This article is missing citations or needs footnotes. Please help add inline citations to guard against copyright violations and factual inaccuracies. (August 2009)

Carbon forms the backbone of biology for all 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 in the universe, it will also be carbon-based. This assumption is referred to by critics as carbon chauvinism.

In cinematic and literary science fiction, a moment when man-made machines cross from nonliving to living, is often posited,^{[citation needed]} 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 filed under "silicon-based life", even though the silicon backing matrix of these processors is not nearly as fundamental to their operation as carbon is for "wet life".

Contents 1 Characteristics of carbon as a basis for life 2 Key carbon-based molecules in the life processes 3 See also 4 References 5 External links

[edit] 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 behavior, but the most frequent alternative suggested is silicon.^{[citation needed]} 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, less amenable to a complete source of life, rather than long chains. However, its compounds are generally highly stable and do not support the ability to readily re-combine in different permutations in a manner that would plausibly support life-like processes.

This speculation of a life based on the chemistry of silicon is clearly distinct from "silicon-based life" in the above sense of artificial intelligence based on electronic processes utilizing silicon integrated circuits.

[edit] 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.

[edit] See also

• Hypothetical types of biochemistry
• CHON

[edit] References

[edit] 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.