Which enzyme fixes CO2 in C3 photosynthesis in the mesophyll cells?

Which enzyme fixes CO2 in C3 photosynthesis in the mesophyll cells?

• A. PEP carboxylase.
• B. Ribulose phosphate.
• C. ATP synthase.
• D. RuBisCO.

Answer: (D)

In C3 photosynthesis, CO2 is fixed directly in the Calvin cycle by the enzyme RuBisCO in the chloroplast stroma of mesophyll cells. RuBisCO catalyzes the carboxylation of ribulose-1,5-bisphosphate (RuBP) with CO2 to form two molecules of 3-phosphoglycerate, which then proceed through the cycle using ATP and NADPH from the light reactions to produce carbohydrate. This is distinct from plants that use PEP carboxylase to fix CO2 in C4 and CAM pathways, a step that helps concentrate CO2 elsewhere to reduce wasteful photorespiration. ATP synthase is involved in generating ATP during the light reactions, not fixing CO2, and ribulose phosphate is a metabolite rather than the CO2-fixing enzyme.

In C3 photosynthesis, CO2 is fixed directly in the Calvin cycle by the enzyme RuBisCO in the chloroplast stroma of mesophyll cells. RuBisCO catalyzes the carboxylation of ribulose-1,5-bisphosphate (RuBP) with CO2 to form two molecules of 3-phosphoglycerate, which then proceed through the cycle using ATP and NADPH from the light reactions to produce carbohydrate. This is distinct from plants that use PEP carboxylase to fix CO2 in C4 and CAM pathways, a step that helps concentrate CO2 elsewhere to reduce wasteful photorespiration. ATP synthase is involved in generating ATP during the light reactions, not fixing CO2, and ribulose phosphate is a metabolite rather than the CO2-fixing enzyme.