Carbon-based life

The Lewis structure of a carbon atom, showing its four valence electrons.

Carbon is a key component of all known life on Earth, representing approximately 45-50% of all dry biomass[1]. Complex molecules are made up of carbon bonded with other elements, especially oxygen and hydrogen and frequently also with nitrogen, phosphorus and sulfur. Carbon is abundant on Earth. It is also lightweight and relatively small in size, making it easier for enzymes to manipulate carbon molecules. It is frequently assumed in astrobiology that if life exists elsewhere in the universe, it will also be carbon-based.[2][3] Critics refer to this assumption as carbon chauvinism. [4]

Characteristics

"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." [5] The most important characteristics of carbon as a basis for the chemistry of life are, that it has four valence bonds, and that the energy required to make or break a bond is at an appropriate level for building molecules, which are stable and reactive. Carbon atoms bond readily to other carbon atoms; this allows the building of arbitrarily long complex molecules and polymers.

Other candidates

There are not many other elements which even appear to be promising candidates for supporting life, for example, processes such as metabolism. The most frequently suggested alternative is silicon.[6] Silicon is in the same group in the Periodic Table of elements, and has four valence bonds, and bonds to itself, generally in the form of crystal lattices rather than long chains. It is considerably more electropositive than carbon. Silicon compounds do not readily recombine into different permutations in a manner that would plausibly support lifelike processes.

Key molecules

The most notable groups of chemicals used in the processes of living organisms include:

  • 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)
  • Nucleic acids, which carry genetic information
  • Carbohydrates, which store energy in a form that can be used by living cells
  • Lipids, which also store energy, but in a more concentrated form, and which may be stored for extended periods in the bodies of animals

Fiction

Silicon has been a theme of non-carbon-based-life since it is somewhat similar to carbon in its chemical characteristics. In cinematic and literary science fiction, when man-made machines cross from non-living to living, this new form would be an example of non-carbon-based life. Since the advent of the microprocessor in the late 1960s, these machines are often classed as "silicon-based life". Another example of "silicon-based life" is the episode "The Devil in the Dark" from Star Trek: The Original Series, in which a living rock creature's biochemistry is based on silicon. Also in The X-Files episode "Firewalker" where silicon based organism was discovered in a volcano.

In the movie adaptation of Arthur C. Clarke's "2010" (1984) a character argues, "Whether we are based on carbon or on silicon makes no fundamental difference; we should each be treated with appropriate respect"[7]. This quote may be the basis of Steve Jobs's quip when he introduced Carbon within MacOS X[8], "Carbon. All life forms will be based on it."

See also

References

  1. ↑ http://www.fao.org/forestry/17111/en/
  2. ↑ "Astrobiology". Biology Cabinet. September 26, 2006. Retrieved 2011-01-17.
  3. ↑ "Polycyclic Aromatic Hydrocarbons: An Interview With Dr. Farid Salama". Astrobiology magazine. 2000. Archived from the original on 2008-06-20. Retrieved 2008-10-20.
  4. ↑ Darling, David. "Carbon-based life". Encyclopedia of Life. Retrieved 14 September 2007.
  5. ↑ Stephen Hawking (1 October 2008). "Life in the Universe, 50th anniversary celebration of NASA". NASA. Retrieved 28 August 2015.
  6. ↑ Pace, NR (2001). "The universal nature of biochemistry" (PDF). Proceedings of the National Academy of Sciences of the United States of America. 98 (3): 805–8. Bibcode:2001PNAS...98..805P. doi:10.1073/pnas.98.3.805. PMC 33372. PMID 11158550.
  7. ↑ "2010: Quotes". IMDb. Retrieved 26 July 2017.
  8. ↑ "WWDC '98". NotesKey. May 11, 1998. Retrieved 26 July 2017.
  • "Encyclopedia of Astrobiology, Astronomy & Spaceflight". Retrieved 2006-03-14.
  • "School of Chemistry, University of Bristol, United Kingdom".
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