🌿 How does photosynthesis work?
How plants turn sunlight, water and air into food and oxygen.
Plants make their own food with sunlight! They take water from the soil and air through their leaves, and with the energy of the sun they turn them into sugar. And they give us the oxygen we breathe.
Photosynthesis is how plants, algae and some bacteria turn light energy into chemical energy. Using chlorophyll, the green pigment, they capture sunlight to turn carbon dioxide and water into glucose (a sugar) and release oxygen. In short: 6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂. The sugar fuels the plant's growth, and almost every food chain on Earth starts here.
Oxygenic photosynthesis has two parts. In the thylakoid membranes, the light-dependent reactions use photosystems II and I to move electrons from water to NADP⁺, making ATP by chemiosmosis and releasing O₂. In the stroma, the Calvin–Benson cycle fixes CO₂ with the enzyme RuBisCO. RuBisCO's oxygenase side reaction causes photorespiration, which C₄ and CAM plants limit by concentrating CO₂. Crops turn only about 1% of sunlight into biomass, far below the theoretical limit. Ancient cyanobacteria filled the air with oxygen in the Great Oxidation Event, about 2.4 billion years ago.
⚡ Quick facts
- RuBisCO is probably the most abundant protein on Earth.
- Ocean phytoplankton make roughly half of the world's oxygen.
- Photosynthesis made Earth's air rich in oxygen about 2.4 billion years ago.