Carbon-based life

For the music group, see Carbon Based Lifeforms.

See also: Organism and Life form

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

Carbon forms the backbone of biology for all of life on Earth. Complex molecules are made up of carbon bonded with other elements, especially oxygen, hydrogen and nitrogen, and carbon is able to bond with all of these because of its four valence electrons. It is often assumed in astrobiology that if life exists somewhere else in the universe, it will also be carbon based.^[1][2] This assumption is referred to by critics as carbon chauvinism.

Characteristics of carbon as a basis for life

The two 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 just at an appropriate level for building molecules which are not only stable, but also reactive. The fact that carbon atoms bond readily to other carbon atoms allows for the building of arbitrarily long and complex molecules.

There are not many other elements which appear to be even promising candidates for supporting life-like metabolism, but the most frequently suggested alternative is silicon.^[3] This is in the same group in the Periodic Table of elements and therefore also has four valence bonds. It also bonds to itself, but generally in the form of crystal lattices rather than long chains. Its compounds are generally highly stable and do not support the ability readily to re-combine in different permutations in a manner that would plausibly support life-like processes.

Key carbon-based molecules in the life processes

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
• Fats, which also store energy, but in a more concentrated form, and which may be stored for extended periods in the bodies of animals.

Fiction

In cinematic and literary science fiction, a moment when man-made machines cross from nonliving to living, it is often posited, this new form would be the first example of non-carbon-based life. Since the advent of the microprocessor in the late 1960s, these machines are often classed as computers (or computer-guided robots) and called "silicon-based life". The speculation of a life based on the chemistry of silicon is clearly distinct from "silicon-based life" in the above sense of artificial intelligence as it is based on electronic processes utilizing silicon integrated circuits.

See also

• Hypothetical types of biochemistry
• CHONPS, mnemonic acronym for the most common elements in living organisms: carbon, hydrogen, oxygen, and nitrogen, phosphorous, sulfur

References

1. ^ "Astrobiology". Biology Cabinet. September 26, 2006. http://biocab.org/Astrobiology.html. Retrieved 2011-01-17. 
2. ^ "Polycyclic Aromatic Hydrocarbons: An Interview With Dr. Farid Salama". Astrobiology magazine. 2000. http://library.thinkquest.org/C003763/index.php?page=interview07. Retrieved 2008-10-20. 
3. ^ Pace, NR (2001). "The universal nature of biochemistry.". 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. http://www.pnas.org/content/98/3/805.full.pdf. 

External links

• "Encyclopedia of Astrobiology, Astronomy & Spaceflight". http://www.daviddarling.info/encyclopedia/C/carbon-based_life.html. Retrieved 2006-03-14. 
• "School of Chemistry, University of Bristol, United Kingdom". http://www.chm.bris.ac.uk/webprojects1997/ClaireO/Welcome.htm. Retrieved 2000-01-01.