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The O₂ evolved in photosynthesis comes from water. Which pair of elements is involved in this reaction?

(a) magnesium and molybdenum Your answer
(b) manganese and chlorine Correct
(c) magnesium and chlorine Your answer
(d) manganese and potassium Your answer

Your answer: Correct answer: (b)

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Explanation

Why (b) is correct

The water-splitting complex of PS II needs manganese and chloride ions (with calcium). Water is split there to give O₂, protons and electrons.

Why the other options are wrong

(a) · your answer Magnesium is in chlorophyll and molybdenum in nitrogen-metabolism enzymes; neither splits water.
(c) · your answer Chlorine is right, but magnesium is part of chlorophyll, not of the water-splitting complex.
(d) · your answer Manganese is right, but potassium is not needed for photolysis.

NCERT point: The splitting of water is associated with PS II, which is located on the inner side of the thylakoid membrane.

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