**1. Introduction**

Metabolomics is the study of metabolites in biofluids, cells, tissues, or organisms [1]. Metabolites and their interactions are collectively referred to as the metabolome [2]. Metabolites are tiny molecules formed as a result of metabolic processes; these molecules are either intermediate or final results of metabolic reactions. Natural enzymes found in the cells of organisms accelerate metabolic processes primary and secondary metabolites are compounds that result from primary and secondary metabolism, respectively. Primary metabolites are essential molecules used by organisms for growth, development, and reproduction; these compounds are produced by cells during the growth phase as a result of metabolism. Because of their importance in maintaining normal physiological functions, primary metabolites are called nuclear metabolites. Vitamins (B2 and B12), lactic acid, amino acids, polyols, alcohols such as ethanol, nucleotides, organic acids, and other substances are examples of primary metabolites [3].

The present chapter examines the meaning and origins or sources of various significant types of secondary metabolites, such as alkaloids, terpenoids, tannins, flavonoids, saponins, cardiac glycosides, phenolic compounds, and others, as well as their impact on human and animal health.

### **2. Secondary metabolites**

Secondary metabolites are organic chemicals created by organisms such as plants, fungi, or bacteria as a result of secondary metabolic processes that result in the creation and accumulation of different chemical compounds known as secondary metabolites. These substances are not essential for the organisms' core metabolic activities [4]. Secondary metabolites are created near the end of the growth phase and hence are not directly engaged in the organism's typical physiologic activities such as growth, development or reproduction. Instead, they boost the organism's survival through the mediation of ecological interaction, which acts as a selection benefit to the organism [4]. Secondary metabolites play crucial roles in interspecies defenses, such as plant defense against herbivory. Secondary metabolites, on the other hand, are used by humans as medications, recreational drugs, flavorings, colors, and so on. Secondary metabolites are generally categorized based on their extensive structural diversity, biosynthesis, and function. Over 2140,000 secondary metabolites have been identified in the literature; secondary metabolites are classified into five types: alkaloids, terpenoids, steroids, polyphenols, fatty-acid-derived compounds, nonribosomal polypeptides, and enzyme cofactors [5].

#### **2.1 Alkaloids**

Plants offer a large reservoir of active ingredients with substantial therapeutic uses such as antiviral, anticancer, analgesic, and antitubercular [5]. Alkaloids are significant secondary metabolites that were identified and utilized as early as 4000 years ago and are widely known for their medicinal potential [6]. Alkaloids are categorized into many groups based on their heterocyclic ring system and biosynthetic precursors, such as indole, purine, quinoline, isoquinoline, tropane, and imidazole, among others [7, 8]. Alkaloids contain antiproliferative, antimicrobial, and antioxidant properties that can be exploited in medication development [9]. Alkaloids' medicinal potential expands their industrial applicability. Numerous studies on the medicinal characteristics of various alkaloids derived from plants have been conducted. Alkaloids are naturally occurring chemical composites that often include basic nitrogen atoms. They could also have some neutral or mildly acidic substances in them [10, 11]. Several synthesized substances are also classified as alkaloids [12]. Alkaloids, in addition to carbon, nitrogen, or hydrogen, may include sulfur and, in rare cases, bromine, phosphorus, or chlorine. The term "alkaloid" was coined in 1819 by German scientist Carl F. W. Meissner, who derived it from the Arabic name al-Qali, which is related to the plant from which soda was initially extracted [13]. Alkaloids are low-molecular-weight compounds that account for around 20% of plant-based secondary metabolites [7]. So far, over 12,000 alkaloids have been isolated from diverse plant species [7]. Alkaloids are primarily solids that are found in higher plants. They are found in the following botanical families: Apocynaceae, Annonaceae, Amaryllidaceae, Berberidaceae, Boraginaceae, Gnetaceae, Liliaceae, Leguminoceae, Lauraceae, Loganiaceae, Magnoliaceae, Menispermaceae, Papaveraceae, Piperaceae, Rutaceae, Rubiaceae, and Ranunculaceae [14].

#### *2.1.1 Phytochemistry and classification of alkaloids*

Alkaloids exhibit a wide range of variety, not only in their botanical and biological origins but also in structure and pharmacological function. In this regard, different categorization schemes are feasible. Alkaloids can be classified structurally based on their chemical precursor, structures, and sources, or on the biological mechanisms

### *Secondary Metabolites: Alkaloids and Flavonoids in Medicinal Plants DOI: http://dx.doi.org/10.5772/intechopen.108030*

employed to acquire the molecule. True alkaloids, protoalkaloids, and pseudoalkaloids are the three main kinds of alkaloids. True alkaloids and protoalkaloids are generated from amino acids, but pseudoalkaloids are not.
