Limiting Factors (Edexcel GCSE Biology: Combined Science)

• Plants do not have unlimited supplies of their raw materials so their rate of photosynthesis is limited by whatever factor is the lowest at that time
• So a limiting factor can be defined as something present in the environment in such short supply that it restricts life processes
• There are three main factors that limit the rate of photosynthesis:
  • Temperature
  • Light intensity
  • Carbon dioxide concentration

• Although water is necessary for photosynthesis, it is not considered a limiting factor as the amount needed is relatively small compared to the amount of water transpired from a plant so there is hardly ever a situation where there is not enough water for photosynthesis
• The number of chloroplasts or the amount of chlorophyll in the chloroplasts can also affect the rate of photosynthesis

Temperature

• The temperature of the environment affects how much kinetic energy all particles have – so temperature affects the speed at which carbon dioxide and water move through a plant
• By affecting how much kinetic energy particles have, temperature also determines enzyme activity
  • The lower the temperature, the less kinetic energy particles have, resulting in fewer successful collisions occurring between reactants and enzymes over a period of time
  • Increasing temperature increases the kinetic energy of particles, increasing the likelihood of successful collisions between reactants and enzymes, which leads to increased formation of products
  • At higher temperatures, however, enzymes that control the processes of photosynthesis can be denatured (where the active site changes shape and is no longer complementary to its substrate) – this reduces the overall rate of photosynthesis

The effect of temperature on the rate of photosynthesis

Light intensity

• The intensity of the light available to the plant affects the rate of photosynthesis
• The more light a plant receives, the faster the rate of photosynthesis
• This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply

Graph showing the effect of light intensity on the rate of photosynthesis. At low light intensities, increasing the intensity will initially increase the rate of photosynthesis. At a certain point, increasing the light intensity stops increasing the rate. The rate becomes constant regardless of how much light intensity increases as something else is limiting the rate.

Carbon dioxide concentration

• Carbon dioxide is one of the raw materials required for photosynthesis
• This means the more carbon dioxide that is present, the faster the reaction can occur
• This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply

A graph showing the effect of the concentration of carbon dioxide on the rate of photosynthesis

Chlorophyll

• The number of chloroplasts (as they contain the pigment chlorophyll which absorbs light energy for photosynthesis) will affect the rate of photosynthesis
• The more chloroplasts a plant has, the faster the rate of photosynthesis
• The number of chloroplasts (or amount of chlorophyll they contain) can be affected by:
  • Diseases (such as tobacco mosaic virus)
  • Lack of nutrients (such as magnesium)
  • Loss of leaves (fewer leaves means fewer chloroplasts)