Photosynthesis

Photosynthesis is a process used by plants and other autotrophic organisms to convert light energy, normally from the sun, into chemical energy that can be used to fuel the organisms' activities. Carbohydrates, such as sugars, are synthesized from carbon dioxide and water (hence the name photosynthesis, from the Greek , phōs ( phōtos), i.e. "light", and synthesis, i.e. "putting together") during the process. Oxygen is also released, mostly as a waste product. Most plants, most algae, and cyanobacteria perform the process of photosynthesis, and are called photoautotrophs. Photosynthesis maintains atmospheric oxygen levels and supplies most of the energy necessary for all life on Earth, except for chemotrophs, which gain energy through oxidative chemical reactions.
Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centres that contain green chlorophyll pigments. In plants, these proteins are held inside organelles called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane. In these light-dependent reactions, some energy is used to strip electrons from suitable substances such as water. This produces oxygen gas and hydrogen ions, which are transferred to a compound called nicotinamide adenine dinucleotide phosphate (NADP⁺), reducing it to NADPH. More light energy is transferred to chemical energy in the generation of adenosine triphosphate (ATP), the "energy currency" of cells.
In plants, algae and cyanobacteria, sugars are produced by a sequence of light-independent reactions called the Calvin cycle, but some bacteria use different mechanisms, such as the reverse Krebs cycle. In the Calvin cycle, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, such as ribulose bisphosphate .Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reduced into triose phosphate. Of every six triose phosphate molecules produced, one is removed to form further carbohydrates and five are "recycled" back into the cycle to regenerate the original carbon dioxide acceptor, RuBP.