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

On hot, dry days, C3 plant leaves close their stomata, so internal O2 rises relative to CO2. Why does net photosynthesis decline under these conditions?

A.Chlorophyll absorbs O2 in place of light, shutting down both photosystems simultaneously.
B.Rubisco increasingly acts as an oxygenase, diverting RuBP and energy into photorespiration instead of carbohydrate production.
C.Closed stomata stop electron flow between Photosystem II and Photosystem I.
D.Rising O2 permanently denatures Rubisco at ordinary leaf temperatures.

Explanation

Core Concept

Rubisco's active site accepts either CO2 or O2, so the local ratio of the two gases decides which reaction prevails. When stomata close, O2 accumulates while CO2 falls, and oxygenation of RuBP rises. Photorespiration releases previously fixed CO2 and consumes ATP and reducing power while producing no sugar. Less carbon therefore reaches the Calvin cycle, lowering net photosynthesis.

Correct Answer

BRubisco increasingly acts as an oxygenase, diverting RuBP and energy into photorespiration instead of carbohydrate production.

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