Carbon-based life

<h2 id="characteristics">Characteristics</h2>
Carbon is capable of forming a vast number of <a href="/facts/Chemical_compound/37FXqqtv">compounds</a>, more than any other element, with almost ten million compounds described to date,<a class="footnote-ref" id="fnref:14" href="#fn:14">14</a> 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 <a href="/facts/Organic_compound/gqCNaN45">organic compounds</a>, has led to a distinction between them and the <a href="/facts/Inorganic_compounds/TfFfzxSz">inorganic compounds</a> that do not contain carbon. The branch of chemistry that studies organic compounds is known as <a href="/facts/Organic_chemistry/djxhxaNg">organic chemistry</a>.<a class="footnote-ref" id="fnref:15" href="#fn:15">15</a>
Carbon is the 15th <a href="/facts/Abundance_of_elements_in_Earth%27s_crust/UDSVHH56">most abundant element in the Earth's crust</a>, and the <a href="/facts/Abundance_of_the_chemical_elements/CE9WehBl">fourth most abundant element in the universe</a> by mass, after <a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a>, <a href="/facts/Helium/yRul93TG">helium</a>, and <a href="/facts/Oxygen/acyn6o8r">oxygen</a>. Carbon's widespread abundance, its ability to form stable bonds with numerous other elements, and its unusual ability to form <a href="/facts/Polymer/eF32E50K">polymers</a> at the temperatures commonly encountered on <a href="/facts/Earth/HLLmDfj0">Earth</a> 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.<a class="footnote-ref" id="fnref:16" href="#fn:16">16</a><a class="footnote-ref" id="fnref:17" href="#fn:17">17</a> It is the second most abundant element in the <a href="/facts/Human_body/WP1QbDTn">human body</a> by mass (about 18.5%) after oxygen.<a class="footnote-ref" id="fnref:18" href="#fn:18">18</a>
The most important characteristics of carbon as a basis for the <a href="/facts/Chemistry/SrNqkGVm">chemistry</a> of <a href="/facts/Cell_(biology)/bLM9UQvy">cellular life</a> are that each carbon atom is capable of forming up to four <a href="/facts/Valence_bond_theory/F04YPkfJ">valence bonds</a> 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.<a class="footnote-ref" id="fnref:19" href="#fn:19">19</a> Carbon atoms bond readily to other carbon atoms; this allows the building of arbitrarily long <a href="/facts/Macromolecule/039ULSLg">macromolecules</a> and <a href="/facts/Polymer/eF32E50K">polymers</a> in a process known as <a href="/facts/Catenation/vm7k4zsv">catenation</a>.<a class="footnote-ref" id="fnref:20" href="#fn:20">20</a><a class="footnote-ref" id="fnref:21" href="#fn:21">21</a><a class="footnote-ref" id="fnref:22" href="#fn:22">22</a> "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 <a href="/facts/Stephen_Hawking/b7dcce7n">Stephen Hawking</a> in a 2008 lecture, "carbon [...] has the richest chemistry."<a class="footnote-ref" id="fnref:23" href="#fn:23">23</a>
<a href="/facts/Norman_Horowitz/h5W7G5zH">Norman Horowitz</a> was the head of the <a href="/facts/Jet_Propulsion_Laboratory/zPXnxRBf">Jet Propulsion Laboratory</a>'s bioscience section for the first U.S. mission, <a href="/facts/Viking_1/lBbblTgw">Viking Lander of 1976</a>, to successfully land an unmanned probe on the surface of <a href="/facts/Mars/AQGtVvz0">Mars</a>. 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 <a href="/facts/Viking_lander_biological_experiments/C0FSMBRH">results of this mission</a> indicated that Mars was presently <a href="/facts/Mars_carbonate_catastrophe/XWjTWKMb">extremely hostile</a> 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.<a class="footnote-ref" id="fnref:24" href="#fn:24">24</a>

<h2 id="key-molecules">Key molecules</h2>
The most notable classes of biological <a href="/facts/Macromolecule/039ULSLg">macromolecules</a> used in the fundamental processes of living organisms include:<a class="footnote-ref" id="fnref:25" href="#fn:25">25</a>

<ul><li><a href="/facts/Protein/Jxmagu3E">Proteins</a>, which are the building blocks from which the structures of living organisms are constructed (this includes almost all <a href="/facts/Enzyme/DSI1Fh7y">enzymes</a>, which <a href="/facts/Catalyst/LPBS7TQC">catalyse</a> organic chemical reactions).<a class="footnote-ref" id="fnref:26" href="#fn:26">26</a></li>
<li><a href="/facts/Amino_acid/WDxYwBny">Amino acid</a>, make up proteins, included the use in <a href="/facts/Genetic_code/zUK0bY1B">genetic code</a> of life.<a class="footnote-ref" id="fnref:27" href="#fn:27">27</a></li>
<li><a href="/facts/Nucleic_acid/2tTgcI0v">Nucleic acids</a>, which carry <a href="/facts/Genetics/bu8wne0A">genetic</a> information.<a class="footnote-ref" id="fnref:28" href="#fn:28">28</a></li>
<li><a href="/facts/Ribonucleic_acid/HxPeNfNj">Ribonucleic acid</a> (RNA), production of proteins.<a class="footnote-ref" id="fnref:29" href="#fn:29">29</a></li>
<li><a href="/facts/Deoxyribonucleic_acid/nB89Iauz">Deoxyribonucleic acid</a> (DNA), nucleic acid in genetic form.<a class="footnote-ref" id="fnref:30" href="#fn:30">30</a></li>
<li><a href="/facts/Peptide/C4ltc7UG">Peptide</a>, building block of proteins.<a class="footnote-ref" id="fnref:31" href="#fn:31">31</a></li>
<li><a href="/facts/Lipid/TdQR87HX">Lipids</a>, which also store energy, but in a more concentrated form, and which may be stored for extended periods in the bodies of animals.<a class="footnote-ref" id="fnref:32" href="#fn:32">32</a></li>
<li><a href="/facts/Phospholipid/u6GhNCF5">Phospholipid</a> used in <a href="/facts/Cell_membrane/vAXSD0DT">cell membrane</a>.<a class="footnote-ref" id="fnref:33" href="#fn:33">33</a></li>
<li><a href="/facts/Carbohydrate/Geuuzxsf">Carbohydrates</a>, which store energy in a form that can be used by living <a href="/facts/Cell_(biology)/bLM9UQvy">cells</a>.<a class="footnote-ref" id="fnref:34" href="#fn:34">34</a></li>
<li><a href="/facts/Lectin/I7Clqx4X">Lectin</a>, for binding proteins.<a class="footnote-ref" id="fnref:35" href="#fn:35">35</a></li>
<li><a href="/facts/Monosaccharide/yzs2d6Eq">Monosaccharide</a>, simple sugars, including <a href="/facts/Glucose/oUSkoLCD">glucose</a> and <a href="/facts/Fructose/pEqWJC2N">fructose</a>.<a class="footnote-ref" id="fnref:36" href="#fn:36">36</a></li>
<li><a href="/facts/Disaccharides/9dnUQggw">Disaccharides</a>, sugar soluble in water, including <a href="/facts/Lactose/lBa5ozYk">lactose</a>, <a href="/facts/Maltose/v43CVtaU">maltose</a>, and <a href="/facts/Sucrose/7y1ke94L">sucrose</a>.<a class="footnote-ref" id="fnref:37" href="#fn:37">37</a></li>
<li><a href="/facts/Starch/gHAqGfJb">Starch</a>, made of <a href="/facts/Amylose/FJBcmQ2v">amylose</a> and <a href="/facts/Amylopectin/xPoJFo52">amylopectin</a>, plants energy storage.<a class="footnote-ref" id="fnref:38" href="#fn:38">38</a></li>
<li><a href="/facts/Glycogen/kWa8fPFE">Glycogen</a>, energy in animals.<a class="footnote-ref" id="fnref:39" href="#fn:39">39</a></li>
<li><a href="/facts/Cellulose/wYPafTU0">Cellulose</a>, a <a href="/facts/Biopolymer/GaCwr4EY">biopolymer</a>, found in the cell walls of plants.<a class="footnote-ref" id="fnref:40" href="#fn:40">40</a></li>
<li><a href="/facts/Fatty_acid/JGg3erIu">Fatty acid</a>, two types, <a href="/facts/Saturated_fat/8pvw306D">saturated fat</a> and <a href="/facts/Unsaturated_fat/87HcP11s">unsaturated fat</a> (oil), are stored energy.<a class="footnote-ref" id="fnref:41" href="#fn:41">41</a></li>
<li><a href="/facts/Essential_fatty_acid/0WQfcE9D">Essential fatty acid</a>, needed but not synthesized by the human body.<a class="footnote-ref" id="fnref:42" href="#fn:42">42</a></li>
<li><a href="/facts/Steroid/4AhMbgXt">Steroid</a>, <a href="/facts/Hormone/WFnBnINJ">hormone</a>, and used in cell membrane.<a class="footnote-ref" id="fnref:43" href="#fn:43">43</a></li>
<li><a href="/facts/Neurotransmitter/LlpYFSU3">Neurotransmitter</a>, are <a href="/facts/Signaling_molecule/6oYAVP9I">signaling molecules</a>.<a class="footnote-ref" id="fnref:44" href="#fn:44">44</a></li>
<li><a href="/facts/Cholesterol/TnfPlYc1">Cholesterol</a>, used in the <a href="/facts/Brain/EKla9NkJ">brain</a> and <a href="/facts/Spinal_cord/XbWLr917">spinal cord</a> of animals.<a class="footnote-ref" id="fnref:45" href="#fn:45">45</a></li>
<li><a href="/facts/Wax/MAXhjzNh">Wax</a>, found in <a href="/facts/Beeswax/dLxg45jP">beeswax</a> and <a href="/facts/Lanolin/VyKnaor3">lanolin</a>. Plant wax used for protection.<a class="footnote-ref" id="fnref:46" href="#fn:46">46</a></li></ul>
<h2 id="water">Water</h2>
Main articles: <a href="/facts/Water/0lkXnJPK">Water</a>, <a href="/facts/Photosynthesis/TMV7w693">Photosynthesis</a>, <a href="/facts/Geochemistry_of_carbon/WQGWFVSK">Geochemistry of carbon</a>, and <a href="/facts/Cell_(biology)/bLM9UQvy">Cell (biology)</a>

<a href="/facts/Liquid_water/0lkXnJPK">Liquid water</a> is essential for carbon-based life. Chemical bonding of carbon molecules requires liquid water.<a class="footnote-ref" id="fnref:47" href="#fn:47">47</a> Water has the <a href="/facts/Chemical_property/uMVxKspS">chemical property</a> to make compound-solvent pairing.<a class="footnote-ref" id="fnref:48" href="#fn:48">48</a> Water provides the reversible <a href="/facts/Hydration_reaction/T1Vb2u1d">hydration</a> of <a href="/facts/Carbon_dioxide/xGzpppEp">carbon dioxide</a>. Hydration of carbon dioxide is needed in carbon-based life. All life on Earth uses the same <a href="/facts/Biochemistry/fa2pWEBp">biochemistry</a> of carbon. Water is important in life's <a href="/facts/Carbonic_anhydrase/je7bk6fk">carbonic anhydrase</a> the interaction of between carbon dioxide and water. Carbonic anhydrase needs a family of carbon base enzymes for the hydration of carbon dioxide and <a href="/facts/Acid%E2%80%93base_homeostasis/y4U6UENE">acid–base homeostasis</a>, that regulates <a href="/facts/PH/RnSv8D7y">PH</a> levels in life. <a class="footnote-ref" id="fnref:49" href="#fn:49">49</a><a class="footnote-ref" id="fnref:50" href="#fn:50">50</a> In <a href="/facts/Plant/X5BMDMvS">plant</a> life, liquid water is needed for <a href="/facts/Photosynthesis/TMV7w693">photosynthesis</a>, the <a href="/facts/Biological_process/jpHldXz2">biological process</a> plants use to <a href="/facts/Energy_transformation/8g4seWcj">convert</a> <a href="/facts/Light_energy/8QHpm6KY">light energy</a> and carbon dioxide into <a href="/facts/Chemical_energy/Y3VP3Ifq">chemical energy</a>.<a class="footnote-ref" id="fnref:51" href="#fn:51">51</a> Water makes up 55% to 60% of the human body by weight.<a class="footnote-ref" id="fnref:52" href="#fn:52">52</a>

<h2 id="other-candidates">Other candidates</h2>
Main article: <a href="/facts/Hypothetical_types_of_biochemistry/EsfhbBXC">Hypothetical types of biochemistry</a>
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 <a href="/facts/Metabolism/WxDrm8k5">metabolism</a>. The most frequently suggested alternative is <a href="/facts/Silicon/cgWx0hdr">silicon</a>.<a class="footnote-ref" id="fnref:53" href="#fn:53">53</a> Silicon, atomic number of 14, more than twice the size of carbon, shares a group in the <a href="/facts/Periodic_table/umdGIfUb">periodic table</a> with carbon, can also form four <a href="/facts/Valence_bond_theory/F04YPkfJ">valence bonds</a>, and also bonds to itself readily, though generally in the form of <a href="/facts/Crystal_lattice/7gQKTsXk">crystal lattices</a> rather than long chains. Despite these similarities, silicon is considerably more <a href="/facts/Electropositive/H7Vq7Uah">electropositive</a> than carbon, and silicon compounds do not readily <a href="/facts/Recombination_(chemistry)/gDRGdeSN">recombine</a> 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.<a class="footnote-ref" id="fnref:54" href="#fn:54">54</a> <a href="/facts/Boron/Pmwgd7Mf">Boron</a> does not react with acids and does not form chains naturally. Thus boron is not a candidate for life.<a class="footnote-ref" id="fnref:55" href="#fn:55">55</a> <a href="/facts/Arsenic/k1c4f44D">Arsenic</a> is <a href="/facts/Toxicity/qa96rs60">toxic</a> to life, and its possible candidacy has been rejected.<a class="footnote-ref" id="fnref:56" href="#fn:56">56</a><a class="footnote-ref" id="fnref:57" href="#fn:57">57</a> 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.<a class="footnote-ref" id="fnref:58" href="#fn:58">58</a><a class="footnote-ref" id="fnref:59" href="#fn:59">59</a><a class="footnote-ref" id="fnref:60" href="#fn:60">60</a>

<h2 id="fiction">Fiction</h2>
Speculations about the chemical structure and properties of hypothetical non-carbon-based life have been a recurring theme in <a href="/facts/Science_fiction/d9hE2Lwt">science fiction</a>. 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 <a href="/facts/Microprocessor/Fx9CaQht">microprocessor</a> 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 "<a href="/facts/The_Devil_in_the_Dark/8mNtG63a">The Devil in the Dark</a>" from <a href="/facts/Star_Trek:_The_Original_Series/BtLu8Sgu">Star Trek: The Original Series</a>, in which a living rock creature's biochemistry is based on silicon.<a class="footnote-ref" id="fnref:61" href="#fn:61">61</a> In the 1994 <a href="/facts/The_X-Files/J6SpBK3H">The X-Files</a> episode "<a href="/facts/Firewalker_(The_X-Files)/kvu1EFN7">Firewalker</a>", in which a silicon-based organism is discovered in a volcano.<a class="footnote-ref" id="fnref:62" href="#fn:62">62</a><a class="footnote-ref" id="fnref:63" href="#fn:63">63</a>
In the <a href="/facts/2010_(film)/ynqFy2hJ">1984 film adaptation</a> of <a href="/facts/Arthur_C._Clarke/2XJ6UGDB">Arthur C. Clarke</a>'s 1982 novel <a href="/facts/2010:_Odyssey_Two/mlKGx6yq">2010: Odyssey Two</a>, a character argues, "Whether we are based on carbon or on silicon makes no fundamental difference; we should each be treated with appropriate respect."<a class="footnote-ref" id="fnref:64" href="#fn:64">64</a>
In <a href="/facts/JoJolion/0pw6kznE">JoJolion</a>, the eighth part of the larger <a href="/facts/JoJo%27s_Bizarre_Adventure/daLmwx0v">JoJo's Bizarre Adventure</a> series, a mysterious race of silicon-based lifeforms "Rock Humans" serve as the primary antagonists.<a class="footnote-ref" id="fnref:65" href="#fn:65">65</a>

<h2 id="gallery">Gallery</h2>

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Carbon_source_(biology)/RGcJRgVG">Carbon source (biology)</a></li>
<li><a href="/facts/Cell_biology/kdIBGHub">Cell biology</a></li>
<li><a href="/facts/CHON/lI4Wve8U">CHONPS</a>, a <a href="/facts/Mnemonic/6HfDn0ks">mnemonic</a> acronym for the order of the most common elements in living organisms: carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur</li>
<li><a href="/facts/Habitable_zone_for_complex_life/hL7fQuzW">Habitable zone for complex life</a></li></ul>

<h2 id="external-links">External links</h2>
<ul><li><a href="http://www.daviddarling.info/encyclopedia/C/carbon-based_life.html">"Encyclopedia of Astrobiology, Astronomy & Spaceflight"</a>. Retrieved 2006-03-14.</li>
<li><a href="http://www.chm.bris.ac.uk/webprojects1997/ClaireO/Welcome.htm">"School of Chemistry, University of Bristol, United Kingdom"</a>.</li></ul>

<h2 id="references">References</h2>

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</ol>

Carbon-based life open-in-new

Carbon-based life