The shape of the lesson
By the end of the lesson, learners can
- Two photosystems, one membrane: Start on non-cyclic.
- From electrons to ATP: Watch the gold protons pile up above the membrane each time an electron passes the carriers, then stream down through ATP synthase.
- Non-cyclic: PSII + PSI, photolysis, ATP + reduced NADP + O2.
- Cyclic: PSI only, ATP only.
- Oxygen comes from water, split by the oxygen-evolving complex.
- H+ pumped into the thylakoid space, back out through ATP synthase.
One picture every idea returns to
The light-dependent stage happens on the thylakoid membranes by photophosphorylation: making ATP using light energy. Light absorbed by a photosystem causes photoactivation of chlorophyll: electrons are excited to a higher energy level and leave it. In non-cyclic photophosphorylation, PSII and PSI are both involved, the oxygen-evolving complex catalyses the photolysis of water to replace the electrons lost from PSII, and both ATP and reduced NADP are made, with oxygen as a by-product. In cyclic photophosphorylation only PSI is involved; electrons return to PSI and only ATP is made. In both, energetic electrons release energy along the electron transport chain; that energy transfers protons into the thylakoid space, and the protons return to the stroma by facilitated diffusion through ATP synthase, providing the energy for ATP synthesis.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Excited electrons, pumped protons.. On the thylakoid membranes, light knocks electrons out of chlorophyll. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Two photosystems, one membrane | Start on non-cyclic. Project tab 4 of the lab (Photosynthesis lab, the light-dependent stage) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: From electrons to ATP | Watch the gold protons pile up above the membrane each time an electron passes the carriers, then stream down through ATP synthase. | Jigsaw |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 18 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in From electrons to ATP, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Oxygen comes from water, split by photolysis, not from carbon dioxide. | And cyclic photophosphorylation makes no reduced NADP and no oxygen: check the counters in the simulator, where only ATP keeps rising. |
| Protons are pumped into the thylakoid space and flow out to the stroma through ATP synthase, so ATP is made on the stroma side, where the Calvin cycle needs it. | Do not mix this up with the mitochondrion, where protons are pumped into the intermembrane space. |
The light-dependent stage: worksheet
6 questions, 18 marks. Written in the style of the papers; no past paper question is reproduced.
State the two types of photophosphorylation that occur in the light-dependent stage and the products of each.
Explain the roles of photosystem II and the oxygen-evolving complex in non-cyclic photophosphorylation.
Explain how the movement of electrons along the electron transport chain in the thylakoid membrane leads to the synthesis of ATP.
Compare cyclic and non-cyclic photophosphorylation, giving three differences.
Isolated chloroplasts are put in the light. The pH of the thylakoid space falls from about 7.5 to 4.5 within a minute, while the stroma rises to about pH 8. Explain these changes and their importance.
Suggest why a chloroplast whose PSII has been damaged can still make some ATP in the light, but cannot fix carbon dioxide for long.
Answer key and marking notes
- Non-cyclic photophosphorylation: ATP and reduced NADP (with oxygen as a by-product).
- Cyclic photophosphorylation: ATP only.
- Light absorbed by PSII causes photoactivation of chlorophyll: electrons are excited and leave the reaction centre.
- These electrons pass along the electron transport chain to PSI.
- The oxygen-evolving complex catalyses photolysis of water: 2H2O → 4H+ + 4e− + O2.
- The electrons replace those lost by PSII; the protons add to the gradient / help reduce NADP; oxygen is released.
- Energetic (excited) electrons release energy as they pass from carrier to carrier.
- This energy is used to transfer protons from the stroma across the thylakoid membrane into the thylakoid space.
- A proton concentration gradient forms (high in the thylakoid space).
- Protons return to the stroma by facilitated diffusion through ATP synthase, providing energy to make ATP from ADP and Pi.
- Cyclic involves PSI only; non-cyclic involves PSI and PSII.
- Non-cyclic includes photolysis of water and releases oxygen; cyclic does not.
- Non-cyclic makes reduced NADP as well as ATP; cyclic makes only ATP (electrons return to PSI rather than going to NADP).
- Energy released by electrons in the electron transport chain transfers protons from the stroma into the thylakoid space (and photolysis releases protons inside), so the thylakoid space becomes more acidic.
- Protons are removed from the stroma (transferred across the membrane and used to reduce NADP), so its pH rises.
- The steep gradient (about 1000 times more H+ inside) drives protons through ATP synthase, providing the energy to make ATP.
- PSI can still carry out cyclic photophosphorylation, pumping protons and making ATP.
- But no electrons flow from water through PSII to NADP, so no reduced NADP is made; GP cannot be reduced to TP, RuBP is not regenerated and fixation soon stops.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which substance is the source of the oxygen released in photosynthesis?
- Awater
- Bcarbon dioxide
- Cglucose
- Dribulose bisphosphate
Which statement about cyclic photophosphorylation is correct?
- Aonly photosystem I is involved and ATP is the only product
- Bphotosystems I and II are both involved and oxygen is released
- Conly photosystem II is involved and reduced NADP is made
- Dno photoactivation of chlorophyll occurs, so no light is needed
What is meant by photoactivation of chlorophyll?
- Aabsorbed light energy raises electrons in chlorophyll to a higher energy level, so they leave it
- Blight energy splits chlorophyll molecules to release electrons and magnesium ions
- Clight energy is used directly by ATP synthase to join ADP and phosphate
- Dchlorophyll is made in the chloroplast only when the plant is in the light
Where in the chloroplast is the concentration of protons highest during the light-dependent stage?
- Ain the thylakoid space
- Bin the stroma
- Cbetween the two membranes of the envelope
- Din the cytoplasm outside the chloroplast
Which products of non-cyclic photophosphorylation are used in the Calvin cycle?
- AATP and reduced NADP
- BATP and oxygen
- Creduced NADP and oxygen
- Doxygen and protons from water
A compound makes the thylakoid membrane permeable to protons. Chloroplasts treated with it are illuminated. Which result is expected?
- Aelectron transport continues but little ATP is made
- BATP synthesis increases because protons move faster
- Celectron transport stops, so no reduced NADP is made
- Doxygen release stops but ATP synthesis continues
Chloroplasts are illuminated with light of a wavelength absorbed by PSI but not by PSII. Which pair of products would be made?
- AATP only, with no oxygen released
- BATP and reduced NADP, with oxygen released
- Creduced NADP only, with oxygen released
- DATP and reduced NADP, with no oxygen released
The pH of the thylakoid space falls from 7.0 to 4.0 when a chloroplast is illuminated. By what factor has the proton concentration increased?
- A1000
- B3
- C300
- D30
Quiz key
The oxygen-evolving complex of PSII catalyses the photolysis of water: 2H2O → 4H+ + 4e− + O2. The oxygen atoms of carbon dioxide end up in organic molecules and water made in the Calvin cycle, not in the oxygen gas. Glucose is a product, not a source of oxygen. RuBP accepts carbon dioxide in the stroma and releases no oxygen.
In cyclic photophosphorylation, electrons excited at PSI pass along the electron transport chain and return to PSI, so only ATP is made. Both photosystems and oxygen release belong to non-cyclic photophosphorylation. PSII is not involved in the cyclic pathway, and reduced NADP is not made because the electrons return to PSI. Light is still needed: chlorophyll in PSI must be photoactivated.
When chlorophyll at a reaction centre absorbs light energy, an electron is excited to a higher energy level and is emitted to an electron acceptor. The chlorophyll molecule is not split. ATP synthase is powered by the proton gradient, not by light directly. The synthesis of chlorophyll is a different process from photoactivation.
Energy released by electrons in the electron transport chain transfers protons from the stroma into the thylakoid space, and photolysis releases more protons there, so the thylakoid space reaches about pH 4-5. The stroma loses protons (pH about 8). The envelope space and the cytoplasm are not part of the chemiosmotic gradient used by ATP synthase in chloroplasts; confusing these with the mitochondrial intermembrane space is a common error.
ATP supplies energy and reduced NADP supplies reducing power for the Calvin cycle in the stroma. Oxygen is a by-product: it diffuses out of the leaf or is used in respiration and plays no part in the Calvin cycle. The protons from water contribute to the gradient and to reducing NADP, but are not used directly by the Calvin cycle.
If protons can leak back across the membrane, no proton gradient builds up, so ATP synthase has no source of energy and little ATP is made; electrons can still flow from water to NADP. ATP synthesis depends on protons passing through ATP synthase, so leaking around it reduces synthesis. Electron flow does not need a gradient, so reduced NADP and oxygen are still produced. Oxygen release continues because photolysis still happens.
With only PSI excited, cyclic photophosphorylation can run: electrons cycle back to PSI and ATP is made. PSII cannot replace electrons by photolysis, so no oxygen is released and no continuous supply of electrons reaches NADP. Oxygen release requires PSII activity. Reduced NADP without ATP, or reduced NADP without photolysis, cannot be sustained because electrons lost from PSI to NADP would not be replaced.
A fall of 3 pH units means [H+] has risen by 103 = 1000 times. 3 is just the change in pH, treating the scale as linear. 300 and 30 mix the logarithmic and linear scales. A gradient this steep stores a lot of energy for ATP synthase.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.