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 Atom.