Magnesium is an essential constituent metal of which important biological molecule?

Introduction / Context:
This question connects basic inorganic chemistry with biology by asking about the role of magnesium in living systems. Many bioinorganic questions focus on which metal ion is central in a particular biomolecule, such as iron in haemoglobin or magnesium in chlorophyll. Knowing these associations is important for both chemistry and biology examinations.

Given Data / Assumptions:

• We are asked which important biological molecule contains magnesium as a constituent metal.
• Options include chlorophyll, DNA, mitochondria, ribosomes and haemoglobin.
• We assume standard school level biology and chemistry content.

Concept / Approach:
Chlorophyll is the green pigment in plants responsible for capturing light energy for photosynthesis. Its core structure is a porphyrin like ring with a central magnesium ion. This is similar in shape to haemoglobin, which has a porphyrin ring with a central iron ion. DNA, mitochondria and ribosomes are important structures or molecules but do not contain magnesium as a central metal ion in the same characteristic way that chlorophyll does.

Step-by-Step Solution:
Step 1: Recall that chlorophyll is a green pigment located in the chloroplasts of plant cells. Step 2: The structural formula of chlorophyll shows a large ring system called a chlorin ring, which is related to porphyrin. Step 3: At the centre of this ring is a magnesium ion that coordinates with nitrogen atoms of the ring. Step 4: This central magnesium ion is crucial for the light absorption properties of chlorophyll, allowing efficient capture of solar energy. Step 5: DNA is a nucleic acid polymer composed of sugar, phosphate and nitrogenous bases; while magnesium ions may stabilise structures in solution, they are not permanent constituent metals in the molecule in the same way. Step 6: Mitochondria and ribosomes are cell organelles, not single molecules with a central metal ion. Step 7: Haemoglobin contains iron at the centre of a heme group, not magnesium. Step 8: Therefore, the biological molecule with magnesium as a constituent metal is chlorophyll.

Verification / Alternative check:
You can verify this by recalling the common exam pairings: iron in haemoglobin, magnesium in chlorophyll, calcium in bones and teeth, and iodine in thyroid hormones. These associations are often listed together in science textbooks. When you see a question pairing a metal and a biomolecule, mentally link magnesium with chlorophyll and iron with haemoglobin to avoid confusion.

Why Other Options Are Wrong:
DNA does not have a central metal ion as part of its basic covalent structure. Mitochondria and ribosomes contain many different proteins and molecules, plus various metal ions, but they are not defined by a single central metal the way chlorophyll is defined by magnesium. Haemoglobin is the oxygen carrying protein in blood and contains iron in the heme group, not magnesium.

Common Pitfalls:
A common mistake is to confuse the metal in chlorophyll with that in haemoglobin, since both structures involve a large ring and a central metal ion. Remember the simple rule: green plants, green pigment, magnesium; red blood, red pigment, iron. Another pitfall is to overthink the role of magnesium in biochemical reactions in general. While magnesium ions can activate enzymes and stabilise nucleic acids, the distinctive structural example in exams is magnesium in chlorophyll.

Final Answer:
Magnesium is a constituent metal of the chlorophyll molecule in plants.