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In CAM plants, the source of CO2 for the Calvin cycle during the day is

  • (a) malic acid Correct
  • (b) 3-PGA Your answer
  • (c) pyruvate Your answer
  • (d) RuBP Your answer
Your answer: Correct answer: (a)
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Explanation

Why (a) is correct

In CAM plants, malic acid stored at night is decarboxylated in the day. The CO2 released enters the Calvin cycle while the stomata stay closed.

Why the other options are wrong

  • (b) · your answer 3-PGA is the product of fixation by RuBisCO, not the source of CO2.
  • (c) · your answer Pyruvate is left over after decarboxylation; it does not supply CO2.
  • (d) · your answer RuBP accepts CO2; it does not supply it.
NCERT point: In the Calvin cycle, RuBisCO joins CO2 to RuBP to form two molecules of 3-PGA.
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