Which of the following natural substances contains iron (Fe)?

Myoglobin is the answer. It is an oxygen-storing protein in muscle cells, and its active site holds a heme group, a porphyrin ring with a central iron atom (Fe2+) that binds O2 reversibly. That iron is why muscle tissue looks dark red, and the heme-iron link is a favourite Biology-linked question in Prelims. Haemoglobin works on the same principle in blood.

• Chlorophyll is wrong because its central metal is magnesium, not iron. Iron appears in plants in other roles, such as in cytochromes, but not in the chlorophyll molecule itself.
• Collagen is wrong. It is a structural fibrous protein of connective tissue, rich in glycine, proline and hydroxyproline, and it contains no metal cofactor.
• Keratin is wrong. It is a structural protein of hair, nails and horn, stabilised by disulphide bonds from cysteine, and has no iron.

Mains angle: the link between dietary iron, haemoglobin and myoglobin matters for nutrition and anaemia questions in GS-II and GS-III health and food security.

Trap: aspirants see chlorophyll as a green pigment with a metal and guess iron; the metal is magnesium.