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

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.

Iron acts primarily as an energy substrate within red blood cells, providing ATP that powers oxygen diffusion through hemoglobin to tissues over time.

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.

Iron is the central atom in the heme group that directly binds oxygen, enabling transport in blood and storage in muscle.