Apentix/AP Biology/Unit 3: Cellular Energetics/Question
AP Biologyeasymcq1 pt

During carbon fixation, Rubisco attaches a molecule of CO2 to which partner?

A.Glyceraldehyde 3 phosphate, the finished three-carbon export of the cycle
B.Oxaloacetate, the four-carbon carrier some plants use to capture CO2 ahead of the cycle
C.Ribulose 1,5-bisphosphate, a five-carbon sugar whose carboxylation yields two three-carbon products
D.3-phosphoglycerate, which collects one more carbon before reduction begins

Explanation

Core Concept

Rubisco's fixation substrate is ribulose 1,5-bisphosphate, the cycle's five-carbon acceptor. Adding CO2 creates an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate. Later reactions then reduce those products and regenerate RuBP so fixation can continue, which is why the pathway runs as a cycle.

Correct Answer

CRibulose 1,5-bisphosphate, a five-carbon sugar whose carboxylation yields two three-carbon products

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