5.2.1(e) - Light-independent stage and the Calvin cycle
The Calvin cycle fixes carbon dioxide in the chloroplast stroma. Follow the carbon atoms from RuBP through GP to TP, then distinguish the energy supplied by ATP from the reducing power supplied by reduced NADP.
Why Light-Independent Still Depends On Light
The light-independent stage is the stage of photosynthesis in which carbon dioxide is fixed into organic molecules. It is also called the Calvin cycle. It takes place in the stroma of the chloroplast, the fluid-filled region around the thylakoid membranes.
The name can be misleading. This stage does not use light energy directly in the way the light-dependent stage does. However, it depends on two products made by the light-dependent stage:
| Product from the light-dependent stage | Role in the light-independent stage |
|---|---|
| ATP | Supplies energy for reactions that produce TP |
| Reduced NADP | Supplies reducing power, often described as hydrogen/electrons, so GP can be reduced to TP |
This means the Calvin cycle cannot continue indefinitely in darkness. If light-dependent reactions stop, the supply of ATP and reduced NADP runs down, so GP cannot keep being converted into TP.
Carbon Fixation
Carbon fixation is the incorporation of inorganic carbon dioxide into an organic molecule.
Carbon Fixation To Form GP
The Calvin cycle begins with a five-carbon compound called ribulose bisphosphate, abbreviated to RuBP. Carbon dioxide combines with RuBP. This is the fixation step: carbon from CO₂ is now part of an organic carbon compound rather than a free inorganic molecule.
The enzyme that catalyses this carbon fixation reaction is ribulose bisphosphate carboxylase, usually called RuBisCO.
RuBisCO
RuBisCO is the enzyme that catalyses the fixation of carbon dioxide to RuBP in the Calvin cycle.
The first stable product is glycerate 3-phosphate, abbreviated to GP. GP is a three-carbon compound. A useful carbon-count check is:
Carbon Count In Carbon Fixation
RuBP contains 5 carbon atoms.
Carbon dioxide contributes 1 carbon atom.
Together, that gives 6 carbon atoms in total. The unstable six-carbon product splits to give two molecules of GP, each with 3 carbon atoms:
5C RuBP + 1C CO₂ -> two 3C GP molecules
RuBisCO catalyses the reaction and is available to catalyse it again; it is not consumed or turned into GP. RuBP is a substrate, so its supply must be regenerated.
RuBisCO fixes CO₂ to RuBP, producing GP.
From GP To TP
GP must be reduced before its carbon can leave the cycle as a useful sugar phosphate. GP is converted into triose phosphate, abbreviated to TP. TP is a three-carbon sugar phosphate.
The conversion of GP to TP uses both products from the light-dependent stage:
| Input used | What happens in the Calvin cycle |
|---|---|
| ATP | Energy is transferred as GP is converted towards TP |
| Reduced NADP | GP is reduced to TP |
When ATP is used, it becomes ADP and inorganic phosphate. When reduced NADP is used, it becomes NADP again. These can return to the light-dependent stage, where ATP and reduced NADP can be made again if light-dependent reactions are running.
Continue this lesson.
You’re previewing NovaLearn without an account. Create a free account to read the rest.
Create free account