The three carbon molecules of PGA (phosphoglycerate) are converted to G3P (glyceraldehyde 3-phosphate) in the Calvin cycle, which is part of the light-independent reactions of photosynthesis. This conversion requires NADPH and ATP, both of which are products of the light-dependent reactions.

Here's how it works:

1. Light-dependent reactions: Light energy is used to create ATP (energy carrier) and NADPH (reducing power).

2. Calvin cycle:

* Carbon fixation: CO2 is incorporated into RuBP (ribulose bisphosphate) to form an unstable 6-carbon molecule that quickly breaks down into two molecules of PGA.

* Reduction: PGA is then reduced to G3P using NADPH and ATP from the light-dependent reactions.

* Regeneration: RuBP is regenerated to continue the cycle.

So, the crucial molecules from the light reactions that power the conversion of PGA to G3P are NADPH and ATP.