Question

Explain the rise in the amount of ribulose bisphosphate after the carbon dioxide concentration is reduced. (1)

a) Reduced carbon dioxide levels stimulate the production of ribulose bisphosphate.
b) There is no change in ribulose bisphosphate levels when carbon dioxide decreases.
c) Carbon dioxide reduction has no impact on ribulose bisphosphate.
d) Ribulose bisphosphate decreases when carbon dioxide decreases.

Answer 1

Reduced carbon dioxide levels lead to an accumulation of ribulose bisphosphate (RuBP) because the Calvin cycle's carboxylation step, catalyzed by RuBisCO, slows down due to fewer CO2 molecules being available to fix to RuBP.

Step-by-step explanation:

The increase in ribulose bisphosphate (RuBP) levels after a reduction in carbon dioxide concentration occurs because RuBP accumulation is a result of decreased carbon fixation. When CO2 levels are low, the enzyme ribulose bisphosphate carboxylase (RuBisCO) has fewer CO2 molecules to catalyze with RuBP, leading to a buildup of unused RuBP. RuBisCO catalyzes the first step of the Calvin cycle by fixing carbon dioxide to RuBP, forming 3-phosphoglycerate (3-PGA). If this carboxylation step slows down because there's less CO2, less RuBP is converted into 3-PGA, and so RuBP accumulates. This corresponds to the statement that reduced carbon dioxide levels stimulate the production of ribulose bisphosphate.

Therefore, between the options provided, the correct answer is:
a) Reduced carbon dioxide levels stimulate the production of ribulose bisphosphate.