Final answer:
The S-shaped O2-binding curve of hemoglobin is due to cooperativity, where the affinity for oxygen depends on already bound oxygen. The curve is plotted as a function of partial pressure of oxygen in the blood and relative Hb-oxygen saturation.
Step-by-step explanation:
The S-shaped O2-binding curve of hemoglobin is due to a phenomenon called cooperativity, in which the affinity of one heme for O2 depends on whether the other hemes are already bound to O2. Cooperativity in hemoglobin requires an interaction between the four heme groups in the hemoglobin tetramer, and depends on the change in structure of the heme group that occurs with oxygen binding. As a result of these factors, the binding of oxygen to hemoglobin can be plotted as a function of the partial pressure of oxygen in the blood versus the relative Hb-oxygen saturation, resulting in an S-shaped curve.