Explain how light energy is used to produce ATP and 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 is absorbed by the chlorophyll and other pigments in the reaction centre of photosystems I and II, exciting electrons to a high‑energy state.
The excited electrons are transferred to the electron transport chain (ETC). As they move down the ETC, energy is released and is used to pump H⁺ ions from the stroma into the thylakoid lumen, creating a proton gradient.
The proton gradient drives ATP synthase, which synthesises ATP from ADP and Pi – a process called photophosphorylation.
At photosystem I the electrons are re‑energised by light and then transferred to NADP⁺ together with a proton from the stroma, reducing it to NADPH.
Thus light energy is converted into the chemical energy of ATP and NADPH, which are used in the Calvin cycle.
The excited electrons are transferred to the electron transport chain (ETC). As they move down the ETC, energy is released and is used to pump H⁺ ions from the stroma into the thylakoid lumen, creating a proton gradient.
The proton gradient drives ATP synthase, which synthesises ATP from ADP and Pi – a process called photophosphorylation.
At photosystem I the electrons are re‑energised by light and then transferred to NADP⁺ together with a proton from the stroma, reducing it to NADPH.
Thus light energy is converted into the chemical energy of ATP and NADPH, which are used in the Calvin cycle.
Examiner tips
- Show the sequence: light absorption → electron excitation → ETC → proton pumping → ATP synthase → ATP; then PSI → NADP⁺ reduction → NADPH
- Use the exact terms: photophosphorylation, proton gradient, electron transport chain, NADPH
Common mistakes
- Confusing the order of photosystems (mixing PSI and PSII roles)
- Omitting the proton gradient step or the role of ATP synthase
- Using vague terms like ‘energy’ instead of ‘photophosphorylation’
Mark scheme (4 marks)
- Light energy is absorbed by photosystems (I and/or II) / by photosynthetic pigments such as chlorophyll
- Excited / high-energy electrons are released from chlorophyll and passed along the electron transport chain
- Energy released from electrons moving along the electron transport chain drives the pumping of protons / H⁺ ions, establishing a proton gradient that drives ATP synthase to produce ATP (by photophosphorylation)
- At photosystem I, re-energised electrons are used to reduce NADP⁺ to NADPH (with H⁺ from the stroma)
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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