Question: What does it mean that hemoglobin binds O2 cooperatively? 
 Options:  <1> That the binding of an O2 molecule to a subunit of hemoglobin drives the binding of other subunits to form a complete protein hemoglobin. <2> That the binding of an O2 molecule to a subunit of hemoglobin increases the affinity of the same subunit to bind more O2 molecules. <3> That the binding of an O2 molecule to a subunit of hemoglobin increases the affinity of other subunits for O2. <4> That the union of an O2 molecule to a protein hemoglobin causes the union of another O2 molecule to another protein different hemoglobin. <5> None of this is true.
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