text: The chloroplast stroma contains many proteins, though the most common and important is RuBisCO, which is probably also the most abundant protein on the planet. RuBisCO is the enzyme that fixes CO into sugar molecules. In  plants, RuBisCO is abundant in all chloroplasts, though in  plants, it is confined to the bundle sheath chloroplasts, where the Calvin cycle is carried out in  plants.