1. Primary vs. Secondary Metabolites & Biosynthetic Pathways

Plants synthesize two distinct classes of chemical compounds: (1) Primary Metabolites — ubiquitous, basic biochemical building blocks (Carbohydrates, Amino Acids, Lipids, Proteins, Nucleic Acids) essential for cellular survival, growth, respiration, and reproduction; and (2) Secondary Metabolites — chemically diverse, specialized organic molecules (Alkaloids, Glycosides, Tannins, Resins, Flavonoids, Volatile Oils) produced in specific plant families for ecological defense against herbivores, insects, microbial pathogens, UV radiation, and inter-plant competition (allelopathy). Secondary metabolites are synthesized from primary metabolic precursors through three major intersecting biosynthetic pathways: (1) The Shikimic Acid Pathway (generates aromatic amino acids Phenylalanine, Tyrosine, Tryptophan $\rightarrow$ Phenylpropanoids, Lignans, Alkaloids, Hydrolyzable Tannins), (2) The Mevalonic Acid / MEP Pathway (generates Isopentenyl pyrophosphate - IPP $\rightarrow$ Monoterpenes, Sesquiterpenes, Steroids, Cardiac Glycosides), and (3) The Acetate-Malonate / Polyketide Pathway (generates Fatty Acids, Anthraquinones, Polyketides).