Photosynthesis is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their activities
Photosynthetic organisms use intracellular organic compounds to store the chemical energy they produce in photosynthesis within organic compounds like sugars, glycogen, cellulose and starches
Some bacteria also perform anoxygenic photosynthesis, which uses bacteriochlorophyll to split hydrogen sulfide as a reductant instead of water, producing sulfur instead of oxygen
Archaea such as Halobacterium also perform a type of non-carbon-fixing anoxygenic photosynthesis, where the simpler photopigment retinal and its microbial rhodopsin derivatives are used to absorb green light and power proton pumps to directly synthesize adenosine triphosphate (ATP)
The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons
Cyanobacteria appeared later and the excess oxygen they produced contributed directly to the oxygenation of the Earth, which rendered the evolution of complex life possible
The average rate of energy captured by photosynthesis globally is approximately 130 terawatts, which is about eight times the current power consumption of human civilization
Coated with a water-resistant waxy cuticle that protects the leaf from excessive evaporation and decreases absorption of ultraviolet or blue light to minimize heating
Transparent epidermis layer allows light to pass through to the palisade mesophyll cells where most photosynthesis takes place
Chemically fixes carbon dioxide in the cells of the mesophyll by adding it to the three-carbon molecule phosphoenolpyruvate (PEP)
Translocates the four-carbon organic acid oxaloacetic acid or malate to specialized bundle sheath cells where RuBisCO and other Calvin cycle enzymes are located
CO2 released by decarboxylation of the four-carbon acids is then fixed by RuBisCO activity to the three-carbon 3-phosphoglyceric acids
Increase in oxygen gas produced by the light reactions of photosynthesis, causing an increase in the oxygenase activity of RuBisCO and decrease in carbon fixation
Plants that do not use PEP-carboxylase in carbon fixation, where the primary carboxylation reaction catalyzed by RuBisCO produces the three-carbon 3-phosphoglyceric acids directly in the Calvin-Benson cycle