Question

In a chlorophyll molecule, the magnesium is situated in the
A. Center of porphyrin
B. Corner of phytol
C. Phytol chamber
D. Isocyclic ring

Answer 1

Hint: Chlorophyll synthesizes carbohydrates from CO2 and water. This process is known as photosynthesis which is very important for all plants. Chloroplasts pigment is what makes green plants green. The basic structure of a chlorophyll molecule is a porphyrin ring, co-ordinated to a central atom. And that central atom is magnesium.

Complete answer:
(A) Centre of porphyrin: Chlorophyll is the green colored pigment which is present in the plastid. It is located in the thylakoid membranes of the plastids. Magnesium is centrally located in the chlorophyll. It consists of a porphyrin ring which has magnesium ions in the center. The two types of chlorophyll are chlorophyll a and b. Chlorophyll a has a porphyrin ring with the side chain of the methyl group and a phytol chain. Chlorophyll b has a porphyrin ring and side chain of the aldehyde group and a phytol chain. It acts like an accessory pigment. Therefore, Magnesium is situated in the centre of porphyrin.
 So, "A" is the correct answer.

(B) Corner of phytol: Chlorophyll molecule is asymmetrical and consist of two parts:
 i- The porphyrin head
 ii- A phytol tail
Phytol is an acyclic diterpene alcohol which is a part of chlorophyll. But as we studied the chlorophyll molecule consists of a central magnesium atom surrounded by a nitrogen-containing structure called a porphyrin ring; attached to the ring is a long carbon–hydrogen side chain, known as a phytol chain. So, "B" is not right.

(C) Phytol chamber: Chlorophyll a contains a magnesium ion encased in a large ring structure known as a chlorin. The chlorin ring is heterocyclic in nature and is derived from pyrrole. Four nitrogen atoms from the chlorine surround and bind the magnesium atom. The magnesium center uniquely defines the structure as a chlorophyll molecule. Therefore, the magnesium is not in the corner of phytol. So, "C" is the wrong answer.

(D) Isocyclic ring: As studied, the molecular structure of chlorophyll is showing its central position and an isocyclic cyclopentanone ring is attached to the third pyrrole group. So, "D" is not the correct answer.

Hence, the correct answer is option (A).

Note: In plants, magnesium is necessary for synthesis of chlorophyll and photosynthesis. Mg2+ has the additional vital role of being the coordinating ion in the chlorophyll molecule. Mg2+ is probably taken up into chloroplasts to the greatest extent during the light-induced development from proplastid to chloroplast or etioplast to chloroplast. At these times, the synthesis of chlorophyll and the biogenesis of the thylakoid membrane stacks absolutely require the divalent cation.