The light-dependent stage
Introduced| English | Meaning |
|---|---|
| photophosphorylation/ˌfəʊtəʊˌfɒsfɔːrɪˈleɪʃn/ | The formation of ATP from ADP and inorganic phosphate using energy captured during photosynthesis. |
| cyclic/ˈsaɪklɪk/ | PSI only → ATP only. |
| non-cyclic/nɒn ˈsaɪklɪk/ | PSI + PSII → photolysis of water (releases O₂) → ATP + reduced NADP. |
| photosystem/ˌfəʊtəʊˈsɪstəm/ | A complex of pigments and proteins that captures light and initiates electron transfer during photosynthesis. |
| photolysis/fəʊˈtɒləsɪs/ | Chemical breakdown caused by light; in photosynthesis, water is split to provide electrons, hydrogen ions and oxygen. |
| ATP synthase/ˌeɪ tiː ˈpiː ˈsɪnθeɪs/ | An enzyme complex that uses the movement of protons down their electrochemical gradient to synthesise ATP. |
Capturing the Sun
- In the thylakoids, light energy is used to make ATP.
- This is photophosphorylation 光合磷酸化, and it comes in two forms.
- One form also splits water and releases the oxygen we breathe.
Water is split in the light-dependent stage by ______.
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Photolysis releases electrons, H⁺ and oxygen — the oxygen we breathe is a by-product.
Cyclic 循环 vs non-cyclic 非循环
- Cyclic photophosphorylation: uses photosystem 光系统 I only; chlorophyll is photoactivated; makes only ATP.
- Non-cyclic photophosphorylation: uses photosystems I and II; the oxygen-evolving complex carries out photolysis 光解 of water (releasing oxygen); makes ATP and reduced NADP.
Non-cyclic photophosphorylation in the light-dependent stage
The light-dependent stage
Step through it. Light excites electrons and splits water; the result is ATP, reduced NADP and oxygen.
Cyclic photophosphorylation:
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Cyclic photophosphorylation uses PSI alone and produces ATP only (no NADP reduced, no water split).
Non-cyclic photophosphorylation produces:
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Using PSI and PSII, it makes ATP and reduced NADP, and photolysis of water releases oxygen.
How the ATP is made
- In both forms:
- energetic electrons pass along an electron transport chain, releasing energy.
- that energy pumps protons across the thylakoid membrane.
- protons flow back into the stroma through ATP synthase ATP合酶, making ATP (chemiosmosis).
Photolysis splits water using light energy, releasing oxygen as a by-product.
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The oxygen released by photolysis is the source of the oxygen in our atmosphere.
In the thylakoid, ATP is made as protons flow back through:
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Protons pumped into the thylakoid space flow back through ATP synthase, driving ATP synthesis.
Match each part of the light-dependent stage to what it does.
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Light excites electrons and splits water; the chain makes ATP; reduced NADP carries hydrogen onward.
Photolysis of water
- Light energy splits water in photolysis: water → hydrogen ions + electrons + oxygen.
- The electrons replace those lost by chlorophyll, and the oxygen is released as a waste product.
You've got it
- the light-dependent stage = photophosphorylation in the thylakoids
- cyclic: PSI only → ATP only
- non-cyclic: PSI + PSII → photolysis of water (releases O₂) → ATP + reduced NADP
- electrons pump protons → flow back through ATP synthase → ATP (chemiosmosis)