In these final five chapters we shall turn to
the most complex and most intricately interwoven collection of chemical
reactions to be found on our planet: a living organism.
We can talk of "a living organism" as
representing all forms of life because, to a remarkable extent,
every living thing on this planet is composed of the same set of
chemical substances and stays alive by carrying out the same kinds
of chemical reactions. We differ in details, but we all are fundamentally
alike. This similarity may have arisen partly because only certain
substances and reactions are suitable as the basis for life, but
another factor is the great probability that all forms of life on
this planet evolved from one or a small number of primitive ancestors
that carried out these particularly suitable reactions. In this
chapter we will be concerned with two of the most fundamental chemical
substances of all forms of life: proteins and nucleic acids.
Right: Heme group (side view) showing Iron