carbon-based life

Carbon is capable of forming a vast number of compounds, more than any other element, with almost ten million compounds described to date,"There are close to ten million known carbon compounds, many thousands of which are vital to organic and life processes." {{cite web|author=Chemistry Operations|date=December 15, 2003|url=http://periodic.lanl.gov/elements/6.html|title=Carbon|publisher=Los Alamos National Laboratory|access-date=2008-10-09|archive-url=https://web.archive.org/web/20080913063402/http://periodic.lanl.gov/elements/6.html |archive-date=2008-09-13}} and yet that is but a fraction of the number of compounds that are theoretically possible under standard conditions. The enormous diversity of carbon compounds, known as organic compounds, has led to a distinction between them and the inorganic compounds that do not contain carbon. The branch of chemistry that studies organic compounds is known as organic chemistry.Clayden, J.; Greeves, N. and Warren, S. (2012) Organic Chemistry. Oxford University Press. pp. 1–15. {{ISBN|0-19-927029-5}}.

Carbon is the 15th most abundant element in the Earth's crust, and the fourth most abundant element in the universe by mass, after hydrogen, helium, and oxygen. Carbon's widespread abundance, its ability to form stable bonds with numerous other elements, and its unusual ability to form polymers at the temperatures commonly encountered on Earth enables it to serve as a common element of all known living organisms. In a 2018 study, carbon was found to compose approximately 550 billion tons of all life on Earth.{{Cite journal|title=The biomass distribution on Earth|first1=Yinon M.|last1=Bar-On|first2=Rob|last2=Phillips|first3=Ron|last3=Milo|date=May 21, 2018|journal=Proceedings of the National Academy of Sciences|volume=115|issue=25|pages=6506–6511|doi=10.1073/pnas.1711842115|pmid=29784790|pmc=6016768|bibcode=2018PNAS..115.6506B |doi-access=free}}{{Cite news|url=https://www.theguardian.com/environment/2018/may/21/human-race-just-001-of-all-life-but-has-destroyed-over-80-of-wild-mammals-study|title=Humans just 0.01% of all life but have destroyed 83% of wild mammals – study|first=Damian |last=Carrington |date=May 21, 2018|access-date=Feb 20, 2019|via=www.theguardian.com|newspaper=The Guardian}} It is the second most abundant element in the human body by mass (about 18.5%) after oxygen.{{Cite book|title=Campbell Biology|last=Reece|first=Jane B.|date=31 October 2013|publisher=Pearson|isbn=9780321775658|edition=10}}

The most important characteristics of carbon as a basis for the chemistry of cellular life are that each carbon atom is capable of forming up to four valence 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.{{Cite web|url=https://www.khanacademy.org/science/ap-biology/chemistry-of-life/elements-of-life/a/carbon-and-hydrocarbons|title=Carbon and hydrocarbons (article)|website=Khan Academy}} Carbon atoms bond readily to other carbon atoms; this allows the building of arbitrarily long macromolecules and polymers in a process known as catenation.Oxford English Dictionary, 1st edition (1889) [http://www.oed.com/view/Entry/30197 s.v. 'chain', definition 4g{{Cite web|url=https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_General_Chemistry_(Petrucci_et_al.)/27%3A_Reactions_of_Organic_Compounds/27.08%3A_Polymers_and_Polymerization_Reactions|title=27.8: Polymers and Polymerization Reactions|date=January 18, 2015|website=Chemistry LibreTexts}}{{Cite web|url=https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/polymers.htm|title=Polymers|website=www2.chemistry.msu.edu}} "What we normally think of as 'life' is based on chains of carbon atoms, with a few other atoms, such as nitrogen or phosphorus", per Stephen Hawking in a 2008 lecture, "carbon [...] has the richest chemistry."{{cite web|author1=Stephen Hawking|title=Life in the Universe, 50th anniversary celebration of NASA|url=http://www.dailygalaxy.com/my_weblog/2011/05/stephen-hawking-on-non-carbon-based-alien-life.html|publisher=NASA|access-date=28 August 2015|date=1 October 2008}}

Norman Horowitz was the head of the Jet Propulsion Laboratory's bioscience section for the first U.S. mission, Viking Lander of 1976, to successfully land an unmanned probe on the surface of Mars. He considered that the great versatility of the carbon atom makes it the element most likely to provide solutions, even exotic solutions, to the problems of survival on other planets. However, the results of this mission indicated that Mars was presently extremely hostile to carbon-based life. He also considered that, in general, there was only a remote possibility that non-carbon life forms would be able to evolve with genetic information systems capable of self-replication and adaptation.Horowitz, N.H. (1986). Utopia and Back and the search for life in the solar system. New York: W.H. Freeman and Company. {{ISBN|0-7167-1766-2}}

The most notable classes of biological macromolecules used in the fundamental processes of living organisms include:{{Cite journal |last1=Molnar |first1=Charles |last2=Gair |first2=Jane |date=2015-05-14 |title=2.3 Biological Molecules |url=https://opentextbc.ca/biology/chapter/2-3-biological-molecules/ |language=en|journal=Introduction to the Chemistry of Life}}

• Proteins, which are the building blocks from which the structures of living organisms are constructed (this includes almost all enzymes, which catalyse organic chemical reactions).
• Amino acid, make up proteins, included the use in genetic code of life.
• Nucleic acids, which carry genetic information.
• Ribonucleic acid (RNA), production of proteins.{{cite web | url=https://learn.genetics.utah.edu/content/basics/rna/ | title =RNA: The Versatile Molecule | publisher =University of Utah | year =2015}}
• Deoxyribonucleic acid (DNA), nucleic acid in genetic form.{{MerriamWebsterDictionary|deoxyribonucleic acid}}
• Peptide, building block of proteins.
• Lipids, which also store energy, but in a more concentrated form, and which may be stored for extended periods in the bodies of animals.
• Phospholipid used in cell membrane.
• Carbohydrates, which store energy in a form that can be used by living cells.
• Lectin, for binding proteins.{{cite journal |author1=URS Rutishauser |author2=Leo Sachs |title=Cell-to-Cell Binding Induced by Different Lectins |journal=Journal of Cell Biology |volume=65 |issue=2 |pages=247–257 |date=May 1, 1975 |doi=10.1083/jcb.65.2.247 |pmid=805150 |pmc=2109424}}
• Monosaccharide, simple sugars, including glucose and fructose.
• Disaccharides, sugar soluble in water, including lactose, maltose, and sucrose.
• Starch, made of amylose and amylopectin, plants energy storage.
• Glycogen, energy in animals.
• Cellulose, a biopolymer, found in the cell walls of plants.
• Fatty acid, two types, saturated fat and unsaturated fat (oil), are stored energy.
• Essential fatty acid, needed but not synthesized by the human body.
• Steroid, hormone, and used in cell membrane.
• Neurotransmitter, are signaling molecules.{{Cite book |last1=Smelser |first1=Neil J. |title=International encyclopedia of the social & behavioral sciences |last2=Baltes |first2=Paul B. |date=2001 |publisher=Elsevier |isbn=978-0-08-043076-8 |edition=1st |location=Amsterdam New York}}
• Cholesterol, used in the brain and spinal cord of animals.
• Wax, found in beeswax and lanolin. Plant wax used for protection.

{{main|Water|Photosynthesis|Geochemistry of carbon|Cell (biology)}}

File:Photosynthesis en.svg in plants. The carbohydrates produced are stored in or used by the plant. Photosynthesis is foundation of food on Earth]]

Liquid water is essential for carbon-based life. Chemical bonding of carbon molecules requires liquid water.{{Cite web|url=https://www.nhm.ac.uk/discover/eight-ingredients-life-in-space.html|title=Eight ingredients for life in space|website=www.nhm.ac.uk}} Water has the chemical property to make compound-solvent pairing.{{Cite journal|url=https://ui.adsabs.harvard.edu/abs/2018SSRv..214...50W|title=The Importance of Water for Life|first1=Frances|last1=Westall|first2=André|last2=Brack|date=March 1, 2018|journal=Space Science Reviews|volume=214|issue=2 |pages=50|via=NASA ADS|doi=10.1007/s11214-018-0476-7|bibcode=2018SSRv..214...50W |s2cid=255068746 }} Water provides the reversible hydration of carbon dioxide. Hydration of carbon dioxide is needed in carbon-based life. All life on Earth uses the same biochemistry of carbon. Water is important in life's carbonic anhydrase the interaction of between carbon dioxide and water. Carbonic anhydrase needs a family of carbon base enzymes for the hydration of carbon dioxide and acid–base homeostasis, that regulates PH levels in life. {{Cite web|url=https://reactome.org/content/detail/R-HSA-1475029|title=Reactome | Reversible hydration of carbon dioxide|website=reactome.org}}{{Cite web|url=https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/carbon-based-life|title=Carbon-Based Life - an overview | ScienceDirect Topics|website=www.sciencedirect.com}} In plant life, liquid water is needed for photosynthesis, the biological process plants use to convert light energy and carbon dioxide into chemical energy.{{Cite web |url=https://www.lexico.com/definition/photosynthesis |title=Photosynthesis |access-date=2023-07-15 |website=Lexico UK English Dictionary |archive-url=https://web.archive.org/web/20220811182212/https://www.lexico.com/definition/photosynthesis |archive-date=2022-08-11 |publisher=Oxford University Press}} Water makes up 55% to 60% of the human body by weight.{{Cite web|url=https://www.usgs.gov/special-topics/water-science-school/science/water-you-water-and-human-body|title=The Water in You: Water and the Human Body | U.S. Geological Survey|website=www.usgs.gov|date=22 October 2019 }}

{{main|Hypothetical types of biochemistry}}

A few other elements have been proposed as candidates for supporting biological systems and processes as fundamentally as carbon does, for example, processes such as metabolism. The most frequently suggested alternative is silicon.{{cite journal | last1 = Pace | first1 = NR | title = The universal nature of biochemistry. | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 98 | issue = 3 | pages = 805–8 | year = 2001 | pmid = 11158550 | pmc = 33372 | doi = 10.1073/pnas.98.3.805 | bibcode=2001PNAS...98..805P| doi-access = free }} Silicon, atomic number of 14, more than twice the size of carbon, shares a group in the periodic table with carbon, can also form four valence bonds, and also bonds to itself readily, though generally in the form of crystal lattices rather than long chains. Despite these similarities, silicon is considerably more electropositive than carbon, and silicon compounds do not readily recombine 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.{{Cite journal|title=On the Potential of Silicon as a Building Block for Life|first1=Janusz Jurand|last1=Petkowski|first2=William|last2=Bains|first3=Sara|last3=Seager|date=June 10, 2020|journal=Life|volume=10|issue=6|pages=84|doi=10.3390/life10060084|doi-access=free |pmid=32532048|pmc=7345352|bibcode=2020Life...10...84P }} Boron does not react with acids and does not form chains naturally. Thus boron is not a candidate for life.{{Cite web|url=https://www.scribd.com/document/422964178/Hydrogen|title=The Boron Family & Its Physical and Chemical Properties | PDF | Carbon | Silicon|website=Scribd}} Arsenic is toxic to life, and its possible candidacy has been rejected.{{Cite journal|url=https://www.nature.com/articles/nature.2012.9861|title=Study challenges existence of arsenic-based life|first=Erika|last=Check Hayden|date=January 20, 2012|journal=Nature|via=www.nature.com|doi=10.1038/nature.2012.9861|s2cid=211729481 }}{{Cite web|url=https://phys.org/news/2012-07-scientists-nasa-arsenic-life-untrue.html|title=Scientists say NASA's 'new arsenic form of life' was untrue|first=Kerry|last=Sheridan|website=phys.org}} 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.{{Cite journal|title=A Principled Approach to the Origin Problem|first1=Masashi|last1=Aono|first2=Norio|last2=Kitadai|first3=Yoshi|last3=Oono|date=February 12, 2015|journal=Origins of Life and Evolution of the Biosphere|volume=45|issue=3|pages=327–338|doi=10.1007/s11084-015-9444-3|pmid=26177711|pmc=4510921|bibcode=2015OLEB...45..327A }}{{Cite web|url=https://gml.noaa.gov/outreach/lesson_plans/The%20Unique%20Carbon%20Atom.pdf|title=The Unique Carbon Atom, National Oceanic and Atmospheric Administration, noaa.gov}}{{Cite web|url=https://oertx.highered.texas.gov/courseware/lesson/1615/overview|title=Biology, The Chemistry of Life, The Chemical Foundation of Life, Carbon|website=OERTX}}

Speculations about the chemical structure and properties of hypothetical non-carbon-based life have been a recurring theme in science 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 the microprocessor 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" from Star Trek: The Original Series, in which a living rock creature's biochemistry is based on silicon.{{Cite web|url=https://www.thecompanion.app/star-trek-the-science-of-the-crystalline-entity/|title=Star Trek | The Science of Silicon-Based Life|date=March 30, 2022|website=The Companion}} In the 1994 The X-Files episode "Firewalker", in which a silicon-based organism is discovered in a volcano.{{Cite book |title=The Truth is Out There: The Official Guide to the X-Files |first=Brian |last=Lowry |publisher=Harper Prism |year=1995 |isbn=0-06-105330-9 }}{{Cite book |title=X-Files Confidential |first=Ted |last=Edwards |publisher=Little, Brown and Company |year=1996 |isbn=0-316-21808-1 |url=https://archive.org/details/xfilesconfidenti00edwa_0 }}

In the 1984 film adaptation of Arthur C. Clarke's 1982 novel 2010: 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."{{cite web|title=2010: Quotes|url=https://www.imdb.com/character/ch0002895/quotes|website=IMDb|access-date=26 July 2017|archive-date=12 January 2017|archive-url=https://web.archive.org/web/20170112043238/http://www.imdb.com/character/ch0002895/quotes|url-status=dead}}

In JoJolion, the eighth part of the larger JoJo's Bizarre Adventure series, a mysterious race of silicon-based lifeforms "Rock Humans" serve as the primary antagonists.{{Cite web|url=https://jojowiki.com/Rock_Organism|title=Rock Organism|website=JoJo's Bizarre Encyclopedia - JoJo Wiki|date=23 November 2023 }}

Auto-and heterotrophs.png|Correlation between the carbon cycle and formation of organic compounds.

Plagiomnium affine laminazellen.jpeg|Plant cell wall (cellulose) and chloroplasts conduct photosynthesis in plant cells and other eukaryotic organisms.

Plant cell wall diagram-en.svg|The arrangement of cellulose and other polysaccharides in a plant cell wall

Celltypes.svg|Cell types: eukaryotic cell (left) and prokaryotic cell (right)

Carbon cycle.jpg|Fast carbon cycle showing the movement of carbon between land, atmosphere

Carbon stored in ecosystems.png|Carbon stored in ecosystems

Where carbon goes when water flows.jpg|Where carbon goes when water flows

Oceanic Food Web.jpg|Oceanic carbon cycle

Simplified diagram of the global carbon cycle.jpg|Simplified diagram of the global carbon cycle

Carbon cycle-cute diagram.svg|Carbon cycle diagram

Dehydrobenzoannulene.jpg|Triple bond of Carbon in Benzene

DIBAL-3D-balls.png|Model of diisobutylaluminium hydride, showing aluminium as pink, bonded to carbon in black, and hydrogen as white in Organoaluminium chemistry

Rasyslami.jpg|Fatty acids made of long chains of carbon

• Carbon source (biology)
• Cell biology
• CHONPS, a mnemonic acronym for the order of the most common elements in living organisms: carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur
• Habitable zone for complex life

{{reflist}}

• {{cite web| url=http://www.daviddarling.info/encyclopedia/C/carbon-based_life.html| title=Encyclopedia of Astrobiology, Astronomy & Spaceflight| access-date=2006-03-14}}
• {{cite web| url=http://www.chm.bris.ac.uk/webprojects1997/ClaireO/Welcome.htm| title=School of Chemistry, University of Bristol, United Kingdom }}

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