The light-independent reaction

• The light-independent reaction, also known as the Calvin cycle, occurs in the stroma of the chloroplast. It depends on the products of the light-dependent reactions: ATP and NADPH.
• 1. Carbon Fixation

  • CO₂ from the atmosphere diffuses into the stroma of the chloroplast.
  • The enzyme ribulose bisphosphate carboxylase-oxygenase (RuBisCO) catalyzes the reaction between carbon dioxide (CO₂) and ribulose bisphosphate (RuBP) (a 5-carbon molecule).
  • This forms an unstable 6-carbon compound, which immediately breaks down into two molecules of glycerate 3-phosphate (GP) (a 3-carbon compound).
• 2. Reduction of GP to Triose Phosphate (TP)

  • The ATP produced in the light-dependent reactions provides energy to convert glycerate 3-phosphate (GP) into triose phosphate (TP).
  • NADPH from the light-dependent reactions donates hydrogen (H⁺), reducing GP to TP.
  • For each GP molecule:
  • ATP → ADP + Pi (energy used)
  • NADPH → NADP⁺ (hydrogen donated)
• 3. Regeneration of RuBP

  • 5/6 of the triose phosphate (TP) molecules produced are used to regenerate ribulose bisphosphate (RuBP).
  • This regeneration process requires energy, which is provided by ATP. ATP → ADP + Pi (energy used to convert TP back to RuBP)
  • Regenerating RuBP ensures that the Calvin cycle can continue, allowing the plant to fix more carbon dioxide.
• 4. Formation of Useful Organic Molecules

  • Only 1/6 of the triose phosphate (TP) molecules is used to produce useful organic compounds like glucose, amino acids, and lipids.
  • After several turns of the Calvin cycle (usually 6), enough TP molecules are produced to form one molecule of glucose.
• The three main phases of the Calvin cycle include:

  1. carbon fixation
  2. reduction
  3. regeneration of RuBP