Trace carbon through the Calvin cycle
Outline carbon fixation by rubisco, GP reduction to TP and ATP-dependent RuBP regeneration.
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.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many carbons are in RuBP?
What you will learn
- Describe CO₂ fixation with RuBP to form GP
- Explain GP reduction and RuBP regeneration
Build the explanation
The Calvin cycle has three linked stages. First, rubisco catalyses carbon dioxide fixation: CO₂ combines with ribulose bisphosphate (RuBP, 5C) to form glycerate 3-phosphate (GP, 3C). The unstable 6C intermediate splits into two GP molecules. Second, GP is reduced to triose phosphate (TP) using ATP and reduced NADP. Third, most TP is rearranged to regenerate RuBP using ATP, allowing the cycle to continue. Some TP can leave the cycle for synthesis of organic molecules.
Apply it carefully
Track carbon, not just names: a 5C RuBP plus one carbon from CO₂ yields an unstable 6C intermediate and then two 3C GP molecules. ATP supplies energy and reduced NADP supplies reducing power. RuBP regeneration is essential; if it stopped, carbon fixation could not continue.
Pause and explain
What are GP and TP used for in the cycle's sequence?
Worked example
Which enzyme catalyses fixation of carbon dioxide with RuBP?
Show the worked solution
- Name the Calvin-cycle enzyme that catalyses the fixation step.
- Rubisco.
Answer Rubisco.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Outline the three stages of the Calvin cycle and name the energy carriers used.
Give me a hint
Move from carbon fixation to reduction and regeneration.
Compare my reasoning
- Rubisco fixes CO₂ with 5C RuBP to form 6C intermediate, which splits into two 3C GP.
- ATP and reduced NADP reduce GP to TP; ATP supports RuBP regeneration from TP.
Rubisco catalyses fixation of CO₂ with 5C RuBP, forming an unstable 6C intermediate that splits into two 3C GP. GP is reduced to TP using ATP and reduced NADP. Most TP is used, with ATP, to regenerate RuBP; some TP can leave for organic synthesis.
Look for these in your work
- Rubisco and fixation stated
- Carbon-number flow correct
- GP reduction inputs correct
- RuBP regeneration stated
A student says six molecules of carbon dioxide each combine directly with a six-carbon RuBP molecule. Correct the carbon bookkeeping for one fixation event.
Give me a hint
Use the 5C and 1C inputs.
Compare my reasoning
- RuBP has five carbons and CO₂ supplies one.
- The 6C intermediate splits into two molecules of 3C GP.
One 5C RuBP combines with one carbon from CO₂, forming an unstable 6C intermediate. This splits into two 3C GP molecules; RuBP is not a six-carbon compound.
Look for these in your work
- RuBP carbon count correct
- CO₂ contribution stated
- GP products correct
A Calvin-cycle preparation has ATP and reduced NADP but no functioning rubisco. Which stage is directly prevented, and what follows for later cycle steps?
Give me a hint
Start with the enzyme's role in carbon fixation.
Compare my reasoning
- Rubisco catalyses CO₂ fixation with RuBP.
- Without fixation, new GP is not produced by that step, limiting downstream TP formation and RuBP cycling.
Carbon fixation is directly prevented because rubisco catalyses the combination of CO₂ with RuBP. Without newly formed GP, the downstream GP-to-TP and RuBP-regeneration sequence cannot proceed as a continuing cycle from that carbon input.
Look for these in your work
- Direct blocked step identified
- Downstream consequence linked
- ATP/coenzyme distinction maintained
Common mistakes
- Calling RuBP a six-carbon molecule
- Omitting ATP or reduced NADP from GP reduction
- Forgetting ATP use in RuBP regeneration
What can you explain now?
How many carbons are in RuBP?
Compare with the explanation
Five
RuBP combines with one carbon from carbon dioxide before GP is formed.
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Make it stick
How do carbon fixation, GP reduction and RuBP regeneration form the Calvin cycle?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.