• The?light-independent?reactions?of photosynthesis are sometimes referred to as the?Calvin cycle
• 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
• There are?three main steps?within the?light-independent reactions
  1. Carbon dioxide?is combined with?ribulose bisphosphate?(RuBP), a 5-carbon (5C) compound; this yields two molecules of?glycerate 3-phosphate?(GP), a 3-carbon (3C) compound
  2. GP is reduced?to?glyceraldehyde 3-phosphate?(GALP), another 3C compound,?in a reaction involving reduced NADP and ATP
  3. RuBP is?regenerated?from GALP in reactions that use ATP

Carbon dioxide and RuBP are combined

• Carbon dioxide combines with a 5C sugar known as?RuBP?in a reaction catalysed by the enzyme?rubisco
• The resulting 6-carbon (6C) compound is?unstable?and splits in two
• This results in two 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,?both produced during the light-dependent reactions, are used to?reduce?the two molecules of?GP?to two 3C molecule known as?GALP
• Some of the carbons in GALP go towards the production of?useful organic molecules such as glucose, while the rest remain in the Calvin cycle to allow the?regeneration of RuBP
  • Two molecules of GALP contain six carbon atoms, five of which are needed to regenerate RuBP; this means that for every turn through the Calvin cycle only one sixth 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

• Five sixths?of the GALP molecules are used to?regenerate RuBP
• This process?requires ATP

The Calvin cycle produces glucose and other important biological molecules.