photosynthesis The chemical process by which green plants and other phototrophs synthesize organic compounds from carbon dioxide and water in the presence of sunlight. In plants and most algae it occurs in the chloroplasts and there are two principal types of reactions. In the light-dependent reactions, which require the presence of light, energy from sunlight is absorbed by photosynthetic pigments (chiefly the green pigment chlorophyll) and used to bring about the photolysis of water: H₂O → 2H⁺ + 2e^– + ½O₂

The electrons released by this reaction pass along a series of electron carrier molecules; as they do so they lose their energy, which is used to convert ADP to ATP in the process of photophosphorylation. The electrons and protons produced by the photolysis of water are used to reduce NADP: 2H⁺ + 2e^– + NADP⁺ → NADPH + H⁺

The ATP and NADPH produced during the light-dependent reactions provide energy and reducing power, respectively, for the ensuing light-independent reactions (formerly called the ‘dark reaction’), which nevertheless cannot be sustained without the ATP generated by the light-dependent reactions. During these reactions carbon dioxide is reduced to carbohydrate in a metabolic pathway known as the Calvin cycle. Photosynthesis can be summarized by the equation: CO₂ + 2H₂O → [CH₂O] + H₂O + O₂

Since virtually all other forms of life are directly or indirectly dependent on plants for food, photosynthesis is the basis for all life on earth. Furthermore virtually all the atmospheric oxygen has originated from oxygen released during photosynthesis.