Final answer:
The oxygen-dissociation curve is an S-shaped graph showing the relationship between partial pressure of oxygen and hemoglobin saturation, illustrating cooperative binding. Not equivalent amounts of oxygen are released with every 20 mm Hg change in PO2 because of the curve's shape, with more oxygen being released at the steeper parts of the curve.
Step-by-step explanation:
The oxygen-dissociation curve illustrates the relationship between the partial pressure of oxygen (PO2) and hemoglobin saturation. It is an S-shaped (sigmoid) curve that reflects how hemoglobin's affinity for oxygen changes with changes in PO2.
This curve demonstrates that as the PO2 increases, hemoglobin becomes more saturated with oxygen up to a certain point, after which the curve levels off, indicating that hemoglobin is nearing full oxygen saturation.
Cooperative binding refers to the phenomenon where the binding of one oxygen molecule to hemoglobin increases the affinity for subsequent oxygen molecules.
This process is represented by the S-shaped curve, where initially the binding is harder, but as more oxygen binds, the process becomes easier until the last oxygen molecule, where binding again becomes more difficult.
For instance, going from a PO2 of 80 to 60 mm Hg might release less oxygen than going from 40 to 20 mm Hg, due to the steeper part of the curve at lower partial pressures where hemoglobin has a reduced affinity for oxygen and releases it more readily.