light-independent reactions

• the reactions produce complex organic molecules such as

  starch for storage

  sucrose for transport

  cellulose for making cell walls
• the light independent reactions do not require energy from light but do require ATP and reduced NADP from the light-dependent reactions
• carbon dioxide and RuBP are combined

  • carbon dioxide combines with a 5C sugar (RuBP) in a reaction catalysed by the enzyme rubisco
  • the resulting 6-carbon (6C) compound is unstable and splits in 2
  • this results in 2 molecules of a 3C compound known as glycerate 3-phosphate (GP)
  • the carbon dioxide has been 'fixed' meaning that it has been removed from the external environment and become part of a molecule inside the plant cell
• reduction of glycerate 3-phosphate

  • energy from ATP and hydrogen from reduced NADP used to reduce the 2 molecules of GP to two 3C molecule (GALP)
  • some carbons in GALP go towards production of useful organic molecules such as glucose, the rest remain in the calvin cycle to allow regeneration of RuBP
  • 2 molecules of GALP contain 6 carbon atoms, 5 of which are needed to regenerate RuBP so for every turn through calvin cycle only 1/6 of a molecule of glucose is produced
  • glucose is a 6-carbon molecule, so six turns of the calvin cycle are required to produce one molecule of glucose
• regeneration of ribulose bisphosphate

  • 5/6 of the GALP molecules used to regenerate RuBP
  • this process requires ATP