Light Independent Reactions

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Understanding:

•  In the light independent reactions a carboxylase catalyses the carboxylation of ribulose bisphosphate

        
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


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Understanding:

•  Glycerate-3-phosphate is reduced to triose phosphate using reduced NADP and ATP

        
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


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Understanding:

•  Triose phosphate is used to regenerate RuBP and produce carbohydrates

•  Ribulose bisphosphate is reformed using ATP

        
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


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