A2 1 structured question, photosynthesis — recurring in most series · 5 marks
Describe how ATP is produced during the light-dependent reaction of photosynthesis.
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A2 · A2 1 Physiology, Co-ordination and Control, and Ecosystems
The light-dependent reaction, explained for A-level Biology
The short answer
The light-dependent reaction happens on the thylakoid membranes. Light excites electrons in chlorophyll; those electrons pass along an electron transport chain, releasing energy that pumps protons into the thylakoid space. Protons flow back through ATP synthase to make ATP, water is photolysed to replace lost electrons, and NADP is reduced.
Step by step, in the order examiners want it
- Light is absorbed by chlorophyll in photosystem II; two electrons are excited to a higher energy level and leave the pigment.
- The electrons pass down a chain of carriers in the thylakoid membrane, losing energy at each transfer.
- That energy pumps protons (H+) from the stroma into the thylakoid space, creating a proton gradient.
- Protons diffuse back into the stroma through ATP synthase; the flow drives phosphorylation of ADP to ATP (chemiosmosis).
- Photolysis of water replaces the electrons lost from photosystem II: 2H₂O → 4H+ + 4e⁻ + O₂. The oxygen is the by-product released.
- At photosystem I the electrons are re-excited and used, with H+, to reduce NADP to reduced NADP.
Where the products go
ATP and reduced NADP are the only two things the light-dependent reaction hands over to the light-independent reaction. Say so explicitly — the mark is usually for naming the destination, not just the product.
In the Calvin cycle, reduced NADP reduces glycerate 3-phosphate (GP) to triose phosphate, and ATP supplies energy both for that reduction and for regenerating ribulose bisphosphate (RuBP).
Cyclic vs non-cyclic photophosphorylation
| Non-cyclic | Cyclic | |
|---|---|---|
| Photosystems used | PSII and PSI | PSI only |
| Electron source | Photolysis of water | Recycled from PSI |
| Products | ATP, reduced NADP, oxygen | ATP only |
Phrases that earn the marks
- electron excited / raised to a higher energy level
- electron transport chain / chain of carriers in the thylakoid membrane
- proton gradient across the thylakoid membrane
- chemiosmosis through ATP synthase
- photolysis of water replaces electrons lost from photosystem II
- NADP is reduced / reduced NADP
Where marks get lost
- Writing 'energy is made'. Energy is transferred; ATP is made.
- Saying oxygen comes from carbon dioxide. It comes from the photolysis of water — this is a standard one-mark trap.
- Naming ATP and reduced NADP as products but never saying they are used in the Calvin cycle.
- Describing photosystems as enzymes. They are pigment–protein complexes in the thylakoid membrane.
Try these light dependent reaction questions yourself
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These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
Exam questions on light dependent reaction, marked
Two A2-standard questions in CCEA paper style, each with the full mark scheme, a full-mark answer written the way you should write it, and the point where most candidates drop marks.
A2 1 structured question, photosynthesis — recurring in most series · 5 marks
Describe how ATP is produced during the light-dependent reaction of photosynthesis.
Command word: Describe — how to answer itMark scheme — 6 creditworthy points
- Light absorbed by chlorophyll in photosystem II / electrons excited to a higher energy level
- Electrons pass along the electron transport chain in the thylakoid membrane
- Energy released is used to pump protons (H⁺) from the stroma into the thylakoid space
- A proton gradient is established across the thylakoid membrane
- Protons move back into the stroma through ATP synthase (chemiosmosis)
- ADP + Pi → ATP / phosphorylation of ADP
Full-mark answer
Light is absorbed by chlorophyll in photosystem II and electrons are excited to a higher energy level. These electrons pass along a chain of electron carriers in the thylakoid membrane, and the energy released at each transfer is used to pump protons from the stroma into the thylakoid space. This creates a proton gradient across the thylakoid membrane. Protons then diffuse back into the stroma through ATP synthase, and the movement of protons drives the phosphorylation of ADP to ATP.
Examiner insight: Five marks are available but there are six marking points, so you can afford to lose one. Students who lose marks almost always omit the proton gradient step — they jump straight from the electron transport chain to ATP synthase. Name the compartments (stroma, thylakoid space) explicitly.
A2 1 application question — isotope-labelling contexts · 3 marks
Explain why the oxygen released by a photosynthesising plant originates from water and not from carbon dioxide.
Command word: Explain — how to answer itMark scheme — 4 creditworthy points
- Water is photolysed / split by light in photosystem II
- 2H₂O → 4H⁺ + 4e⁻ + O₂ (equation or description)
- Carbon dioxide is used in the light-independent reaction / fixed by RuBP to form GP, so its oxygen stays in the carbohydrate
- Isotope-labelled H₂¹⁸O produces ¹⁸O₂ (allow as evidence point)
Full-mark answer
Water is photolysed in photosystem II to replace the electrons lost from chlorophyll: 2H₂O → 4H⁺ + 4e⁻ + O₂, so the oxygen released is a by-product of splitting water. Carbon dioxide is not split; it is fixed by RuBP in the light-independent reaction and its oxygen atoms remain in GP and then in the carbohydrate produced. Experiments using H₂¹⁸O show that the ¹⁸O appears in the oxygen released, confirming water as the source.
Examiner insight: The third marking point is where most answers stop short. You must say what happens to the carbon dioxide, not only what happens to the water.
These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
Download the official CCEA paper and mark schemeCommon questions
- Where does the light-dependent reaction take place?
- On the thylakoid membranes of the chloroplast, with the proton gradient built up in the thylakoid space (lumen). The light-independent reaction happens in the stroma.
- What are the products of the light-dependent reaction?
- ATP, reduced NADP and oxygen. ATP and reduced NADP pass to the Calvin cycle; oxygen is released as a by-product of photolysis.
- Why is it called photophosphorylation?
- Because light energy (photo-) drives the addition of a phosphate group to ADP (phosphorylation) to make ATP.
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