5.6.7 The Light-Independent Stage

Question 1

What is the LIS sometimes refferd to as?

Answer 1

The calvin cycle

Question 2

What complex organic molecules does this stage produce?

Answer 2

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)

Question 3

Why can the LIS not indefinitely continue to take place in darkness?

Answer 3

The light-independent stage does not require energy from light and 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.

Question 4

What are the three main steps in the calvin cycle?

Answer 4

• 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

Question 5

Describe carbon fixation.

Answer 5

• 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

Question 6

Describe the reduction of gylcerate 3-phosphate.

Answer 6

• 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

Question 7

Describe the regeneration of ribulose biphosphate.

Answer 7

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