Explain how the absorption of light by photosystems I and II leads to the reduction of NADP⁺ to NADPH 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 absorbed by pigments in PSII excites electrons to a higher energy level.
These excited electrons travel through the electron transport chain, losing energy that is used to produce ATP by photophosphorylation.
The electrons reach PSI, where a second absorption of light re‑energises them to an even higher energy level.
The re‑energised electrons are then transferred to NADP⁺, reducing it to NADPH with the addition of protons (H⁺).
These excited electrons travel through the electron transport chain, losing energy that is used to produce ATP by photophosphorylation.
The electrons reach PSI, where a second absorption of light re‑energises them to an even higher energy level.
The re‑energised electrons are then transferred to NADP⁺, reducing it to NADPH with the addition of protons (H⁺).
Examiner tips
- Use the exact sequence: PSII excitation → electron transport → ATP synthesis → PSI re‑excitation → NADP⁺ reduction.
- Mention the role of protons in forming NADPH.
- Keep the answer concise and use the terminology "electron transport chain", "photophosphorylation" and "re‑energised".
Common mistakes
- Confusing the order of PSII and PSI excitation.
- Omitting the role of ATP synthesis or proton addition.
- Using vague terms like "light energy” without specifying the photosystems involved.
Mark scheme (4 marks)
- Light energy is absorbed by pigments in photosystem II, causing excitation/boosting of electrons to a higher energy level
- Excited electrons from PSII pass along an electron transport chain, losing energy used to produce ATP (by photophosphorylation)
- Electrons from the transport chain reach PSI, where light energy is absorbed again, re-energising/boosting the electrons to an even higher energy level
- The re-energised electrons from PSI are used to reduce NADP⁺ to NADPH (with the addition of H⁺/protons)
Key terms in this question
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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