Overview of Plant Secondary Metabolites: Their Types, Classification, and Significance

Abstract

Plant-based metabolites find use in several fields such as agriculture and pharmaceuticals and are categorized as primary metabolites and secondary metabolites. Secondary metabolites do not get involved in cell death but affect the survival of the organism in the long term. They consist of more than 8,000 phenyl propanoids, 40,000 terpenoids, and 12,000 alkaloids. They are known for their biotechnological and biological applications and are classified according to chemical structure into terpenoids, phenolic compounds, and nitrogen-containing compounds. This chapter presents an overview of secondary metabolites, their chemical structures, and their significance. Terpenoids are associated with colour and taste and are synthesized through the shikimate metabolic pathway. Polyphenols possess anti-inflammatory, antioxidant, and antihepatotoxic properties, whereas nitrogen-containing secondary metabolites consist of alkaloids, glucosides, glucosinolates, non-protein amino acids, and polyamines. Alkaloids may be heterocyclic or non-heterocyclic, exhibit pharmacological and cytotoxic properties, and also serve as nitrogen storage. Secondary metabolites have various applications in pharmaceutical, nutritional, and agricultural fields and have been explored to increase productivity through cell culturing and signalling pathways. Fruits and vegetables are sources of bioactive secondary metabolites such as phytochemicals, amino acids, and vitamins. Their stability depends on factors such as light, temperature, humidity, and carbon dioxide content, which are discussed in this chapter.