Three-carbon molecules of PGA are converted to G3P small sugar molecules by ATP and NADPH, which come from the light reaction.
Photosynthesis, a process in which green plants use energy from the sun to make their own food, involves two types of reactions, light reactions in light stage and light independent reactions in dark stage.
During the light reactions ATP and NADH are generated from the electron transport chains, which are then taken to the second stage (light independent stage) of photosynthesis. During the Light independent stage the energy stored in ATP and NADH is used to convert 3-carbon PGA to G3P.