Primary and Secondary Metabolites

One of the most exciting parts of biological chemistry is taking a living organism apart chemically — isolating thousands of compounds, both small and large, working out their structures, and where possible making them synthetically in the laboratory.

If you listed every biomolecule found in an organism, the list would run to thousands of organic compounds — amino acids, sugars and many more. For reasons connected to metabolism, all of these biomolecules can be described more generally as metabolites.

Primary metabolites

In animal tissues you find all the familiar categories of compounds — amino acids, sugars, fatty acids, nucleotides and the like. These are the primary metabolites. Their defining feature is that they have identifiable functions and play known, well-understood roles in the normal physiological processes of the organism.

Secondary metabolites

The situation changes when you analyse plant, fungal and microbial cells. Alongside the primary metabolites, these cells contain thousands of additional compounds that are quite different in kind. Examples include:

• Alkaloids (such as morphine and codeine)
• Flavonoids
• Rubber
• Essential oils (such as lemon grass oil)
• Antibiotics
• Coloured pigments (such as carotenoids and anthocyanins)
• Scents
• Gums
• Spices

These are called secondary metabolites. Other examples that fall into this broad group are terpenoids (monoterpenes, diterpenes), toxins such as abrin and ricin, lectins such as concanavalin A, and drugs such as vinblastin and curcumin.

Unlike the primary metabolites, the roles and functions of many secondary metabolites in the host organism are still not fully understood. Even so, a great number of them are extremely valuable to human welfare — rubber, drugs, spices, scents and pigments are all secondary metabolites that people use directly. Some secondary metabolites also have ecological importance in the life of the organism and its surroundings, a topic explored further in later chapters. …