Explain how the products of the light-dependent reactions are used in the Calvin cycle to produce triose phosphate (TP).

IB DP Biology Standard Level (2023 syllabus) — C1.3 Photosynthesis · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

1. CO₂ is fixed by RuBP via rubisco to give 3‑phosphoglycerate (3‑PGA). 2. ATP from the light reactions phosphorylates 3‑PGA to 1‑,3‑bisphosphoglycerate. 3. NADPH donates electrons (hydrogen) to reduce 1‑,3‑bisphosphoglycerate to glyceraldehyde‑3‑phosphate (G3P), a triose phosphate. 4. Some G3P is used to regenerate RuBP (requiring ATP) while the rest exits the cycle to build sugars.

Examiner tips

Use the exact terms ATP, NADPH, RuBP, rubisco, CO₂, 3‑PGA, G3P; link each to the step it supplies. Show the flow from CO₂ fixation to reduction and regeneration. Mention that ATP is for phosphorylation and NADPH for reduction. Include that some TP leaves the cycle for biosynthesis.

Common mistakes

Confusing ATP with ADP or omitting the role of NADPH; not stating that RuBP is regenerated; writing that CO₂ is directly reduced to G3P without mentioning rubisco or 3‑PGA.

Mark scheme (4 marks)

ATP from the light-dependent reactions provides energy for the phosphorylation of glycerate-3-phosphate (G3P) / for the reduction step in the Calvin cycle
NADPH acts as a reducing agent / donates hydrogen/electrons to reduce G3P to triose phosphate
Carbon dioxide is first fixed into G3P / RuBP accepts CO₂ to form G3P via rubisco, before ATP and NADPH are used
Some TP is used to regenerate RuBP (using ATP) so the cycle can continue / some TP leaves the cycle to form organic molecules such as glucose

Key terms in this question

Calvin cycle · triose phosphate (TP)