The Oxygen Dissociation Curve (Cambridge (CIE) AS Biology): Revision Note

The oxygen dissociation curve

• The oxygen dissociation curve shows the rate at which oxygen associates and 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

    • This is 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 become associated with water, or to dissolve, at a constant rate, providing a straight line on a graph

• However, with haemoglobin, oxygen binds at different rates as the pO₂ changes

  • This results in a curve when plotted

  • It can be said that haemoglobin's affinity for oxygen changes at different partial pressures of oxygen

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 (in 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 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 Oxygen Dissociation 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

    • This explains 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

    • This makes it easier for the next oxygen molecules to bind

    • This speeds up binding of the remaining oxygen molecules and explains the steeper part of the curve in the middle of the graph

    • The shape change of haemoglobin leading to easier oxygen binding is known as cooperative binding

  • As the haemoglobin molecule approaches saturation it takes longer for the fourth oxygen molecule to bind due to the shortage of remaining binding sites

    • This explains the levelling off of the curve in the top right corner of the graph