Sunlight, water, and air, combined inside a green leaf, become sugar. The reaction has been running on Earth for more than 3 billion years — first in cyanobacteria, then in the chloroplasts of every plant, alga, and seaweed. Photosynthesis is the primary energy event of the biosphere: nearly every calorie consumed by every animal, every drop of fossil fuel burned by every engine, every breath of oxygen drawn by every aerobic organism is downstream of a leaf catching a photon. The Great Oxidation Event (~2.4 billion years ago), during which photosynthesis pumped enough oxygen into Earth's atmosphere to transform the planet's surface chemistry, is the most consequential single event in biological history.
The overall reaction: 6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂. Photosynthesis happens in two stages. The light reactions (in the thylakoid membranes of chloroplasts): chlorophyll absorbs photons (chlorophyll a absorbs red and blue, reflecting green — which is why plants are green); the absorbed energy excites electrons, which are passed down an electron transport chain (Photosystem II → cytochrome b6f → Photosystem I); water is split as the electron source (4 H₂O → O₂ + 4 H⁺ + 4 e⁻) — this is the source of atmospheric oxygen; the proton gradient drives ATP synthase; NADPH is produced as a high-energy electron carrier. The Calvin cycle (in the chloroplast stroma): the enzyme RuBisCO (the most abundant protein on Earth) fixes CO₂ onto a 5-carbon sugar; ATP and NADPH from the light reactions reduce the products to glucose precursors; the cycle regenerates RuBisCO's substrate. RuBisCO is famously inefficient — it catalyzes only ~3 reactions per second and confuses oxygen for carbon dioxide about 25% of the time, an evolutionary holdover from when atmospheric oxygen was negligible. Different photosynthetic strategies: C3 plants use the basic Calvin cycle; C4 plants (corn, sugarcane, ~25% of terrestrial productivity) concentrate CO₂ before delivering it to RuBisCO, increasing efficiency in hot, dry conditions; CAM plants (cacti, succulents) open their stomata at night and run the Calvin cycle by day with stomata closed. Net primary productivity is on the order of 100 billion tons of carbon per year globally, half from terrestrial vegetation and half from oceanic phytoplankton. Fossil fuels are buried photosynthetic biomass: burning them now releases photosynthetic energy stored over deep time, on a timescale a million times faster than it was sequestered.
Climate change makes photosynthesis a critical lever: terrestrial plants and oceanic phytoplankton currently absorb about a quarter of human CO₂ emissions, but whether this carbon sink will grow, plateau, or shrink as the climate warms is a major open question. Crop improvement through enhanced photosynthesis is a long-running effort with recent breakthroughs: RIPE (Realizing Increased Photosynthetic Efficiency) has produced 25% yield gains in soybean through engineered photoprotection (2024), and engineering of more efficient RuBisCO variants has shown promise. Artificial photosynthesis — synthetic systems that capture solar energy and use it to split water or fix CO₂ — is a major research area aimed at carbon-neutral fuels. Solar panels are artificial photosynthesis without the carbon-fixing step, converting photons to electricity at much higher efficiency (~20–25%) than biological photosynthesis (~1–6% under field conditions).