OCR AS Biology

Revision Notes

3.2.12 Adult & Fetal Haemoglobin

The Oxygen Dissociation Curve

  • The oxygen dissociation curve describes the relationship between the partial pressure of oxygen and the percentage saturation of haemoglobin

The Oxygen Dissociation Curve, downloadable AS & A Level Biology revision notes

The oxygen dissociation curve. The S-shaped curve is indicative of cooperative binding.

  • It’s often easier to understand the curve by starting in the top right-hand corner of the graph. This area shows what happens in the lungs
  • The partial pressure of oxygen is high and so haemoglobin picks up oxygen rapidly, forming oxyhaemoglobin
  • The bottom left-hand portion of the graph shows what happens next
  • As oxygen reaches where it’s needed in respiring tissues, the partial pressure of oxygen in the tissues is comparatively low
  • As a result, oxygen diffuses out into the body cells down a concentration gradient to the tissues

Explaining the Oxygen Dissociation Curve

  • A small change in the partial pressure of oxygen can have a very large impact on the percentage saturation of haemoglobin
  • This is because haemoglobin has such a high affinity for oxygen
  • The partial pressure of oxygen in the lungs is high, so haemoglobin picks up oxygen rapidly
  • In respiring tissues, the partial pressure of oxygen is low, so oxygen is dropped off rapidly
  • This ensures that oxygen is picked up where there’s plenty of it and delivered to where it is needed in respiring tissues

Foetal haemoglobin

  • The haemoglobin of a developing foetus has different properties to that of adult haemoglobin
    • It is more efficient at absorbing oxygen at lower concentrations
  • This is vital as it allows for a baby to obtain oxygen from its mother’s blood
    • The oxygen concentration in the mother’s blood is much lower than the oxygen concentration of the air inside the lungs
  • After birth, a baby begins to produce adult haemoglobin
    • It is better suited for the uptake of oxygen from air

Foetal and adult haemoglobin, downloadable AS & A Level Biology revision notes

The foetal haemoglobin has a higher affinity for oxygen, this is why it’s oxygen dissociation curve lies further to the left.

Different types of haemoglobin

  • Haemoglobin is a quaternary protein
    • It is made up of four globin polypeptides and four haem groups
  • The structure of haem is identical in all types of haemoglobin
  • The globin chains however can differ substantially between species
    • The globin polypeptides determine the precise properties of haemoglobin
  • There are a wide range of haemoglobin types that exist
    • They vary in their oxygen-binding properties
    • They bind to and release oxygen in different conditions
  • Environmental factors can have a major impact on the evolution of haemoglobin within a species

Effects of altitude

  • The partial pressure of oxygen is lower at higher altitudes
  • Species living at high altitudes have haemoglobin that is adapted to these conditions
  • For example, llamas have haemoglobin that binds very readily to oxygen
  • This is beneficial as it allows them to obtain a sufficient level of oxygen saturation in their blood when the partial pressure of oxygen in the air is low

Exam Tip

You may be shown the oxygen dissociation curves of different types of haemoglobin and asked to explain how they are adapted to the environment the animal is living in.

Remember that the curve furthest to the left represents the haemoglobin with the highest affinity for oxygen.

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