Limiting Factors: Extended (CIE IGCSE Biology)

• If a plant is given unlimited sunlight, carbon dioxide and water and is at a warm temperature, the limit on the rate (speed) at which it can photosynthesise is its own ability to absorb these materials and make them react
• However, most often 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 which 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

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

Light intensity

• 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

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
• The factors which could be limiting the rate when the line on the graph is horizontal include temperature not being high enough or not enough carbon dioxide. 

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

The effect of carbon dioxide concentration on the rate of photosynthesis

• The factors which could be limiting the rate when the line on the graph is horizontal include temperature not being high enough or not enough light