Non-Cyclic Electron Transport and Photophosphorylation
Biology · Photosynthesis in Higher Plants · NEET
In non-cyclic photophosphorylation, electrons flow one way from water to NADP+ through both Photosystem II (P680) and Photosystem I (P700). It makes three products: ATP, NADPH and oxygen. Memory hook: "Non-cyclic = No return, Needs both, gives NADPH" — because the electron never comes back and both photosystems work in series.
Non-cyclic electron transport: electrons flow one-way from water through PS II and PS I to NADP+, producing ATP, NADPH and O2. Because the electron never returns, the path is "non-cyclic."
Your doubts, answered
How many photosystems work in non-cyclic photophosphorylation?
Both PS II (P680) and PS I (P700) work together in series. The electron starts at PS II, travels to PS I, and finally reaches NADP+. This is the most common NEET trap: cyclic uses only PS I, but non-cyclic needs BOTH photosystems.
Why is it called 'non-cyclic'?
Because the electron does not come back to where it started. It leaves water, moves through PS II and PS I, and ends up on NADP+ to make NADPH. Since the electron is used up and does not return in a circle, the path is called non-cyclic.
Where do the electrons for this pathway come from?
From water. When water is split (photolysis) at PS II, it releases electrons, protons (H+) and oxygen. These electrons replace the ones lost by P680, so water is the electron donor for the whole non-cyclic chain.
What are the exact products of non-cyclic photophosphorylation?
Three products: ATP, NADPH (NADPH + H+), and oxygen (O2). A frequent NEET question asks which is NOT a product — NADH is the wrong one, because NADH belongs to respiration, not the light reaction.
What is the role of plastoquinone here?
Plastoquinone carries electrons from PS II to the cytochrome b6f complex. This movement of protons across the membrane helps build the proton gradient. NEET 2020 tested exactly this transfer step.
Does non-cyclic photophosphorylation make ATP and NADPH both?
Yes. Non-cyclic makes both ATP and NADPH + H+. Cyclic makes only ATP (no NADPH). This one-line difference is directly asked in NEET as an 'incorrect statement' question.
⚠️ The NEET trap ✗ Non-cyclic photophosphorylation involves only PS I, like the cyclic pathway. ✓ Non-cyclic involves BOTH PS II and PS I. It is CYCLIC photophosphorylation that uses only PS I (P700) in the stroma lamellae. 🧠 Read the photosystem count carefully: non-cyclic = two photosystems (both), cyclic = one (PS I only). NTA swaps these two lines in almost every 'incorrect statement' question.
Real NEET questions
2019
Which of the following statements is incorrect?
A · Grana lamellae have both PS I and PS II
B · Cyclic photophosphorylation involves both PS I and PS II ✓
C · Both ATP and NADPH + H+ are synthesized during non-cyclic photophosphorylation
D · Stroma lamellae have PS I only and lack NADP reductase
Solution: Cyclic photophosphorylation uses ONLY PS I (P700); the electron is cycled back within PS I and only ATP is made. Involving both PS I and PS II is a feature of NON-cyclic photophosphorylation. So statement B is incorrect. NCERT Class 11 Ch 11.
2018
Which of the following is not a product of light reaction of photosynthesis?
A · NADPH
B · NADH ✓
C · ATP
D · Oxygen
Solution: The light reaction (non-cyclic path) makes ATP, NADPH and O2 (from water splitting). NADH is not formed in photosynthesis at all — it belongs to respiration. So NADH is the odd one out. NCERT p.138.
2020
In light reaction, plastoquinone facilitates the transfer of electrons from
A · PS-I to NADP+
B · PS-I to ATP synthase
C · PS-II to Cyt b6f complex ✓
D · Cyt b6f complex to PS-I
Solution: In the non-cyclic (Z-scheme) path, electrons from PS II are carried by plastoquinone to the cytochrome b6f complex, before reaching PS I. So the transfer is PS-II to Cyt b6f. NCERT p.139.
Solved Photosynthesis in Higher Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Is water the electron donor in non-cyclic photophosphorylation?
Yes. Water is split at PS II to give electrons, protons and O2. These electrons flow through PS II and PS I to NADP+, so water is the ultimate electron donor.
Which final acceptor gets the electron in the non-cyclic path?
NADP+ is the final electron acceptor. It picks up electrons and a proton to become NADPH + H+, which is later used in the Calvin cycle.
Does non-cyclic photophosphorylation release oxygen?
Yes. Oxygen is released because water is split at PS II. Cyclic photophosphorylation does not split water, so it releases no oxygen.
Why is non-cyclic important for the dark reaction?
It supplies both ATP and NADPH. The Calvin cycle needs both of these to fix CO2, and only the non-cyclic path can provide NADPH.
Where in the chloroplast does non-cyclic photophosphorylation happen?
In the grana lamellae (thylakoids of the grana), because these membranes have both PS I and PS II. Stroma lamellae have only PS I and run the cyclic path.
