Photosynthesis

The process by which plants, algae and some bacteria convert light energy, water and carbon dioxide into chemical energy stored as sugar, releasing oxygen as a byproduct. It is the foundation of nearly every food web on Earth, since almost all other life either eats a photosynthetic organism directly or eats something that did, and it is the source of essentially all atmospheric oxygen and, over geological time, the fossil fuels formed from ancient photosynthetic life. Most plants use a standard photosynthetic pathway that loses significant water through open leaf pores during the heat of the day, but desert plants such as the saguaro cactus instead use a variant called CAM photosynthesis, opening their pores only at night to take in carbon dioxide while conserving water, then completing the light-driven reactions internally during the following day.