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
  

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 G-3-P

• Glycerate-3-phosphate (GP) is converted into triose phosphate (TP) using NADPH and ATP
  

• The NADPH and ATP are generated by the light dependent reactions (via non-cyclic photophosphorylation)

• Reduction by NADPH transfers hydrogen atoms to the compound, while the hydrolysis of ATP provides energy

• As six molecules of GP were produced via carbon fixation, six molecules of TP are similarly produced per cycle

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