Carbon fixation in photosynthesis occurs in the stroma of the chloroplast, which is the fluid-filled space surrounding the thylakoid membranes inside the chloroplast
. Specifically, during the Calvin cycle (also called the C3 cycle), the enzyme RuBisCO catalyzes the fixation of carbon dioxide (CO2) by attaching it to ribulose bisphosphate (RuBP), producing 3-phosphoglycerate (3-PGA), the first stable compound formed in carbon fixation
. In C4 plants, the initial fixation of CO2 occurs in the mesophyll cells' chloroplasts , where CO2 is fixed into a four-carbon compound by the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase). This compound is then transported to bundle sheath cells where the Calvin cycle takes place
. In summary:
- In most plants (C3), carbon fixation occurs in the chloroplast stroma of mesophyll cells during the Calvin cycle.
- In C4 plants, initial carbon fixation occurs in the mesophyll cell chloroplasts , followed by the Calvin cycle in the bundle sheath cell chloroplasts.
This compartmentalization helps optimize carbon fixation efficiency and reduce photorespiration in C4 plants
