What critical role does Iron play in vertebrates?

Iron plays a critical role in oxygen transport within vertebrates, mainly through its involvement in hemoglobin—the protein found in red blood cells that carries oxygen from the lungs to tissues throughout the body. Hemoglobin contains iron atoms bound to heme groups, which allow it to efficiently absorb and release oxygen. The coordination of iron with oxygen is essential for maintaining optimal oxygen levels in bodily tissues, thus supporting cellular metabolism and energy production.

Other roles, such as functioning as a neurotransmitter, being a structural component, or facilitating DNA replication, do not accurately reflect the primary and well-known biological function of iron in relation to vertebrate physiology. While iron does play varied roles in biological processes—such as acting as a cofactor in enzymatic reactions—its most definitive and critical function in vertebrates is indeed tied to its capacity to bind and transport oxygen in hemoglobin.