Carbon-based life
forms the key component for almost all known naturally occurring 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. 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 often assumed in astrobiology
that if life exists somewhere else in the universe, it
will also be carbon based. 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
complex molecules and polymers.
There are not many other elements which even appear to be
promising candidates for supporting life - for example, processes such as metabolism
- but the most frequently suggested alternative is silicon. 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. Silicon
compounds are generally stable but do not support the ability to readily
re-combine in different permutations in a manner that would plausibly support
lifelike 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
- 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
also has 4 bonding sites and is just below carbon on the periodic table of the elements. This
means silicon is very similar to carbon in its chemical characteristics. In
cinematic and literary science fiction, when man-made machines cross from
nonliving 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,
where a living rock creature's biochemistry is based on silicon.
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