CIE A Level Biology (9700) 2019-2021

Revision Notes

8.1.8 The Bohr Shift

The Bohr Shift

  • Changes in the oxygen dissociation curve as a result of carbon dioxide levels are known as the Bohr shift or Bohr effect
  • The Bohr effect explains how the ability of haemoglobin to bind to, and release its oxygen changes
  • When the partial pressure of carbon dioxide is high, in respiring tissues for example, haemoglobin’s affinity for oxygen is reduced
  • This is a helpful change, because it means that haemoglobin gives up its oxygen much more readily
  • This occurs because CO2 lowers the pH of the blood (by forming carbonic acid), which causes haemoglobin to release its oxygen
  • Carbon dioxide levels in the lungs are comparatively very low, haemoglobin’s affinity for oxygen is increased, which makes it easier for oxygen to bind to haemoglobin
  • The higher the partial pressure of oxygen, the greater the saturation of haemoglobin with oxygen, and vice versa
  • The more carbon dioxide there is, the line on the graph shifts to the right (the lower of the two lines)
    This shows that more dissociation has occurred, as the percentage saturation of oxygen is lower
  • This is an important change, as it means that where there is a lot of carbon dioxide, such as at respiring tissues, haemoglobin gives up its oxygen to the nearby tissues

Exam Tip

Take care to distinguish between questions based on adult haemoglobin and foetal haemoglobin, as their oxygen dissociation graphs are different. This is because foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin.

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