Trace non-cyclic photophosphorylation
Explain PSI, PSII, photoactivation, water photolysis, oxygen release, ATP and reduced-NADP synthesis.
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 two photosystems take part in non-cyclic photophosphorylation?
What you will learn
- Describe water photolysis at the oxygen-evolving complex
- Relate electron movement to ATP and reduced NADP production
Build the explanation
Non-cyclic photophosphorylation involves photosystems II and I. Light photoactivates chlorophyll in PSII and PSI. The oxygen-evolving complex catalyses photolysis of water, replacing electrons lost from PSII and releasing protons and oxygen. Energised electrons pass along carriers, releasing energy used to transfer protons across the thylakoid membrane. Electrons eventually reduce NADP to reduced NADP; proton flow back through ATP synthase drives ATP synthesis. The electrons do not return to their starting photosystem.
Apply it carefully
Water supplies replacement electrons, and its splitting releases oxygen; carbon dioxide is not split to release oxygen in this stage. Keep reduced NADP distinct from NADP. Avoid adding unrequired carrier names or claiming that electron flow alone directly forms carbohydrate.
Pause and explain
What is produced when NADP accepts hydrogen/electrons in this stage?
Worked example
Which molecule is split by the oxygen-evolving complex in non-cyclic photophosphorylation?
Show the worked solution
- Identify the source of replacement electrons at PSII.
- Water is photolysed.
Answer Water; photolysis also releases oxygen and protons.
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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 non-cyclic photophosphorylation, including water photolysis and its products.
Give me a hint
Trace electrons from water through PSII to PSI and then to NADP.
Compare my reasoning
- PSII and PSI are photoactivated; the oxygen-evolving complex splits water to replace electrons, releasing oxygen.
- Electron transfer supports proton pumping and ATP synthesis; electrons reduce NADP to reduced NADP.
Non-cyclic photophosphorylation uses PSI and PSII. Light photoactivates chlorophyll; the oxygen-evolving complex catalyses water photolysis, supplying electrons and releasing oxygen and protons. Electron transfer supports proton pumping and ATP synthesis, and electrons reduce NADP to reduced NADP.
Look for these in your work
- Both photosystems named
- Water photolysis and oxygen stated
- ATP synthesis linked
- Reduced NADP stated
A student says oxygen released in photosynthesis comes from carbon dioxide splitting. Correct the source for the light-dependent stage.
Give me a hint
Identify the reactant split by the oxygen-evolving complex.
Compare my reasoning
- The oxygen-evolving complex photolyses water.
- Water supplies electrons and releases oxygen; carbon dioxide is fixed later in the Calvin cycle.
In the light-dependent stage, the oxygen-evolving complex photolyses water, releasing oxygen and supplying electrons. Carbon dioxide is fixed in the Calvin cycle; it is not the source of the oxygen released by water photolysis in this account.
Look for these in your work
- Water identified as source
- Oxygen-evolving complex named
- Calvin cycle separated
A non-cyclic electron route continues, but NADP is unavailable. Predict which product may be directly limited and explain one consequence for the linked stages.
Give me a hint
Track the terminal electron acceptor and distinguish it from proton-driven ATP synthesis.
Compare my reasoning
- Reduced-NADP production is limited because NADP is the acceptor.
- Electron flow may be affected; ATP synthesis also depends on the proton gradient, so its response cannot be inferred quantitatively from the information alone.
Reduced-NADP production is directly limited because NADP is required as the electron acceptor. The effect on electron flow and ATP depends on the coupled conditions and cannot be quantified from this information alone.
Look for these in your work
- Directly affected product identified
- Electron acceptor role stated
- ATP inference qualified
Common mistakes
- Using carbon dioxide as the oxygen source
- Omitting PSII or the oxygen-evolving complex
- Saying electrons return in the non-cyclic route
What can you explain now?
Which two photosystems take part in non-cyclic photophosphorylation?
Compare with the explanation
PSI and PSII
Both are photoactivated in the non-cyclic route.
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Make it stick
How does non-cyclic photophosphorylation produce oxygen, ATP and reduced NADP?
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