Photosynthesis

Photosynthesis (/ˌfoʊtəˈsɪnθəsɪs/ FOH-tə-SINTH-ə-sis)^[1] 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. Photosynthesis is usually used to refer to oxygenic photosynthesis, a process that produces oxygen. To use this stored chemical energy, the organisms' cells metabolize the organic compounds through another process called cellular respiration. Photosynthesis plays a critical role in producing and maintaining the oxygen content of the Earth's atmosphere, and it supplies most of the biological energy necessary for complex life on Earth.^[2]

Some bacteria also perform anoxygenic photosynthesis, which uses bacteriochlorophyll to split hydrogen sulfide as a reductant instead of water. This process produces sulfur 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 "energy currency" of cells. Such archaeal photosynthesis might have been the earliest form of photosynthesis that evolved on Earth, going back as far as the Paleoarchean, preceding that of cyanobacteria (see Purple Earth hypothesis).

While the details may differ between species, the process always begins when light energy is absorbed by the reaction centers, proteins that contain photosynthetic pigments or chromophores. In plants, these proteins are chlorophylls (a porphyrin derivative that absorbs the red and blue spectrums of light, thus reflecting a green color) held inside chloroplasts, abundant in leaf cells. 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 important molecules that participate in energetic processes: reduced nicotinamide adenine dinucleotide phosphate (NADPH) and ATP.

In plants, algae, and cyanobacteria, sugars are synthesized by a subsequent sequence of light-independent reactions called the Calvin cycle. In this process, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, such as ribulose bisphosphate (RuBP).^[3] Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reduced and removed to form further carbohydrates, such as glucose. In other bacteria, different mechanisms like the reverse Krebs cycle are used to achieve the same end.

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.^[4] Cyanobacteria appeared later; the excess oxygen they produced contributed directly to the oxygenation of the Earth,^[5] which rendered the evolution of complex life possible. Today, the average rate of energy captured by photosynthesis globally is approximately 130 terawatts,^[6][7][8] which is about eight times the current power consumption of human civilization.^[9] Photosynthetic organisms also convert around 100–115 billion tons (91–104 Pg petagrams, or a billion metric tons), of carbon into biomass per year.^[10][11] Photosyntesis was first discovered in 1779 by Jan Ingenhousz; he showed that plants need light, not just air, soil, and water.

Photosynthesis is vital for climate processes, as it captures carbon dioxide from the air and then binds it in plants, harvested products and soil. Cereals alone are estimated to bind 3,825 Tg (teragrams) or 3.825 Pg (petagrams) of carbon dioxide every year, i.e. 3.825 billion metric tons.^[12]