▶️ Answer/Explanation
Solution
(a)
Suberin:
- Forms a hydrophobic barrier in the Casparian strip of endodermal cells
- Forces water to switch from apoplast to symplast pathway, enabling selective mineral absorption
- Prevents backflow of water and toxins into root cortex
Lignin:
- Reinforces xylem vessel walls through secondary thickening
- Prevents collapse under tension during transpiration
- Creates waterproof conduits for efficient water transport
Explanation: Suberin regulates water entry into the vascular cylinder, while lignin maintains xylem structural integrity under negative pressure.
(b)
- Laccase uses induced fit – its active site conformationally changes to bind monolignol substrates
- Forms enzyme-substrate complexes with phenolic compounds (monolignols)
- Oxidizes monolignols via copper ion cofactors, enabling radical coupling
- Lowers activation energy for polymerization into lignin’s complex 3D structure
- Regenerates after releasing the lignin polymer
Explanation: The diagram shows laccase’s catalytic cycle where it facilitates oxidative coupling of monolignol precursors into the heterogeneous lignin polymer.