Carbon fixation at Higher Level: An AHL statement: the first step of the light-independent reactions (3.3.15-3.3.17).
Practise this as you read
- Outline carbon fixation.
- State the reactants and products.
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The big idea: In the stroma, the enzyme Rubisco joins carbon dioxide to a five-carbon compound, RuBP. The product splits at once into two glycerate 3-phosphate (GP) molecules. This is carbon fixation: inorganic carbon becomes part of an organic compound.
The points to remember
- Carbon fixation happens in the stroma, the first step of the Calvin cycle.
- RuBP (5C) + CO₂ → 2 glycerate 3-phosphate (3C).
- The enzyme is Rubisco (ribulose bisphosphate carboxylase).
- Rubisco is slow, so leaves contain a great deal of it: the most abundant enzyme on Earth.
- Carbon fixation does not need light directly, but it needs RuBP, which needs ATP.
Remember it as: Five plus one makes two threes.
Real example: the most common protein. Rubisco can make up a large share of the soluble protein in a leaf: plants invest heavily in it because each molecule fixes only a few carbon dioxide molecules per second.
Trap: Carbon dioxide joins RuBP, not glucose; the first stable product is glycerate 3-phosphate, not triose phosphate.
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How it is asked: Expect an Outline of carbon fixation, a question on the numbers of carbon atoms, or a data question on RuBP and GP levels.
IB-style questionOutline[3 marks]
Outline the process of carbon fixation in the Calvin cycle.
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