01/05/2017
Photosynthesis is a process used by plants and other organisms to convert
light energy into chemical energy that can later be released to fuel the organisms' activities (energy transformation ). This chemical energy is stored in carbohydrate molecules , such as sugars, which are synthesized from carbon dioxide and water – hence the name photosynthesis , from the Greek φῶς, phōs , "light", and
σύνθεσις , synthesis , "putting together". [1][2][3] In most cases,
oxygen is also released as a waste product. Most plants , most algae, and
cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Photosynthesis is largely responsible for producing and maintaining the
oxygen content of the Earth's atmosphere, and supplies all of the organic compounds and most of the energy necessary for life on Earth.[4]
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, producing oxygen gas. The hydrogen freed by the splitting of water is used in the creation of two further compounds that act as an immediate energy storage means: reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), the "energy currency" of cells.
In plants, algae and cyanobacteria, long-term energy storage in the form of sugars is produced by a subsequent sequence of light-independent reactions called the
Calvin cycle; some bacteria use different mechanisms, such as the
reverse Krebs cycle , to achieve the same end. In the Calvin cycle, atmospheric carbon dioxide is
incorporated into already existing organic carbon compou
