15. The precursor for most of phenolic secondary metabolites is
    (1) Phenyl alanine
    (2) Isopentyl pyrophosphate
    (3) Mevalonate
    (4) Geranyl phosphate

    Correct answer: (1) Phenyl alanine

    Explanation:

    • Phenylalanine is the primary precursor for most phenolic secondary metabolites in plants. It is produced through the shikimic acid pathway and enzymatically converted by phenylalanine ammonia-lyase (PAL) into cinnamic acid, which is further processed into a wide variety of phenolic compounds such as flavonoids, tannins, lignins, and more.

    • Isopentenyl pyrophosphate is a five-carbon building block used to synthesize terpenes, not phenolics.

    • Mevalonate and geranyl phosphate are precursors involved in terpene biosynthesis pathways, unrelated to phenolics.

    Option-wise:

    • (1) Phenylalanine — Correct.

    • (2) Isopentenyl pyrophosphate — Incorrect; terpene precursor.

    • (3) Mevalonate — Incorrect; terpene biosynthesis precursor.

    • (4) Geranyl phosphate — Incorrect; terpene precursor.

Phenylalanine is the fundamental precursor of phenolic secondary metabolites in plants, entering the phenylpropanoid pathway via phenylalanine ammonia-lyase, leading to the biosynthesis of diverse compounds such as flavonoids, tannins, and lignins, essential for plant protection and development