Final answer:
During the second phase of the Calvin cycle, which is the reduction phase, 3-PGA molecules are converted into G3P using energy from ATP and NADPH.
Step-by-step explanation:
The second phase of the Calvin cycle is characterized by the reduction process. During this stage, the enzyme RuBisCO has already catalyzed the initial step of carbon fixation, bonding carbon dioxide to RuBP and producing a short-lived unstable 6-carbon compound that quickly splits into two molecules of 3-phosphoglycerate (3-PGA). In the reduction phase, these 3-PGA molecules are then converted into glyceraldehyde-3-phosphate (G3P) with the help of energy from ATP and electrons from NADPH, both of which are products of the light-dependent reactions in photosynthesis. This crucial phase reduces 3-PGA into G3P, a three-carbon sugar, which can eventually be used to form glucose and other carbohydrates. It is important to note that although G3P formation takes place, the actual direct answer to which process occurs during the second phase is the reduction of 3-PGA to G3P.