From Light-Dependent to Light-Independent Reactions
- The light-dependent reactions occur in the thylakoid membranes and produce ATP and NADPH (covered in C1.3.14).
- These products now fuel the light-independent reactions (Calvin cycle) in the stroma of the chloroplast, which uses ATP and NADPH to convert inorganic carbon (CO₂) into organic molecules.
- The first and crucial step of this cycle is carbon fixation, catalyzed by the enzyme Rubisco.
Definition
Carbon fixation
The process of converting inorganic carbon dioxide (CO₂) into organic molecules.
Rubisco is the most abundant enzyme on Earth, with plants producing it in massive quantities.
The Carbon Fixation Reaction
Substrates and Product
- Substrates:
- RuBP (ribulose bisphosphate): A 5-carbon sugar molecule that acts as the CO₂ acceptor.
- CO₂ (carbon dioxide): The inorganic carbon source from the atmosphere.
- Product:
- Glycerate 3-phosphate (G3P): A 3-carbon organic molecule (also called 3-phosphoglycerate or 3-PGA).
How the Reaction Works
- Step 1: Rubisco catalyzes the attachment of CO₂ (1C) to RuBP (5C).
- Step 2: This forms an unstable 6-carbon intermediate compound.
