(e) the fixation of carbon dioxide and the light- independent stage of photosynthesis

To include how the products of the light-dependent stage are used in the light-independent stage (Calvin cycle) to produce triose phosphate (TP) with reference to ribulose bisphosphate (RuBP), ribulose bisphosphate carboxylase (RuBisCO) and glycerate 3-phosphate (GP) – no other biochemical detail is required.

The light-independent stage and what does it produce

• referred to as the Calvin cycle

• This stage produces 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)

Where can the light independent stage take place?

• does not require energy from light

• can therefore take place in light or darkness. However, as it requires inputs of ATP and reduced NADP from the light-dependent stage,

• it cannot continue indefinitely in darkness, as these inputs will run out

What are the three main steps within the Calvin cycle

• Rubisco catalyses the fixation of carbon dioxide by combination with a molecule of ribulose biphosphate (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

Regeneration of ribulose bisphosphate

• Five-sixths of the triose phosphate (TP) molecules are used to regenerate ribulose bisphosphate (RuBP)

• This process requires ATP