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Calvin and colleagues determined the pathway of carbohydrate synthesis in plants by studying the incorporation of radioactive carbon dioxide into biological compounds. Suppose that photosynthesis is proceeding at a steady pace in a typical experiment with the lights on, and carbon dioxide is being combined with ribulose-bisphosphate (RuBP) to produce 3-phosphoglycerate (3PG). Then suddenly the source of carbon dioxide is eliminated. What changes in the concentrations of 3PG and RuBP would occur?

A. 3PG levels rise, RuBP levels fall.

B. 3PG levels fall, RuBP levels rise.

Elimination of a source of CO2 would block the action of RUBISCO, which requires CO2 as a substrate. There would be no further production of 3-phosphoglycerate (3PG).

The remaining reactions of the Calvin Cycle are not blocked by the absence of CO2. Intermediates in the cyclic pathway would be converted to ribulose-1,5-bisphosphate (RuBP), and the level of RuBP would rise.

Calvin and associates used this experimental strategy to identify RuBP as the acceptor of CO2 during carbohydrate biosynthesis.

C. 3PG levels rise, RuBP levels rise.

D. 3PG levels fall, RuBP levels fall.

E. 3PG levels rise, RuBP levels stay the same.