Iron has many functions but is a key component in the production of chlorophyll, the site of photosynthesis.
Without enough iron, plants can not produce sufficiently chlorophyll, leading to delayed growth of plants marked by interveinal chlorosis. Iron is also a key component of cytochrome — a hemeprotein that plays a critical role in the production of ATP — the currency of cell metabolism.
In this ability, it is irreplaceable for both plants and animals. Iron also plays a vital role in many other proteins and reactions.