9477 / 2027

Lesson 3 of 5 / Photosynthesis and energy capture

Use a proton gradient to make ATP

Why must thylakoid membranes remain intact?

I can explain photophosphorylation, chemiosmosis and cyclic electron flow.

  • I can explain light-dependent energy conversion in chloroplasts.Syllabus

    Syllabus 9477, Core 3(c). Explain light-dependent energy conversion in chloroplasts.

  • I can compare chemiosmosis in photosynthesis and respiration.Syllabus

    Syllabus 9477, Core 3(l). Compare chemiosmosis in photosynthesis and respiration.

About 7 min

Make a guess. You are not marked.

A chemical makes thylakoid membranes leaky to protons. Light still drives electron flow. What happens to ATP production?

  1. Nothing changes, because electrons are still flowing.
  2. It rises, because protons move faster.
  3. It falls, because no proton gradient builds up.
Show the answer

It falls, because no proton gradient builds up.

ATP synthase needs protons flowing back down a gradient. A leaky membrane lets the gradient vanish.

The key idea

A proton gradient across the thylakoid membrane drives ATP synthase as protons return from lumen to stroma.

Explore the idea

Store energy across a membrane

Thylakoid lumenStromaH+H+H+H+H+H+H+H+H+H+H+H+PumpingATP synthaseADP + Pi -> ATP

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
Water -> PSII -> PSINADP accepts electronsATP + NADPH; O2 released

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. 1

    Name the compartments

    Do not say merely inside and outside.

  2. 2

    Trace proton movement

    Uphill accumulation then downhill return.

  3. 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.

Watch out for this

A student says: "ATP is produced because protons collect forever and never return." What is wrong with this?

Show the answer

ATP synthesis depends on their controlled downhill return through ATP synthase.

Check your understanding

Which is produced by cyclic photophosphorylation without net NADP reduction?

  1. Oxygen from a required new water-splitting step in that cycle
  2. Reduced NADP as a net product
  3. ATP
Show the answer

ATP

Cyclic electron flow supports proton-gradient-driven ATP synthesis.

Quick recall

Card 1 of 3. Answer in your head, then check.

Why must thylakoid membranes remain intact?

You can return to this lesson any time.