The electron transport chain in photosynthesis is a series of protein complexes embedded in the thylakoid membranes of chloroplasts that transfer electrons from water to a final acceptor, producing the ATP and NADPH that plants use to build sugars. It runs on light energy, splits water molecules, releases oxygen as a byproduct, and generates a proton gradient that powers the cell’s energy currency. This chain is the engine behind nearly all oxygen on Earth and, indirectly, the fuel source for almost every living thing.
How the Chain Works as a Whole
The electron transport chain operates during the light-dependent reactions of photosynthesis. Electrons are pulled from water molecules on one end and delivered to a carrier molecule called NADPH on the other. Along the way, the energy from those electrons is used to pump protons (hydrogen ions) across the thylakoid membrane, creating a concentration difference. That stored energy then drives an enzyme called ATP synthase, which produces ATP.
The pathway is often called the “Z-scheme” because when you plot the energy levels of each electron carrier on a diagram, the path zigzags up and down in a shape resembling the letter Z. Two separate light-absorbing complexes, Photosystem II and Photosystem I, each boost electrons to higher energy levels using photons. Between and after these boosts, the electrons step downhill through a series of carriers, releasing usable energy at each step.
Photosystem II: Where Water Splits
The chain starts at Photosystem II (PSII), which sits primarily in the tightly stacked regions of the thylakoid membrane called grana. Its reaction center contains a special pair of chlorophyll molecules that absorb light most strongly at 680 nanometers, earning the designation P680. When a photon hits P680, charge separation happens on a picosecond timescale, launching an energized electron to a nearby acceptor molecule.
That electron vacancy in P680 is immediately filled by stripping electrons from water. A cluster of manganese and calcium atoms called the oxygen-evolving complex cycles through a series of oxidation states, and after accumulating four positive charges over four light flashes, it splits two water molecules into one molecule of oxygen, four protons, and four electrons. The protons are released into the thylakoid interior (the lumen), contributing to the proton gradient. The oxygen diffuses out of the leaf. This single reaction is the source of virtually all breathable oxygen in the atmosphere.
Once the energized electron leaves P680, it passes through a series of acceptors within PSII itself before landing on a mobile carrier called plastoquinone. Plastoquinone picks up two electrons and two protons from the surrounding fluid, becoming plastoquinol, and then shuttles them to the next major complex.
The Cytochrome b6f Complex
Plastoquinol delivers its cargo to the cytochrome b6f complex, which is distributed relatively evenly across both stacked and unstacked thylakoid regions. This complex acts as a proton pump. It oxidizes plastoquinol on the lumen side of the membrane, splitting the two electrons into separate paths. One electron moves through an iron-sulfur protein and then to a small copper-containing protein called plastocyanin, which ferries it toward Photosystem I. The other electron cycles back through the complex in a process called the Q-cycle, which moves additional protons from the stroma into the lumen.
This Q-cycle is critical because it roughly doubles the number of protons pumped per pair of electrons passing through, amplifying the proton gradient without consuming extra light energy. Water molecules within the complex play a direct role in shuttling protons and exchanging with the quinone molecules at the catalytic sites.
Photosystem I: Boosting Electrons Again
Photosystem I (PSI) is concentrated in the unstacked stromal lamellae and grana margins of the thylakoid membrane, physically separated from the bulk of PSII. Its reaction center, P700, absorbs light at 700 nanometers. When a photon energizes P700, the resulting high-energy electron is passed through a chain of internal carriers to a small iron-sulfur protein called ferredoxin on the stromal side of the membrane.
The electron that P700 lost is replaced by plastocyanin arriving from the cytochrome b6f complex. This completes the linear connection between the two photosystems.
Making NADPH
Ferredoxin carries its high-energy electron to an enzyme that catalyzes one of the final steps in converting light into chemical energy. This enzyme accepts two electrons, delivered one at a time by two separate ferredoxin molecules, and combines them with a proton to reduce NADP+ into NADPH. The reaction requires the formation of a short-lived complex between each ferredoxin molecule and the enzyme before the electron transfers. NADPH then moves into the stroma, where it provides the reducing power for the Calvin cycle to fix carbon dioxide into sugar.
ATP Synthase and the Proton Gradient
All the proton pumping along the chain, from water splitting in PSII and proton translocation at cytochrome b6f, creates a steep concentration gradient across the thylakoid membrane. The lumen becomes acidic relative to the stroma. Protons can only escape back to the stroma through ATP synthase, a molecular turbine also located in the unstacked thylakoid regions alongside PSI.
ATP synthase has two main parts: a membrane-embedded ring and a catalytic head that protrudes into the stroma. Protons enter through a channel from the lumen, bind to a specific amino acid on the ring, ride the ring nearly a full turn, then exit through a second channel into the alkaline stroma. This rotation of the ring drives a central shaft that causes shape changes in the catalytic head, squeezing ADP and phosphate together to form ATP. Each full revolution produces three ATP molecules. The high pH of the stroma helps strip protons off the ring, ensuring the rotation only goes in one direction.
Linear vs. Cyclic Electron Flow
The pathway described above, from water through both photosystems to NADPH, is called linear (or non-cyclic) electron flow. It produces both ATP and NADPH. But the Calvin cycle and other cellular processes sometimes need extra ATP without additional NADPH. Plants handle this through cyclic electron flow.
In cyclic flow, only Photosystem I participates. Electrons energized by P700 pass through ferredoxin but instead of going to NADP+, they loop back to the cytochrome b6f complex, which pumps more protons into the lumen and returns the electrons to PSI via plastocyanin. No water is split, no oxygen is released, and no NADPH is made. The sole product is ATP. This allows the plant to fine-tune the ATP-to-NADPH ratio depending on metabolic demand.
Efficiency of the Chain
Under optimal conditions, open PSII reaction centers operate at a photochemical efficiency of roughly 0.8, meaning about 80% of absorbed photons successfully drive electron transfer rather than being lost as heat or fluorescence. The overall maximum quantum yield for carbon fixation in C3 plants is around 0.095 molecules of CO2 fixed per absorbed photon, or about 0.106 molecules of O2 released per photon. These numbers reflect the combined efficiency of the light reactions and the Calvin cycle working together.
Efficiency drops under stress. High light, drought, or temperature extremes can reduce the fraction of functional reaction centers and force more energy to dissipate as heat. Plants use cyclic electron flow and other protective mechanisms to prevent damage to the chain under these conditions, sacrificing some productivity for safety.
Where Each Complex Sits in the Membrane
The physical layout of the chain matters for its function. PSII and its associated light-harvesting antenna proteins pack tightly into the stacked grana, where only about 20% of protein complexes are mobile. PSI and ATP synthase cluster in the unstacked stromal lamellae and grana margins, where about 50% of complexes can move freely. Cytochrome b6f bridges both regions, consistent with its role as the link between the two photosystems. Plastoquinone and plastocyanin are the mobile messengers that connect spatially separated complexes, diffusing through the membrane lipid layer and the lumen respectively.