C1.3.15 Carbon fixation by Rubisco (HL) Notes

From Light-Dependent to Light-Independent Reactions

1. The light-dependent reactions occur in the thylakoid membranes and produce ATP and NADPH (covered in C1.3.14).
2. 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.
3. The first and crucial step of this cycle is carbon fixation, catalyzed by the enzyme Rubisco.

The Carbon Fixation Reaction

Substrates and Product

1. Substrates:
   1. RuBP (ribulose bisphosphate): A 5-carbon sugar molecule that acts as the CO₂ acceptor.
   2. CO₂ (carbon dioxide): The inorganic carbon source from the atmosphere.
2. Product:
   1. Glycerate 3-phosphate (G3P): A 3-carbon organic molecule (also called 3-phosphoglycerate or 3-PGA).

How the Reaction Works

1. Step 1: Rubisco catalyzes the attachment of CO₂ (1C) to RuBP (5C).
2. Step 2: This forms an unstable 6-carbon intermediate compound.
3. Step 3: The 6-carbon compound immediately splits into two molecules of glycerate 3-phosphate (G3P), each with 3 carbons.

Rubisco’s Abundance Compensates for Its Slow and Inefficient Nature

Despite being essential, Rubisco has significant limitations:

1. Slow Reaction Rate
   1. Most enzymes process thousands of substrate molecules per second.
   2. Rubisco fixes only about 3 CO₂ molecules per second—extremely slow by enzyme standards.
2. Ineffective at Low CO₂ Concentrations
   1. Rubisco works efficiently only when CO₂ concentrations are relatively high.
   2. At low CO₂ levels, its activity drops significantly.
   3. This is a problem because atmospheric CO₂ concentration is relatively low (~0.04%).
3. Can Bind Oxygen Instead of CO₂
   1. Rubisco can mistakenly bind O₂ instead of CO₂ (this is called photorespiration and reduces efficiency).
   2. This further limits its effectiveness.