Which of the following statement is true regarding binding of O2 to hemoglobin?

The deoxygenated subunits of the hemoglobin tetramer exist in the T ("tense") conformational state. As erythrocytes enter the capillaries of the lung alveoli, the paial pressure of O2 has increased sufficiently to allow binding to hemoglobin. When one mole of O2 binds, it causes a shift in the overall conformation of the other subunits to a more R ("relaxed") conformational state. This change in conformation leads to higher affinity of the remaining monomers for O2. Each deoxygenated monomer has a progressively increased affinity for O2 as more oxygen binds. This cooperative binding is observed as a sigmoidal curve when plotting the paial pressure of oxygen versus moles of O2 bound. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 35. Gas Transpo & pH. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds),Ganong's Review of Medical Physiology, 24e.