Distinguish cyclic and non-cyclic photophosphorylation
State that both cyclic and non-cyclic photophosphorylation occur in the light-dependent stage and compare their outcomes.
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 route uses both PSI and PSII?
What you will learn
- Name cyclic and non-cyclic photophosphorylation
- Compare photosystems and products at syllabus level
Build the explanation
Photophosphorylation is ATP synthesis during the light-dependent stage. It occurs in two forms: cyclic and non-cyclic. Cyclic photophosphorylation involves photosystem I (PSI) and makes ATP. Non-cyclic photophosphorylation involves both PSI and photosystem II (PSII), photolysis of water by the oxygen-evolving complex, and production of ATP and reduced NADP; oxygen is released from water. The routes therefore differ in photosystems and products.
Apply it carefully
Do not infer that cyclic photophosphorylation releases oxygen or forms reduced NADP in the syllabus model. Both routes can synthesize ATP; only the non-cyclic route described here includes PSII and water photolysis, producing reduced NADP and oxygen.
Pause and explain
Which photophosphorylation route releases oxygen from water?
Worked example
Which route uses PSI only and synthesizes ATP?
Show the worked solution
- Compare the photosystems involved.
- Cyclic photophosphorylation uses PSI only.
Answer Cyclic photophosphorylation.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
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.
Compare cyclic and non-cyclic photophosphorylation by photosystems and products.
Give me a hint
Identify which one includes PSII and water photolysis.
Compare my reasoning
- Cyclic: PSI only; ATP is synthesised.
- Non-cyclic: PSI and PSII; water is split and ATP and reduced NADP are synthesised, with oxygen released.
Cyclic photophosphorylation involves PSI only and synthesises ATP. Non-cyclic photophosphorylation involves PSI and PSII; water photolysis supplies electrons and produces oxygen, and ATP and reduced NADP are synthesised.
Look for these in your work
- Photosystems compared
- ATP in both stated
- Non-cyclic products stated
A sample produces ATP but no detectable oxygen or reduced NADP during the tested light response. Which route is more consistent with the syllabus model, and what cannot be concluded from this evidence alone?
Give me a hint
Use the product pattern but consider detection limits and other conditions.
Compare my reasoning
- ATP without the two non-cyclic products is consistent with cyclic photophosphorylation.
- The evidence does not prove that no non-cyclic activity occurred if product detection or conditions were limited.
The pattern is consistent with cyclic photophosphorylation, which uses PSI and synthesises ATP. It does not prove that non-cyclic activity is absent unless the detection method and conditions are known to detect its products reliably.
Look for these in your work
- Likely route identified
- Evidence limit considered
- No absolute claim
A learner says cyclic photophosphorylation must replace non-cyclic photophosphorylation whenever light intensity falls. Correct this inference.
Give me a hint
The route definitions do not specify a universal switching rule.
Compare my reasoning
- Cyclic and non-cyclic routes differ in photosystems and products.
- A change in light intensity alone does not establish which route replaces the other without more conditions or evidence.
Cyclic and non-cyclic photophosphorylation are distinct light-dependent routes with different photosystem involvement and products. The stated syllabus outcomes do not imply that lower light automatically switches one route for the other; additional conditions and evidence are needed.
Look for these in your work
- Route difference retained
- Unsupported automatic switch rejected
- Evidence need stated
Common mistakes
- Saying cyclic photophosphorylation uses PSII
- Assigning oxygen release to the cyclic route
- Treating ATP as unique to non-cyclic photophosphorylation
What can you explain now?
Which route uses both PSI and PSII?
Compare with the explanation
Non-cyclic photophosphorylation
The non-cyclic route also involves water photolysis and reduced NADP production.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Which photosystems and products distinguish cyclic from non-cyclic photophosphorylation?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.