Photosynthesis
Photosynthesis is the process by which green plants, algae and cyanobacteria transform light energy into chemical energy, converting carbon dioxide and water into sugars and releasing oxygen. It runs in two linked stages: the light-dependent reactions in the thylakoid membrane, where water is split and sunlight drives electron transport that makes ATP and NADPH, and the Calvin–Benson cycle in the stroma, where the enzyme Rubisco fixes CO2 into sugars. The oxygen released comes from water, not carbon dioxide — a point settled with 18O-labelled water in 1941. Photosynthesizers split into oxygenic forms (plants, algae, cyanobacteria) and anoxygenic bacteria such as green and purple sulfur bacteria, which use hydrogen sulfide instead of water. Land plants add further strategies — C3, C4 (Hatch–Slack) and CAM — that differ in how they supply CO2 to Rubisco. Photosynthesis sets the scale of the biosphere: global net primary production is estimated at about 104.9 petagrams of carbon per year, and roughly half of Earth's oxygen production comes from the ocean. Efficiency figures vary with definition: the theoretical maximum conversion of solar energy to biomass is put at 4.6% for C3 and 6% for C4 photosynthesis, while realized field efficiencies are far lower.