After one molecule of oxygen has bound to Haemoglobin, it is easier for a second molecule of oxygen to bind. Explain Why?

Binding of the first molecule of oxygen changes the tertiary/quarternary structure of Haemoglobin - this can be referred to as a conformational shift. This change in structure then exposes another binding site on the Haemoglobin (where there is a second heam group) for the next oxygen molecule to bind to. This explains why binding of the first molecule enhances its ability to bind the second molecule.