Carbon Reactions

Principles Module 30–2


  • Discuss how energy from the light reactions is used to form C-C bonds
  • Describe the three major steps of the carbon reactions, including the inputs and outputs of each

Key points

  • Carbon fixation involves the incorporation of atmospheric CO2 into stable carbohydrate molecules through the investment of energy
  • The carbon reactions are made up of 3 major steps
    • Carboxylation is catalyzed by the enzyme RuBisCO, the most abundant enzyme on earth
      • combines 1 CO2 with a 5-carbon substrate
      • the resulting 6-carbon product is highly unstable, splits into 2 molecules of the 3-carbon molecule, phosphoglycerate (PGA)
    • Reduction is a 2-step process in which much of the energy of the light reactions is invested
      • PGA is phosphorylated (using ATP) and reduced (using NADPH)
      • the resulting 3-carbon molecule is the carbohydrate glyceraldehyde 3-phosphate (G3P)
    • Regeneration is a multi-step pathway leading to the re-formation of the initial 5-carbon substrate, making this a metabolic cycle
      • G3P is used along with the remainder of the ATP from the light reactions

Questions for Practice

  • What is the main product of the carbon reactions? What is it used to make?
  • The carbon reactions form a metabolic cycle – what does this mean?
  • How is the energy from light transferred to carbon to form carbohydrates? Be specific.