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Photosynthesis is a process used by plants, algae, and some bacteria to convert light energy, usually from the sun, into chemical energy in the form of glucose, or sugar. This process is crucial for the production of oxygen and the sustenance of life on Earth. The term "photosynthesis" is derived from the Greek words "photos," meaning light, and "synthesis," meaning putting together.

The process occurs in two stages: the light-dependent reactions and the Calvin cycle (also known as the light-independent reactions). In the light-dependent reactions, light is absorbed by pigments such as chlorophyll, which then excites electrons. These excited electrons are used to power the synthesis of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), which are energy-rich molecules. Water molecules are split during this process, releasing oxygen as a byproduct.

In the Calvin cycle, the energy from ATP and NADPH is used to fix carbon dioxide into organic molecules, ultimately producing glucose. This cycle consists of three main phases: carbon fixation, reduction, and regeneration. The glucose produced can be used by the plant for energy and growth, or it can be converted into other organic compounds.

Photosynthesis plays a vital role in the Earth's ecosystem by providing the primary source of energy for most life forms. It is estimated that photosynthesis by plants and other organisms releases approximately 120 billion metric tons of oxygen into the atmosphere each year. Additionally, photosynthesis is a key component of the carbon cycle, as it removes carbon dioxide from the atmosphere and stores it in organic matter.