ture. However, other aspects of their structure as well as how they are synthesized have features in common. Understanding these common features makes the structure of these large molecules surprisingly simple to learn and understand.

All macromolecules are polymers (poly means "many"), large molecules formed by joining together small molecules, the subunits. Each different class of macromolecule is composed of different subunits, although the subunits of the same macro-molecule have a similar structure.

The synthesis of macromolecules involves two steps: first, the subunits are synthesized from different precursor metabolites and then, they are joined together, one by one. The synthesis of the various subunits is very complex and involves almost 100 different chemical reactions. An overview of how some of these sub-units are synthesized is given in chapter 6.

The overall process of joining two subunits involves a chemical reaction in which H2O is removed, a reaction termed dehydration synthesis (figure 2.12a). When the macromolecule is broken down into its subunits, the reverse reaction occurs, and H2O is added back, a hydrolytic reaction, or hydrolysis (figure 2.12b). This type of reversible reaction, involving the removal and addition of H2O molecules, is common to the synthesis and degradation of all macromolecules and requires the action of specific enzymes.

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