Q.Describe two distinct biological roles each of magnesium and calcium in the human body/plants.

Magnesium.

1. Central ion of chlorophyll. $\text{Mg}^{2+}$ occupies the centre of the porphyrin ring in chlorophyll, the green pigment that captures light energy for photosynthesis in plants -- exactly the structural role iron plays at the centre of the haem ring in haemoglobin. Without this coordinated magnesium ion, chlorophyll cannot carry out its light-capturing function, making magnesium indispensable to plant life and, through the food chain, to essentially all life that depends on photosynthesis.
2. Enzyme cofactor. Magnesium ions are required as a cofactor by a very large number of enzymes, especially those that catalyse reactions involving ATP (such as kinases), making $\text{Mg}^{2+}$ essential to cellular energy metabolism generally.

Calcium.

1. Structural mineral of bone and teeth. $\text{Ca}^{2+}$ is, by mass, the most abundant mineral cation in the human body, present overwhelmingly as the mineral hydroxyapatite, $\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2$, which forms the hard structural material of bones and teeth. …

List two well-established biological roles each for Mg (chlorophyll centre, enzyme cofactor) and Ca (bone/tooth mineral, clotting/muscle contrac …

Confusing magnesium's chlorophyll role with iron's haemoglobin role (both are metal-centred porphyrin/porphyrin-like rings, but for different metals and diff …