Photosynthesis referred to the process of generally capable of capturing light energy and then converting it into chemical energy.
Atmospheric carbon dioxide is fixed by plants in form of carbohydrates during the dark reaction of photosynthesis.
The carboxylation of RuBP by the addition of CO2 is the first step of the Calvin Cycle.
This reaction is catalyzed by the enzyme RuBisCO. It results in the formation of two molecules of 3-phosphoglyceric acid (3PGA), which further gets reduced in a series of reactions to produce glucose and RuBP is regenerated.