Water is the raw material that powers the light-dependent reactions of photosynthesis, serving as the source of the electrons and protons that drive the entire process and releasing the oxygen we breathe as a byproduct. When sunlight hits a leaf, water molecules are split apart inside a protein complex called Photosystem II, and that single reaction underpins nearly all oxygen production on the planet. But water’s importance to photosynthesis extends well beyond the splitting reaction itself, touching everything from how gases enter a leaf to how plants survive drought and how Earth’s atmosphere became breathable in the first place.
Where Water Enters the Picture
Photosynthesis has two broad stages. In the first, light energy is captured and used to generate chemical energy carriers. In the second, those carriers fuel the assembly of sugar from carbon dioxide. Water’s starring role is in the first stage, specifically inside Photosystem II, a large protein complex embedded in the thylakoid membranes of chloroplasts. When light energy reaches a special pair of chlorophyll molecules at the heart of Photosystem II (known collectively as P680), an electron is boosted to a high energy state and passed along a chain of acceptors. That leaves P680 one electron short, and it reclaims that electron by pulling it from water.
Splitting a single water molecule isn’t enough. The full reaction strips four electrons from two water molecules, releasing one molecule of oxygen gas and four protons. Those protons contribute to a concentration gradient across the thylakoid membrane that ultimately drives ATP production, while the electrons travel down the electron transport chain to help produce NADPH. Both ATP and NADPH then power the sugar-building reactions in the second stage. Without water donating those electrons, the entire chain stalls and photosynthesis stops.
The Cluster That Breaks Water Apart
The actual splitting of water takes place at a small metal cluster called the oxygen-evolving complex, or OEC. This cluster contains four manganese atoms and one calcium atom bridged by oxygen atoms, often written as Mn₄CaO₅. It sits on the interior side of Photosystem II, positioned to interact with water molecules that diffuse in through specific channels in the protein.
The OEC doesn’t rip water apart in one step. Instead, it cycles through a series of progressively more oxidized states, labeled S₀ through S₄, in what is known as the Kok cycle. Each time light hits P680 and an electron is removed, the OEC advances one step. After four successive light-driven oxidations, the cluster has accumulated enough oxidizing power to extract four electrons from two water molecules all at once, releasing O₂ and resetting to S₀. Researchers have tracked these transitions using X-ray absorption spectroscopy, confirming that manganese atoms in the cluster change their oxidation states at each step.1PubMed. Oxidation states of the manganese cluster during the flash-induced S-state cycle of the photosynthetic oxygen-evolving complex The final transition from S₃ to S₀, where oxygen is actually released, involves spectral changes detectable within a few hundred microseconds, consistent with the formation of the oxygen–oxygen bond and reduction of manganese back to a lower oxidation state.2PubMed Central. Time-resolved Mn Kα emission reveals early redox dynamics in the S3 to S0 transition of the photosystem II Kok cycle
What makes this cluster remarkable is how precisely the surrounding protein tunes its behavior. The amino acid residues that cradle the Mn₄CaO₅ cluster don’t just hold it in place. They actively influence which water molecule gets selected as a substrate, how it is positioned, and how it is stabilized during the reaction. Computational studies using quantum-mechanical modeling have shown that the flexibility of even a single amino acid side chain near the cluster can shift how water approaches and binds, affecting the overall efficiency of the splitting reaction.3PubMed Central. Conformational Flexibility of D1-Glu189: A Crucial Determinant in Substrate Water Selection, Positioning, and Stabilization within the Oxygen-Evolving Complex of Photosystem II The hydrogen-bond network around the cluster also matters: the protein and its bound water molecules create a landscape of proton pathways that make the splitting reaction thermodynamically feasible at the modest voltages that biological chlorophyll can generate.4ScienceDirect. Mechanism of light induced water splitting in Photosystem II of oxygen evolving photosynthetic organisms
How Water Molecules Move Through Photosystem II
Water doesn’t just float randomly into the active site. High-resolution structural studies have identified specific channels through the Photosystem II protein that ferry water molecules in and shuttle protons out. At least three major channels have been characterized, each running from the protein surface down toward the OEC. They go by shorthand names based on nearby oxygen or chloride atoms in the structure: the O1 channel, the O4 channel, and the Cl1 channel.
Not all channels serve the same function. Analysis of how water molecules behave during the S₂-to-S₃ transition found that the O1 channel is the likely route through which fresh water enters the active site, while the Cl1 channel appears to serve as the exit path for protons released during the splitting reaction. One amino acid in the Cl1 channel acts as a kind of gate, flipping between positions to let protons pass outward while preventing them from flowing backward and disrupting the reaction.5PubMed Central. Structural dynamics in the water and proton channels of photosystem II during the S2 to S3 transition This level of molecular plumbing is essential. Without organized water intake and proton removal, the OEC would quickly become clogged by its own products and unable to cycle.
Getting Water to Where It’s Needed
Before water can be split inside a chloroplast, it has to reach the leaf cells in the first place. Plants pull water from the soil through their roots, move it up the stem through xylem vessels, and distribute it to mesophyll cells in the leaves. Once inside a leaf cell, water enters the chloroplasts by diffusion. Most of a plant’s water, though, never makes it to the light reactions. The vast majority is lost through transpiration: evaporation from tiny pores called stomata on the leaf surface.
Stomata present a tradeoff. They need to be open for carbon dioxide to enter the leaf and feed the sugar-building reactions, but every moment they stay open, water vapor escapes. Plants regulate stomatal opening and closing based on water availability, light intensity, carbon dioxide concentration, and other cues. Species with high maximum stomatal conductance tend to close their stomata at relatively mild levels of leaf dehydration, while species with lower conductance can tolerate more drying before they shut down. A study across diverse plant species found a consistent tradeoff: species that let more gas flow through their stomata when water is plentiful also seal up more readily when conditions turn dry.6PubMed Central. A stomatal safety-efficiency trade-off constrains responses to leaf dehydration In practical terms, a plant can’t simply evolve wider-open stomata and fix more carbon for free. Greater gas exchange capacity comes coupled to greater vulnerability to water loss.
What Drought Does to the Light Reactions
When soil moisture drops and leaves begin to dehydrate, photosynthesis doesn’t fail because the OEC runs out of water to split. The amount of water consumed by the light reactions is tiny compared to what transpiration burns through. The real problem is indirect. Stomata close to conserve water, which starves the leaf of CO₂. Without enough CO₂ to consume the ATP and NADPH produced by the light reactions, excess energy builds up in the photosynthetic machinery. That surplus energy generates reactive oxygen species, which damage membranes, proteins, and the photosystems themselves.
Research on soybeans exposed to drought combined with intense light showed exactly this cascade: chlorophyll content dropped, levels of hydrogen peroxide and markers of membrane damage rose, and the performance of Photosystem II declined.7PubMed Central. Interactive effects of drought and high radiation on photosystem II performance and redox homeostasis in soybean (Glycine max L.) The combination of water deficit and strong sunlight is particularly destructive because the light keeps pumping energy into a system that has nowhere safe to send it. Drought alone slows things down; drought plus high light actively breaks things.
Plants have a built-in safety valve for this situation called non-photochemical quenching, or NPQ. When light energy exceeds what the photosynthetic machinery can productively use, NPQ diverts the surplus into harmless heat. Under drought, the dynamics of NPQ change as the plant ramps up its protective dissipation.8PubMed Central. Non-Photochemical Quenching under Drought and Fluctuating Light Some drought-tolerant species are especially good at this. Certain mosses, for example, can survive drying to a fraction of their normal water content by rapidly accelerating the rate at which they dump excess excitation energy as heat. When rehydrated, the quenching reverses and photosynthesis resumes within minutes.9PubMed. Dissipation of excess excitation energy by drought-induced nonphotochemical quenching in two species of drought-tolerant moss: desiccation-induced acceleration of photosystem II fluorescence decay In crop species like chickpea, varieties that maintain stronger NPQ under drought also tend to maintain more biomass, suggesting that efficient energy dissipation is a genuine survival advantage rather than just a laboratory curiosity.10PubMed Central. Implementation of theoretical non-photochemical quenching (NPQ((T))) to investigate NPQ of chickpea under drought stress with High-throughput Phenotyping
C3 Versus C4 and the Water Efficiency Gap
Not all plants handle the water-photosynthesis tradeoff the same way. Most plants use what’s called C3 photosynthesis, where CO₂ is fixed directly by the enzyme rubisco. This works well under moderate conditions, but rubisco also reacts with oxygen in a wasteful process called photorespiration, which burns energy without producing sugar. Because C3 plants need a relatively high concentration of CO₂ inside the leaf to outcompete photorespiration, they tend to keep their stomata open wider and lose more water per unit of carbon gained.
C4 plants, which include crops like maize, sorghum, and sugarcane, evolved a workaround. They pre-concentrate CO₂ around rubisco using a preliminary fixation step in specialized cells, so rubisco rarely encounters oxygen. This means C4 plants can keep their stomata partially closed and still fix carbon efficiently, losing less water in the process. Under both normal watering and drought conditions, C4 grasses show higher photosynthetic efficiency and water-use efficiency compared to their C3 counterparts, along with anatomical features like thinner leaves and higher stomatal density that aid water conservation.11International Journal For Multidisciplinary Research. Comparative Study of Photosynthetic Adaptations in C3 and C4 Grasses under Water Stress Conditions This efficiency gap is one reason researchers have been trying to engineer C4 traits into major C3 crops like rice, which could potentially improve yields in water-limited environments.12Journal of Experimental Agriculture International. Engineering C4 Photosynthesis Pathways into C3 Crops to Improve Nutrient Use Efficiency: A Review
A third strategy, CAM photosynthesis, takes water conservation even further. Cacti and other succulent plants open their stomata only at night, when temperatures are cooler and humidity is higher, storing CO₂ as organic acids until daytime when light is available to drive the reactions. CAM plants can thrive in deserts precisely because they’ve almost completely decoupled gas exchange from the hottest, driest part of the day.
How Water Oxidation Changed the Planet
For roughly the first billion years of photosynthesis on Earth, organisms used molecules other than water as their electron source. Early photosynthetic bacteria extracted electrons from hydrogen sulfide, ferrous iron, or organic compounds. These are easier to oxidize than water, but they’re far less abundant. The evolution of a system capable of splitting water was a transformative innovation: it gave photosynthetic organisms access to an essentially unlimited electron donor. The cyanobacteria that developed this ability released oxygen as waste, and over hundreds of millions of years, that oxygen accumulated in the atmosphere, eventually making aerobic life possible.13PubMed Central. The origin of atmospheric oxygen on Earth: the innovation of oxygenic photosynthesis
How this capability first arose is still debated, but phylogenetic analyses suggest that the ability to oxidize metals, and eventually water, may have developed in stages. An ancestral reaction center that could bind metals could have carried out a crude form of light-driven oxidation well before the modern, highly tuned version of the OEC appeared.14PubMed Central. Origin and Evolution of Water Oxidation before the Last Common Ancestor of the Cyanobacteria Today’s Mn₄CaO₅ cluster, with its precise amino-acid scaffolding and dedicated water channels, represents the product of billions of years of refinement on what was likely a far messier original reaction. Researchers have even proposed that the modern assembly process, in which the manganese cluster is built and activated by light, might recapitulate aspects of how water oxidation evolved in the first place.15PubMed Central. Reconstructing the Origin of Oxygenic Photosynthesis: Do Assembly and Photoactivation Recapitulate Evolution?
Desiccation Tolerance in Extreme Environments
In deserts and on exposed rock surfaces, photosynthetic organisms face conditions where liquid water is available only intermittently. Biological soil crusts, a community of cyanobacteria, algae, mosses, and lichens that form a living skin on desert soils, include species that can dry down to roughly five to ten percent of their normal water content, suspend all metabolism, and then resume photosynthesis within minutes to a couple of hours of getting wet again.5PubMed Central. Structural dynamics in the water and proton channels of photosystem II during the S2 to S3 transition This isn’t just “surviving” drought in the way a wheat plant might. These organisms essentially shut off, glass-like, and reboot when rehydrated, with photosynthetic components still functional.
How they pull this off involves several tricks. Some produce sugars like trehalose that replace water molecules around proteins and membranes during desiccation, maintaining structural integrity. Others rearrange their photosynthetic antennae to dump energy as heat the moment drying begins, preventing oxidative damage to a system that can’t use its light energy productively without water. The drought-induced quenching described earlier in mosses is one example. The speed of recovery upon rehydration suggests that the photosynthetic machinery doesn’t need to be rebuilt from scratch each time. Instead, it’s preserved in a protected state, waiting for water to return.
Water, Warming, and the Future of Photosynthesis
Climate projections add an uncomfortable twist to the water-photosynthesis relationship. As the atmosphere warms and vapor pressure deficit increases, plants face rising pressure to close their stomata even when soil water is technically still available. The air itself becomes hungrier for moisture, pulling water out of leaves faster than roots can replace it. This constrains transpiration and, with it, both carbon uptake and the evaporative cooling that keeps leaf temperatures manageable.
Modeling work suggests that the biggest projected increases in canopy temperature relative to air temperature will occur in regions where rising vapor pressure deficit increasingly limits plant growth and transpiration. In other words, leaves will get hotter than the surrounding air because they’re losing their ability to cool themselves through evaporation. This means standard air-temperature projections underestimate the thermal stress that photosynthesis will face in coming decades.16PubMed Central. Vegetation responses to air dryness amplify future land surface warming For crops and natural ecosystems alike, the availability of water won’t just limit how much carbon gets fixed. It will also determine whether leaves stay cool enough for their enzymes to function at all. The interplay between water supply, atmospheric dryness, and temperature is shaping up to be one of the more consequential bottlenecks in twenty-first-century plant productivity.
Heavy Water and the Limits of Substitution
An odd corner of research highlights just how sensitive photosynthesis is to the precise identity of its water molecules. Heavy water, or deuterium oxide (D₂O), is chemically similar to ordinary water but has a slightly higher mass because each hydrogen atom carries an extra neutron. When the model plant Arabidopsis thaliana was grown in heavy water, it rapidly incorporated deuterium into sucrose, confirming that photosynthesis pulls hydrogen atoms directly from the surrounding water and routes them into sugar.17PubMed Central. Divergent Molecular Responses to Heavy Water in Arabidopsis thaliana Compared to Bacteria and Yeast The plant essentially swapped its building material on the fly. This experiment is a neat demonstration that the hydrogen atoms in a plant’s sugars genuinely originate from the water it absorbs, not from any other source. It also underscores the intimate chemical dependence of the entire photosynthetic output on the water supply: change the water, and you change the sugar.