Mg and Fe are needed for plants in the

Magnesium is a structural component of chlorophyll. A chlorophyll molecule consists of a single magnesium atom present in the center which is attached to four pyrrole rings. Iron is a functional component for the synthesis of chlorophyll.
The chlorophyll molecule is made up of a squarish tetrapyrrolic ring known as head and phytol alcohol called the tail. The magnesium atom is present in the central position of the tetrapyrrolic ring. The four pyrrole rings of porphyrin head are linked together by methine (CH =) groups forming a ring system.

When central Mg is replaced by Fe, the chlorophyll becomes a green pigment called 'cytochrome' which is used in photosynthesis (Photophosphorylation) and respiration both.