Connect light-stage products to carbon fixation
Explain how ATP and reduced NADP from the light-dependent stage support the Calvin cycle and synthesis of organic molecules.
A Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; practical investigations need supervised work and human subject-teacher review remains pending.
Before you begin
- Chloroplasts contain photosynthetic pigments, and enzymes catalyse metabolic reactions.
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Quick readiness check
Which Calvin-cycle intermediate is reduced using ATP and reduced NADP?
What you will learn
- Name ATP and reduced NADP as products transferred from the light-dependent stage
- Relate their use to Calvin-cycle reactions and organic synthesis
Build the explanation
The light-dependent stage transfers light energy into chemical energy in ATP and reduced NADP. In the Calvin cycle, ATP supplies energy and reduced NADP supplies hydrogen and reducing power as glycerate 3-phosphate (GP) is reduced to triose phosphate (TP). ATP is also used as ribulose bisphosphate (RuBP) is regenerated. TP can leave the cycle and contribute to synthesis of carbohydrates, lipids and amino acids; GP can contribute to some amino-acid synthesis.
Apply it carefully
Do not say light energy directly fixes carbon dioxide or that ATP and reduced NADP are carbon atoms. Carbon enters the Calvin cycle in carbon dioxide; ATP and reduced NADP support the reactions that rearrange and reduce carbon compounds.
Pause and explain
What provides the carbon atoms incorporated into Calvin-cycle products?
Worked example
Which two light-stage products support the Calvin cycle?
Show the worked solution
- One carries usable energy and one provides reducing power.
- Name ATP and reduced NADP.
Answer ATP and reduced NADP.
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Explain how the light-dependent stage supports production of organic molecules in the Calvin cycle.
Give me a hint
Include ATP, reduced NADP and carbon dioxide as distinct inputs or roles.
Compare my reasoning
- The light-dependent stage makes ATP and reduced NADP.
- ATP and reduced NADP support GP reduction and RuBP regeneration; carbon enters from CO₂.
The light-dependent stage produces ATP and reduced NADP. ATP supplies energy and reduced NADP supplies reducing power during Calvin-cycle reactions, including reduction of GP to TP; carbon dioxide supplies the carbon incorporated into organic molecules.
Look for these in your work
- Both light-stage products named
- Roles separated
- Carbon source identified
A learner says reduced NADP provides the carbon used to make carbohydrate. Correct the explanation.
Give me a hint
Separate a reducing coenzyme from the carbon substrate.
Compare my reasoning
- Carbon dioxide supplies carbon to the Calvin cycle.
- Reduced NADP provides hydrogen and reducing power as GP is reduced to TP.
Carbon dioxide supplies carbon. Reduced NADP transfers hydrogen and reducing power to Calvin-cycle reactions, including reduction of GP to TP; it does not provide the carbohydrate's carbon atoms.
Look for these in your work
- Carbon source corrected
- Reduced coenzyme role stated
- GP-to-TP step named
A leaf is illuminated briefly, then the Calvin cycle continues while supplied ATP and reduced NADP remain available. Explain why carbon fixation can continue temporarily without current light.
Give me a hint
Trace stored chemical intermediates from the light-dependent stage.
Compare my reasoning
- Light supports ATP and reduced-NADP synthesis in the thylakoid stage.
- The Calvin cycle can use existing ATP and reduced NADP in the stroma until those supplies are depleted or other conditions limit it.
The light-dependent stage normally supplies ATP and reduced NADP. If these molecules remain available, Calvin-cycle reactions can use them for a time in the stroma; continued fixation is not guaranteed once their supply or other requirements become limiting.
Look for these in your work
- Products linked to light
- Temporary continuation reasoned
- Limit qualified
Common mistakes
- Treating ATP as the carbon source
- Confusing NADP with reduced NADP
- Claiming carbon fixation is powered directly by light photons
What can you explain now?
Which Calvin-cycle intermediate is reduced using ATP and reduced NADP?
Compare with the explanation
Glycerate 3-phosphate (GP)
GP is reduced to triose phosphate (TP).
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Make it stick
How do ATP and reduced NADP connect the light-dependent stage to Calvin-cycle products?
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