The site of CO₂ fixation in chloroplasts is:
Chloroplasts compartmentalize photosynthesis into light harvesting and carbon assimilation stages across membrane systems. Thylakoid membranes, intricate network of flattened discs stacking into grana interconnected by stroma lamellae, house photosystems II and I, cytochrome b6f complex and ATP synthase that convert light energy into ATP and NADPH through non-cyclic and cyclic electron transport, proton pumping and water splitting at oxygen-evolving complex. Those products are consumed in the aqueous phase surrounding thylakoids, the stroma, where the Calvin-Benson-Bassham cycle operates. Stromal soluble enzymes, including Rubisco large and small subunits, phosphoglycerate kinase, glyceraldehyde-3-phosphate dehydrogenase, fructose-1,6-bisphosphatase, aldolase and transketolase, fix CO2 onto ribulose-1,5-bisphosphate to generate 3-phosphoglycerate, reduced to triose phosphates using ATP and NADPH. Stroma also contains plastid DNA, 70S ribosomes, starch synthases, nitrite reductase, enzymes for fatty acid and amino acid synthesis, and antioxidant systems. High pH near 8.0 and Mg2+ elevation in illuminated stroma activate Rubisco via carbamylation and Rubisco activase, optimizing carbon fixation and preventing wasteful photorespiration.
Ref: Taiz et al., Plant Physiology and Development, 6th ed., Chapter 8: Photosynthesis – The Calvin Cycle in Stroma.