CIE IGCSE Biology

Revision Notes

6.3 Limiting Factors

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What is a Limiting Factor?

  • 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, IGCSE & GCSE Biology revision notesThe 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, IGCSE & GCSE Biology revision notesThe 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, IGCSE & GCSE Biology revision notesThe 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

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.

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Changing Glasshouse Condition

  • The knowledge about limiting factors and how they affect the rate of photosynthesis can be used to help control factors in glass houses to ensure maximum crop yields for farmers
  • Growing crops outside does not allow farmers to control any of these factors to increase growth of plants
  • In a glass house, several conditions can be manipulated to increase the rate of photosynthesis, including:
    • artificial heating (enzymes controlling photosynthesis can work faster at slightly higher temperatures – only used in temperature countries such as the UK)
    • artificial lighting (plants can photosynthesise for longer)
    • increasing carbon dioxide content of the air inside (plants can photosynthesise quicker)
    • regular watering
  • When considering the use of glasshouses and manipulating conditions like this, farmers need to balance the extra cost of providing heating, lighting and carbon dioxide against the increased income
  • In tropical countries where temperatures are much hotter, glasshouses may still be used to control other conditions however they may need to be ventilated to release hot air and avoid temperatures rising too high, which could cause the denaturation of the enzymes controlling the photosynthesis reaction

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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