5.6.7 The Light-Independent stage

Question 1

What is the light independent stage often referred to as?

Answer 1

The Calvin Cycle

Question 2

Why can the light independent stage not continue indefinitely in darkness?

Answer 2

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 3

What are the three main steps within the Calvin cycle?

Answer 3

Carbon fixation/carboxylation (CO2 fixation with RuBP)
Reduction (Reduction of glycerate-3-phosphate to triose phosphate)
Regeneration (Reformation of CO2 acceptor molecule - RuBP)

Question 4

Describe the process of carbon fixation/carboxylation

Answer 4

1. CO2 combines with Ribulose biphosphate (5C) - a CO2 acceptor. This is catalysed by RuBisCO
2. RuBP becomes carboxylated, forming an unstable intermediate six-carbon compond that immediately breaks down
3. The product is two molecules of GP (glycerine-3-phosphate). CO2 has now been fixed
   RuBiSCO = 8 active sites where CO2 is fixed

Question 5

Describe the process of reduction

Answer 5

4. GP is reduced, using hydrogens from the NADP made during the light dependent stage, to triose phosphate (TP).
   Energy from ATP, also made during the LDR, is used at this stage at the rate of two molecules of ATP for every molecule of carbon dioxide fixed during stage 3

Question 6

Describe the process of regeneration

Answer 6

5. In 10 of every 12 TP molecules, the atoms are rearranged to regenerate six molecules of RuBP. This process requires phosphate groups.
   Chloroplasts contain only low levels of RuBP, as it is continually being regenerated. The remaining two of the 12 molecules of TP are the product