**1. Introduction**

The metabolism can be defined as the sum of all the biochemical reactions carried out by an organism. Metabolites are the intermediates and products of metabolism and are usually restricted to small molecules. The term "secondary" introduced by A. Kossel in 1891 implies that while primary metabolites are present in every living cell capable of dividing, the secondary metabolites are present only incidentally and are not of paramount significance for organism's life. Though secondary metabolites are derived from primary metabolism, they do not make up basic molecular skeleton of the organism. Its absence does not immediately curtail the life of an organism, a feature contrary to primary metabolite, but survival of the organism is impaired to a larger extent. Its presence and synthesis are observed in ecologically disadvantaged species within a phylogenetic group [1].

The difference between primary and secondary metabolite is ambiguous since many of the intermediates in primary metabolism is overlapping with the intermediates of secondary metabolites [2]. Amino acids though considered a product of primary metabolite are definitely secondary metabolite too. Contrary to the observation that sterols are secondary metabolites that are indispensable part of many structural framework of a cell. The mosaic nature of an intermediate indicates common biochemical pathway being shared by primary and secondary metabolism [3]. The secondary metabolites serve as a buffering zone into which excess C and N can be shunted into to form inactive part of primary metabolism. The stored C and N can revert back to primary metabolite by the metabolic disintegration of secondary metabolite when on demand. There is dynamism and a delicate balance between the activities of the primary and secondary metabolism (**Figure 1**) being influenced by growth, tissue differentiation and development of the cell or body, and also external pressures [4].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

have different variety of unrelated structures, while steroids have a common tetracyclic carbon skeleton and are modified terpenoids that are biosynthesized from the triterpene lanosterol.

An Introductory Chapter: Secondary Metabolites http://dx.doi.org/10.5772/intechopen.79766 5

There are over 12,000 known compounds of alkaloids, and their basic structures consist of

Around 10,000 compounds are identified and are biosynthesized from simple acyl precursors

These amino acids derived compounds are biologically synthesized by a multifunctional

(i) competitive weapons against other livings such as animals, plants, insects, and

The other functions include interference in spore formation (not obligatory) and germination [5]. Predominantly, the secondary metabolites are used for variety of biological activities like antimicrobial and antiparasitic agents, enzyme inhibitors and antitumor agent, immunosup-

The major sources of secondary metabolites are plants (80% of secondary metabolite), bacteria, fungi, and many marine organisms (sponges, tunicates, corals, and snails) (**Table 1**) [8].

basic amine group and are derived biosynthetically from amino acids.

Enzyme cofactors are nonprotein, low-molecular enzyme component [6].

The major functions of the secondary metabolites including antibiotics are:

**2.3. Fatty acid-derived substances and polyketides**

enzyme complex without direct RNA transcription.

**3. Functions of secondary metabolites**

(iii) agents for symbiotic relation with other organisms

(vi) agents of communication between organisms

**4. Sources of secondary metabolites**

**2.4. Nonribosomal polypeptides**

**2.5. Enzyme cofactors**

microorganisms

(ii) metal transporting agents

(iv) reproductive agent and (v) differentiation effectors

pressive agents, etc. [7].

such as propionyl CoA, acetyl CoA, and methylmalonyl CoA.

**2.2. Alkaloids**

**Figure 1.** Schematic diagram representing integration of primary and secondary metabolism.

Hence, secondary metabolites or natural products can be defined as a heterogeneous group of natural metabolic products that are not essential for vegetative growth of the producing organisms, but they are considered differentiation compounds conferring adaptive roles, for example, by functioning as defense compounds or signaling molecules in ecological interactions, symbiosis, metal transport, competition, and so on [5]. The multitude of secondary metabolite secretions is harvested by human kind to improve their health (antibiotics, enzyme inhibitors, immunomodulators, antitumor agents, and growth promoters of animals and plants), widen the pyramid of healthy nutrition (pigments and nutraceuticals), enhancing agricultural productivity (pesticides, insecticides, effectors of ecological competition and symbiosis and pheromones), and hence impacting economics our society in a certain positive way. They are a source of antibiotics.
