Photosystem I (PSI) and Photosystem II (PSII) are two large protein complexes embedded in the internal membranes of chloroplasts, and together they drive the light-dependent reactions of photosynthesis. PSII absorbs light energy and uses it to split water molecules, releasing the oxygen we breathe, while PSI absorbs light energy a second time and uses it to produce the high-energy electron carrier NADPH. Working in sequence, connected by a chain of electron-shuttling molecules, these two photosystems convert sunlight into the chemical energy that powers nearly all life on Earth. The way they cooperate, break down, and get repaired is more dynamic and interesting than the static diagrams in most textbooks suggest.
Where They Sit Inside the Chloroplast
The two photosystems are not randomly scattered across chloroplast membranes. They occupy distinct neighborhoods. Inside a chloroplast, the internal membrane system called the thylakoid forms stacked discs (grana) connected by unstacked stretches (stroma lamellae). Classic fractionation work showed that PSII is concentrated in the stacked grana regions, while PSI is found almost exclusively in the unstacked, stroma-exposed thylakoid membranes.1PubMed Central. Localization of different photosystems in separate regions of chloroplast membranes This physical separation matters: it keeps the two photosystems from competing for the same incoming light and helps the cell regulate how much energy each one receives. It also means that the electrons freed by PSII have to travel through a relay of mobile carriers to reach PSI, which sits some distance away.
Photosystem II and the Splitting of Water
PSII is where photosynthesis does something no other biological system can do: rip electrons out of water. Each PSII unit is a large complex of more than 20 protein subunits, loaded with pigments including chlorophylls, carotenoids, and pheophytins, plus a collection of lipids and metal cofactors.2Oxford University Press / Plant and Cell Physiology. Photosystem II: commonality and diversity with emphasis on the extrinsic subunits – Section: Overview of the Structure and Assembly of PSII / Charge separation and electron transfer in PSII The pigments act as an antenna: they absorb photons across a range of visible wavelengths and funnel that energy inward to a special pair of chlorophyll molecules called P680.
When the collected energy reaches P680, it pushes P680 into an excited state. This triggers charge separation, the moment when an electron is ejected and photochemistry truly begins. The energized electron hops first to a pheophytin molecule, then to a bound plastoquinone called QA, and finally to a second, exchangeable plastoquinone QB. Once QB picks up two electrons and two protons, it detaches from PSII and floats off into the membrane as plastoquinol, carrying its cargo toward the next station in the chain.2Oxford University Press / Plant and Cell Physiology. Photosystem II: commonality and diversity with emphasis on the extrinsic subunits – Section: Overview of the Structure and Assembly of PSII / Charge separation and electron transfer in PSII
Meanwhile, P680 has lost an electron and needs a replacement. That replacement comes from water. Buried within PSII is a cluster of four manganese ions, one calcium ion, and five oxygen atoms, arranged in a shape often described as a “distorted chair.” This manganese cluster is the oxygen-evolving center (OEC), the catalytic heart of water splitting.2Oxford University Press / Plant and Cell Physiology. Photosystem II: commonality and diversity with emphasis on the extrinsic subunits – Section: Overview of the Structure and Assembly of PSII / Charge separation and electron transfer in PSII It cycles through a series of oxidation states, accumulating enough oxidizing power to strip four electrons from two water molecules. The byproducts are molecular oxygen (O₂) and protons (H⁺). The oxygen escapes as a gas; the protons contribute to a concentration gradient that will later drive ATP production. Assembling this manganese cluster in the first place requires a light-driven process called photoactivation, where metal ions are incorporated step by step into the active site.3PubMed. From manganese oxidation to water oxidation: assembly and evolution of the water-splitting complex in photosystem II
Photosystem I and the Production of NADPH
PSI picks up where PSII left off. It is a massive complex in its own right: in cyanobacteria, a single PSI trimer has a molecular mass of roughly 1 megadalton, with each monomer packing around 89 chlorophylls, 21 carotenoids, three iron-sulfur clusters, and two phylloquinones, among other cofactors.4PubMed Central. The structure of Photosystem I acclimated to far-red light illuminates an ecologically important acclimation process in photosynthesis In plants, the antenna system is somewhat different, but the core architecture is conserved.
The central event in PSI is similar in concept to PSII but different in detail. Light energy is funneled to a special pair of chlorophyll molecules called P700. When excited, P700 ejects an electron into an internal chain that includes chlorophyll monomers and phylloquinones, ending at a series of iron-sulfur clusters on the stromal side of the membrane. Research on the primary charge separation in PSI suggests it involves a symmetric arrangement in which P700 is electronically coupled to two flanking chlorophylls, forming a kind of shared excited state before the electron commits to one branch of the transfer chain.5PubMed Central. Current state of the primary charge separation mechanism in photosystem I of cyanobacteria
The electron that exits PSI is handed to a small iron-sulfur protein called ferredoxin, which in turn delivers it to an enzyme called ferredoxin-NADP⁺ reductase (FNR). FNR uses the electron (along with a proton) to reduce NADP⁺ to NADPH, the final product of the light reactions’ electron transport chain.6PubMed. Tethering ferredoxin-NADP+ reductase to photosystem I promotes photosynthetic cyclic electron transfer NADPH is a potent reducing agent that the Calvin cycle uses to convert carbon dioxide into sugars.
The Bridge Between the Two Photosystems
PSII and PSI are not directly connected. Electrons travel between them through a relay that includes plastoquinol, the cytochrome b6f complex, and the small copper-containing protein plastocyanin. The cytochrome b6f complex sits at the center of this relay and has been called the “heart of the hub” connecting the two photosystems.7PubMed. The cytochrome b(6)f complex: plastoquinol oxidation and regulation of electron transport in chloroplasts
Plastoquinol arrives at cytochrome b6f loaded with electrons from PSII. The complex oxidizes plastoquinol and passes an electron through a high-potential chain involving an iron-sulfur cluster and a heme group, eventually reaching plastocyanin.8Biochimica et Biophysica Acta (BBA) – Bioenergetics. Cytochrome b6f – Orchestrator of photosynthetic electron transfer – Section: 3. The modified Q-cycle and electron transfer mechanics Plastocyanin then diffuses along the thylakoid lumen to PSI, where it donates its electron to re-reduce P700 after it has fired. This entire path, from water splitting at PSII through cytochrome b6f to NADPH production at PSI, is called linear electron transport. Along the way, protons are pumped into the thylakoid lumen at two points: when PSII splits water and when cytochrome b6f oxidizes plastoquinol. Those protons accumulate on one side of the membrane, creating a gradient.
How the Proton Gradient Makes ATP
The proton gradient built up during electron transport is not just a byproduct. It is the energy source for ATP synthesis. Protons concentrated in the thylakoid lumen flow back across the membrane through chloroplast ATP synthase (sometimes called CF₀CF₁), a rotary molecular machine. As protons pass through it, the enzyme spins and catalyzes the conversion of ADP and inorganic phosphate into ATP.9PubMed Central. Proton gradient across the chloroplast thylakoid membrane governs the redox regulatory function of ATP synthase The light-induced proton gradient across the thylakoid membrane is essential for this process.10PubMed Central. Proton Gradients and Proton-Dependent Transport Processes in the Chloroplast ATP and NADPH together are the two currencies the Calvin cycle spends to fix carbon dioxide into sugar. Without both products, the cycle stalls.
Cyclic Electron Flow Around PSI
Linear electron transport produces ATP and NADPH in a fixed ratio. But the Calvin cycle and other chloroplast processes do not always need those two products in exactly that ratio. When the cell needs extra ATP without making more NADPH, it can run a shortcut called cyclic electron flow. In this mode, electrons leaving PSI via ferredoxin loop back to the cytochrome b6f complex instead of going to NADP⁺. This extra pass through b6f pumps more protons into the lumen, driving additional ATP synthesis without generating any NADPH.
Cyclic electron flow appears to be regulated by the cell’s ATP status. Work on isolated chloroplasts showed that the two main pathways for cyclic flow in plants, one involving a complex called NDH and the other involving PGR5/PGRL1, are both inhibited by ATP. The NDH pathway is inhibited at a higher ATP threshold, while the PGR5/PGRL1 pathway kicks in only when ATP levels drop more severely.11PubMed. Regulation of photosynthetic cyclic electron flow pathways by adenylate status in higher plant chloroplasts This creates a tiered system: moderate ATP shortfall activates one pathway, severe shortfall activates both. Whether cyclic flow actually ramps up in a living leaf depends on conditions. Under high light and when photorespiration is high (meaning ATP demand is elevated), cyclic electron flow increases. Under low light, researchers found that cyclic flow did not shift much despite changes in the modeled ATP-to-NADPH demand.12PubMed Central. The Response of Cyclic Electron Flow around Photosystem I to Changes in Photorespiration and Nitrate Assimilation
The enzyme FNR may serve as a physical switch between linear and cyclic modes. When FNR is associated with PSI, electrons flow toward NADPH. When it associates with the cytochrome b6f complex instead, electrons are redirected into the cyclic pathway.6PubMed. Tethering ferredoxin-NADP+ reductase to photosystem I promotes photosynthetic cyclic electron transfer
Balancing the Two Photosystems With State Transitions
Because PSII and PSI absorb light at slightly different wavelengths, shifting light conditions can easily throw their activity out of balance. If PSII gets too much light relative to PSI, the plastoquinone pool becomes over-reduced, and electron flow backs up. Plants handle this with a short-term fix called a state transition. When PSII is being over-excited, a kinase phosphorylates part of PSII’s antenna system (the mobile light-harvesting complex II, or LHCII). The phosphorylated LHCII physically detaches from PSII and migrates to PSI, boosting PSI’s light-harvesting capacity at PSII’s expense.13PubMed. Balance of power: a view of the mechanism of photosynthetic state transitions
Recent quantitative work put numbers on this redistribution. Under conditions that favor the “State 2” configuration, about 12% of the most highly phosphorylated LHCII moves from stacked to unstacked thylakoids. This reduces the number of chlorophylls feeding PSII from roughly 216 to 182, while increasing the PSI antenna from about 187 chlorophylls to 223. The shift precisely synchronizes electron transport rates in both photosystems.14PubMed Central. How state transitions balance photosynthetic electron transport in plants – a quantitative study In microalgae like Chlamydomonas, state transitions have a much larger amplitude than in land plants, and they serve mainly to redistribute energy between the two photosystems rather than to dissipate it as heat.15PubMed. State transitions redistribute rather than dissipate energy between the two photosystems in Chlamydomonas
When Light Becomes Too Much
Light drives photosynthesis, but too much of it is dangerous. Excess light energy that cannot be used productively generates reactive oxygen species that damage proteins and membranes. Plants have evolved a safety valve called non-photochemical quenching (NPQ), which harmlessly converts surplus absorbed light energy into heat before it can do damage. The dominant rapid form of NPQ, called qE, is triggered by a drop in pH inside the thylakoid lumen, the same acidification that drives ATP synthesis. When the lumen becomes acidic enough, a protein called PsbS binds to antenna complexes and induces a conformational change that turns them into energy quenchers. At the molecular level, energy is transferred from a cluster of chlorophyll molecules to a carotenoid (lutein) in a twisted conformation that can rapidly dissipate the energy as heat.16PubMed Central. Nonphotochemical quenching in plants: Mechanisms and mysteries – Section: Identikit of the quencher/quenchers Researchers have also found evidence for a second quenching pathway involving a chlorophyll excimer, where two chlorophylls interact and funnel energy into a charge-transfer state that safely de-excites.17PubMed. Excimer and non-photochemical quenching in LHCII
PSII Damage and the Repair Cycle
PSII takes the brunt of photodamage. The very chemistry that makes it special, extracting electrons from water with a powerful oxidant, also makes its reaction-center protein D1 a prime target for destruction. Under strong or sustained light, D1 breaks down faster than almost any other protein in the chloroplast. Photosynthetic organisms deal with this through a dedicated repair cycle. The damaged D1 protein is selectively removed and replaced with a freshly synthesized copy, all while the rest of PSII stays largely intact.
Two families of enzymes handle the demolition job. FtsH is a processive protease that chews through D1 from one end. Deg proteases, located on the lumenal side of the membrane, cut D1 at an internal loop, producing fragments that FtsH can then finish degrading. Studies in Arabidopsis showed that these two proteases work cooperatively: when FtsH is absent, Deg-generated D1 fragments accumulate, confirming that Deg cleavage assists FtsH in breaking down the damaged protein.18PubMed Central. Cooperative D1 Degradation in the Photosystem II Repair Mediated by Chloroplastic Proteases in Arabidopsis Research further supports a two-step model of photodamage: initial damage to the manganese cluster by light (particularly blue light) is followed by secondary damage to D1 itself, triggering Deg-mediated fragmentation.19PubMed. D1 fragmentation in photosystem II repair caused by photo-damage of a two-step model Even the chlorophyll molecules bound to D1 need to be dealt with during repair. An enzyme called chlorophyllase (CLH1) helps strip chlorophyll from the damaged D1 protein, facilitating FtsH-mediated degradation, particularly in young leaves.20PubMed. Arabidopsis CHLOROPHYLLASE 1 protects young leaves from long-term photodamage by facilitating FtsH-mediated D1 degradation in photosystem II repair
PSI, by contrast, is far more stable and does not need this kind of constant repair. Its damage tends to show up under different stress conditions, as discussed below.
How Environmental Stress Affects the Photosystems
Heat, drought, and intense light each hit the photosystems differently. PSII’s oxygen-evolving complex is one of the most heat-sensitive components in the whole photosynthetic apparatus. In barley varieties subjected to temperatures up to 45°C, the OEC was progressively inactivated, and this could be tracked by changes in chlorophyll fluorescence patterns. An interesting twist: drought stress actually increased PSII’s heat tolerance in both drought-sensitive and drought-tolerant barley varieties, essentially pre-hardening the system.21PubMed. Drought stress effects on photosystem I content and photosystem II thermotolerance analyzed using Chl a fluorescence kinetics in barley varieties differing in their drought tolerance
PSI, while more robust against light damage, can be vulnerable to drought. The same barley study found that drought stress led to a loss of PSI reaction centers, detected through changes in the OJIP fluorescence transient. The extent of PSI loss correlated with how drought-tolerant a given variety was. Studies on citrus rootstocks subjected to combinations of high light, heat, and water stress showed that PSII efficiency dropped sharply under multi-stress conditions and sometimes failed to recover even after stress was removed, particularly in the more sensitive genotype.22PubMed Central. Reduction of heat stress pressure and activation of photosystem II repairing system are crucial for citrus tolerance to multiple abiotic stress combination These findings underline why crop breeders pay close attention to photosystem resilience when developing varieties for hotter, drier climates.
Antenna Diversity Across Different Organisms
Not all photosynthetic organisms harvest light the same way. Cyanobacteria, red algae, and glaucophyte algae use a massive external antenna structure called the phycobilisome, which sits on the surface of the thylakoid membrane and funnels energy primarily into PSII. Phycobilisomes are excellent at capturing green and yellow-orange light that chlorophyll alone absorbs poorly, giving these organisms an advantage in aquatic environments where red and blue light are filtered out by water. Despite the phycobilisome’s efficiency, it was completely lost in green algae and land plants, which instead evolved a family of membrane-embedded chlorophyll-binding antenna proteins (the LHC family) to do the light-harvesting job.23PubMed Central. What Happened to the Phycobilisome?
Some cyanobacteria have pushed adaptation even further. Certain species that grow in far-red light, such as Fischerella thermalis, remodel their PSI to incorporate chlorophylls that absorb at longer wavelengths than normal. The structure of far-red-light-adapted PSI from this organism revealed a trimer of roughly 1 MDa containing 36 subunits per trimer, with an overall architecture remarkably similar to standard cyanobacterial PSI despite low sequence identity in some subunits.4PubMed Central. The structure of Photosystem I acclimated to far-red light illuminates an ecologically important acclimation process in photosynthesis Another cyanobacterium, Acaryochloris marina, uses chlorophyll d instead of chlorophyll a in its PSI, shifting the primary electron donor from P700 to P740 and allowing it to thrive in near-infrared light.24PubMed. Theoretical Model of the Far-Red-Light-Adapted Photosystem I Reaction Center of Cyanobacterium Acaryochloris marina Using Chlorophyll d and the Effect of Chlorophyll Exchange
Where Did the Two Photosystems Come From?
The evolutionary origin of the two photosystems is one of the more fascinating puzzles in biology. The two systems did not arise by one duplicating from the other. Sequence analysis shows that PSI’s reaction center is related to the reaction center found in green sulfur bacteria, while PSII’s reaction center shares ancestry with the reaction center of purple bacteria.25PubMed. Photosynthetic reaction center genes in green sulfur bacteria and in photosystem 1 are related The two different ancestral reaction centers were apparently brought together in the lineage that became cyanobacteria, wired in series, and combined with a newly evolved water-oxidizing complex to create the first oxygen-producing photosynthetic system.
PSII’s own heterodimeric core, made of the D1 and D2 proteins, arose from a gene duplication event distinct from the one that produced the core of anoxygenic Type II reaction centers.26PubMed Central. Thinking twice about the evolution of photosynthesis Phylogenetic analyses of D1 sequences have revealed several early-diverging forms, including “super-rogue” and “rogue” variants, that predate the conventional D1 proteins used in modern water-splitting PSII. This suggests that the water-oxidation capability was not present from the beginning but evolved gradually within the D1 family.27Molecular Biology and Evolution. Origin and Evolution of Water Oxidation before the Last Common Ancestor of the Cyanobacteria In other words, there was likely a version of PSII that did not split water before the modern oxygen-evolving version took over, a sobering reminder that something as fundamental as breathable oxygen depended on a specific evolutionary innovation in a metal cluster.
Mimicking the Photosystems for Clean Energy
PSII’s ability to split water using abundant metals and sunlight has made it a blueprint for artificial photosynthesis research. The goal is to build synthetic catalysts that do what the manganese cluster does: use sunlight to break water into oxygen and hydrogen, with the hydrogen serving as a clean fuel. PSII’s reliance on first-row transition metals like manganese rather than rare precious metals is a key design feature that researchers hope to replicate for globally scalable technology.28PubMed Central. Artificial photosynthesis: understanding water splitting in nature
Some groups have built working systems that directly mimic PSII’s functional architecture. One approach used a bismuth vanadate semiconductor as a light absorber, a layered nickel-iron hydroxide as a hole storage layer (analogous to PSII’s charge-accumulating OEC), partially oxidized graphene as a stand-in for the tyrosine residue that shuttles charges in natural PSII, and a molecular cobalt cubane as the oxygen-evolving catalyst. This integrated system achieved a solar-to-hydrogen efficiency of 2.0% with a notably low onset potential, demonstrating that the biomimetic strategy can deliver practical gains in charge separation and transfer.29PubMed. Mimicking the Key Functions of Photosystem II in Artificial Photosynthesis for Photoelectrocatalytic Water Splitting Two percent may sound modest, but natural photosynthesis itself converts only a small fraction of incoming solar energy into stored chemical energy, so even incremental progress in artificial systems is encouraging. The real challenge is durability: natural PSII solves its own fragility with the D1 repair cycle, but synthetic catalysts do not yet have anything equivalent, and they degrade under the harsh oxidizing conditions that water splitting demands.