Photosynthesis is the process that converts light energy into chemical energy stored in sugar, using carbon dioxide and water, and releasing oxygen as a by-product. Nearly every food chain on Earth depends on it, and the oxygen in your lungs was put there by it.
It is also one of the most elegant reactions in biology, because it does something chemistry does not do easily: it uses visible light to split water — a molecule that does not want to split.
The equation
The overall reaction, written in its simplified form:
6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2
Six molecules of carbon dioxide and six of water become one molecule of glucose and six of oxygen.
Two things are worth noticing. The carbon in the sugar comes entirely from CO2 — the plant is building matter out of air. And the oxygen released comes entirely from water, not from carbon dioxide, which is a fact that took decades to establish.
Where it happens
Photosynthesis takes place in chloroplasts, organelles containing stacks of membrane discs called thylakoids suspended in a fluid called the stroma. Two locations, two stages:
- Thylakoid membranes — the light-dependent reactions. Pigments are embedded in the membrane itself
- Stroma — the light-independent reactions (the Calvin cycle)
The pigments are key. Chlorophyll a absorbs blue-violet and red light most strongly and reflects green — which is the entire explanation for why plants look green. Accessory pigments — chlorophyll b and carotenoids — absorb wavelengths chlorophyll a misses, widening the usable spectrum, and are responsible for the yellows and oranges that appear when chlorophyll breaks down in autumn.
Stage one: the light-dependent reactions
This is where light energy is captured and converted. The sequence:
- Photosystem II (P680) absorbs a photon. An electron is excited to higher energy and captured by a primary acceptor
- Water is split to replace that electron — photolysis: 2 H2O → 4 H+ + 4 e− + O2. This reaction is the sole source of atmospheric oxygen. Every oxygen molecule you have ever breathed came from this step
- Electron transport chain — electrons pass through plastoquinone, the cytochrome b6f complex and plastocyanin. As they move, protons are pumped into the thylakoid interior
- Photosystem I (P700) re-energises the electrons with a second photon
- NADP+ reductase reduces NADP+ to NADPH
- Chemiosmosis — the accumulated proton gradient drives ATP synthase, producing ATP
Outputs: ATP, NADPH and O2. The first two are energy currencies; the oxygen is waste from the plant’s perspective.
Stage two: the Calvin cycle
Now in the stroma, no light is directly required — but the products of stage one are, which is why plants cannot photosynthesise in the dark for long.
- Carbon fixation — the enzyme RuBisCO attaches CO2 to a five-carbon sugar, ribulose bisphosphate (RuBP)
- Reduction — the unstable six-carbon intermediate immediately splits into two molecules of 3-phosphoglycerate, which ATP and NADPH convert into glyceraldehyde-3-phosphate (G3P)
- Regeneration — most G3P is used to regenerate RuBP so the cycle can continue; the remainder is exported to build glucose, sucrose, starch and, crucially, everything else the plant is made of
RuBisCO is the most abundant protein on Earth — plausibly 25% of all leaf protein — for two reasons. It is extraordinarily slow, catalysing only a handful of reactions per second, and it frequently mistakes O2 for CO2, initiating photorespiration, a wasteful process that releases previously fixed carbon. The plant compensates by making enormous quantities of it.
How plants coped with that flaw
| Type | Strategy | Examples |
|---|---|---|
| C3 | Default. No compensation for photorespiration | Wheat, rice, most trees |
| C4 | Fixes CO2 into a 4-carbon compound in outer cells, then releases it inside bundle-sheath cells where RuBisCO sees high CO2 — spatial separation | Maize, sugarcane, sorghum |
| CAM | Opens stomata at night to collect CO2 as malate, stores it, then fixes it during the day with stomata closed — temporal separation. Conserves water | Cacti, pineapple, succulents |
C4 and CAM evolved independently more than sixty times, which tells you how strongly photorespiration and water loss have shaped plant evolution.
What limits the rate
Blackman’s law of limiting factors: the rate is constrained by whichever factor is in shortest supply. Raise it and another becomes limiting.
- Light intensity — rate rises linearly, then plateaus once the photosystems are saturated
- CO2 concentration — the limiting factor for C3 plants under normal conditions (about 0.04% of the atmosphere is quite low for RuBisCO)
- Temperature — enzymes have an optimum; too high and denaturation, plus increased photorespiration
- Water — shortage closes stomata, cutting CO2 supply. This is why drought stunts growth even with abundant light
- Minerals — nitrogen for enzymes and chlorophyll, magnesium at the centre of the chlorophyll molecule, iron as a cofactor
How efficient is it?
Lower than you would guess. Typical crop plants convert only about 1–2% of incident sunlight into biomass. The theoretical ceiling for C3 photosynthesis is around 11%, constrained by the fraction of sunlight plants can absorb and the metabolic cost of using it.
That gap — between 2% actual and 11% theoretical — is one of the active frontiers of agricultural research, and the reason artificial photosynthesis and engineered RuBisCO attract serious funding.
Why it matters beyond plants
- Oxygenic photosynthesis was a planetary event. Cyanobacteria invented it, and the oxygen they produced accumulated until it triggered the Great Oxidation Event roughly 2.4 billion years ago — transforming an anaerobic atmosphere, causing a mass extinction of obligate anaerobes, and making complex aerobic life eventually possible
- Fossil fuels are stored photosynthesis. Coal, oil and gas are ancient sunlight captured by organisms hundreds of millions of years ago
- Global scale: terrestrial and marine primary production together fix on the order of 100 billion tonnes of carbon annually — roughly half of it by phytoplankton
- Food webs start here. Herbivores capture some of that chemical energy; carnivores capture it second-hand. There is no third source
For how that oxygen ultimately connected to the evolution of complex life, see our explainer on what evolution is.
Frequently asked questions about photosynthesis
Do plants photosynthesise at night?
No — without light, the light-dependent reactions stop, and the Calvin cycle runs out of ATP and NADPH. What plants do at night is respiration, consuming oxygen and releasing CO2 exactly as animals do. Plants respire continuously, day and night; they only photosynthesise in light. CAM plants are the exception in an interesting way: they open their stomata at night to capture CO2, but still need light to convert it.
Why are plants green rather than black?
Black would absorb all wavelengths and maximise energy capture — so the fact that plants are green is an evolutionary question. One strong explanation is photoprotection: absorbing every photon in full sunlight risks overloading the photosystems and generating damaging reactive oxygen species, so reflecting green light acts as a partial filter. Another is that early photosynthesisers may have been adapted to light filtered through water where green was abundant. The pigments themselves likely originated in cyanobacteria.
Do indoor plants improve air quality meaningfully?
Not at any realistic scale. The influential NASA chamber study used sealed chambers with artificial light and plant-appropriate loads; subsequent work has shown that you would need an implausible number of plants per square metre to reproduce the effect in an occupied room with ventilation. Plants remain worth having — just for the reasons they are actually good at.
Where does the mass of a tree come from?
Almost entirely from the air. Around 95% of dry plant biomass is derived from CO2 fixed during photosynthesis; the remainder is water and minerals drawn from soil. A tree gains mass by assembling carbon from the atmosphere — a fact that surprises almost everyone the first time they hear it.
Is the equation misleading?
Slightly, yes — it is a summary rather than a mechanism. It does not show that the oxygen comes from water, does not indicate that light is required at two specific points, implies glucose is the direct product when G3P is, and omits the ATP and NADPH cycle entirely. It is the correct net result; it is not the chemistry.
This article explains general science concepts. For the light physics behind pigment absorption, see why the sky is blue, and for the biology of how these pathways arose, what evolution is.
