<P> In C plants , the first step in the light - independent reactions of photosynthesis involves the fixation of CO by the enzyme RuBisCO into 3 - phosphoglycerate . However , due to the dual carboxylase and oxygenase activity of RuBisCo , some part of the substrate is oxidized rather than carboxylated , resulting in loss of substrate and consumption of energy , in what is known as photorespiration . In order to bypass the photorespiration pathway , C plants have developed a mechanism to efficiently deliver CO to the RuBisCO enzyme . They utilize their specific leaf anatomy where chloroplasts exist not only in the mesophyll cells in the outer part of their leaves but in the bundle sheath cells as well . Instead of direct fixation to RuBisCO in the Calvin cycle , CO is incorporated into a 4 - carbon organic acid , which has the ability to regenerate CO in the chloroplasts of the bundle sheath cells . Bundle sheath cells can then utilize this CO to generate carbohydrates by the conventional C pathway . </P>
in photosynthesis the carbon in co2 is initially fixed to what molecule
3 - phosphoglycerate