Which occurs when a hemoglobin molecule binds oxygen? a) Oxygen molecules in the lungs bind irreversibly to the protein’s heme groups. b) The second oxygen molecule binds more easily than the first oxygen molecule. c) The α subunits bind oxygen while the β subunits control cooperativity. d) Ionic bonds form between subunits which changes the quaternary structure.

     b) The second oxygen molecule binds more easily than the first oxygen molecule. 
Explanation:
Once the first oxygen molecule binds to hemoglobin, it becomes easier to bind a second and third oxygen molecule to hemoglobin compared to the first molecule. This is explained by the fact that once bound by oxygen, the hemoglobin molecule changes its conformation. However, it is difficult to bind the fourth oxygen. Structurally, when oxygen atom binds hemoglobin molecule, on one monomer, the tetramer’s conformation changes from being T state to being a R (relaxed) state which is why the second atom is bound easily.