Apentix/AP Biology/Unit 3: Cellular Energetics/Question
AP Biologyhardmcq1 pt

Isolated chloroplasts are illuminated in a solution containing CO2, then the light is switched off while CO2 remains available. Which outcome follows within minutes, and why?

A.Fixation accelerates because accumulated sugar supplies extra chemical energy to rubisco
B.Fixation continues unchanged because rubisco itself does not absorb light
C.Fixation declines sharply because the ATP and NADPH pools run down and RuBP regeneration stalls
D.Fixation halts permanently because rubisco denatures in darkness

Explanation

Core Concept

When light is removed, the chloroplast stops continuously producing ATP and NADPH through the light reactions. The Calvin cycle can continue only briefly as those pools are consumed. ATP is required for RuBP regeneration, while NADPH provides reducing power primarily during the reduction phase; depletion of both ultimately causes carbon fixation to fall sharply.

Correct Answer

CFixation declines sharply because the ATP and NADPH pools run down and RuBP regeneration stalls

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