Evaluate the role of partial pressure of oxygen in modulating hemoglobin's oxygen affinity and its function as a homotrophic allosteric activator.

• A. Partial pressure of oxygen decreases hemoglobin's affinity for oxygen, acting as a homotrophic allosteric inhibitor.
• B. Partial pressure of oxygen has no effect on hemoglobin's affinity for oxygen.
• C. Decreasing partial pressure of oxygen enhances hemoglobin's affinity for oxygen, acting as a homotrophic allosteric activator.
• D. Increasing partial pressure of oxygen enhances hemoglobin's affinity for oxygen, acting as a homotrophic allosteric activator.

Answer: (D)