The reactants of photosynthesis are carbon dioxide and water, and the products are glucose and oxygen. Light energy drives the entire process, so it is sometimes listed as a reactant as well, though technically it is the energy input rather than a chemical substance consumed. The familiar summary equation captures this neatly: six molecules of carbon dioxide plus six molecules of water, powered by sunlight, yield one molecule of glucose and six molecules of oxygen. What that tidy summary conceals is a sprawling chain of reactions split across two distinct stages, each with its own inputs, outputs, and molecular machinery.
Where Each Reactant Goes
A common misconception is that the oxygen released during photosynthesis comes from carbon dioxide. It does not. The oxygen gas that exits a leaf comes entirely from water molecules. During the first stage of photosynthesis, water is split apart inside a protein complex called Photosystem II. This reaction strips electrons and protons from water and releases oxygen as a byproduct. The electrons harvested from water then travel through a series of protein carriers, ultimately providing the reducing power needed to build sugar.
Carbon dioxide, meanwhile, enters the picture in the second stage. It is captured by an enzyme and chemically attached to an existing carbon-containing molecule, then progressively reduced using the energy carriers generated in the first stage. The end result is a three-carbon sugar that can be assembled into glucose, sucrose, starch, or other organic molecules the plant needs. So water supplies the oxygen and the electrons, while carbon dioxide supplies the carbon atoms that become sugar. Both are consumed, but in different stages and for different purposes.
The Light-Dependent Reactions
The first stage of photosynthesis takes place in the thylakoid membranes inside chloroplasts. When a chlorophyll molecule absorbs a photon of light, the energy bumps an electron to a higher energy state, and that excited electron is passed along a chain of carrier proteins. Two large protein complexes, Photosystem II and Photosystem I, work in series. Photosystem II handles the water-splitting step, while Photosystem I re-energizes the electrons so they can ultimately be used to produce NADPH, a molecule that carries reducing power to the next stage.
The water-splitting reaction in Photosystem II is catalyzed by a small metal cluster containing four manganese atoms and one calcium atom. This cluster cycles through a series of intermediate states, accumulating enough oxidizing power to extract four electrons from two water molecules in a single coordinated event, releasing one molecule of oxygen gas in the process.1PubMed. Investigation of the high-spin S(2) states in photosystem II using Mn X-ray absorption spectroscopy with insights from XFEL crystallography Researchers have mapped this cluster in extraordinary detail using cryo-electron microscopy and X-ray crystallography, revealing exactly how nearby amino acids and water molecules are arranged to allow the reaction to proceed.2PubMed. Cryo-EM structure of photosystem II D1-V185T mutant from Thermosynechococcus vestitus Recent crystallographic work has even captured the moment when an additional water molecule is introduced into the cluster during one of the intermediate steps, shedding light on exactly how oxygen-oxygen bonds form.3PubMed Central. An additional water is introduced into the manganese cluster during the formation of the S3 state of photosystem II
As electrons flow from water through the two photosystems, protons are pumped across the thylakoid membrane, creating a concentration gradient. That gradient drives an enzyme called ATP synthase, which produces ATP. At the end of the electron chain, Photosystem I uses a second dose of light energy to push the electrons onto NADPH. The net outputs of the light-dependent reactions, then, are ATP, NADPH, and oxygen. The oxygen diffuses out of the leaf. The ATP and NADPH move on to the next stage.
Not All Light Is Equal
Chlorophyll absorbs red and blue wavelengths of light most efficiently, which is why leaves appear green: green light is mostly reflected. But photosynthesis is not limited to chlorophyll alone. Cyanobacteria and red algae, for instance, use accessory pigment structures called phycobilisomes. These contain specialized pigments called bilins that capture green and yellow wavelengths, precisely the wavelengths chlorophyll misses.4PubMed Central. Phycobilisomes: modular light-harvesting systems of cyanobacteria and red algae The energy these accessory pigments absorb is funneled into the photosystems and used in the same light-dependent reactions. This is one reason cyanobacteria thrive in deep or murky water where only green light penetrates.
Measured carefully, the peak quantum yield for carbon fixation in plants is roughly one molecule of carbon dioxide fixed per eleven absorbed photons, with the best efficiency occurring at red-orange wavelengths around 620 to 640 nanometers.5PubMed Central. Photosynthetic Quantum Yield Dynamics: From Photosystems to Leaves That might sound wasteful, but keep in mind that each photon carries only a tiny packet of energy, and the chemistry of splitting water and fixing carbon requires multiple coordinated electron transfers.
The Calvin Cycle and Carbon Fixation
The second stage of photosynthesis, often called the Calvin cycle, takes place in the fluid-filled interior of the chloroplast called the stroma. Here, carbon dioxide from the atmosphere is attached to a five-carbon molecule by an enzyme called RuBisCO, widely considered one of the most abundant proteins on Earth.6PubMed Central. A short history of RubisCO: the rise and fall (?) of Nature’s predominant CO2 fixing enzyme The resulting six-carbon intermediate immediately splits into two three-carbon molecules, which are then reduced using the ATP and NADPH produced in the light-dependent reactions.
Some of those three-carbon molecules are recycled to regenerate the five-carbon acceptor molecule, keeping the cycle going. The rest are exported from the chloroplast. Carbon leaves the chloroplast as a three-carbon sugar phosphate, which is exchanged across the chloroplast membrane for inorganic phosphate in a one-for-one swap.7PubMed Central. The end game(s) of photosynthetic carbon metabolism Once in the cytoplasm, these three-carbon units are assembled into sucrose, which is a phosphate-free molecule that can be transported throughout the plant. Starch, by contrast, is built and stored inside the chloroplast itself for use later, especially at night when photosynthesis shuts down.
The Calvin cycle does not directly require light, which is why it was historically called the “dark reactions.” That label is misleading, though. The cycle depends on a continuous supply of ATP and NADPH, which are only produced when light is hitting the photosystems. In practice, the Calvin cycle runs during the day and slows dramatically at night. Some of its enzymes are even directly activated by light-dependent signals and switch off in the dark.
The ATP and NADPH Budget Does Not Quite Balance
One underappreciated detail is that the ratio of ATP to NADPH produced by the light reactions does not perfectly match what the Calvin cycle needs. The light reactions produce ATP and NADPH at a ratio of roughly 1.3 to 1, but carbon fixation requires them at a ratio closer to 1.5 to 1.8PubMed Central. Glycogen deficiency impairs diurnal energy metabolism and cell division in Synechocystis That means the Calvin cycle needs proportionally more ATP than the linear electron flow from water to NADPH can provide. Plants bridge this gap through several supplementary pathways, including cyclic electron flow around Photosystem I, which generates extra ATP without producing more NADPH. This kind of fine-tuning is invisible in the summary equation but essential for keeping the whole system running.
What Else Plants Need Beyond Carbon Dioxide and Water
The classic equation lists only carbon dioxide, water, and light. But a plant that had nothing else would photosynthesize very poorly, if at all. Photosynthesis depends on at least 14 mineral elements that serve as structural components, enzyme activators, or electron carriers.9PubMed Central. Plant nutrition for sustainable development and global health
Nitrogen, for example, is a core part of every chlorophyll molecule and every protein in the photosynthetic machinery. Magnesium sits at the center of each chlorophyll ring. Manganese is essential for the water-splitting cluster in Photosystem II. Iron appears in multiple electron carriers. Phosphorus is needed for ATP and NADPH. Sulfur is part of the iron-sulfur proteins that shuttle electrons through Photosystem I. Even trace elements like copper, zinc, molybdenum, and boron play specific roles in electron transport, carbon metabolism, or photoprotection.10Frontiers in Photobiology. The essential nutrient elements in photosynthesis A deficiency in any one of these can bottleneck the entire process, which is why fertilizer makes such a dramatic difference in crop yields.
C4 and CAM Plants Run the Same Chemistry With a Different Front End
The standard Calvin cycle works well in cool, moist conditions, but RuBisCO has a well-known flaw: it can grab oxygen instead of carbon dioxide, triggering a wasteful side reaction called photorespiration. In hot, dry environments where plants close their stomata to conserve water, carbon dioxide levels inside the leaf drop and oxygen levels rise, making photorespiration worse. Two groups of plants have independently evolved workarounds.
C4 plants, which include maize, sugarcane, and many tropical grasses, use a preliminary carbon-fixation step in their outer leaf cells. An enzyme called PEP carboxylase grabs carbon dioxide and attaches it to a three-carbon molecule, producing a four-carbon acid (hence “C4”).11PubMed Central. Diffusion of CO(2) across the Mesophyll-Bundle Sheath Cell Interface in a C(4) Plant with Genetically Reduced PEP Carboxylase Activity That four-carbon acid is shuttled to inner bundle-sheath cells, where it releases its carbon dioxide directly to RuBisCO. Because the bundle-sheath cells accumulate high concentrations of carbon dioxide, RuBisCO rarely encounters oxygen, and photorespiration is effectively suppressed.12Australian Journal of Plant Physiology. Analysis of Bundle Sheath Conductance and C4 Photosynthesis using a PEP-Carboxylase Inhibitor PEP carboxylase exerts substantial control over the rate of the whole pathway; experiments on mutant plants with reduced PEP carboxylase activity show clear drops in carbon fixation capacity, especially in bright light.13PubMed. Control of C4 photosynthesis: effects of reduced activities of phosphoenolpyruvate carboxylase on CO2 assimilation in Amaranthus edulis L.
CAM plants, which include cacti, agaves, and many succulents, take a different approach by separating the two fixation steps in time rather than space. They open their stomata at night, when temperatures are cooler and humidity is higher, and use PEP carboxylase to capture carbon dioxide into organic acids that are stored in cell vacuoles. During the day, the stomata close to prevent water loss, and the stored acids release their carbon dioxide internally for the Calvin cycle to use.14PubMed Central. Rhythmic Mechanisms Governing CAM Photosynthesis in Kalanchoe fedtschenkoi: High-Resolution Temporal Transcriptomics The overall reactants and products are the same: carbon dioxide and water go in, sugar and oxygen come out. But the timing and cellular logistics differ dramatically.
When Water Is Not the Electron Donor
The photosynthesis most people learn about is oxygenic photosynthesis, meaning it produces oxygen. But several groups of bacteria carry out anoxygenic photosynthesis, which uses the same basic logic of capturing light energy to fix carbon but substitutes a different electron donor for water. Green sulfur bacteria, for instance, use hydrogen sulfide instead of water. They oxidize hydrogen sulfide to elemental sulfur and fix carbon dioxide through a pathway called the reverse citric acid cycle rather than the Calvin cycle.15PubMed Central. Anoxygenic photosynthesis with emphasis on green sulfur bacteria and a perspective for hydrogen sulfide detoxification of anoxic environments Because no water is split, no oxygen is released. The “product” replacing oxygen is sulfur.
Purple sulfur bacteria follow a similar strategy but are even more versatile. Some can use iron sulfide minerals like pyrite as both their electron and sulfur source, essentially growing on rock with nothing but light and carbon dioxide.16PubMed Central. Evidence for autotrophic growth of purple sulfur bacteria using pyrite as electron and sulfur source These organisms play important roles in the sulfur cycle of oxygen-free environments like deep lakes and hydrothermal vents. Their existence is a reminder that the familiar equation with water and oxygen is specific to plants, algae, and cyanobacteria. Photosynthesis as a broader phenomenon is more flexible than the textbook version suggests.
Photorespiration and Stress
RuBisCO’s tendency to react with oxygen instead of carbon dioxide creates a real metabolic cost. The resulting product has to be recycled through a complex salvage pathway that spans three different organelles and consumes energy without producing sugar. Under normal conditions, this photorespiration wastes a measurable fraction of the carbon a plant fixes. Under stress, the picture shifts in interesting ways.
Drought forces stomata shut, which restricts carbon dioxide entry and makes photorespiration worse initially. But in silver fir seedlings subjected to prolonged severe drought, photorespiration actually decreased because the electron transport chain itself was impaired, reducing the flow of energy to RuBisCO and limiting all reactions. Heat stress, on the other hand, increased photorespiration substantially in well-watered seedlings, but did not further increase it in drought-stressed seedlings because the drought had already suppressed the system’s overall capacity.17PubMed. Drought and heat stress interactions modify photorespiration and hydrogen peroxide content in silver fir These interacting effects mean that the net output of photosynthesis under real-world conditions can be far lower than the theoretical maximum, and predicting how a forest or a crop field will respond to a changing climate requires understanding more than just the summary equation.
How Photosynthesis Shaped the Atmosphere
The oxygen in Earth’s atmosphere is almost entirely a product of photosynthesis, but the relationship between the evolution of oxygen-producing cyanobacteria and the actual accumulation of atmospheric oxygen is surprisingly complicated. Geological evidence suggests that oxygenic photosynthesis may have evolved hundreds of millions of years before the Great Oxidation Event, the point roughly 2.4 billion years ago when atmospheric oxygen rose to detectable levels. Early cyanobacteria living in thin microbial mats on land could have generated enough oxygen locally to drive chemical weathering of rocks and mobilize sulfur and trace metals into the oceans, all while the atmosphere itself remained essentially oxygen-free.18PubMed Central. Benthic perspective on Earth’s oldest evidence for oxygenic photosynthesis
Why the long delay? One proposal is that environmental conditions during the early Archean, including high concentrations of dissolved nickel and urea in the oceans, inhibited the growth and spread of cyanobacteria for an extended period.19Communications Earth & Environment. Biogeochemical impact of nickel and urea in the great oxidation event Only when those conditions changed could cyanobacterial populations expand enough for oxygen to overwhelm the chemical sinks that had been consuming it. The reactants and products of photosynthesis, in other words, eventually transformed the entire planet, but it took geological time and the right environmental conditions for the oxygen product to accumulate.
Artificial Photosynthesis
Given how central photosynthesis is to life on Earth, researchers have spent decades trying to build artificial versions that could convert sunlight, water, and carbon dioxide into fuels or useful chemicals on demand. The goal is to mimic the core logic: use light energy to split water and then use the resulting electrons to reduce carbon dioxide into energy-dense molecules.20PubMed Central. Artificial photosynthetic processes using carbon dioxide, water and sunlight: can they power a sustainable future? In principle, this would create a closed carbon loop, pulling carbon dioxide from the air and storing solar energy in liquid fuels that could be burned later without adding new carbon to the atmosphere.
The challenge is catalysis. Natural photosynthesis relies on exquisitely tuned protein-metal complexes, like the manganese cluster that splits water, and no synthetic catalyst yet matches their performance at scale and cost. Current artificial systems can carry out individual steps: some split water efficiently, others reduce carbon dioxide to carbon monoxide or methanol. But integrating both halves into a single, durable, affordable device remains an unsolved engineering problem.21PubMed Central. Artificial Photosynthesis: Current Advancements and Future Prospects The fact that a leaf manages all of this with water, air, and sunlight, using catalysts made from common elements like manganese and iron, is part of what makes the natural process so remarkable and so hard to replicate.