All life is based on the element carbon. Carbon is the major chemical constituent of most organic matter, from fossil fuels to the complex molecules DNA and RNA that control genetic reproduction in organisms.
Yet by weight, carbon is not one of the most abundant elements within the Earth's crust. In fact, the lithosphere is only 0.032 % carbon by weight.
In comparison, oxygen and silicon respectively make up 45.2 % and 29.4 % of the Earth's surface rocks.
There are two important properties of carbon that make it a suitable element to form the compounds in living things:
Firstly, carbon atoms can link together to form stable chains of great length. (This process is called catenation.) Carbon atoms bind strongly to each other and so often form very large molecules which are built around a carbon 'backbone'.
The covalent bond between two carbon atoms is strong so that the backbones are stable. In all of these compounds simple sub-units called monomers are linked together by< condensation reactions. Carbohydrates, proteins and nucleic acids are formed when large numbers of these sub-units bond together to form huge macromolecules called polymers.
Secondly, it is a valency IV element - each atom of carbon can form four covalent bonds. The arrangement of electrons around the carbon atom allows it to make 4 single bonds (or 2 double bonds, or one triple and one single bonds) with other elements. The branched chains of carbon atoms can form an almost limitless number of possible compounds.
This provides the variety of chemistry that allows living things to achieve great complexity. Although between 70% and 90% of a cell is composed of water, most of the rest consists of carbon-based compounds.
All of the compounds that distinguish the chemicals of living organisms from inanimate matter, such as proteins, DNA and carbohydrates, are made primarily of carbon atoms.
Scientists have occasionally speculated that life could be based on an element other than carbon. Silicon, being the lightest element with an electronic structure analogous to that of carbon (having a half-filled outer shell with 4 unpaired electrons, is the most likely candidate mentioned.
However, carbon's tendency to form the long chains and rings that form the basis for organic compounds that at some level of complexity begin to self-replicate is unique.
Also, because older stars naturally produce carbon, along with nitrogen and oxygen (its neighbors on the periodic table), it is relatively abundant in the universe.
Many astrophysicists who study the spectra of stars believe that complex chains and even rings of carbon appear in such unlikely places as stellar envelopes (e.g., in the form of PAHs, polycyclic aromatic hydrocarbons).
When such compounds reach cooler regions of space where they can bond with readily available hydrogen, organic compounds as we know them are naturally formed. Although other elements may form complex, covalently bonded structures, none has the rich molecular variety of carbon.
It is the chemistry of carbon that allows us to consider the possibility of life "as we know it" in other parts of the Galaxy and the Universe beyond.
We do not know whether Earth-like conditions exist elsewhere; but if they do, it is highly likely that life forms, if they exist, will be based on carbon.
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