carbon chain is able to link to a wide range of chemical elements
and molecules. This, combined with carbon's chaining ability,
enables the synthesis of organic molecules that constitute metabolism
in its simplest form. This provides an enormous diversity
of molecules based on this simple structure. Thousands of
molecules are possible using just carbon and hydrogen. When
oxygen, nitrogen, calcium, phosphorus and sulfur are included, this
rises to tens of millions. Importantly, from the life perspective,
some such molecules have shapes and chemistries that allow them
to replicate and to serve as catalysts in other chemical reactions.
One such, is DNA.
we have seen, carbon and its associated "elements of life" can make
up a vast range of molecules, many of them highly complex.
This branch of chemistry is therefore an important one and it has
its own name - organic chemistry, so
called because it deals mainly with the the chemistry of life.
It plays an important role in economic activity, particularly in
the manufacture of industrial chemicals and pharmaceuticals, all
branches of medicine and in food production, both on the farm and
in the factory.
because of the geometry of the way carbon works as the backbone
of organic molecules, it is possible to create molecules that are
right- or left-handed. This ability is called chirality.
It should be noted that not all organic molecules are chiral: some
can be symmetrical. The chiral
molecule, however, has properties that are crucial to the working
of the chemistry of life. As far as life on Earth is concerned,
all normal chiral molecules associated with life processes are left-handed.
terms of basic chemistry, left- and right- chiral molecules are
identical, but their behavior can be vastly different. This
is because the chemistry of life depends on the ability of molecules
to form atomic bonds at several points simultaneously, to create
new compounds. The molecules, however, are three-dimensional
structures and to make the bonds work they must be turned round
and fitted, much as a jigsaw puzzle would be put together.
The consequence is that molecules must be the correct shape, in
order to form the required bonding, as well as being chemically
of the computer jigsaw puzzle game, Tetris, will appreciate the
problem. Some pieces are handed - they are chiral -
and however they are turned around and manipulated, they will not
fit into the available slots. In fact Tetris would soon become
incredibly and boringly easy, if the chiral pieces were not present.
in terms of bio-chemistry, you can see that it is impossible for
the "wrong-handed" molecule to be used in a particular process,
even if it is chemically correct. This characteristic of some
organic molecules is key in the processes of life, particularly
in replication, such as in the behavior of DNA.
us now return to the humble carbon atom and think about its role
when considering life elsewhere, off planet Earth. The unique
characteristics of carbon give it a versatility and functionality
like no other atom in the periodic table. These are why carbon
is the basis of life here. Additionally, carbon is only beaten
in abundance by hydrogen, helium and oxygen, at least within the
Solar System. It is ten times more abundant than silicon and
a million times more abundant than boron, which have also been suggested
as alternative base elements for living molecules.
interstellar space, radio
astronomers have detected just over eighty different molecular
compounds. Over seventy of these contain carbon, while only
eight contain silicon. The chemical characteristics
of carbon are of course, universal, so apply as much in distant
galaxies as they do here. As the abundance of carbon
appears to be similar out in deep
space too, then carbon must be the best candidate as the basis
of life everywhere.