Which occurs when a hemoglobin molecule binds oxygen in the lungs?
A. Cooperative binding of O₂ causes the four subunits to dissociate Salt bridges between subunits break as the first O₂ binds.
B. Each heme group can reversibly bind four O₂ molecules simultaneously.
C. The first O2 molecule binds more easily than the last O₂ molecule.
D. An iron atom is removed from a porphyrin group.