Light-Dependent Reactions of Photosynthesis, Step by Step
AP Biology / first-year college · 12 flashcards · 7 quiz questions · updated 2026-08-19
The light-dependent reactions happen in the thylakoid membrane and produce three things: ATP, NADPH and the oxygen you are breathing right now. Everything they make is spent by the Calvin cycle in the stroma.
The trap in this topic is the numbering. Photosystem II acts before photosystem I — they are numbered in the order they were discovered, not the order they work.
The path of an electron
- •Light strikes photosystem II (P680). An excited electron leaves the reaction centre.
- •Photolysis replaces it: water is split into 2 electrons, 2 H⁺ and ½ O₂. This is the only source of the oxygen released by plants.
- •The electron travels down the chain — plastoquinone, the cytochrome b₆f complex, plastocyanin — and the energy released pumps H⁺ into the thylakoid lumen.
- •Light strikes photosystem I (P700) and re-energises the electron.
- •Ferredoxin passes it to NADP⁺ reductase, which reduces NADP⁺ to NADPH in the stroma.
Chemiosmosis: where the ATP comes from
Pumping protons into the lumen builds a steep concentration gradient — the lumen can reach pH 5 while the stroma sits near pH 8. Protons flow back through ATP synthase, and that flow drives phosphorylation of ADP.
Because the energy originally came from light, this is called photophosphorylation. The mechanism is the same chemiosmosis used by mitochondria; only the source of the gradient differs.
Cyclic vs non-cyclic photophosphorylation
| Non-cyclic | Cyclic | |
|---|---|---|
| Photosystems used | PSII then PSI | PSI only |
| Electron source | Water | Recycled from ferredoxin |
| Oxygen released | Yes | No |
| NADPH made | Yes | No |
| ATP made | Yes | Yes |
| When it dominates | Normal conditions | When the cell needs extra ATP relative to NADPH |
What the Calvin cycle does with the products
The Calvin cycle runs in the stroma and does not need light directly — calling it the 'dark reaction' is misleading, since it stops in darkness once ATP and NADPH run out.
Fixing one CO₂ costs 3 ATP and 2 NADPH. Making one glucose takes six turns: 18 ATP and 12 NADPH.
Structure that makes it work
- •Thylakoids are flattened sacs stacked into grana — stacking maximises the membrane area holding photosystems.
- •The lumen is a small enclosed space, so a modest number of pumped protons makes a steep gradient.
- •Chlorophyll a is the reaction-centre pigment; accessory pigments (chlorophyll b, carotenoids) widen the range of wavelengths absorbed and pass energy inward.
- •Chlorophyll absorbs red and blue light and reflects green — which is why leaves look green.
Common mistakes
- ✗Saying the oxygen released comes from carbon dioxide. It comes from splitting water.
- ✗Assuming PSI acts before PSII because of the numbers.
- ✗Calling the Calvin cycle the 'dark reaction' as though it happens at night — it needs the ATP and NADPH the light reactions supply.
- ✗Placing ATP synthesis in the stroma. The enzyme sits in the thylakoid membrane; the ATP is released into the stroma.
- ✗Writing that NADPH is made in the lumen. NADP⁺ reductase faces the stroma, which is where the Calvin cycle needs it.
Flashcards
Tap a card to reveal the answer.
Where do the light-dependent reactions occur?⌄
In the thylakoid membrane of the chloroplast.
What are the three products of the light reactions?⌄
ATP, NADPH and O₂.
Where does the released oxygen come from?⌄
Photolysis — the splitting of water at photosystem II.
Which photosystem acts first?⌄
Photosystem II (P680), despite the numbering.
What does photosystem I produce?⌄
Re-energised electrons that ferredoxin passes to NADP⁺ reductase, forming NADPH.
How is the proton gradient built?⌄
Electron transport pumps H⁺ from the stroma into the thylakoid lumen; photolysis also releases H⁺ into the lumen.
What is photophosphorylation?⌄
ATP synthesis driven by a light-generated proton gradient flowing through ATP synthase.
What does cyclic photophosphorylation produce?⌄
ATP only — no NADPH and no oxygen, because PSI recycles its own electrons.
What does the Calvin cycle need per CO₂ fixed?⌄
3 ATP and 2 NADPH.
Why are thylakoids stacked into grana?⌄
Stacking packs more photosystem-bearing membrane into the same volume.
Which wavelengths does chlorophyll absorb best?⌄
Red and blue; green is reflected, which is why leaves appear green.
What is the role of accessory pigments?⌄
They absorb wavelengths chlorophyll a misses and transfer that energy to the reaction centre.
Practice quiz
Answer first, then open the explanation.
1. The oxygen released during photosynthesis comes from:
- A. Carbon dioxide
- B. Water
- C. Glucose
- D. ATP
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B. Water
Photolysis at photosystem II splits water into electrons, protons and oxygen.2. Which sequence is correct?
- A. PSI → PSII → NADPH
- B. PSII → PSI → NADPH
- C. PSII → NADPH → PSI
- D. PSI → NADPH → PSII
Show answer
B. PSII → PSI → NADPH
Photosystem II passes electrons down the chain to photosystem I, which reduces NADP⁺.3. Where is the proton concentration highest during the light reactions?
- A. The stroma
- B. The thylakoid lumen
- C. The cytosol
- D. The intermembrane space
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B. The thylakoid lumen
Protons are pumped into the lumen, which becomes far more acidic than the stroma.4. Cyclic photophosphorylation produces:
- A. ATP only
- B. NADPH only
- C. ATP and NADPH
- D. ATP, NADPH and O₂
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A. ATP only
Electrons cycle back from ferredoxin through the chain to PSI, driving proton pumping but reducing no NADP⁺.5. How many ATP and NADPH does the Calvin cycle use to make one glucose?
- A. 3 ATP and 2 NADPH
- B. 6 ATP and 6 NADPH
- C. 18 ATP and 12 NADPH
- D. 36 ATP and 24 NADPH
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C. 18 ATP and 12 NADPH
Six turns at 3 ATP and 2 NADPH each.6. ATP synthase in the chloroplast is located in:
- A. The stroma
- B. The thylakoid membrane
- C. The outer chloroplast membrane
- D. The lumen
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B. The thylakoid membrane
It spans the thylakoid membrane and releases ATP into the stroma, where the Calvin cycle uses it.7. Why do leaves appear green?
- A. Chlorophyll absorbs green light most strongly
- B. Green light has too little energy to be absorbed
- C. Chlorophyll reflects green light while absorbing red and blue
- D. Accessory pigments emit green light
Show answer
C. Chlorophyll reflects green light while absorbing red and blue
Absorption peaks in red and blue; the green wavelengths are reflected to your eye.
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