Talk about the mechanism of C3 plants.

The first step of the Calvin cycle consists of fixing a molecule of CO2 (carboxylation) on the ribulose 1,5-diphosphate by the Rubisco , to give a compound of 3 carbon atoms , PGA. A large majority of plants, including trees work under this mechanism. The CO2 fixed by Rubisco comes initially from the diffusion of atmospheric CO2 through stomata, then in a dissolved form, through the leaf cells to the chloroplast stroma.