Apentix/AP Biology/Unit 3: Cellular Energetics/Question
AP Biologymediummcq1 pt

During the Calvin cycle in the chloroplast stroma, most of the glyceraldehyde 3-phosphate (G3P) molecules produced are immediately used to do what?

A.They are exported whole to the mitochondria and oxidized completely to carbon dioxide.
B.Regenerate ribulose bisphosphate (RuBP) so the cycle can continue fixing carbon dioxide.
C.They donate electrons directly to plastoquinone in the thylakoid membrane.
D.They combine with oxygen to regenerate ATP without light input.

Explanation

Core Concept

Fixing three CO2 molecules yields six G3P, but five of those six reenter the regeneration phase to rebuild RuBP, the five-carbon CO2 acceptor. Rebuilding RuBP consumes additional ATP supplied by the light reactions. Only the remaining single G3P leaves as net product for synthesizing glucose, starch, and other organic molecules. This recycling keeps Rubisco supplied with substrate so carbon fixation continues.

Correct Answer

BRegenerate ribulose bisphosphate (RuBP) so the cycle can continue fixing carbon dioxide.

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