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Oxygen Dissociation

Biology: Cambridge International A Level, topic 8.2.2

Dissociation Curves

The relationship between the percentage saturation of haemoglobin and oxygen partial pressure of the surrounding tissues can be shown in a dissociation curve.

Low pO2

Low pO2

  • When partial pressure is low, haemoglobin has a low affinity for oxygen.
  • The percentage saturation of haemoglobin is low because oxygen dissociates from the haemoglobin.
Increasing pO2

Increasing pO2

  • As pO2 increases, affinity of haemoglobin for oxygen increases slightly.
  • When the first molecule of O2 binds to haemoglobin, the protein undergoes a conformational change.
  • The change in shape allows the other O2 molecules to bind to haemoglobin more easily.
  • The percentage saturation of haemoglobin increases quickly.
Plateau in percentage saturation

Plateau in percentage saturation

  • As more molecules of O2 bind to haemoglobin, it becomes more difficult for more O2 molecules to bind.
  • The percentage saturation of haemoglobin begins to plateau.
S-shaped curve

S-shaped curve

  • The increasing affinity of haemoglobin with increasing pO2 in this way creates an S-shaped curve.
  • The S-shaped curve is called the dissociation curve.
Higher altitudes

Higher altitudes

  • The partial pressure of oxygen is lower at higher altitudes. This makes it difficult to saturate haemaglobin with oxygen. It can be difficult to deliver enough oxygen to tissues.
  • To compensate for this, people who live at higher altitudes have a higher red blood cell count. This increases the amount of haemoglobin in blood.
  • The hormone erythropoietin stimulates red blood cell production at low partial pressures of oxygen. (You do not need to remember the name of this hormone).
  • This process is called acclimation.

The Bohr Effect

The partial pressure of carbon dioxide (pCO2) also influences the affinity of haemoglobin for oxygen. This is called the Bohr effect.

High pCO2

High pCO2

  • Respiring cells use oxygen in respiration and produce carbon dioxide.
  • The respiring cells have low pO2 and high pCO2.
  • When pCO2 is high, the rate of oxygen dissociation increases.
Bohr effect

Bohr effect

  • The increased dissociation of oxygen causes a shift in the oxyhaemoglobin dissociation curve to the right.
  • The shift in the dissociation curve means that oxygen will dissociate from haemoglobin at a lower pO2 than normal.
  • This is called the Bohr effect.

Test yourself on Oxygen Dissociation

  • Which of the following statements about the dissociation curve are true?

  • What is the impact on haemoglobin when the first molecule of oxygen binds?

  • What is the shape of the dissociation curve?

  • Which of the following statements about the Bohr effect are true?

  • How does a high pCO2 impact haemoglobin affinity for oxygen?

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