Water provides the electrons for the light reactions of photosynthesis. When light energy strikes a chlorophyll complex called P680 in Photosystem II, it triggers a chain of events that splits water molecules apart, pulling electrons from them and feeding those electrons into the photosynthetic electron transport chain. This water-splitting reaction also releases oxygen as a byproduct, which is why plants produce the oxygen we breathe.
How Water Gets Split
Splitting water is not easy. Water is an extremely stable molecule, and breaking it apart to extract electrons requires a powerful oxidizing agent. The chlorophyll complex P680 in Photosystem II has an oxidation potential of roughly 1.2 volts, making it one of the strongest biological oxidants known. That’s well above the approximately 0.82 volts needed to pull electrons from water. This remarkable power comes partly from the protein environment surrounding P680: the chlorophyll sits inside a hydrophobic (water-repelling) pocket of nonpolar amino acids, which amplifies its ability to strip electrons away from nearby molecules.
But P680 doesn’t rip electrons directly from water on its own. Instead, when light excites P680 and it loses an electron to the transport chain, it becomes so electron-hungry that it pulls an electron from a nearby structure called the oxygen-evolving complex (OEC). The OEC then replenishes itself by taking electrons from water.
The Oxygen-Evolving Complex
The oxygen-evolving complex is a small cluster of four manganese atoms and one calcium atom embedded in Photosystem II. It acts as a kind of molecular battery, accumulating enough oxidizing power to break apart water. The process works in a four-step cycle, with each step triggered by one photon of light hitting P680. Scientists label these steps S0 through S4.
At each step, the manganese cluster gives up one electron to replace the electron that P680 just lost. As the cluster cycles from S0 to S3, it becomes progressively more oxidized. On the fourth step (S3 to S4 and back to S0), the cluster has accumulated enough energy to split two water molecules at once. This produces one molecule of oxygen gas, releases four protons into the interior of the thylakoid membrane, and resets the cycle back to S0. The protons released at each stage follow a specific pattern: one proton during the first transition, zero during the second, one during the third, and two during the final oxygen-releasing step.
So while we say “water provides the electrons,” the OEC is the molecular machinery that makes it happen. Without this manganese-calcium cluster, plants would have no way to access water’s electrons.
Where the Electrons Go Next
Once extracted from water, electrons travel through a series of carriers embedded in the thylakoid membrane. The path looks like this:
- Photosystem II to plastoquinone: After light excites P680 and the electron moves through a short chain within Photosystem II, it lands on plastoquinone, a small mobile molecule that carries it to the next stop.
- Cytochrome b6f complex: Plastoquinone delivers electrons here. This complex uses the electron’s energy to pump protons across the membrane, building up a concentration gradient that will later drive the production of ATP.
- Plastocyanin to Photosystem I: A small copper-containing protein called plastocyanin shuttles the electron from cytochrome b6f to Photosystem I, where a second photon of light boosts it to an even higher energy level.
- Final destination: The re-energized electron ultimately reduces NADP+ to NADPH, a molecule that carries chemical energy into the Calvin cycle to help build sugars.
Each electron from water therefore gets two energy boosts (one at each photosystem) before reaching its final destination. Because splitting two water molecules requires four electrons and four photons at Photosystem II alone, plus four more at Photosystem I, the entire process of water oxidation consumes at least eight photons of light.
Why Water and Not Something Else?
Water wasn’t always the electron source for photosynthesis. Billions of years before plants evolved, ancient bacteria used other molecules to feed their simpler photosynthetic systems. Some used hydrogen sulfide, others used molecular hydrogen or dissolved iron. These forms of photosynthesis, called anoxygenic photosynthesis, still exist today in certain bacteria living in hot springs, deep-sea vents, and sulfur-rich environments. Green sulfur bacteria, for instance, oxidize hydrogen sulfide to elemental sulfur instead of oxidizing water to oxygen.
The switch to water as an electron donor was a pivotal moment in Earth’s history. Water is everywhere, giving organisms that could use it a massive advantage. But water is also much harder to split than hydrogen sulfide or hydrogen gas, which is why Photosystem II needed to evolve the high-powered P680 chlorophyll and the manganese-based OEC. The payoff was enormous: cyanobacteria, the first organisms to crack water splitting, flooded the atmosphere with oxygen roughly 2.4 billion years ago. Every plant, alga, and cyanobacterium on Earth today still uses essentially the same molecular machinery to pull electrons from water and power the light reactions.