Three-carbon molecules of PGA (phosphoglycerate) are converted to energy-rich G3P (glyceraldehyde 3-phosphate) sugar by reduction.

Here's a breakdown:

* PGA is a product of the carbon fixation step in photosynthesis. It's a low-energy molecule.

* G3P is a high-energy molecule that can be used to build carbohydrates like glucose.

* Reduction is the process of gaining electrons. In this case, PGA gains electrons and a phosphate group from ATP and NADPH. This reduction provides the energy needed to convert PGA into G3P.

This process is part of the Calvin cycle, the light-independent reactions of photosynthesis.