Explain how non-cyclic photophosphorylation results in the production of ATP, NADPH, and oxygen during the light-dependent reactions of photosynthesis.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Light energy is absorbed by chlorophyll in photosystem II, exciting electrons to a higher energy level.
These excited electrons travel through the electron transport chain, releasing energy that pumps protons into the thylakoid lumen and drives ATP synthesis by chemiosmosis (photophosphorylation).
The loss of electrons from PSII is compensated by photolysis of water, which splits water into protons, electrons and oxygen; the oxygen is released as a by‑product.
The electrons that reach photosystem I are further excited and ultimately reduce NADP⁺ to NADPH, using protons from the stroma (or from photolysis).
These excited electrons travel through the electron transport chain, releasing energy that pumps protons into the thylakoid lumen and drives ATP synthesis by chemiosmosis (photophosphorylation).
The loss of electrons from PSII is compensated by photolysis of water, which splits water into protons, electrons and oxygen; the oxygen is released as a by‑product.
The electrons that reach photosystem I are further excited and ultimately reduce NADP⁺ to NADPH, using protons from the stroma (or from photolysis).
Examiner tips
- Use the exact terms ‘photosystem II’, ‘electron transport chain’, ‘chemiosmosis’, ‘photolysis’, ‘NADP⁺ to NADPH’.
- Show the sequence: PSII → electron transport → ATP → water → O₂ → PSI → NADPH.
- Include the role of protons in both ATP synthesis and NADPH formation.
Common mistakes
- Mixing up the order of PSII and PSI or omitting the water‑splitting step.
- Using ‘photosynthesis’ instead of ‘light‑dependent reactions’ or failing to mention oxygen as a by‑product.
Mark scheme (4 marks)
- Light energy is absorbed by photosystem II (PSII), causing photoexcitation of chlorophyll / electrons are excited to a higher energy level.
- Excited electrons pass along the electron transport chain, releasing energy used to synthesise ATP by chemiosmosis / photophosphorylation.
- Electrons from PSII are replaced by photolysis of water, which splits water into protons, electrons, and oxygen / oxygen is released as a by-product of photolysis.
- Excited electrons from PSI are used to reduce NADP⁺ to NADPH (with protons from the stroma / from photolysis).
Key terms in this question
non-cyclic photophosphorylation · NADPH
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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