Explain chemiosmosis in the thylakoid membrane
Trace electron transport, proton movement and ATP synthesis during photophosphorylation.
Section 13.1 · Photosynthesis as an energy transfer process
A Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; practical investigations need supervised work and human subject-teacher review remains pending.
Before you begin
- Chloroplasts contain photosynthetic pigments, and enzymes catalyse metabolic reactions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What provides energy for ATP synthesis by chloroplast ATP synthase?
What you will learn
- Explain how electron transport creates a thylakoid proton gradient
- Relate proton return through ATP synthase to ATP synthesis
Build the explanation
During photophosphorylation, energetic electrons release energy as they pass through an electron transport chain in the thylakoid membrane. This energy is used to transfer protons across the membrane into the thylakoid space. A proton gradient forms. Protons then return from the thylakoid space to the stroma by facilitated diffusion through ATP synthase. Their flow provides energy for phosphorylation of ADP to ATP. This chemiosmotic mechanism links light-driven electron transport to ATP synthesis.
Apply it carefully
Keep the proton direction correct: accumulation is in the thylakoid space and return flow is to the stroma through ATP synthase. Do not say protons return freely through the lipid membrane in this model or that light directly phosphorylates ADP.
Pause and explain
Across which membrane is the photophosphorylation proton gradient established?
Worked example
Which direction do protons move through ATP synthase in the chloroplast?
Show the worked solution
- They move down the gradient from the thylakoid space.
- Protons return to the stroma.
Answer From the thylakoid space to the stroma through ATP synthase.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Trace how electron transport at thylakoids leads to ATP synthesis by chemiosmosis.
Give me a hint
Include proton accumulation, return direction and ATP synthase.
Compare my reasoning
- Electrons release energy along the transport chain, driving proton transfer into the thylakoid space.
- Protons return to the stroma through ATP synthase, driving ADP phosphorylation.
Energetic electrons pass along the thylakoid electron transport chain and release energy used to move protons into the thylakoid space. Protons return by facilitated diffusion through ATP synthase to the stroma, and this flow supplies energy to synthesise ATP from ADP and phosphate.
Look for these in your work
- Electron energy used
- Gradient direction correct
- Return through ATP synthase
- ATP synthesis linked
A compound makes the thylakoid membrane freely permeable to protons. Predict the effect on chemiosmotic ATP synthesis.
Give me a hint
Ask whether a proton gradient can remain and whether protons pass through ATP synthase.
Compare my reasoning
- Protons leak across the membrane and dissipate the gradient.
- Less proton flow is coupled through ATP synthase, so chemiosmotic ATP synthesis decreases.
The proton gradient would dissipate as protons leak across the thylakoid membrane outside ATP synthase. Less proton flow would be coupled through ATP synthase, reducing ATP synthesis by chemiosmosis.
Look for these in your work
- Gradient dissipation predicted
- ATP-synthase route distinguished
- Effect explained
A diagram shows protons moving from the stroma into the thylakoid space through ATP synthase, with ATP formed. Identify the direction error and correct it.
Give me a hint
Separate the proton-pumping direction from the ATP-synthase return path.
Compare my reasoning
- Protons are pumped into the thylakoid space by electron-transfer energy.
- They return through ATP synthase from thylakoid space to stroma, driving ATP formation.
The ATP-synthase arrow is reversed. Electron transport builds the proton gradient by moving protons into the thylakoid space; protons return through ATP synthase to the stroma, supporting ATP synthesis.
Look for these in your work
- Pumping direction stated
- Return direction corrected
- ATP consequence retained
Common mistakes
- Reversing proton flow through ATP synthase
- Saying protons accumulate in the stroma
- Claiming light directly makes ATP without a gradient
What can you explain now?
What provides energy for ATP synthesis by chloroplast ATP synthase?
Compare with the explanation
Protons flowing down their gradient through ATP synthase
Proton flow is coupled to ADP phosphorylation.
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Make it stick
How does a thylakoid proton gradient power ATP synthesis?
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