Oxygen Dissociation Curves

Haemoglobin is a quaternary protein which means that it is made up of multiple polypeptide chains

Each haemoglobin can carry 4 oxygen molecules which means that it can carry 8 oxygen atoms

The reaction between haemoglobin and oxygen is reversible which means that oxygen can be released at respiring tissues

Partial pressure of a gas is the pressure that the gas produces individually in a system

Haemoglobins affinity for oxygen depends on oxygens partial pressure

An oxygen dissociation curve shows how saturated with oxygen the haemoglobin is at any given pO2

The curve is S shaped because when the first oxygen binds to the haemoglobin it causes a shape change of the molecule which means that haemoglobin’s oxygen affinity increases.

While this is the case, after the 3rd oxygen binds to a haemoglobin molecule the affinity for oxygen is substantially reduced. This means that haemoglobin can pick up a 2nd and 3rd oxygen very easily but finds it more difficult to bind its 1st and 4th due to shape changes that affect the affinity

At a place with a high partial pressure of oxygen, haemoglobins affinity is high, this means that it will go through a process called loading where the binding oxygen molecules to the ferrous ion becomes a lot easier and therefore the haemoglobin molecule can more easily “load” oxygen molecules

At a place with a low partial pressure of oxygen, haemoglobin affinity is low, this means that it will undergo unloading, where it releases the oxygen into the tissues that need it

Places of high partial pressure of oxygen include the lungs, where oxygen concentration is high

Places of low partial pressure of oxygen include respiring tissue, where oxygen concentration is low