Magnesium is a component part of

Magnesium is the central atom of the chlorophyll molecule located in the centre of the porphyrin head, with each molecule containing 6.7% magnesium.
Chlorophyll pigment in the leaves absorbs the sunlight that excites the electrons in the molecules, enabling them to be transferred to other molecules. Chlorophyll and magnesium are connected in this step because magnesium helps the molecule absorb light and keeps the electrons in an excited state so that they can be transferred.