photosynthesis

Photosynthesis is the process by which photoautotrophic organisms convert light energy into chemical energy, using carbon dioxide and water to synthesize organic compounds and releasing oxygen. In plants, algae, and cyanobacteria, chloroplasts contain pigments such as chlorophyll that capture light for energy conversion. Photosynthesis forms the base of most food chains and drives the Earth's oxygen supply.

The process occurs in two linked sets of reactions. The light-dependent reactions take place in the thylakoid membranes and require light. They use photosystems II and I to drive electron transport, generate ATP and NADPH, and split water to release oxygen. The Calvin cycle, or light-independent reactions, occurs in the stroma and uses CO2 along with ATP and NADPH to synthesize glyceraldehyde-3-phosphate, which is ultimately converted into glucose and other sugars.

Most land plants perform C3 photosynthesis, but some use C4 or CAM pathways to reduce photorespiration in hot or dry environments. In addition to chlorophyll a and b, pigments such as carotenoids and phycobiliproteins extend the range of usable light. The rate of photosynthesis is influenced by light intensity and quality, carbon dioxide concentration, temperature, and water availability.

Photosynthesis is a central process in the global carbon cycle and a foundational source of organic matter for nearly all organisms, while its byproduct—oxygen—replenishes the atmosphere. Historical studies of photosynthesis contributed to core concepts in chemistry and metabolism, including carbon fixation and energy transduction in cells.