Photosynthesis

Schematic of photosynthesis in plants. The carbohydrates produced are stored in or used by the plant.
Composite image showing the global distribution of photosynthesis, including both oceanic phytoplankton and terrestrial vegetation. Dark red and blue-green indicate regions of high photosynthetic activity in the ocean and on land, respectively.

Photosynthesis (/ˌftəˈsɪnθəsɪs/ 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. Photosynthesis is usually used to refer to oxygenic photosynthesis, a form of photosynthesis where the photosynthetic processes produce oxygen as a byproduct and synthesize carbohydrate molecules like sugars, starches, glycogen, and cellulose to store the chemical energy. To use the chemical energy stored in these organic compounds, the organisms' cells metabolize the organic compounds through another process called cellular respiration. Photosynthesis is largely responsible for 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 use bacteriochlorophyll to split hydrogen sulfide as a reductant instead of water, and sulfur is produced as a byproduct 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). Such archaeal photosynthesis might have been the earliest form of photosynthesis evolved on Earth, going back as far as the Paleoarchean, preceding that of cyanobacteria (see Purple Earth hypothesis).

Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centers that contain photosynthetic pigments or chromophores. In plants, these proteins are chlorophyll (a porphyrin derivative that absorbs the red and blue spectrums of light, thus reflecting a green color) 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 serve as short-term stores of energy to later drive other reactions: reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), the "energy currency" of cells.

In plants, algae, and cyanobacteria, sugars are synthesized by a subsequent sequence of light-independent reactions called the Calvin cycle. In the Calvin cycle, 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 such as 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 capture 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] That plants receive some energy from light — in addition to air, soil, and water — was first discovered in 1779 by Jan Ingenhousz.

Photosynthesis is vital for climate processes, as it captures carbon dioxide from the air and then binds carbon in plants and further in soils and harvested products. 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]

  1. ^ "Photosynthesis". Lexico UK English Dictionary. Oxford University Press. Archived from the original on 2022-08-11. Retrieved 2023-07-15.
  2. ^ Bryant DA, Frigaard NU (Nov 2006). "Prokaryotic photosynthesis and phototrophy illuminated". Trends in Microbiology. 14 (11): 488–496. doi:10.1016/j.tim.2006.09.001. PMID 16997562.
  3. ^ Reece J, Urry L, Cain M, Wasserman S, Minorsky P, Jackson R (2011). Biology (International ed.). Upper Saddle River, NJ: Pearson Education. pp. 235, 244. ISBN 978-0-321-73975-9. This initial incorporation of carbon into organic compounds is known as carbon fixation.
  4. ^ Olson JM (May 2006). "Photosynthesis in the Archean era". Photosynthesis Research. 88 (2): 109–117. Bibcode:2006PhoRe..88..109O. doi:10.1007/s11120-006-9040-5. PMID 16453059. S2CID 20364747.
  5. ^ Buick R (Aug 2008). "When did oxygenic photosynthesis evolve?". Philosophical Transactions of the Royal Society of London, Series B. 363 (1504): 2731–2743. doi:10.1098/rstb.2008.0041. PMC 2606769. PMID 18468984.
  6. ^ Nealson KH, Conrad PG (Dec 1999). "Life: past, present and future". Philosophical Transactions of the Royal Society of London, Series B. 354 (1392): 1923–1939. doi:10.1098/rstb.1999.0532. PMC 1692713. PMID 10670014.
  7. ^ Whitmarsh J, Govindjee (1999). "The photosynthetic process". In Singhal GS, Renger G, Sopory SK, Irrgang KD, Govindjee (eds.). Concepts in photobiology: photosynthesis and photomorphogenesis. Boston: Kluwer Academic Publishers. pp. 11–51. ISBN 978-0-7923-5519-9. Archived from the original on 2010-08-14. Retrieved 2012-07-07. 100×1015 grams of carbon/year fixed by photosynthetic organisms, which is equivalent to 4×1018 kJ/yr = 4×1021 J/yr of free energy stored as reduced carbon.
  8. ^ Steger U, Achterberg W, Blok K, Bode H, Frenz W, Gather C, Hanekamp G, Imboden D, Jahnke M, Kost M, Kurz R, Nutzinger HG, Ziesemer T (2005). Sustainable development and innovation in the energy sector. Berlin: Springer. p. 32. ISBN 978-3-540-23103-5. Archived from the original on 2016-09-02. Retrieved 2016-02-21. The average global rate of photosynthesis is 130 TW.
  9. ^ "World Consumption of Primary Energy by Energy Type and Selected Country Groups, 1980–2004". Energy Information Administration. July 31, 2006. Archived from the original (XLS) on November 9, 2006. Retrieved 2007-01-20.
  10. ^ Field CB, Behrenfeld MJ, Randerson JT, Falkowski P (Jul 1998). "Primary production of the biosphere: integrating terrestrial and oceanic components". Science. 281 (5374): 237–240. Bibcode:1998Sci...281..237F. doi:10.1126/science.281.5374.237. PMID 9657713. Archived from the original on 2018-09-25. Retrieved 2018-04-20.
  11. ^ "Photosynthesis". McGraw-Hill Encyclopedia of Science & Technology. Vol. 13. New York: McGraw-Hill. 2007. ISBN 978-0-07-144143-8.
  12. ^ Frankelius P (July–August 2020). "A proposal to rethink agriculture in the climate calculations". Agronomy Journal. 112 (4): 3216–3221. Bibcode:2020AgrJ..112.3216F. doi:10.1002/agj2.20286. S2CID 219423329.

From Wikipedia, the free encyclopedia · View on Wikipedia

Developed by Nelliwinne