A suspension of algae is incubated in a flask in the presence of both light and CO₂. When it is transferred to the dark, the reduction of 3-phosphoglycerate to glyceraldehyde 3-phosphate is blocked, and the concentration of ribulose bisphosphate (RuBP) declines. Why does the RuBP concentration decline? A) Ribulose bisphosphate is synthesized from glyceraldehyde 3-phosphate. B) Glyceraldehyde 3-phosphate is converted to glucose. C) Ribulose bisphosphate is used to synthesize 3-phosphoglycerate. D) Both a and b E) Both a and c

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Quizplus · Accepted Answer

In the Calvin cycle, RuBP is used to fix CO2 in order to form 3-phosphoglycerate. This reaction is catalyzed by the enzyme Rubisco, which requires both RuBP and CO2 to function. If the reduction of 3PG to G3P is blocked, then there is no need for additional RuBP to be synthesized from G3P. On the other hand, if G3P is converted to glucose, then RuBP would still be required for the Calvin cycle to continue. Therefore, the decline in RuBP concentration is more likely due to the fact that it is not being used in the absence of light and the reduction of 3PG to G3P.

A Suspension of Algae Is Incubated in a Flask in the Presence