Magnesium is present in

Magnesium (Mg) is a vital mineral element that serves as the central metal atom in the chlorophyll molecule [c1][t1]. Chlorophyll, the green pigment in plants and algae, utilizes a porphyrin ring structure to trap light energy for photosynthesis, with magnesium situated at its core [t4][t7]. While chlorophyll and hemoglobin share remarkably similar structural frameworks, they differ in their central elements: chlorophyll contains magnesium, whereas hemoglobin contains iron (Fe) [t2][t6][t8]. Regarding the other options, Ascorbic acid (Vitamin C) is an organic compound (C6H8O6) that does not contain a central metal atom. Vitamin B12 (cobalamin) is unique among vitamins for containing a metal ion, but that metal is cobalt, not magnesium [t4]. Therefore, magnesium is specifically an essential constituent of chlorophyll, acting as an activator for various enzymes in plant metabolism [c1][t9].

Sources

1. [1] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 25: Agriculture > Nitrogen (N) > p. 363
2. [2] https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/chlorophyllase