5.4 Photosynthesis: Light-Independent Stage

• The light-independent reactions of photosynthesis are sometimes referred to as the Calvin cycle
• The reactions eventually allow for the production of 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 3C molecules of GP to two 3C molecules 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

• Intermediate molecules of the Calvin cycle, such as glyceraldehyde 3-phosphate (GALP), are used to produce various other biological molecules needed by plants, such as:
  • Hexose sugars e.g. glucose 
    • Glucose can enter the respiration reactions during which ATP is produced
    • Hexose sugars can be converted into other hexose sugars e.g. glucose can be converted to sucrose for transport in the phloem
    • Hexose sugars can be joined to make polysaccharides such as starch and cellulose
  • Glycerol can be used for building lipid molecules such as triglycerides and phospholipids
  • Fatty acids which form the tails of lipid molecules such as triglycerides and phospholipids
  • Nucleic acids form the basis of DNA and RNA
    • Phosphates from the soil are combined with the molecules of the Calvin cycle to produce nucleic acids
  • Acetyl coenzyme A is important coenzyme in respiration 
  • Amino acids which can be used in protein synthesis for building polypeptides
    • Nitrates from the soil need to be combined with the molecules of the Calvin cycle for amino acids to be produced
• Many of the molecules produced are used to build new plant biomass; these molecules are passed on to consumers when plant tissue is eaten

The products of photosynthesis include amino acids, polysaccarides, lipids and nucleic acids