For the chloroplasts described in Problem 23, how does the level of 3PG vary after the light is turned off?

The first phase of the Calvin cycle involves the reaction of ${\mathbf{CO}}_{\mathbf{2}}$with the ribulose 1, 5 bisphosphate (RuBP) to produce glyceraldehyde-3-phosphate (GAP).

In the Calvin cycle, ribose 5 phosphate (Ru5P) undergoes a phosphorylation reaction to generate ribulose-1, 5-bisphosphate (RuBP). Further, RuBP undergoes a carboxylation reaction to form 3-phosphoglycerate (3PG), which is then converted to 1, 3 bisphosphoglycerate (BPG). 1, 3 biphosphoglycerate undergoes a chemical reaction to form the product glyceraldehyde-3-phosphate (GAP).

In the absence of sunlight, $\text{ATP}$and $\text{NADPH}$levels drop down as they are already used up in the Calvin cycle. Further, RuBP levels decrease as it is also used up by RuBP carboxylase.

In this case, the level of 3PG will shoot up as the RuBP is used in the carboxylase reaction. Hence, the levels of 3PG will increase if the light is turned off.