The Oxygen Dissociation Curve (HL) (HL IB Biology)

• The oxygen dissociation curve shows the rate at which oxygen associates, and also dissociates, with haemoglobin at different partial pressures of oxygen (pO₂)
  
  • Partial pressure of oxygen refers to the pressure exerted by oxygen within a mixture of gases; it is a measure of oxygen concentration
  • Haemoglobin is referred to as being saturated when all of its oxygen binding sites are taken up with oxygen; so when it contains four oxygen molecules
• The ease with which haemoglobin binds and dissociates with oxygen can be described as its affinity for oxygen
  • When haemoglobin has a high affinity it binds easily and dissociates slowly
  • When haemoglobin has a low affinity for oxygen it binds slowly and dissociates easily
• In other liquids, such as water, we would expect oxygen to becomes associated with water, or to dissolve, at a constant rate, providing a straight line on a graph, but with haemoglobin oxygen binds at different rates as the pO₂ changes; hence the resulting curve
  • It can be said that haemoglobin's affinity for oxygen changes at different partial pressures of oxygen

The oxygen dissociation curve

Interpreting the curve

• When the curve is read from left to right, it provides information about the rate at which haemoglobin binds to oxygen at different partial pressures of oxygen
  • At low pO₂ (the bottom left corner of the graph) oxygen binds slowly to haemoglobin; this means that haemoglobin cannot pick up oxygen and become saturated as blood passes through the body's oxygen-depleted tissues
    • Haemoglobin has a low affinity for oxygen at low pO₂, so saturation percentage is low
  • At medium pO₂ (in the central region of the graph) oxygen binds more easily to haemoglobin and saturation increases quickly; at this point on the graph a small increase in pO₂ causes a large increase in haemoglobin saturation
  • At high pO₂ (in the top right corner of the graph) oxygen binds easily to haemoglobin; this means that haemoglobin can pick up oxygen and become saturated as blood passes through the lungs
    • Haemoglobin has a high affinity for oxygen at high pO₂, so saturation percentage is high
    • Note that at this point on the graph increasing the pO₂ by a large amount only has a small effect on the percentage saturation of haemoglobin; this is because most oxygen binding sites on haemoglobin are already occupied
• When read from right to left, the curve provides information about the rate at which haemoglobin dissociates with oxygen at different partial pressures of oxygen
  • In the lungs, where pO₂ is high, there is very little dissociation of oxygen from haemoglobin 
  • At medium pO₂, oxygen dissociates readily from haemoglobin, as shown by the steep region of the curve; this region corresponds with the partial pressures of oxygen present in the respiring tissues of the body, so ready release of oxygen is important for cellular respiration
    • At this point on the graph a small decrease in pO₂ causes a large decrease in percentage saturation of haemoglobin, leading to easy release of plenty of oxygen to the cells
  • At low pO₂ dissociation slows again; there are few oxygen molecules left on the binding sites, and the release of the final oxygen molecule becomes more difficult, in a similar way to the slow binding of the first oxygen molecule

Explaining the curve

• The curved shape of the oxygen dissociation curve for haemoglobin can be explained as follows
  • Due to the shape of the haemoglobin molecule it is difficult for the first oxygen molecule to bind to haemoglobin
    • This means that binding of the first oxygen occurs slowly, explaining the relatively shallow curve at the bottom left corner of the graph
  • After the first oxygen molecule binds to haemoglobin, the haemoglobin protein changes shape, or conformation, making it easier for the next oxygen molecules to bind due to cooperative binding
    • This speeds up binding of the remaining oxygen molecules and explains the steeper part of the curve in the middle of the graph
  • As the haemoglobin molecule approaches saturation it takes longer for the fourth oxygen molecule to bind
    • This is due to the shortage of remaining binding sites, explaining the levelling off of the curve in the top right corner of the graph