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A plant is suddenly moved from light to darkness. Which change is expected in the chloroplast within a few seconds?

  • (a) both RuBP and 3-PGA rise Your answer
  • (b) 3-PGA rises and RuBP falls Correct
  • (c) RuBP rises and 3-PGA falls Your answer
  • (d) both remain unchanged Your answer
Your answer: Correct answer: (b)
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Explanation

Why (b) is correct

In darkness, ATP and NADPH run out. RuBP is still carboxylated to 3-PGA for a short time, but 3-PGA cannot be reduced and RuBP cannot be regenerated. So 3-PGA rises and RuBP falls.

Why the other options are wrong

  • (a) · your answer RuBP falls, because it is not regenerated.
  • (c) · your answer This is the reverse.
  • (d) · your answer Levels change quickly when light stops.
NCERT point: Such experiments helped Calvin work out the cycle.
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