Lesson 3 of 5 / Photosynthesis and energy capture
Use a proton gradient to make ATP
Why must thylakoid membranes remain intact?
In this lesson: Explain photophosphorylation, chemiosmosis and cyclic electron flow.
About 7 min
The key ideaA proton gradient across the thylakoid membrane drives ATP synthase as protons return from lumen to stroma.
Explore the idea
Store energy across a membrane
Energy from light-driven electron transfer supports proton movement towards the thylakoid lumen. Their controlled return to the stroma down the electrochemical gradient drives ATP synthesis.
Water oxidation also releases protons in the lumen, while NADP reduction consumes stromal protons. ATP is made on the stromal side.
H+ symbols show a qualitative difference, not measured concentrations or an ATP-per-proton ratio. The gradient also has an electrical component. Pi means inorganic phosphate. Electron carriers and ATP synthase are simplified; the leak bypasses the normal coupling route.
Compare cyclic and non-cyclic flow
Non-cyclic flow needs replacement electrons from water and ends in NADP reduction. Its proton gradient supports ATP formation, and water oxidation releases oxygen.
Explanation
Water oxidation adds protons to the thylakoid lumen, and electron transport moves additional protons from stroma towards the lumen. Reduction of NADP also consumes stromal protons. Together these processes support an electrochemical proton gradient across the membrane.
The lipid membrane restricts uncontrolled proton movement. Protons return down their electrochemical gradient through ATP synthase, coupling this movement to phosphorylation of ADP with inorganic phosphate. ATP is produced on the stromal side, where it can be used by the Calvin cycle.
If the membrane becomes freely permeable to protons, the gradient dissipates. Electron transfer may still occur for a time, but ATP synthesis by this coupling is reduced. This separates the roles of electron transport, membrane integrity and ATP synthase.
In cyclic photophosphorylation, electrons excited at photosystem I return through an electron-transfer route rather than reducing NADP. This can support additional ATP formation without net reduced NADP or oxygen production from that cyclic route. It helps adjust the balance of ATP and reducing power; it does not replace the need for non-cyclic flow to supply both.
Step by step
- 1
Name the compartments
Do not say merely inside and outside.
- 2
Trace proton movement
Uphill accumulation then downhill return.
- 3
Predict an uncoupler effect
A leaky membrane loses stored gradient energy.
Worked example
Work through the evidence
A chemical makes thylakoid membranes permeable to protons without directly blocking electron carriers. Predict ATP formation.
One way to explain it
ATP formation falls because protons bypass ATP synthase and the electrochemical gradient is dissipated, weakening chemiosmotic coupling.
Why this answer works
- Electron transfer and ATP synthesis are connected but distinct.
- The membrane stores the gradient only if uncontrolled leakage is limited.
Is this true? "ATP is produced because protons collect forever and never return."
ATP synthesis depends on their controlled downhill return through ATP synthase.