Photosystem II (PSII) takes in water molecules and light energy, then splits those water molecules apart into three products: oxygen gas, protons (hydrogen ions), and electrons. It is the only biological system on Earth that can break water’s strong chemical bonds, and it does so using the most powerful oxidizing agent found in any living cell. Every breath of oxygen you take exists because of this reaction happening continuously inside plant cells, algae, and cyanobacteria.
The Overall Reaction
The matter transformation in PSII can be summarized simply: two water molecules enter, and out come one molecule of oxygen gas, four protons, and four electrons. The energy to drive this comes from photons of light absorbed by a cluster of chlorophyll molecules called P680. When P680 absorbs light, it becomes excited and loses an electron, creating P680+, which has an estimated redox potential of roughly +1,210 millivolts. That makes it the strongest oxidizing agent in biology, powerful enough to rip electrons away from water, one of the most stable molecules around.
This reaction happens inside the thylakoid membrane of chloroplasts. The protons released from water splitting are dumped into the thylakoid lumen (the interior of the membrane), where they build up and create a concentration gradient. For every electron moved through the system, one proton is released into the lumen from water splitting alone. Additional protons are transported across the membrane at later stages, bringing the total to about three protons per electron in the full linear electron flow pathway.
Where Water Gets Split: The Manganese Cluster
Water doesn’t just fall apart on its own. The actual bond-breaking happens at a specific metal cluster called the oxygen-evolving complex (OEC), embedded in the core of PSII. This cluster contains four manganese atoms and one calcium atom bridged by oxygen atoms. It’s often written as Mn4CaO5. The manganese ions do the heavy lifting, cycling through different oxidation states as they accumulate enough energy to crack water molecules open.
The cluster works through a cycle of five states, labeled S0 through S4, known as the Kok cycle (named after the researcher who first described it in 1970). Each time P680 absorbs a photon and loses an electron, that “hole” is passed to the manganese cluster through a nearby tyrosine amino acid, nudging the cluster one step further along the cycle. Four photons are needed to complete the full sequence:
- S0 to S1: The cluster absorbs the first oxidizing equivalent. One manganese ion is oxidized.
- S1 to S2: A second oxidizing equivalent is stored. The cluster now holds two “charges” worth of oxidizing power.
- S2 to S3: A third equivalent is absorbed. A water molecule binds to the cluster during this transition.
- S3 to S4 to S0: The fourth and final photon drives the cluster to S4, a transient state that exists only briefly. At this point, the cluster has enough oxidizing power to form the oxygen-oxygen bond. Two oxygen atoms (derived from the two water molecules) are joined together, molecular oxygen is released, and the cluster resets to S0.
After oxygen leaves, a fresh water molecule moves into the vacant site on the cluster. This incoming water transfers a proton to a neighboring hydroxide ion, restoring the cluster’s original structure and preparing it for the next cycle. The calcium ion in the cluster plays a key role here: current evidence suggests that a water molecule bound to calcium is the one that attacks a highly reactive manganese-bound oxygen during the critical bond-forming step.
Where the Electrons Go
Once an electron is stripped from water and passed through P680, it doesn’t just sit there. It travels through a chain of acceptor molecules embedded in PSII’s protein structure. The first stop is pheophytin, a chlorophyll-like molecule that has had its central magnesium atom removed. From pheophytin, the electron moves to a tightly bound plastoquinone molecule called QA, and then to a second, loosely bound plastoquinone called QB.
QB is where things get interesting from a matter perspective. After accepting two electrons (from two separate rounds of light absorption), QB picks up two protons from the stroma (the fluid outside the thylakoid membrane). This converts it from plastoquinone into plastoquinol, a fully reduced molecule that detaches from PSII and floats through the membrane to the next complex in the chain. So the electrons extracted from water inside the lumen are ultimately paired with protons grabbed from the stroma, packaging them into a mobile carrier. This is one of the key ways PSII moves matter: protons from one side of the membrane and electrons from the other are combined into a single molecule that shuttles them downstream.
Light Energy Becomes Chemical Potential
The matter transformations in PSII are really about converting light energy into two forms of chemical potential. First, there’s the proton gradient: protons released from water splitting accumulate inside the thylakoid lumen, while protons are consumed from the stroma when plastoquinol forms. This creates a steep difference in proton concentration across the membrane, which later powers the enzyme that makes ATP.
Second, the electrons themselves carry energy. They entered the system as part of extremely stable water molecules. By absorbing light, PSII boosted those electrons to a much higher energy level, making them capable of driving chemical reactions downstream. Those energized electrons will eventually help reduce NADP+ to NADPH at Photosystem I, providing the chemical reducing power needed to build sugars from carbon dioxide.
Why PSII Constantly Damages Itself
Splitting water is so chemically violent that PSII regularly destroys its own components. The D1 protein, which sits at the heart of the reaction center and holds many of the key cofactors, is the primary casualty. It degrades faster than almost any other protein in the chloroplast, and the cell has evolved an elaborate repair cycle to deal with this.
When D1 is damaged, the entire PSII complex migrates from the tightly stacked grana regions of the thylakoid to the unstacked stroma-exposed regions. There, the complex partially disassembles, specialized protein-cutting enzymes called FtsH and Deg break down the damaged D1, a freshly made copy is inserted, and the whole complex reassembles and migrates back. Under normal light, FtsH handles most of the cleanup through steady, continuous degradation. Under intense light that causes more severe damage, the Deg enzymes make initial cuts that help FtsH work faster, creating a cooperative “escape pathway” for rapid repair.
This constant cycle of damage and repair is the price of using the most powerful oxidant in biology. The cell essentially treats D1 as a disposable part, rebuilding it over and over to keep the water-splitting machinery running.
Summing Up the Matter Flow
If you trace every atom through PSII, the accounting looks like this: two water molecules (4 hydrogen atoms and 2 oxygen atoms) are the raw material. The two oxygen atoms leave as O2 gas. The four hydrogen atoms are split into four protons and four electrons. The protons are deposited in the thylakoid lumen, contributing to the gradient that drives ATP production. The electrons pass through pheophytin and plastoquinone carriers, eventually leaving PSII as part of plastoquinol (which also picks up two protons from the stroma). No carbon is involved, no sugar is made here. PSII’s sole job is to dismantle water and convert its matter into the charged particles and mobile carriers that power everything else in photosynthesis.