Factors affecting rates of Photosynthesis (part 2): Grade 9 Understanding for IGCSE Biology 2.20 2.23

In the previous post on photosynthesis, you revised how there were four environmental factors that can affect rates of photosynthesis in a plant:

  • light intensity
  • light wavelength
  • temperature
  • carbon dioxide concentration

This post will explain the results from experiments with Elodea in which one factor is altered (the independent variable) and the other three are kept exactly the same (control variables)

Light intensity


The independent variable (light intensity) is on the x axis and the dependent variable (number of bubbles per minute) is on the  y axis.

How do we explain the pattern in this graph?

As the light intensity increases the rate of photosynthesis increases.  This is because a higher light intensity gives more energy to the chloroplasts and so more reactions can happen per second and the rate goes up.  But beyond the orange dot on the graph, the increases in rate slows down until at around 12 units of light, adding more light has no effect on the rate.  At these high light intensities some other factor is now the limiting factor as opposed to light intensity.  The limiting factor remember is the factor in the shortest supply.   So perhaps above 12 units of light photosynthesis is limited by the concentration of carbon dioxide.  The only way to find the limiting factor is to repeat the experiment with more carbon dioxide and see whether the rate is higher above 12 units.

Light wavelength

Although this graph is not perfect, it does show how the rate of photosynthesis varies at different light wavelength.

Spectrum of visible light and rates of photosynthesis

Rates of photosynthesis peak in the blue-violet and red parts of the visible spectrum with a much lower rate in green light.  The reason for this is that chlorophyll pigments do not absorb green light well.

Carbon Dioxide concentration


The pattern is similar to the light intensity relationship.  When carbon dioxide concentrations are low, it is the limiting factor for photosynthesis and so increasing the concentration will increase the rate.  As the graph levels off, some other factor is now the limiting factor – perhaps light intensity or temperature.


Temperature is a factor that affects photosynthesis because of enzymes.  Many reactions in photosynthesis are catalysed by enzymes and enzymes all have an optimum temperature.


This pattern is not explained by limiting factors.  At low temperatures the rate is low because the enzymes and the substrate molecules are moving really slowly.  This means there are few collisions between the substrate and the active site of the enzyme.  As temperature increases, the rate increases as there are more collisions and more enzyme-substrate complexes are formed per second.  But high temperatures denature enzymes:  the bonds that hold the enzyme in its precious 3-D shape are broken and the enzyme molecule unravels.  So the active site may either change shape or may be lost as a catalyst.  This slows the rate down to an extremely low rate.

One comment

  1. Pingback: Glasshouses and Polytunnels: A* understanding for iGCSE Biology 5.1 5.2 | PMG Biology

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