Secondary plant substances

Definition

Secondary plant compounds can be described as chemical compounds produced in the secondary metabolism of plants. These can fulfil a number of biological functions and are thus of great benefit to most plants.

Ecological function

For a long time it was unclear exactly what role they played. It was assumed that secondary metabolic pathways were intended to render the unnecessary or toxic by-products or end products of primary metabolism harmless. Currently, however, it is assumed that secondary plant metabolites are essential for the environment.

Presumably, the plant secondary metabolites have evolved as a result of close interaction, especially with their predators and with their environmental influences.

It is estimated that 60,000 of these compounds exist. Accordingly, plants have an enormous number of defence mechanisms at their disposal. They also need them because, unlike animals, they are sessile.

The plant uses many of these substances as chemical defence mechanisms against predators. On the other hand, the colouring and flavouring substances of the plant secondary metabolites attract pollen-spreading insects and fruit eaters that spread seeds. In this way, the secondary substances fulfil several functions for the plant. They protect it from enemies and they provide it with nutrients or propagation material.

The plants that occur in nature contain a large number of different substances. In most cases, the concentration of these substances in the plant part is already sufficient to exert a toxic effect on pests and thus effectively repel them. Secondary plant substances can be divided into different substance classes.

Production

Secondary plant substances are only produced in certain cells and they differ from primary plant substances in that they are not absolutely necessary for the life of the plant.

The secondary metabolism of the plant includes all biosynthetic pathways that convert products of the anabolic and catabolic metabolism. These are mainly carbohydrates, amino acids and carboxylic acids.

The decision as to whether a substance is a primary or a secondary product can only be made on the basis of the function it assumes in the plant organism. This is because primary and secondary metabolism often use the same reaction steps and common enzyme systems.

Secondary plant substances play a decisive role in taxonomy, as their production takes place very differently in different plant species. They are produced under fixed conditions, both temporally and spatially. Moreover, their production also depends on suitable environmental conditions. It is important to analyse secondary plant compounds thoroughly in order to understand how cell differentiation takes place in the plant.

Alkaloids and terpenes, for example, produce chemically completely different structures. This is precisely why they are of outstanding importance in pharmacology as well as for the production of industrial chemicals for humans.