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 (/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 metabolism. Photosynthesis usually refers to oxygenic photosynthesis, a process that produces oxygen. Photosynthetic organisms store the chemical energy so produced within intracellular organic compounds (compounds containing carbon) like sugars, glycogen, cellulose and starches. To use this stored chemical energy, an organism's cells metabolize the organic compounds through 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, 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 "energy currency" of cells. Such archaeal photosynthesis might have been the earliest form of photosynthesis that evolved on Earth, as far back 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 green) 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 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 using 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. The average rate of energy captured by global photosynthesis is approximately 130 terawatts,[6][7][8] which is about eight times the total power consumption of human civilization.[9] Photosynthetic organisms also convert around 100–115 billion tons (91–104 Pg petagrams, or billions of metric tons), of carbon into biomass per year.[10][11] Photosynthesis was 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 binds it into plants, harvested produce and soil. Cereals alone are estimated to bind 3,825 Tg or 3.825 Pg of carbon dioxide every year, i.e. 3.825 billion metric tons.[12]

  1. ^ "Photosynthesis". lexico.com (Lexico UK English Dictionary). Oxford University Press. Archived from the original on 2022-08-11. Retrieved 2023-07-15.
  2. ^ Bryant, Donald A.; Frigaard, Niels-Ulrik (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, Jane B.; Urry, Lisa A.; Cain, Michael L.; Wasserman, Steven A.; Minorsky, Peter V.; Jackson, Robert B.; Campbel, Neil A. (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, John; Govindjee (1999). "Chapter 2: The photosynthetic process". In Singhal G.S.; Renger G.; Sopory S.K.; Irrgang K.D.; 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. It is estimated that photosynthetic organisms remove 100×1015 grams of carbon/year fixed by photosynthetic organisms. This is equivalent to 4×1018 kJ/yr of free energy stored in reduced carbon. (in Part 8: "Global photosynthesis and the atmosphere")
  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.