Photosynthetic pigments: which metal ion serves as the central ligand coordinated in chlorophyll molecules of plants?

Introduction / Context:
Chlorophylls are tetrapyrrole pigments crucial for light harvesting in photosystems. The identity of the central metal ion is a staple concept in biochemistry and plant physiology and helps distinguish chlorophylls from related heme-type porphyrins.

Given Data / Assumptions:

• We refer to plant chlorophylls a and b.
• The macrocycle is a chlorin ring coordinating a divalent cation.
• Function involves photon capture and energy transfer within the thylakoid membranes.

Concept / Approach:
Chlorophylls coordinate Mg2+ at the center of the porphyrin-like ring. In contrast, heme proteins coordinate Fe2+/Fe3+. During photosynthesis, chlorophylls embedded in thylakoid membranes participate in exciton transfer and charge separation events in photosystems I and II.

Step-by-Step Solution:

Verification / Alternative check:
Structural diagrams of chlorophyll a show Mg2+ chelated by the nitrogen atoms of the chlorin ring, with a phytol tail anchoring the pigment in the membrane.

Why Other Options Are Wrong:

• A/B/D/E: These metals participate in photosynthesis elsewhere (e.g., Mn in the oxygen-evolving complex, Fe in cytochromes), but not as the central ion of chlorophyll.

Common Pitfalls:
Confusing roles of metals in photosystems versus pigments; Mg is for chlorophyll, Mn cluster for water-splitting, Fe for electron carriers.

Final Answer:
Magnesium (Mg)