Photosynthesis
Photosynthesis is the process by which plants, algae and cyanobacteria capture energy from sunlight and store it in sugar molecules, using carbon dioxide and water and releasing oxygen. Overall it can be summarised as 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. In chloroplasts, light-dependent reactions split water and make ATP and NADPH, and the Calvin cycle uses them to turn carbon dioxide into sugar.
Electron flow, energy carriers and carbon fixation
Light harvested by antenna pigments is funnelled to reaction-centre chlorophyll a. Photosystem II (P680) passes excited electrons along the electron transport chain to photosystem I (P700), which reduces NADP⁺ to NADPH; P680 is re-reduced with electrons extracted from water, releasing O₂ (two H₂O split per O₂). Proton pumping and water splitting build a proton gradient into the thylakoid lumen that ATP synthase uses to make ATP.
In the stroma, RuBisCO catalyses the reaction of CO₂ with RuBP to give two 3-PGA molecules per CO₂; ATP and NADPH reduce 3-PGA to G3P. Three turns fix enough carbon to export one G3P, and the remaining five G3P regenerate three RuBP.
Full explanation — the complete reference version every reading depth is based on
What photosynthesis is
Photosynthesis turns light energy into chemical energy. Plants, algae and cyanobacteria — photoautotrophs, or 'self-feeders using light' — take in carbon dioxide and water, use the energy of sunlight to build carbohydrates, and release oxygen. Animals, fungi and most other bacteria cannot do this; they are heterotrophs that ultimately depend on the sugars photosynthesis makes.
Overall summary of photosynthesis: carbon dioxide and water, using light energy, give glucose and oxygen.
Where it happens
- In plants, most photosynthesis happens in the leaf's middle layer of cells, the mesophyll.
- Carbon dioxide enters and oxygen leaves through stomata — tiny pores opened and closed by guard cells.
- Inside each cell, chloroplasts hold stacks of membrane discs called thylakoids; chlorophyll sits in those membranes.
- The fluid around the thylakoids is the stroma, where sugars are built.
- Chlorophyll a absorbs blue and red light but not green, which is why leaves look green.
The two stages
- Light-dependent reactions (thylakoid membranes): light energy is captured by photosystems and stored in the energy carriers ATP and NADPH. To replace the electrons it loses, photosystem II pulls electrons from water, splitting it and releasing oxygen.
- Light-independent reactions — the Calvin cycle (stroma): the enzyme RuBisCO fixes carbon dioxide onto a five-carbon molecule, RuBP; ATP and NADPH then reduce the products to a three-carbon sugar, G3P, and the cycle regenerates RuBP.
Worked example: counting carbon
Each turn of the Calvin cycle fixes one carbon dioxide molecule, so it takes three turns to gain the three carbon atoms of one exported G3P, and six turns to gain the six carbons of one glucose. Checking the overall equation the same way: six CO₂ supply 6 carbon atoms, matching the 6 carbons in C₆H₁₂O₆; and because splitting two water molecules makes one O₂, making six O₂ needs twelve water molecules to be split — more than the six in the simplified equation, because that equation shows only the net balance. (The carbon and oxygen arithmetic is ours; the step rules come from the cited textbook.)
Why it matters and how we know
Photosynthesis is the route by which the Sun's energy enters most food chains, and it is a source of the oxygen many organisms need. Combining models of land and ocean production, one study estimated global net primary production at about 104.9 billion tonnes (petagrams) of carbon per year, with land and oceans contributing roughly equally — so tiny ocean algae and cyanobacteria matter as much as forests. Melvin Calvin's research on how plants assimilate carbon dioxide earned the 1961 Nobel Prize in Chemistry.
Where this connects
Photosynthesis happens inside cells and is a transformation of energy: light energy ends up stored in chemical bonds, and its balanced equation is a chemical reaction like any other. Its partner process, cellular respiration, breaks sugars back down using oxygen to release usable energy — the overall reactions are reverses of each other, and plants do both. The sunlight that drives it comes from nuclear fusion in the Sun, a star.
Assumptions and limits
- The summary equation is a net balance of atoms, not a description of the steps: the real process has many steps, and the oxygen released comes from water, not from carbon dioxide.
- Plant pigments use only light of about 400–700 nm, the photosynthetically active range.
- Global production figures such as 104.9 billion tonnes of carbon a year come from models driven by satellite data, so they are estimates with uncertainty, not direct counts.
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Related concepts
- Chemical Reactions — Related to
- Reaction Energy — Related to
- Stars — Related to
- Respiration — Contrasts with
- Chemical Reactions — Application of
- Light — Related to
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Sources and methodology
- Photosynthesis captures energy from sunlight and stores it in the chemical bonds of carbohydrate molecules, using carbon dioxide and water as substrates and releasing oxygen. (awaiting scientific review)
- The synthesis of glucose by photosynthesis can be summarised as 6CO₂ + 6H₂O + energy → C₆H₁₂O₆ + 6O₂, the reverse of the equation for the breakdown of glucose. (awaiting scientific review)
- Plant pigments absorb only light with wavelengths from about 400 nm to 700 nm, a range plant physiologists call photosynthetically active radiation. (awaiting scientific review)
- Plants, algae and cyanobacteria are photoautotrophs: they use light energy to make their own food, whereas heterotrophs such as animals and fungi rely on sugars made by photosynthetic organisms. (awaiting scientific review)
- In plants, photosynthesis takes place mostly in the leaf's middle layer, the mesophyll, inside chloroplasts, where chlorophyll is embedded in the membranes of stacked thylakoids surrounded by a fluid called the stroma. (awaiting scientific review)
- Carbon dioxide and oxygen are exchanged through small, regulated openings called stomata, each flanked by guard cells that open and close it; on a hot, dry day the guard cells close the stomata to conserve water. (awaiting scientific review)
- Chlorophyll a absorbs light from both ends of the visible spectrum (blue and red) but not green, so green light is reflected or transmitted and chlorophyll looks green. (awaiting scientific review)
- Photosynthesis has two stages: the light-dependent reactions convert light energy into chemical energy stored in ATP and NADPH, and the light-independent reactions (the Calvin cycle) use that ATP and NADPH to build sugars from carbon dioxide. (awaiting scientific review)
- In the light-dependent reactions, photosystem II replaces the electrons it loses by extracting electrons from water, splitting water molecules and releasing oxygen; two water molecules must be split to form one O₂ molecule. (awaiting scientific review)
- In the Calvin cycle, the enzyme RuBisCO joins carbon dioxide to the five-carbon molecule RuBP, and three turns of the cycle are needed to export one three-carbon G3P molecule (six turns for one six-carbon glucose). (awaiting scientific review)
- 'Dark reaction' is the most outdated name for the Calvin cycle and is misleading, because it wrongly implies that the reactions happen only at night or are independent of light. (awaiting scientific review)
- Although the light-independent reactions do not use light as a reactant, they require the products of the light-dependent reactions, and several of their enzymes are activated by light. (awaiting scientific review)
- Global net primary production is estimated at about 104.9 petagrams of carbon per year, with roughly equal contributions from land and oceans. (awaiting scientific review)
- Melvin Calvin was awarded the 1961 Nobel Prize in Chemistry for his research on carbon dioxide assimilation in plants. (awaiting scientific review)
- The Nobel Prize in Chemistry 1961 (Melvin Calvin) — Other (unclassified)
Claims marked “awaiting scientific review” cite the sources listed but have not yet been signed off by a scientific reviewer.
Content status: published 1 October 2026.
- Scientific review: this version has not yet been signed off by a scientific reviewer.
- The Advanced explanation has not yet been reviewed for age suitability.