4. Which of the following acts as a branch point for the bioosynthesis of sesquiterpene and triterpenes?
   (1) Farnesyl pyrophosphate
   (2) Geranyl pyrophosphate
   (3) Isopentyl pyrophosphate
   (4) Hydroxyrnethylglularyl -CoA

   Correct answer: (1) Farnesyl pyrophosphate

   Explanation:

   • Farnesyl pyrophosphate (FPP) is a central intermediate and branch point in terpene biosynthesis.

   • FPP is a 15-carbon isoprenoid and acts as the precursor for sesquiterpenes (C15) and also condenses head-to-head to form squalene, the precursor for triterpenes (C30).

   • Geranyl pyrophosphate (GPP) is a 10-carbon intermediate used for monoterpene synthesis, not sesquiterpene or triterpene.

   • Isopentenyl pyrophosphate (IPP) is a 5-carbon building block in terpene biosynthesis but not the branch point itself.

   • Hydroxymethylglutaryl-CoA (HMG-CoA) is an early precursor in the mevalonic acid pathway but not the branch point for sesquiterpene and triterpene synthesis.

Farnesyl pyrophosphate is the pivotal molecule in terpene biosynthesis, serving as the precursor for sesquiterpenes and allowing dimerization to squalene, the triterpene precursor, thereby controlling the flow of carbon into these important classes of plant secondary metabolites.