Trace cyclic photophosphorylation
Explain photoactivation of chlorophyll in PSI and ATP synthesis during cyclic 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
Which photosystem is involved in cyclic photophosphorylation?
What you will learn
- Describe photoactivation in PSI
- Explain why electron return makes the pathway cyclic and ATP-forming
Build the explanation
Light energy photoactivates chlorophyll in photosystem I (PSI), raising an electron to a higher energy level. As the electron moves through carriers and returns to PSI, released energy supports proton transfer across the thylakoid membrane. Proton movement back through ATP synthase drives ATP synthesis. Because the electron returns to the photosystem, the route is cyclic. In the syllabus account, this pathway involves PSI only and synthesises ATP.
Apply it carefully
Do not add PSII, water photolysis, oxygen release or reduced NADP formation to the cyclic pathway described here. The key cycle is electron return to PSI; the useful output identified in this outcome is ATP.
Pause and explain
What is synthesised during cyclic photophosphorylation?
Worked example
Why is the pathway called cyclic?
Show the worked solution
- Follow the photoactivated electron to its destination.
- It returns to PSI.
Answer It is cyclic because the electron from PSI returns to PSI after passing through carriers.
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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.
Outline cyclic photophosphorylation from light absorption to ATP synthesis.
Give me a hint
Include PSI, photoactivation, electron return and proton flow.
Compare my reasoning
- Light photoactivates chlorophyll in PSI; its electron passes through carriers and returns to PSI.
- Energy released supports proton transfer; proton return through ATP synthase drives ATP synthesis.
Light photoactivates chlorophyll in PSI, raising an electron to a higher energy level. The electron passes along carriers and returns to PSI. Energy released supports proton pumping across the thylakoid membrane; protons return through ATP synthase, driving ATP synthesis.
Look for these in your work
- PSI only
- Photoactivation and return
- Proton gradient linked
- ATP output stated
A diagram of cyclic photophosphorylation ends with an electron leaving PSI and never returning. Identify the missing feature.
Give me a hint
The return step defines the cycle.
Compare my reasoning
- The electron should pass through carriers and return to PSI.
- Energy released along the route supports proton transfer and ATP synthesis.
The diagram should show the photoactivated electron passing through carriers and returning to PSI. Energy released during electron transfer supports proton pumping, and proton flow through ATP synthase drives ATP synthesis.
Look for these in your work
- Electron return added
- Energy transfer included
- ATP synthesis linked
A cyclic pathway synthesises ATP, but the experiment detects neither oxygen nor reduced NADP. Explain why that can fit the pathway rather than indicate failure.
Give me a hint
Compare the measured products with the expected product list.
Compare my reasoning
- Cyclic photophosphorylation uses PSI and synthesises ATP.
- Oxygen and reduced NADP are not listed as its products in this syllabus model.
The pattern can fit cyclic photophosphorylation because its stated output is ATP using PSI only. Oxygen release and reduced-NADP synthesis are associated with the non-cyclic pathway, not required outputs of the cyclic model.
Look for these in your work
- ATP expected
- Missing products are not required
- Cyclic and non-cyclic distinguished
Common mistakes
- Leaving the electron outside PSI
- Claiming cyclic flow splits water
- Adding reduced NADP as an expected cyclic product
What can you explain now?
Which photosystem is involved in cyclic photophosphorylation?
Compare with the explanation
PSI only
The photoactivated electron returns to PSI.
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Make it stick
How does the cyclic electron route support ATP synthesis?
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