Chemiosmosis at Higher Level: An AHL statement: the light-dependent reactions make ATP by the same chemiosmosis as respiration (3.2.15).
Practise this as you read
- Describe how light energy is used to make ATP.
- Compare chemiosmosis in chloroplasts and mitochondria.
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The big idea: Excited electrons lose energy as they pass along electron carriers in the thylakoid membrane. The carriers use it to pump protons into the thylakoid space. The protons flow back through ATP synthase, which makes ATP: chemiosmosis. Making ATP with light energy is called photophosphorylation.
The points to remember
- Electrons from photosystem II pass along a chain of carriers to photosystem I.
- The energy released pumps protons from the stroma into the thylakoid space.
- Photolysis adds more protons inside; the small space makes a steep proton gradient.
- Protons diffuse back through ATP synthase, which makes ATP from ADP and phosphate.
- The ATP is made in the stroma, where the Calvin cycle uses it.
Remember it as: Pump in, flow out, ATP made.
Real example: the same machine in mitochondria. ATP synthase is almost the same protein in chloroplasts and mitochondria: chemiosmosis is one mechanism used in both photosynthesis and respiration (3.2.15).
Trap: The protons are pumped into the thylakoid space; ATP is made as they flow out, into the stroma.
The same mechanism as in respiration, run by light:
| Chloroplast (photosynthesis) | Mitochondrion (respiration) | |
|---|---|---|
| Energy source | Light, through excited electrons | Reduced NAD and FAD from food |
| Protons pumped into | The thylakoid space | The intermembrane space |
| ATP made in | The stroma | The matrix |
| Same mechanism | Chemiosmosis through ATP synthase | Chemiosmosis through ATP synthase |
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How it is asked: Expect a long Describe question on how light energy is used to make ATP, or data on the proton gradient.
IB-style questionDescribe[7 marks]
Describe how plant cells use light energy to produce ATP.
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