• Alkaloids and their modes of action
• Carbohydrates and their modes of action
• Cyanogenic glycosides and their modes of action
• Glucosinolates and their modes of action
• Non-protein amino acids (NPAAs) and their modes of action
• Phenolic compounds: Phenylpropanoids, coumarins, flavonoids and tannins and their modes of action
• Polyacetylenes, thiophenes, alkamides, fatty acids and waxes and their modes of action
• Polyketides and their modes of action
• Terpenoids and their modes of action

Secondary metabolites, or “natural products” are low-molecular weight compounds that do not play a role in primary plant metabolism. They constitute the active ingredients of medicinal plants. Although approximately only 20% of higher plants have been investigated in some depth so far, several ten thousands of secondary metabolites (see Table below) have already been isolated and their structures determined by mass spectrometry, nuclear magnetic resonance or X-ray diffraction. Three major groups of secondary metabolites can be recognised: nitrogen-containing substances, terpenes and phenolics. Over 14 000 nitrogen-containing secondary metabolites have been described so far. Alkaloids, amines, non-protein amino acids, cyanogenic glycosides and glucosinolates are the main compounds in this group.

Overview of numbers and representative structures of secondary metabolites

Most animals can run or fly away in case of danger when they are threatened by predators or can use an immune system against invading microbes or parasites. Both these means are apparently not available to plants when attacked by herbivores, microbes (bacteria, fungi) and even other plants competing for light, space and nutrients. Plants have evolved numerous defence strategies. The major one is the production of secondary metabolites, which have no direct function in growth or development or primary processes such as photosynthesis, respiration, assimilation of nutrients, transport or differentiation but serve for protection against viruses, bacteria, fungi, competing plants and importantly, against herbivores (e.g. nematodes, slugs and snails, arthropods, vertebrates). In addition, secondary metabolites can serve as signal compounds to attract animals for pollination (fragrant monoterpenes; coloured anthocyanins or carotenoids) and seed dispersal. In addition, some secondary metabolites concomitantly exhibit physiological functions, for example they can serve as mobile and toxic nitrogen transport and storage compounds or for UV-protection. Besides chemical defence a number of plants use mechanical and morphological features for protection, such as thorns, spikes, glandular and stinging hairs (often filled with noxious chemicals), or develop an almost impenetrable bark (especially woody perennials).

We have to know the secondary metabolites present in a medicinal plant and their biological activities in order to understand and evaluate the effects and efficacy of phytomedicines.