Photosynthesis uses light to split water and build sugar from carbon dioxide. The oxygen released is the leftover from the water, not from the carbon dioxide — and the carbon in a tree came out of the air.
"Plants turn sunlight into food" compresses three separate claims into one, and it is worth pulling them apart. Light is captured, but it is not converted into food; it is used to drive chemistry. Water is split, and the oxygen atoms left over are what a plant releases — which is why isotopically labelled water shows up in the oxygen and labelled carbon dioxide does not. And carbon dioxide is fixed into sugar in a second set of reactions that do not need light directly at all, which is why they can be separated experimentally from the first. The consequences are larger than the biology. Almost all the oxygen in the atmosphere is water that has been taken apart by light, and almost all the dry mass of every plant — every trunk, every leaf, every acorn — is carbon that was in the air. Photosynthesis is also, unexpectedly, flawed: the enzyme that fixes carbon also reacts with oxygen and wastes energy doing it, and plants have independently invented workarounds more than forty-five times.
Developed record · 58% complete · reviewed 2026-08-10
What this page covers
A process rather than an organism. Oxygenic photosynthesis occurs in plants, algae and cyanobacteria; the machinery is shared, having arrived in plants and algae through the capture of a cyanobacterium.
Quick facts
Oxygen source
Water, not carbon dioxide
A tree’s dry mass
Mostly carbon, taken from the air
Respiration
Runs day and night; photosynthesis stops in the dark
The summary equation — carbon dioxide plus water plus light gives sugar plus oxygen — is accurate and hides the two most interesting things about the process. It does not say where the oxygen comes from, and it makes the whole thing look like one reaction when it is two, running in different places and at different speeds.
The oxygen a plant releases comes from splitting water, not from carbon dioxide
Established
Specialists would state this without hedging. Multiple independent lines of evidence agree.
Algae supplied with water enriched in oxygen-18 released oxygen carrying the isotopic signature of the water, while algae supplied with labelled carbon dioxide did not. Oxygen evolution can also be separated experimentally from carbon fixation: isolated illuminated chloroplasts release oxygen with an artificial electron acceptor and no carbon dioxide present.
Who this applies to
oxygenic photosynthesis, demonstrated in algae and general to plants
Studied in
Chlorella, Plantae
You may have heard
“Plants breathe in carbon dioxide and breathe out oxygen”
The two halves are unrelated in the way the phrase implies. Carbon dioxide goes in and its carbon is built into sugar; the oxygen coming out is water that light has taken apart. The atmosphere we breathe is, atom for atom, split water.
Why we rate it this way, and what the caveats are
EstablishedHigh confidence
Isotopic labelling gives a direct atomic answer, independently corroborated by the separation of oxygen evolution from carbon fixation in isolated chloroplasts.
How far it can be extended
The water-splitting complex is conserved across cyanobacteria, algae and plants, so the isotope result generalises on shared machinery.
Caveats
Demonstrated in algae; generalisation rests on the machinery being shared.
Early isotope-ratio measurements carried substantial uncertainty.
Says nothing about how water is split, which took decades more to work out.
Still unanswered
Can the water-splitting complex be reproduced synthetically at useful efficiency?
Hill, 1939 · Proceedings of the Royal Society B: Biological Sciences
Showed oxygen evolution occurs with no carbon dioxide present, separating the two halves of the process.
How we know
Taking the carbon dioxide away and getting oxygen anyway
Is photosynthesis one reaction, or more than one? And does making oxygen require carbon dioxide at all?
Photosynthesis had been treated as a single process: light plus water plus carbon dioxide in, sugar plus oxygen out. Hill isolated chloroplasts from leaves, illuminated them in a suspension containing an artificial chemical that would accept electrons, and excluded carbon dioxide from the system entirely. If oxygen production required carbon dioxide, nothing should happen.
What happened
Illuminated chloroplasts released oxygen and reduced the artificial acceptor, with no carbon dioxide present and no sugar produced.
What it shows
Photosynthesis is at least two separable processes. Oxygen evolution is light-driven and independent of carbon fixation, which immediately implies the oxygen is not coming from carbon dioxide — pointing at water, and setting up the isotope experiment that confirmed it two years later.
What it does not show
It does not identify the source of the oxygen, only rule out one candidate; the isotope work was still required. It uses an artificial electron acceptor rather than the plant’s own, and isolated chloroplasts are damaged relative to those in a living cell.
The controls — what makes this evidence rather than a story
Carbon dioxide was excluded, which is the whole manipulation — its absence is what makes the result interpretable.
Unilluminated chloroplast suspensions established that the reaction is light-dependent.
An artificial electron acceptor stood in for the plant’s own, showing the reaction needs somewhere to put electrons rather than needing carbon dioxide specifically.
Labelling the atoms to find where oxygen comes from
Photosynthesis consumes water and carbon dioxide and releases oxygen. Both inputs contain oxygen — so which one does the released gas come from?
No amount of measuring how much gas comes off can answer this, because the two candidate sources are chemically indistinguishable once mixed. The solution was to mark the atoms. Algae were supplied with water enriched in the heavy isotope oxygen-18 while their carbon dioxide carried ordinary oxygen; then the labelling was reversed, with heavy carbon dioxide and ordinary water. In each case the oxygen gas released was collected and its isotopic composition measured.
What happened
The released oxygen carried the isotopic signature of the water in both conditions, and never that of the carbon dioxide.
What it shows
The oxygen released in photosynthesis comes from splitting water. It rewrites the summary equation people memorise: plants are not taking carbon dioxide apart to free its oxygen, and the oxygen in the atmosphere is water that light has taken apart.
What it does not show
It says nothing whatever about how the water is split, which took several more decades and is still not fully reproducible synthetically. It was performed on algae, so its application to plants rests on the machinery being shared rather than on direct testing. Early isotope-ratio measurement carried substantial uncertainty.
The controls — what makes this evidence rather than a story
Reversing which input carries the label is the control: if the result tracked the label rather than the source, both conditions would show enrichment.
Ordinary unlabelled cultures established the baseline isotope ratio.
Cultures were otherwise identical, so only the isotopic composition of one input differed.
The natural reading of the equation is that carbon dioxide is taken apart and its oxygen released. It is not. Water is split, its hydrogen is used to do chemistry, and the oxygen is discarded as waste. Every breath anyone has ever taken is water that light pulled apart.
Light reactions, in the thylakoid membranes: light splits water, releasing oxygen and generating the chemical energy carriers ATP and NADPH.
Carbon fixation, in the stroma: those carriers are spent building sugar from carbon dioxide, in a cycle that needs no light directly.
That the two are separable is not a textbook convenience — it was shown by taking carbon dioxide away entirely and finding that illuminated chloroplasts still released oxygen.
Words used here
Chloroplast
The compartment inside a plant cell where photosynthesis happens. It was once a free-living cyanobacterium, captured by an ancestral cell and never released.
Chlorophyll
The pigment that absorbs light for photosynthesis. It absorbs red and blue strongly and reflects green — which is why leaves look green.
ATP and NADPH
Short-lived chemical energy carriers. The light reactions make them; carbon fixation spends them.
A leaf is green because chlorophyll absorbs red and blue light efficiently and reflects green. The colour of a plant is the light it is not using.
That is mildly strange, because green is near the peak of the sunlight reaching the ground — a pigment tuned to absorb it would seem to have an advantage. There is no settled answer. Suggestions include the evolutionary history of the pigment, the risk of absorbing more energy than the machinery can safely dissipate, and the fact that a leaf is thick enough to catch most of the green light eventually anyway.
Autumn colour follows directly. Chlorophyll is expensive and unstable, and a deciduous tree dismantles it before dropping a leaf, recovering the nitrogen. The yellows and oranges were present all along, masked; the reds are largely made fresh as the leaf shuts down.
Words used here
Carotenoid
Accessory pigments, yellow and orange, present in leaves year-round and revealed when chlorophyll is withdrawn.
The most important enzyme on Earth is not very good at its job.
Photosynthesis has a design flaw, and plants have independently worked around it more than forty times
Well supported
Good evidence backs this, though some details remain open.
Rubisco, the carbon-fixing enzyme, also reacts with oxygen, producing photorespiration that wastes fixed carbon and increases with temperature. C4 photosynthesis concentrates carbon dioxide around Rubisco in specialised cells, suppressing this. It has arisen independently more than forty-five times across flowering plant lineages, favoured by high light, high temperature and declining atmospheric carbon dioxide.
Who this applies to
flowering plants; C4 is concentrated in grasses and warm-climate lineages
Studied in
Poaceae, Angiospermae
Why we rate it this way, and what the caveats are
Well supportedHigh confidence
The biochemical inefficiency is directly measurable, the anatomical and biochemical signature of C4 is unambiguous, and the repeated independent origins are established phylogenetically.
How far it can be extended
Independent origins have been reconstructed across many unrelated lineages using phylogeny and anatomy.
Caveats
Estimates of the number of independent origins have been revised upward and will move again.
C4 costs energy; it pays only under specific conditions, which is why most plants are C3.
CAM is a separate workaround, addressing water loss rather than temperature.
Still unanswered
Can C4 be engineered into rice, and what would it be worth?
How much did fire and grazing contribute to C4 grassland expansion?
Benson et al., 1950 · Journal of the American Chemical Society
The C3 pathway that C4 supplements rather than replaces.
Rubisco, the enzyme that grabs carbon dioxide, cannot reliably tell it apart from oxygen. When it grabs oxygen instead — photorespiration — the plant spends energy undoing the mistake. This gets worse as temperature rises and worse as carbon dioxide gets scarcer, and it is generally read as a legacy of an enzyme that evolved when the atmosphere had almost no oxygen in it.
Three ways of running carbon fixation
Pathway
How it works
Where it wins
C3
Rubisco fixes carbon dioxide directly, in the same cells that catch light
Cool, moist, bright-enough conditions. Most plants, most trees, wheat and rice
C4
Carbon dioxide is captured first by a different enzyme and concentrated around Rubisco in separate cells
Hot, high-light, open habitats. Maize, sugarcane, most tropical grasses
CAM
Stomata open at night to take in carbon dioxide, stored as acid and released to Rubisco by day
Deserts, where opening stomata in daylight would be fatal. Cacti, agaves, many orchids
C4 costs energy to run, which is why it has not simply replaced C3 — it pays only where photorespiration is expensive enough to be worth suppressing. That it has been invented independently more than forty-five times is a measure of how much pressure there is.
Words used here
Rubisco
The enzyme that fixes carbon dioxide. Slow, imprecise, and probably the most abundant protein on Earth.
Photorespiration
The wasteful reaction that happens when Rubisco grabs oxygen instead of carbon dioxide.
Stomata
Adjustable pores in a leaf. They must open to let carbon dioxide in, and water escapes while they are open — the central trade-off in plant life.
Not light, most of the time — and the answer explains why a hot afternoon slows a plant down.
Photosynthesis needs light, carbon dioxide and water, and at any given moment one of them is the constraint. Adding more of the others does nothing until you relieve the one that is binding, which is why glasshouse growers who raise the CO₂ concentration see a response and gardeners who move a plant into brighter sun sometimes do not.
Light: limiting at dawn, dusk and in shade. Above a saturation point more light adds nothing and eventually does damage.
Carbon dioxide: at around 0.04% of the air, this is the usual limit for a well-lit plant on a normal day.
Temperature: the enzymes have an optimum, and beyond it the rate falls rather than levelling off.
Water: not as a raw material — the quantity used in the reaction is tiny — but because shortage closes the stomata.
That last point is the one that ties the section together. Stomata are the pores a leaf opens to let CO₂ in, and every molecule that enters costs water vapour going out. On a hot dry afternoon a plant closes them to avoid wilting, and photosynthesis stops for lack of carbon dioxide while standing in full sun. The limit is not light and not really water either: it is the trade-off between them.
C4 and CAM plants are two different responses to the same squeeze. C4 plants concentrate CO₂ around the enzyme so it keeps working while the stomata are barely open — maize, sugarcane and most tropical grasses. CAM plants go further and open their stomata only at night, storing carbon as an acid until morning, which is why a cactus can survive where nothing else does and why CAM plants grow slowly.
Three ways of fixing carbon
Type
How it handles the trade-off
Where it wins
C3
Stomata open by day; no concentrating step
Cool, moist, moderate light — most trees and temperate plants
C4
Concentrates CO₂ internally so less stomatal opening is needed
Opens stomata at night and stores carbon until morning
Deserts and epiphytic perches — cacti, agaves, many orchids
C4 photosynthesis has evolved independently more than sixty times. It is a workaround for a flaw in the enzyme at the centre of the process, and evolution has kept finding the same fix.
Words used here
Stoma
A pore on a leaf, opened and closed by two guard cells. Every molecule of CO₂ that enters costs water vapour leaving.
Limiting factor
Whichever requirement is currently in shortest supply. Increasing anything else has no effect until it is relieved.
CAM
Crassulacean acid metabolism: taking in carbon dioxide at night and using it the following day. Slow, and very water-efficient.
Plants respire day and night. Photosynthesis is the part that stops in the dark
Established
Specialists would state this without hedging. Multiple independent lines of evidence agree.
Photosynthesis requires light and ceases in darkness. Respiration — the oxidation of sugars to release energy — runs continuously in plant cells, as in animal cells. In daylight photosynthesis typically exceeds respiration so there is net oxygen release and net carbon fixation; at night only respiration operates, so a plant is a net consumer of oxygen.
Who this applies to
plants generally
Studied in
Plantae
You may have heard
“Plants take in carbon dioxide and give out oxygen”
Only in the light, and only on balance. Plants respire continuously like any other organism, so at night the exchange runs the other way. The familiar summary describes the daytime net result, not what a plant is doing.
Why we rate it this way, and what the caveats are
EstablishedHigh confidence
Foundational plant physiology, established since the eighteenth century and measurable directly as gas exchange.
How far it can be extended
Both processes are universal in plants; the light dependence of one and not the other is basic physiology.
Caveats
CAM plants open their stomata at night to take in carbon dioxide, but still fix it in the light.
A plant in a dark room consumes oxygen; the quantities involved are far too small to matter in a bedroom.
Net gas exchange depends on light level, temperature and species.
Still unanswered
How do plants balance respiration against photosynthesis under stress?
Hill, 1939 · Proceedings of the Royal Society B: Biological Sciences
Oxygen evolution demonstrated as a strictly light-driven reaction.
A tree is built mostly out of air — the carbon in wood came from carbon dioxide
Established
Specialists would state this without hedging. Multiple independent lines of evidence agree.
The dry mass of a plant is predominantly carbon, hydrogen and oxygen. Carbon enters exclusively as atmospheric carbon dioxide fixed in photosynthesis; hydrogen and oxygen derive from water. Soil supplies mineral nutrients required in comparatively small quantities, not the bulk of the material.
Who this applies to
plants generally, trees most strikingly
Studied in
Plantae
You may have heard
“Trees get their mass from the soil”
Van Helmont tested this in the 1640s by growing a willow in weighed soil for five years. The tree gained 74 kg; the soil lost about 60 g. He concluded the mass came from water, which was half right — most of the dry mass is carbon, and it came out of the air.
Why we rate it this way, and what the caveats are
EstablishedHigh confidence
Established by the carbon-fixation work and by elemental analysis of biomass. Van Helmont’s seventeenth-century willow experiment showed the soil loses almost no mass, though he drew the wrong conclusion from it.
How far it can be extended
Elemental composition of plant biomass is general, and the carbon source is established by isotope tracing.
Caveats
Soil nutrients are essential; the claim concerns bulk mass, not necessity.
A large fraction of a living tree’s mass is water, which does come through the roots.
Mineral nutrient limitation constrains growth even though it contributes little mass.
Still unanswered
How will rising atmospheric carbon dioxide alter growth where nutrients limit it?
The water transport that must accompany carbon fixation, and the ceiling it imposes.
Van Helmont’s willow is the experiment worth knowing here. In the 1640s he planted a willow in a weighed quantity of dried soil, watered it for five years, and weighed everything again. The tree had gained about 74 kg; the soil had lost roughly 60 g. He concluded the mass came from water — half right, and a far better answer than the prevailing view that plants eat soil.
The full answer needed another three hundred years. Most of a tree’s dry mass is carbon, and that carbon arrived as gas. A mature oak is, in a literal and non-metaphorical sense, several tonnes of assembled air.
Two and a half centuries, and every step overturned the previous reading.
1648
First observation
Van Helmont weighs a willow
A willow grown for five years in weighed soil gained around 74 kg while the soil lost about 60 g. Van Helmont concluded plants are made from water — wrong in detail, and the first demonstration that they are not made from soil.
1772
Landmark experiment
Priestley finds that plants restore "injured" air
Air made unbreathable by a burning candle or a dying mouse was restored by a sprig of mint, so that a candle would burn in it again. Performed before oxygen had a name.
Priestley’s results had been maddeningly irreproducible. Ingenhousz showed the restoration happens only in light, and that plants in darkness do the opposite — which explained every failed replication at once.
Changes how the 1772 result reads
The effect was real and the conditions were unstated. Adding light turned an unreliable curiosity into a repeatable phenomenon, and revealed that plants respire continuously while photosynthesising only when lit.
1939
Landmark experiment
Hill splits the process in two
Isolated chloroplasts, illuminated with an artificial electron acceptor and no carbon dioxide at all, still released oxygen. Oxygen evolution and carbon fixation are separate reactions.
Changes how the 1779 result reads
Photosynthesis had been treated as a single reaction consuming carbon dioxide and emitting oxygen. Removing carbon dioxide entirely and still getting oxygen showed there were two processes, and that the oxygen was not coming from the carbon dioxide.
Algae fed radioactive carbon dioxide were killed at intervals from seconds upward and their products separated by chromatography and read off photographic film — a stop-motion reconstruction of a metabolic cycle.
Synthesis of the evidence that C4 photosynthesis suppresses photorespiration and has evolved independently dozens of times, driven by falling atmospheric carbon dioxide and hot open habitats.
Changes how the 1950 result reads
The Calvin cycle had been treated as the way plants fix carbon. C4 and CAM showed it is one route among several, and that a substantial fraction of plants bolt a carbon-concentrating stage in front of it because the enzyme at its centre is unreliable.
Why is chlorophyll green when green light is the most abundant?
Why it matters: A pigment tuned to the peak of available sunlight would seem obviously better, and no explanation for the apparent mismatch is agreed.
Can C4 photosynthesis be engineered into rice?
Why it matters: It is one of the largest ongoing efforts in plant science, and the yield implications for a staple crop are enormous.
Can the water-splitting complex be reproduced synthetically?
Why it matters: Splitting water with sunlight at useful efficiency would change energy storage. Plants have been doing it for billions of years and we cannot match it.