CIE A Level Biology (9700) 2019-2021

Revision Notes

13.2.1 Limiting Factors of Photosynthesis

Limiting Factors of Photosynthesis

  • Plants need several factors for photosynthesis to occur:
    • the presence of photosynthetic pigments
    • a supply of carbon dioxide
    • a supply of water
    • light energy
    • a suitable temperature
  • If there is a shortage of any of these factors, photosynthesis cannot occur at its maximum possible rate
  • The main external factors that affect 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, its rate of photosynthesis will be reduced, even if the other two factors are at the optimum level

Exam Tip

Light intensity, CO2 concentration and temperature are the three limiting factors of photosynthesis that you need to learn. Although a lack of water can reduce the rate of photosynthesis, water shortages usually affect other processes in the plant before affecting photosynthesis.

Limiting Factors of Photosynthesis: Effects

  • Changes in light intensity, carbon dioxide concentration and temperature are all limiting factors that affect the rate of photosynthesis:

Light intensity

  • When temperature and carbon dioxide concentration remain constant, changes in light intensity affect the rate of photosynthesis
  • The rate of photosynthesis increases as light intensity increases:
    • The greater the light intensity, the more energy supplied to the plant and therefore the faster the light-dependent stage of photosynthesis can occur
    • This produces more ATP and reduced NADP for the Calvin cycle (light-independent stage), which can then also occur at a greater rate
    • During this stage of the graph below, light intensity is said to be a limiting factor of photosynthesis
  • At some point, if light intensity continues to increase, the relationship above will no longer apply and the rate of photosynthesis will reach a plateau
  • At this point, light intensity is no longer a limiting factor of photosynthesis – another factor is limiting the rate of photosynthesis
  • The factors which could be limiting the rate when the line on the graph is horizontal include temperature being too low or too high, or not enough carbon dioxide

Carbon dioxide concentration

  • The rate of photosynthesis increases as carbon dioxide concentration increases:
    • Carbon dioxide is one of the raw materials required for photosynthesis
    • It is required for the light-independent stage of photosynthesis, when CO2 is combined with the five-carbon compound ribulose bisphosphate (RuBP)
    • This means the more carbon dioxide that is present, the faster this step of the Calvin cycle can occur and the faster the overall rate of photosynthesis
  • This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is in short supply
  • The factors which could be limiting the rate when the line on the graph is horizontal include temperature being too low or too high, or not enough light

Temperature

  • As temperature increases the rate of photosynthesis increases as the reaction is controlled by enzymes
  • However, as the reaction is controlled by enzymes, this trend only continues up to a certain temperature beyond which the 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
  • However, the Calvin cycle is affected by temperature, as the light-independent reactions are enzyme-controlled reactions (eg. rubisco catalyses the reaction between CO2 and the five-carbon compound ribulose bisphosphate)

Exam Tip

Interpreting graphs of limiting factors can be confusing for many students, but it’s quite simple.

In the section of the graph where the rate is increasing (the line is going up), the limiting factor is whatever the label on the x-axis (the bottom axis) of the graph is.

In the section of the graph where the rate is not increasing (the line is horizontal), the limiting factor will be something other than what is on the x-axis – choose from temperature, light intensity or carbon dioxide concentration.

Limiting Factors of Photosynthesis: Increasing Crop Yields

  • An understanding of limiting factors on the rate of photosynthesis can be used to increase crop yields in protected environments, such as glasshouses
  • In the most sophisticated glasshouses, for example, sensors can be used to monitor the light intensity, the humidity of the atmosphere and the carbon dioxide concentration around the crops
  • All these factors can be managed by a computer and their levels adjusted to ensure the crop can photosynthesis at the highest rate possible
  • This maximises the yield of the crop

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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