AP Syllabus focus:
‘Electron transfer through the thylakoid electron transport chain creates a proton gradient across the thylakoid membrane.’
Electron transport in chloroplast thylakoid membranes converts light-driven electron movement into a transmembrane proton (H⁺) gradient. This gradient stores potential energy by separating charge and pH between the thylakoid lumen and stroma.
This diagram summarizes linear electron flow across the thylakoid membrane (PSII → plastoquinone pool → cytochrome b6f → plastocyanin → PSI → ferredoxin → FNR) and shows how electron transfer is coupled to proton translocation into the thylakoid lumen. The lumen-side proton buildup (plus stromal proton consumption during NADPH formation) creates the electrochemical gradient that powers ATP synthase. Source
Location and Purpose of the Thylakoid Electron Transport Chain
The electron transport chain (ETC) is embedded in the thylakoid membrane, which separates two compartments:
Stroma (outside thylakoids)
Thylakoid lumen (inside thylakoids)
A key outcome of electron transfer is the buildup of H⁺ in the lumen, making it more acidic than the stroma.
Proton gradient: A difference in H⁺ concentration (and associated electrical charge) across a membrane that stores potential energy for cellular work.
This gradient is the direct product of electron transfer steps that are spatially organised in the thylakoid membrane.
Components and Direction of Electron Flow
Core carriers in linear electron transport
In typical linear electron flow, excited electrons move through a series of carriers:
Photosystem II (PSII) donates electrons to the ETC after excitation by light
Plastoquinone (PQ) accepts electrons from PSII and transports them within the membrane
Cytochrome b6f complex transfers electrons onward and is a major site of H⁺ movement into the lumen
Plastocyanin (PC) carries electrons along the lumenal side to Photosystem I (PSI)
Ferredoxin (Fd) carries electrons from PSI on the stromal side
NADP⁺ reductase (FNR) helps transfer electrons to NADP⁺, forming NADPH
Electron transfers are redox reactions: carriers alternate between reduced and oxidised states as electrons pass “down” the chain.
Why electron transfer can build a gradient
Electron movement is coupled to H⁺ relocation because:
Some complexes physically move H⁺ across the membrane
Chemical reactions associated with electron transfer add or remove H⁺ from specific compartments
How the Proton Gradient Is Generated Across the Thylakoid Membrane
H⁺ accumulates in the lumen through three main contributions, all tied to electron transfer.
This figure emphasizes the compartment logic of chemiosmosis in chloroplasts: protons accumulate in the thylakoid lumen while the stroma becomes relatively depleted of . By showing PSII water oxidation, cytochrome b6f–linked proton movement, and stromal NADPH formation together, it visually explains why electron transfer steepens the transmembrane proton gradient. Source
1) Water splitting adds H⁺ to the lumen
At PSII, water is oxidised to replace electrons lost by the reaction centre. This process releases:
Electrons (to keep PSII running)
H⁺ directly into the thylakoid lumen
Oxygen gas as a byproduct
Because the released H⁺ stays inside the lumen, it immediately strengthens the gradient.
2) Cytochrome b6f increases lumenal H⁺ during electron transfer
As electrons move from PQ toward PC, the cytochrome b6f complex is a key site where electron transfer is linked to net H⁺ movement into the lumen. Functionally, it:
Accepts electrons from reduced PQ
Contributes to increasing H⁺ on the lumen side during transfer to PC
This step is central because it couples the energy released by electron transfer to the physical separation of protons across the membrane.
3) NADP⁺ reduction lowers stromal H⁺
At the end of linear flow, electrons are used to reduce NADP⁺ to NADPH on the stromal side. This process effectively:
Removes (consumes) H⁺ from the stroma
Further increases the H⁺ difference between stroma and lumen
Together, adding H⁺ to the lumen and depleting H⁺ from the stroma makes the gradient steeper.
Properties of the Gradient and What Maintains It
Two linked gradients: pH and charge
The thylakoid gradient has:
A chemical component (lumen becomes more acidic than stroma)
An electrical component (separation of charge across the membrane)
Why the gradient can persist
The gradient is maintained because:
The thylakoid membrane is relatively impermeable to ions, limiting H⁺ diffusion back to the stroma
Electron transfer continuously supplies H⁺ to the lumen during illumination
The gradient represents stored energy that can be tapped by membrane proteins, with its magnitude depending on light-driven electron flow and membrane integrity.
FAQ
It uses plastoquinone/plastoquinol chemistry to move more H⁺ into the lumen per pair of electrons.
This can increase the H⁺ gain beyond simple electron hand-offs between carriers.
Researchers can track pH-sensitive dyes or observe electrochromic shifts in thylakoid pigments.
These signals change as membrane potential and lumenal acidity change.
Counter-ion movements (e.g., $Cl^-$ flux) and buffering in the lumen can reduce excessive charge separation.
This allows a strong $\Delta pH$ to persist.
Greater H⁺ permeability dissipates the gradient, so less energy is stored as a proton difference.
Electron transfer may continue, but the gradient-dependent energy capture is reduced.
It routes electrons back to the PQ/cytochrome b6f segment, increasing H⁺ accumulation via b6f without producing NADPH.
This can steepen the gradient when extra proton-driving force is needed.
Practice Questions
Explain how electron transfer in the thylakoid electron transport chain results in a higher concentration of H⁺ in the thylakoid lumen than in the stroma. (3 marks)
Water splitting associated with PSII releases H⁺ into the thylakoid lumen (1).
Electron transfer through cytochrome b6f leads to net movement/accumulation of H⁺ in the lumen (1).
Reduction of NADP⁺ on the stromal side consumes H⁺ (or reduces stromal H⁺), increasing the gradient (1).
Describe the path of electrons from PSII to NADP⁺ and explain two distinct ways this electron flow contributes to formation of a proton gradient across the thylakoid membrane. (6 marks)
Electrons leave excited PSII and are transferred to plastoquinone (PQ) (1).
Electrons pass to cytochrome b6f and then to plastocyanin (PC) (1).
Electrons reach PSI, are re-excited, then transferred to ferredoxin (Fd) and finally to NADP⁺ via NADP⁺ reductase (FNR) to form NADPH (1).
H⁺ are added to the lumen from water oxidation at PSII (1).
Electron transfer via cytochrome b6f is coupled to net H⁺ accumulation/movement into the lumen (1).
NADP⁺ reduction consumes H⁺ in the stroma, increasing the transmembrane H⁺ difference (1).
