How We Discovered Photosynthesis: A Timeline

Where does a tree's wood come from? For most of history, people assumed plants simply took their food from the soil. Working out what really happens took more than three centuries and the work of many scientists, each testing an idea, finding part of the answer and leaving a new question for the next. The story is a good example of how science moves forward: careful measurement, a wrong conclusion, and then better experiments that correct it.

The Flemish scientist Jan Baptist van Helmont planted a willow weighing about 5 pounds (roughly 2.3 kg) in 200 pounds (about 91 kg) of dried soil and gave it only rainwater for five years. The tree grew to about 169 pounds (roughly 77 kg), yet the soil lost only about 2 ounces (around 57 g). His account was published in 1648, after his death. He concluded that the new wood came from water alone. He was right that it did not come from the soil, but he missed the air.

In 1771 the English chemist Joseph Priestley found that a candle burned out in a sealed jar and a mouse could not survive in the 'injured' air. A sprig of mint left in the jar 'restored' it. In 1779 the Dutch physician Jan Ingenhousz showed that plants only restore air in light, and only their green parts do it.

The Swiss pastor and naturalist Jean Senebier showed in 1782 that plants take in 'fixed air', which we now call carbon dioxide, and give out oxygen. In 1804 another Swiss scientist, Nicolas-Théodore de Saussure, used careful measurements to show that water is also a reactant: a plant's gain in mass comes from carbon dioxide and water together. By then, all the ingredients of the modern word equation were known.

In the 1880s the German scientist Theodor Engelmann shone a spectrum of light across a strand of the green alga Spirogyra. Bacteria that need oxygen gathered most thickly beside the parts lit by red and blue light, showing which colours chlorophyll uses to make oxygen.

In 1905 the English plant scientist Frederick Blackman measured photosynthesis in pondweed while changing light, temperature and carbon dioxide. He proposed that the factor in shortest supply limits the whole rate, the idea of the limiting factor that students still use today.

New tools in the 20th century let scientists follow individual atoms. In 1941, Samuel Ruben, Martin Kamen and colleagues in California gave algae water containing a heavy form (isotope) of oxygen. The heavy oxygen turned up in the oxygen gas released, showing that the oxygen comes from water, not from carbon dioxide.

From 1948 to 1954, Melvin Calvin, Andrew Benson, James Bassham and their team fed algae carbon dioxide made with radioactive carbon-14 and tracked where the carbon went. They mapped the cycle of reactions that builds carbon dioxide into sugar, now called the Calvin cycle. Calvin received the 1961 Nobel Prize in Chemistry for this work.