• Energy from?ATP?and?hydrogen from reduced NADP?are passed from the light-dependent stage to the light-independent stage of photosynthesis
• The energy and hydrogen are used during the?light-independent?reactions (known collectively as the?Calvin cycle) to produce complex organic molecules, including (but not limited to)?carbohydrates, such as:
  • Starch?(for storage)
  • Sucrose?(for translocation around the plant)
  • Cellulose?(for making cell walls)

   

• This stage of photosynthesis does not, in itself, require energy from light (hence?light-independent) and can therefore take place in light or darkness. However, as it requires inputs of ATP and reduced NADP from the light-dependent stage, the Calvin cycle cannot continue indefinitely in darkness, as these inputs will run out
• There are?three main steps?within the?Calvin cycle:
  • Rubisco?catalyses the?fixation of carbon dioxide?by combination with a molecule of?ribulose bisphosphate?(RuBP), a 5C compound, to yield two molecules of?glycerate 3-phosphate?(GP), a 3C compound
  • GP is reduced?to?triose phosphate?(TP) in a reaction involving reduced NADP and ATP
  • RuBP is?regenerated?from TP in reactions that use ATP

   

Carbon fixation

• Carbon dioxide combines with a five-carbon (5C) sugar known as?ribulose bisphosphate?(RuBP)
• An?enzyme?called?rubisco?(ribulose bisphosphate carboxylase) catalyses this reaction
• The resulting six-carbon (6C) compound is?unstable?and splits in two
• This gives two molecules of a three-carbon (3C) compound known as?glycerate 3-phosphate?(GP)
• The carbon dioxide has been?‘fixed’?(it has been removed from the external environment and has become part of the plant cell)
• Glycerate 3-phosphate (GP) is not a carbohydrate but the next step in the Calvin cycle converts it into one

Reduction of glycerate 3-phosphate

• Energy from ATP?and?hydrogen from reduced NADP?– both produced during the light-dependent stage of photosynthesis – are used to?reduce glycerate 3-phosphate?(GP) to a phosphorylated three-carbon (3C) sugar known as?triose phosphate?(TP)
• One-sixth?of the triose phosphate (TP) molecules are used to produce useful organic molecules needed by the plant:
  • Triose phosphates can condense to become?hexose phosphates?(6C), which can be used to produce?starch, sucrose or cellulose
  • Triose phosphates can be converted to?glycerol?and glycerate 3-phosphates to?fatty acids, which join to form?lipids for cell membranes
  • Triose phosphates can be used in the?production of amino acids for protein synthesis

   

Regeneration of ribulose bisphosphate

• Five-sixths?of the triose phosphate (TP) molecules are used to?regenerate ribulose bisphosphate?(RuBP)
• This process?requires ATP

The Calvin cycle

• Intermediate molecules of the Calvin cycle (such as?glycerate 3-phosphate?and?triose phosphate) are used to produce other molecules
• Glycerate 3-phosphate (GP)?is used to produce some amino acids
• Triose phosphate (TP)?is used to produce:
  • Hexose phosphates?(6C), which can be used to produce?starch, sucrose or cellulose
  • Lipids?for?cell membranes
  • Amino acids?for?protein synthesis

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