Brent Cornell

        
The light independent reactions use the chemical energy derived from light dependent reactions to form organic molecules

• The light independent reactions occur in the fluid-filled space of the chloroplast called the stroma

The light independent reactions are collectively known as the Calvin cycle and involve three main steps:

• Carboxylation of ribulose bisphosphate
• Reduction of glycerate-3-phosphate
• Regeneration of ribulose bisphosphate

Step 1:  Carbon Fixation

• The Calvin cycle begins with a 5C compound called ribulose bisphosphate (or RuBP)
• An enzyme, RuBP carboxylase (or Rubisco), catalyses the attachment of a CO₂ molecule to RuBP
• The resulting 6C compound is unstable, and breaks down into two 3C compounds – called glycerate-3-phosphate (GP)
  
• A single cycle involves three molecules of RuBP combining with three molecules of CO₂ to make six molecules of GP

        
Step 2:  Reduction of Glycerate-3-Phosphate

• Glycerate-3-phosphate (GP) is converted into triose phosphate (TP) using NADPH and ATP
• Reduction by NADPH transfers hydrogen atoms to the compound, while the hydrolysis of ATP provides energy
  
• Each GP requires one NADPH and one ATP to form a triose phosphate – so a single cycle requires six of each molecule

    
Step 3:  Regeneration of RuBP

• Of the six molecules of TP produced per cycle, one TP molecule may be used to form half a sugar molecule
• Hence two cycles are required to produce a single glucose monomer, and more to produce polysaccharides like starch
• The remaining five TP molecules are recombined to regenerate stocks of RuBP  (5 × 3C = 3 × 5C) 
• The regeneration of RuBP requires energy derived from the hydrolysis of ATP