Early experiments

Joseph Priestley (1770) found that a candle or a mouse in a closed bell jar soon failed, but a mint plant placed inside restored the air so the candle burned and the mouse survived. He later discovered oxygen in 1774.

Jan Ingenhousz ran Priestley's set-up once in darkness and once in sunlight, and found that a plant can purify air fouled by a candle or an animal only when sunlight is present.

Working with an aquatic plant, Ingenhousz saw small bubbles form around its green parts in bright sunlight but not in the dark; he later identified the gas as oxygen, showing that only the green parts of a plant release oxygen.

Around 1854 Julius von Sachs gave evidence that growing plants make glucose, usually stored as starch, in their green parts; his later work placed chlorophyll inside special bodies in the cell, afterwards named chloroplasts.

T.W. Engelmann split light into a spectrum with a prism and shone it on Cladophora, a green alga, which sat in a suspension of aerobic bacteria; these gathered wherever O₂ was being released. Most of them collected in the blue and red bands, and this gave the first action spectrum of photosynthesis, which roughly matches the absorption spectra of chlorophylls a and b.

By the mid-nineteenth century the overall process was understood: green parts, in light, make carbohydrate from CO₂ and water and release oxygen.

Working with purple and green bacteria, Cornelius van Niel showed that photosynthesis depends on light, and that in it CO₂ is reduced to carbohydrate by hydrogen taken from a suitable oxidisable compound: 2H₂A + CO₂ → 2A + CH₂O + H₂O.

Green plants use H₂O as the hydrogen donor, oxidising it to O₂; bacteria that use H₂S produce sulphur or sulphate in place of O₂. So the O₂ that green plants release comes from water rather than CO₂, a point later confirmed with radioisotopic techniques.

The corrected overall equation for green plants is 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂ (in light).