photosynthesis are:

  • light intensity

  • carbon dioxide concentration

  • temperature

• These are known as limiting factors of photosynthesis

• If any one of these factors is below the optimum level for the plant, the rate of photosynthesis will be reduced, even if the other two factors are at their optimum

Light intensity

• The rate of photosynthesis increases as light intensity increases:

  • At high light intensity more electrons undergo photoionisation and more ATP NADPH are produced

  • The Calvin cycle can progress at a faster rate and more organic compounds can be produced

• If light intensity continues to increase, the rate of photosynthesis will eventually level off, or reach a plateau

  • At this point light intensity is no longer a limiting factor, and another factor is limiting the rate of photosynthesis, e.g. temperature or carbon dioxide concentration

Carbon dioxide concentration

• The rate of photosynthesis increases as carbon dioxide concentration increases:

  • When more CO₂ is present, more can combine with RuBP during carbon fixation

  • More GP will be produced, which can be converted into triose phosphate and then into organic compounds

• This trend will continue until another factor becomes limiting

Temperature

• The rate of photosynthesis increases as temperature increases

  • Photosynthesis is an enzyme-controlled reaction, so an increase in kinetic energy will increase reaction rate

  • Examples of enzymes in photosynthesis include:

    • rubisco

    • ATP synthase

• This trend continues up to a certain temperature, beyond which enzymes begin to denature and the rate of reaction decreases

• For most metabolic reactions, temperature has a large effect on reaction rate

• For photosynthesis, temperature has no significant effect on the light-dependent reactions, as these are driven by energy from light rather than the kinetic energy of the reacting molecules

• Temperature can also impact rate of photosynthesis for other reasons, e.g.:

  • increasing temperature causes stomata on the leaf to close in order to reduce water loss; this reduced gas exchange and so influence availability of CO₂

  • the light-dependent reaction relies on a proton gradient forming across the thylakoid membrane; changes in temperature may affect the permeability of the thylakoid membrane

Agricultural practices and limiting factors

• An understanding of the effect of limiting factors on photosynthesis can be used to increase crop yields in controlled environments, such as glasshouses

  • In the most sophisticated glasshouses sensors can be used to monitor light intensity, humidity and carbon dioxide concentration

  • All these factors can be managed by a computer and their levels adjusted to ensure the crop can photosynthesis at the highest rate

• Farmers have to find a balance between crop yield and the cost of maintaining 24-hour lighting and year-round heating