Interdependence at Higher Level: An AHL statement that joins 3.3.9-3.3.17: how the two stages depend on each other.
Practise this as you read
- Explain changes in GP and RuBP in the dark.
- Identify the molecules passed between the stages.
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The big idea: The light-dependent reactions make ATP and reduced NADP; the Calvin cycle uses them and returns ADP + Pi and NADP. Neither can keep going without the other: in the dark, the Calvin cycle soon stops too.
The points to remember
- Light-dependent reactions → ATP and reduced NADP → Calvin cycle.
- Calvin cycle → ADP + Pi and NADP → light-dependent reactions.
- In the dark: no ATP or reduced NADP, so GP is not reduced and RuBP is not regenerated.
- So GP builds up, TP falls and RuBP falls in the dark.
- Without carbon dioxide, NADP is not returned, so the light-dependent reactions slow too.
Remember it as: Light makes the fuel; the cycle sends back the empties.
Real example: the lollipop experiment. Melvin Calvin's team fed radioactive carbon dioxide to algae in a flat 'lollipop' flask and switched the light off: glycerate 3-phosphate rose and RuBP fell, showing how the two stages are linked.
Trap: The 'light-independent' reactions still stop in the dark: they need the ATP and reduced NADP made in the light.
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How it is asked: Expect a data question on GP, TP and RuBP when the light or carbon dioxide changes, or a summary diagram to label.
IB-style questionExplain[3 marks]
Explain why the concentration of glycerate 3-phosphate rises and RuBP falls when the light is switched off.
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