Photosynthesis
How plants, algae and cyanobacteria convert light, water and CO₂ into glucose and oxygen — the reaction that powers almost every food chain on Earth.
Primary sources: NCBI Bookshelf: Photosynthesis · Khan Academy AP Biology
01 · The overall reaction
Light in, glucose out.
Photosynthesis takes place mainly in the chloroplasts of plant cells. The overall equation summarises a complex series of reactions:
The reaction is endothermic — it stores energy from light in the chemical bonds of glucose. That glucose fuels cellular respiration in the plant and, through the food chain, in animals too. See our cellular respiration guide for what cells do with that glucose.
The oxygen released is a by-product of splitting water (photolysis) — not from splitting CO₂. This was established by Ruben and Kamen using isotopically labelled oxygen-18 in 1941.
02 · The four stages
Two locations, four steps.
Photosynthesis is divided into light-dependent reactions (in the thylakoid membranes) and light-independent reactions (in the stroma).
01
Light absorption
Thylakoid membranes
Chlorophyll and accessory pigments absorb photons. The energy excites electrons in Photosystem II, which are passed along an electron transport chain.
02
Photolysis of water
Thylakoid lumen
Water molecules are split (2H₂O → 4H⁺ + 4e⁻ + O₂) to replace the electrons lost from Photosystem II. The oxygen is released as a by-product.
03
ATP and NADPH synthesis
Thylakoid membranes
Electron flow drives proton pumping across the thylakoid membrane. The proton gradient powers ATP synthase (photophosphorylation). NADP⁺ is reduced to NADPH at Photosystem I.
04
Calvin cycle (CO₂ fixation)
Stroma
RuBisCO fixes CO₂ onto RuBP (5C) to form two molecules of 3-PGA (3C). ATP and NADPH reduce 3-PGA to G3P, which is used to build glucose and regenerate RuBP.
03 · Pigments
Different pigments capture different wavelengths.
A single pigment cannot absorb all useful wavelengths of visible light. Plants use multiple pigments to broaden their light-harvesting range:
- Chlorophyll a — the primary pigment; absorbs red (680 nm) and blue-violet (430 nm) light. Found in all photosynthetic organisms.
- Chlorophyll b — an accessory pigment; absorbs blue (450 nm) and orange-red (640 nm); passes energy to chlorophyll a.
- Carotenoids (carotene and xanthophylls) — absorb blue-green (400–500 nm); the yellow-orange colours seen in autumn leaves when chlorophyll breaks down.
The absorption spectrum shows which wavelengths a pigment absorbs; the action spectrum shows which wavelengths drive photosynthesis. They closely match, confirming that absorbed light drives the reaction.
04 · Limiting factors
Three factors control the rate.
At any moment, the factor furthest below its optimum limits the overall rate — Blackman's Law of Limiting Factors (1905):
- Light intensity — increasing light raises the rate until another factor becomes limiting. Measured in lux or μmol photons m⁻² s⁻¹.
- CO₂ concentration — atmospheric CO₂ is about 420 ppm (2026). Increasing it raises photosynthesis rate in greenhouses.
- Temperature — enzyme-catalysed steps (especially RuBisCO in the Calvin cycle) are temperature-sensitive. Optimal around 25–30 °C for most C3 plants; above ~40 °C enzymes begin to denature.
Research into improving photosynthetic efficiency — for food security and bioenergy — is one of the most active areas of plant science. See our photosynthesis efficiency news article and the cellular respiration guide to see what happens to the glucose once it is made.
Questions about photosynthesis
- The overall equation is: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Carbon dioxide and water are the raw materials; glucose and oxygen are the products. Light energy (absorbed by chlorophyll) drives the reaction. It is the reverse of aerobic respiration. See our cellular respiration guide for the comparison.
- Light-dependent reactions (thylakoid membranes): light hits chlorophyll, energising electrons; water is split (photolysis), releasing O₂; ATP and NADPH are produced. Light-independent reactions / Calvin cycle (stroma): CO₂ is fixed by RuBisCO and reduced using ATP and NADPH to produce G3P, which is used to build glucose. See NCBI Bookshelf on photosynthesis.
- Chlorophyll a (absorbs 680 and 700 nm) is the primary pigment. Chlorophyll b, xanthophylls and carotenoids are accessory pigments that absorb different wavelengths and transfer energy to chlorophyll a. Green light is mostly reflected — which is why plants look green. The action spectrum shows which wavelengths drive photosynthesis most effectively.
- Blackman's Law of Limiting Factors states that the rate of a physiological process is limited by the factor that is most deficient. For photosynthesis: light intensity (more light → faster up to saturation), CO₂ concentration (more CO₂ → faster), and temperature (enzyme-controlled steps; too hot denatures RuBisCO). See our cellular respiration guide for how the products of photosynthesis are used.
Now see what cells do with glucose
Photosynthesis stores energy in glucose. Cellular respiration releases it. The two are mirror processes.