13.2.1 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

• 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

The effect of light intensity on the rate of photosynthesis

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

The effect of carbon dioxide concentration on the rate of photosynthesis

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

The effect of temperature on the rate of photosynthesis

• 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)