Transport of oxygen by haemoglobin

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Oxygen dissociation curves:

The graph of the relationship between the saturation of haemoglobin with oxygen and the partial pressure of oxygen

1.

  • Shape of the haemoglobin molecule makes it difficult for the first oxygen molecule to bind to one of the sites on its four polypeptide chains because they are closely united
  • Therefore at low concentration gradients, little oxygen binds to the haemoglobin
  • The gradient of the curve is shallow initially

2.

  • Howver, the binding of the first oxygen molcule changes the quaternary structure of the haemoglobin molecule, causing it to change shape
  • This change makes it easier for the other subunits to bind to an oxygen molecule

3.

  • It therefore takes a smaller increase in the partial pressure of oxygen to bind the second oxygen molecule than it did to bind to the first one
  • This is called positive cooperativity because binding of the first molecule makes the binding of the second one easier and so on
  • The gradient of the curve steepens

4.

  • After the binding of the third molecule, the situation changes
  • It becomes harder for the haemoglobin to bind to the fourth oxygen molecule
  • This is simply due to probability
  • With the majority of the binding sites occupied, it is less likely that a single oxygen molecule will find an empty site to bind to
  • The gradient of the curve reduces and the graph flattens off

The many different oxygen dissociation curves are better understood if two facts are always kept in mind -

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