Role of Plastoquinone and Plastocyanin in Electron Transport
Biology · Photosynthesis in Higher Plants · NEET
In the light reaction, plastoquinone (PQ) carries electrons from Photosystem II (PS-II) to the cytochrome b6f complex, and plastocyanin (PC) carries electrons from the cytochrome b6f complex to Photosystem I (PS-I). Memory hook: order is PS-II to PQ to Cyt b6f to PC to PS-I, so "PQ comes before PC" just like Q comes before C in the alphabet. This is the middle part of the Z-scheme and is a favourite NEET one-mark question.
Order of mobile carriers in the Z-scheme: plastoquinone (PS-II to Cyt b6f), plastocyanin (Cyt b6f to PS-I) and ferredoxin (PS-I to NADP+).
Your doubts, answered
Does plastoquinone or plastocyanin come first?
Plastoquinone comes first. Electrons leave PS-II, are picked up by plastoquinone, and are handed to the cytochrome b6f complex. Only after the Cyt b6f complex does plastocyanin take over and pass electrons to PS-I. Simple order: PS-II to PQ to Cyt b6f to PC to PS-I.
From where to where does plastoquinone move electrons?
Plastoquinone (PQ) moves electrons from PS-II to the cytochrome b6f complex. This is exactly what NEET 2020 asked, and the correct option was PS-II to Cyt b6f complex. Do not write PS-I; plastoquinone acts on the PS-II side.
What does plastocyanin do?
Plastocyanin (PC) is a small copper-containing protein that carries electrons from the cytochrome b6f complex to PS-I. So plastocyanin fills the gap between Cyt b6f and PS-I, delivering the electron that PS-I will re-energise using 700 nm light.
Why is plastoquinone important for the proton gradient?
As plastoquinone shuttles electrons across the thylakoid membrane, it also moves protons (H+) from the stroma into the thylakoid lumen. This adds to the proton gradient that ATP synthase later uses to make ATP by chemiosmosis. So PQ has a double job: electron carrier plus proton pump.
Is plastocyanin a protein or a lipid-soluble molecule?
Plastocyanin is a protein and it contains copper (Cu). Plastoquinone, on the other hand, is a lipid-soluble (mobile) molecule inside the membrane. This chemical difference is a common NEET trap, so remember: PC = copper protein, PQ = mobile quinone.
⚠️ The NEET trap ✗ Plastoquinone transfers electrons from PS-I to NADP+ (or from Cyt b6f to PS-I). ✓ Plastoquinone transfers electrons from PS-II to the cytochrome b6f complex. The carrier between Cyt b6f and PS-I is plastocyanin, and the transfer from PS-I to NADP+ is done by ferredoxin. 🧠 Match the carrier to its exact gap: PQ = PS-II to Cyt b6f, PC = Cyt b6f to PS-I, Fd = PS-I to NADP+.
Real NEET questions
NEET 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 Z-scheme, electrons excited from PS-II are picked up by an acceptor and passed down the electron transport chain. Plastoquinone is the mobile carrier that ferries these electrons from PS-II into the cytochrome b6f complex. Option A (PS-I to NADP+) is done by ferredoxin, and option D (Cyt b6f to PS-I) is done by plastocyanin. Hence the answer is PS-II to Cyt b6f complex.
Solved Photosynthesis in Higher Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the correct sequence of electron carriers in the Z-scheme?
PS-II to plastoquinone (PQ) to cytochrome b6f complex to plastocyanin (PC) to PS-I to ferredoxin (Fd) to NADP+. Remembering this chain answers most NEET light-reaction questions.
Which carrier links the two photosystems?
The cytochrome b6f complex, together with plastoquinone (before it) and plastocyanin (after it), links PS-II to PS-I. Plastocyanin is the last carrier that actually delivers electrons to PS-I.
Does plastocyanin contain a metal?
Yes, plastocyanin contains copper (Cu). This makes it a copper-protein electron carrier. Plastoquinone does not contain a metal; it is a lipid-soluble quinone.
Which carrier passes electrons from PS-I to NADP+?
Ferredoxin passes electrons from PS-I to NADP+, which is then reduced to NADPH. This is the final downhill step of the non-cyclic (Z-scheme) pathway.
Why should NEET students memorise these carriers?
NEET repeatedly asks one-mark questions on which carrier works in which gap (as in NEET 2020). Knowing PQ, Cyt b6f, PC and ferredoxin in order gives quick, guaranteed marks in the Photosynthesis chapter.
