History
In 1771 English chemist Joseph Priestley found that air in a sealed jar where a candle had burned out could again support a mouse if a sprig of mint was placed inside. In 1779 Dutch-born physician Jan Ingenhousz showed this "purifying" happened only in green parts of plants and only in sunlight, and Jean Senebier showed plants take in carbon dioxide. In the 1930s Cornelis van Niel argued that the oxygen comes from water rather than CO₂, which isotope experiments confirmed in 1941.
How it works
Photosynthesis runs in two stages. In the light reactions on the thylakoid membranes, chlorophyll absorbs light, splits water, releases oxygen and produces the energy carriers ATP and NADPH. In the Calvin cycle in the stroma, the enzyme RuBisCO uses that energy to fix carbon dioxide into sugar. In the 1950s Melvin Calvin traced radioactive carbon-14 through algae to map the cycle, winning the 1961 Nobel Prize in Chemistry. C4 plants like corn and CAM plants like cacti use variations adapted to heat and drought.
Impact on Earth and climate
About 2.4 billion years ago, photosynthesis by cyanobacteria triggered the Great Oxidation Event, the first big rise in atmospheric oxygen, paving the way for complex life. Today land plants and ocean phytoplankton absorb vast amounts of carbon each year, acting as sinks that slow the greenhouse effect. That makes protecting and restoring forests central to net-zero plans, and climate swings such as El Niño affect how much plankton photosynthesize. Scientists are also pursuing "artificial photosynthesis" to turn sunlight into fuel.
FAQ
- Do plants give off oxygen at night?
- No. Without light, photosynthesis stops and plants only respire, using a little oxygen. The amount is tiny, so bedroom plants are harmless.
- Why are leaves green?
- Chlorophyll absorbs mostly red and blue light and reflects green.
Sources reviewed: 2026-10-26
References
- Ingenhousz, J. (1779). Experiments upon Vegetables, Discovering Their Great Power of Purifying the Common Air in the Sun-shine. London: P. Elmsly.
- Calvin, M. (1961). The path of carbon in photosynthesis. Nobel Lecture, December 11, 1961.
- Lyons, T. W., Reinhard, C. T., & Planavsky, N. J. (2014). The rise of oxygen in Earth's early ocean and atmosphere. Nature, 506, 307–315.