Carbon-based life is the concept thatcarbon is the core element for all known forms oflife, and a core component of all complex biological molecules with carbon representing approximately 45–50% of all drybiomass. Complex biologicalmolecules consist of carbon atomsbonded with otherelements, in particularoxygen andhydrogen but frequently alsonitrogen,phosphorus, andsulfur (collectively known asCHNOPS).[1][2] In these molecules carbon's four valence electrons and subsequent propensity to form four bonds lends to it's function as the "skeleton" of organic molecules. In addition to molecular level properties of carbon, life on Earth also relies on large scale processes known as thecarbon cycle, involving geological and atmospheric processes.
While is it widely believed that any life found elsewhere in the universe will most likely also be carbon-based, silicon and boron have been discussed as alternatives because of their abilities to form 4 or 5 bonds respectively. These possibilities have resulted in science-fiction literature stories that include life based silicon or boron or other alternative elements.
Carbon's widespreadabundance, its ability to form stable bonds with numerous other elements, and its unusual ability to formpolymers at the temperatures commonly encountered onEarth enables it to serve as a common element of all known living organisms.[citation needed] In a 2018 study, carbon was found to compose approximately 550 billion tons of all life on Earth.[3][4] Approximately 50% of the Earth'sbiomass as dry weight (i.e. without water) is carbon.[5] The carbon content of different kinds of biomass varies from 42%-71%.[6]
The most important characteristics of carbon as a basis for thechemistry ofcellular life are that each carbon atom is capable of forming up to fourvalence bonds with other atoms simultaneously, and that the energy required to make or break a bond with a carbon atom is at an appropriate level for building large and complex molecules which may be both stable and reactive.[7] Carbon atoms bond readily to other carbon atoms; this allows the building of arbitrarily longmacromolecules andpolymers in a process known ascatenation.[8][9][10] "What we normally think of as 'life' is based on chains of carbon atoms, with a few other atoms, such as nitrogen or phosphorus", perStephen Hawking in a 2008 lecture, "carbon [...] has the richest chemistry."[11]
Because it is lightweight and relatively small in size, carbon molecules are easy forenzymes to manipulate.Carbonic anhydrase is a key enzyme for all carbon-based life.[12]
The most notable classes of biologicalmacromolecules used in the fundamental processes of living organisms include:[13]
Liquid water is essential for carbon-based life. Chemical bonding of carbon molecules requires liquid water.[18] Water has thechemical property to make compound-solvent pairing.[19] Water provides the reversiblehydration ofcarbon dioxide. Hydration of carbon dioxide is needed in carbon-based life. All life on Earth uses the samebiochemistry of carbon. Water is important in life'scarbonic anhydrase the interaction of between carbon dioxide and water. Carbonic anhydrase needs a family of carbon base enzymes for the hydration of carbon dioxide andacid–base homeostasis, that regulatesPH levels in life.[20] Inplant life, liquid water is needed forphotosynthesis, thebiological process plants use toconvertlight energy and carbon dioxide intochemical energy.[21] Water makes up 55% to 60% of the human body by weight.[22]
Thecarbon cycle is abiogeochemical cycle that is important in maintaining life on Earth over a long time span. The cycle includescarbon sequestration andcarbon sinks.[23][24]Plate tectonics are needed for life over a long time span, and carbon-based life is important in the plate tectonics process.[25] Iron- and sulfur-basedAnoxygenic photosynthesis life forms that lived from 3.80 to 3.85 billion years ago on Earth produced an abundance ofblack shale deposits. These shale deposits increase heat flow and crust buoyancy, especially on the sea floor, helping to increase plate tectonics.Talc is another organic mineral that helps drive plate tectonics.[26][27]Inorganic processes also help drive plate tectonics.[28] Carbon-basedphotosynthesis life caused a rise inoxygen on Earth. This increase of oxygen helped plate tectonics form the first continents.[29]
It is frequently assumed inastrobiology that if life exists elsewhere in theUniverse, it will also be carbon-based.[30][31] Critics, likeCarl Sagan in 1973, refer to this assumption ascarbon chauvinism.[32] A few other elements have been proposed as candidates for supporting biological systems and processes as fundamentally as carbon does, for example, processes such asmetabolism. The most frequently suggested alternative issilicon.[33] Silicon, atomic number of 14, more than twice the size of carbon, shares a group in theperiodic table with carbon, can also form fourvalence bonds, and also bonds to itself readily, though generally in the form ofcrystal lattices rather than long chains. Despite these similarities, silicon is considerably moreelectropositive than carbon, and silicon compounds do not readilyrecombine into different permutations in a manner that would plausibly support lifelike processes. Silicon is abundant on Earth, but as it is more electropositive and in a water based environment it forms Si–O bonds rather than Si–Si bonds.[34]Boron does not react with acids and does not form chains naturally. Thus boron is not a candidate for life.[35]Arsenic istoxic to life, and its possible candidacy has been rejected.[36][37] In the past (1960s–1970s) other candidates for life were plausible, but with time and more research, only carbon has the complexity and stability to make large molecules and polymers essential for life.[38][39][40]
Speculations about the chemical structure and properties of hypothetical non-carbon-based life have been a recurring theme inscience fiction. Silicon is often used as a substitute for carbon in fictional lifeforms because of its chemical similarities. In cinematic and literary science fiction, when man-made machines cross from non-living to living, this new form is often presented as an example of non-carbon-based life. Since the advent of themicroprocessor in the late 1960s, such machines are often classed as "silicon-based life". Other examples of fictional "silicon-based life" can be seen in the 1967 episode "The Devil in the Dark" fromStar Trek: The Original Series, in which a living rock creature's biochemistry is based on silicon.[41] In the 1994The X-Files episode "Firewalker", in which a silicon-based organism is discovered in a volcano.[42][43]
In the1984 film adaptation ofArthur C. Clarke's 1982 novel2010: Odyssey Two, a character argues, "Whether we are based on carbon or on silicon makes no fundamental difference; we should each be treated with appropriate respect."[44]
InJoJolion, the eighth part of the largerJoJo's Bizarre Adventure series, a mysterious race of silicon-based lifeforms "Rock Humans" serve as the primary antagonists.[45]
Most biological molecules are made from covalent combinations of six important elements, whose chemical symbols are CHNOPS. ... Although more than 25 types of elements can be found in biomolecules, six elements are most common. These are called the CHNOPS elements; the letters stand for the chemical abbreviations of carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur.
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