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A C3 plant is kept in light in air containing 50% O2 instead of the normal 21%. What will happen to its net photosynthesis?

  • (a) It will stop completely and the plant will release only O2. Your answer
  • (b) It will decrease because photorespiration increases. Correct
  • (c) It will not change. Your answer
  • (d) It will increase. Your answer
Your answer: Correct answer: (b)
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Explanation

Why (b) is correct

More O2 makes RuBisCO bind O2 more often, so photorespiration increases and net photosynthesis decreases.

Why the other options are wrong

  • (a) · your answer Photosynthesis does not stop completely.
  • (c) · your answer Raising O2 does change the rate.
  • (d) · your answer It decreases, not increases.
NCERT point: High O2 : CO2 ratio favours photorespiration.
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