Final answer:
Hemoglobin will only transport oxygen when its iron is in the Fe²⁺ state. The iron's oxidation state influences hemoglobin's ability to bind oxygen and changes its color upon oxygen release.
Step-by-step explanation:
Hemoglobin is a complex protein crucial for transporting oxygen through the blood. It consists of four protein subunits: two alpha and two beta globins. Each subunit contains a heme ligand with iron at its center. For hemoglobin to attach to oxygen, the iron must be in the Fe²⁺ oxidation state. When iron is oxidized to Fe³⁺, oxygen transport is prevented. This iron center changes its oxidation state depending on the presence or absence of oxygen, which also leads to a change in the three-dimensional shape of the hemoglobin molecule.
Thus, hemoglobin can only transport oxygen when the iron is in its divalent (Fe²⁺) state. The color of the heme complex changes from a red to a bluish red when oxygen is not bound to the iron, indicating a loss of oxygen from the hemoglobin.