Which statement most accurately describes the role of iron in hemoglobin and myoglobin?

• A. Iron is stored in hemoglobin as a reserve mineral and is released as free elemental iron into plasma when tissues need additional oxygen during exercise, which is the main regulatory mechanism for oxygen delivery.
• B. Iron acts primarily as an energy substrate within red blood cells, providing ATP that powers oxygen diffusion through hemoglobin to tissues over time.
• C. Iron chemically alters the shape of oxygen molecules in the lungs, converting O2 into a soluble form that dissolves in plasma and is later reconverted in tissues.
• D. Iron is the central atom in the heme group that directly binds oxygen, enabling transport in blood and storage in muscle.

Answer: (D)