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A photosynthesising plant is releasing more 18O than normal. The plant must have been supplied with

  • (a) C6H12O6 containing 18O Your answer
  • (b) CO2 containing 18O Your answer
  • (c) H2O containing 18O Correct
  • (d) O3 Your answer
Your answer: Correct answer: (c)
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Explanation

Why (c) is correct

The oxygen released in photosynthesis comes from water, not from CO2. So if more 18O is being released, the plant must have been given water labelled with 18O.

Why the other options are wrong

  • (a) · your answer Glucose is a product, not a source of the oxygen released.
  • (b) · your answer Oxygen from CO2 ends up in sugar and water, not in the O2 that is released.
  • (d) · your answer Ozone is not used in photosynthesis.
NCERT point: Van Niel's idea, later proved with isotopes: the O2 evolved by green plants comes from H2O.
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