3.3.9Inquiry Process
Photophosphorylation — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 124 of 160
Section 1 of 5Non-Cyclic Photophosphorylation
The one-way flow of electrons from water to NADP⁺ is often drawn as the Z-scheme.
1
Photosystem II absorbs light, which excites electrons in its reaction centre so that they leave chlorophyll and are accepted by a primary electron acceptor.
2
Photolysis of water replaces the lost electrons: 2H₂O → 4H⁺ + 4e⁻ + O₂, so oxygen is released as a by-product and protons build up inside the thylakoid space.
3
The excited electrons pass along an electron transport chain between the photosystems, and each transfer releases a little energy used to pump H⁺ from stroma into thylakoid space.
4
Photosystem I absorbs light and re-excites the electrons, which are passed to ferredoxin and then to NADP reductase.
5
NADP reductase uses the electrons and H⁺ from the stroma to reduce NADP⁺ to NADPH, the final electron acceptor of the pathway.
Section 2 of 5Chemiosmosis and ATP Synthesis
Chemiosmosis and ATP Synthesis
Proton gradient
- Water splitting and proton pumping make the thylakoid space far more acidic than the stroma, storing energy as a concentration and charge difference.
ATP synthase
- Protons flow back down their gradient through channels in ATP synthase, which rotates and drives ADP + Pᵢ → ATP.
Chemiosmosis
- The coupling of proton movement down a gradient to ATP synthesis is called chemiosmosis, the same principle as in the mitochondrion.
Light-dependent
- Because light energy is what ultimately drives the gradient,
- making ATP this way is photophosphorylation, not oxidative phosphorylation.
Location
- All of these components sit in the thylakoid membrane, and the stroma next door receives the ATP and NADPH for the Calvin cycle.
Section 3 of 5Cyclic Photophosphorylation
Photosystem I only
- Electrons leaving photosystem I return to it through the electron transport chain instead of being passed to NADP⁺.
ATP only
- The cycling electrons still pump protons so ATP is made, but no NADPH is formed and no water is split.
No oxygen
- Since photosystem II is not involved and photolysis does not occur, cyclic photophosphorylation does not release oxygen.
Why it matters
- The Calvin cycle needs more ATP than NADPH, so cyclic flow helps balance the supply when non-cyclic flow alone is not enough.
| Feature | Non-cyclic | Cyclic |
|---|---|---|
| Photosystems | II and I | I only |
| Electron source | Water | Photosystem I itself |
| Products | ATP, NADPH, O₂ | ATP only |
| Photolysis | Yes | No |
Section 4 of 5Worked Example
Question
- Per molecule of glucose made, the Calvin cycle uses 18 ATP and 12 NADPH.
- How many electrons must come from water, and how many water molecules are split, to make the 12 NADPH?
- Each NADPH carries two electrons from photosystem I, so 12 × 2 = 24 electrons are needed.
- Photolysis gives 2H₂O → 4H⁺ + 4e⁻ + O₂, so one water molecule yields 2 electrons.
- Water molecules split = 24 ÷ 2 = 12, and oxygen released = 12 ÷ 2 = 6 O₂.
Answer 24 electrons are needed, which means 12 H₂O are split and 6 O₂ are released.
Section 5 of 5Exam Checklist & Tips
3.3.9 – Key Points for the Exam
- Photophosphorylation Synthesis of ATP from ADP and Pᵢ using energy from light, via a proton gradient across the thylakoid membrane.
- Photolysis Light-driven splitting of water that supplies electrons, protons and oxygen to the light-dependent reactions.
- Chemiosmosis Flow of protons down their gradient through ATP synthase, coupled to ATP formation.
- NADP reductase Enzyme that transfers electrons and H⁺ to NADP⁺ to form NADPH at the end of the chain.
- Electron transport chain Series of carriers in the thylakoid membrane that pass electrons and pump protons.
Common Mistakes
- Saying that oxygen comes from carbon dioxide; it comes from the photolysis of water.
- Confusing the thylakoid space with the stroma when naming where protons accumulate.
- Stating that cyclic photophosphorylation produces NADPH; it makes ATP only.
- Calling the process oxidative phosphorylation, which is the mitochondrial equivalent.
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