Q.35 Pectin is a polymer of
- Glucose
- Glucuronic acid
- Galacturonic acid
- Iduronic acid
Pectin is a polymer primarily of galacturonic acid, making option 3 correct. Galacturonic acid forms the homogalacturonan backbone (~65% of pectin), distinguishing it from glucose, glucuronic acid, or iduronic acid polymers.
Introduction
Pectin polymer galacturonic acid defines this key plant cell wall heteropolysaccharide, crucial for middle lamella rigidity and fruit gelling. This guide analyzes all MCQ options, explains structural differences from other sugar acids, and covers biosynthesis for biology exam prep.
Pectin Structure
Pectin comprises ~65% α-(1→4)-linked D-galacturonic acid in homogalacturonan (HG), often methyl-esterified at C-6. Rhamnogalacturonan I (RG-I) adds rhamnose breaks with neutral sugar side chains (arabinose, galactose). HG backbone provides strength; degree of esterification affects gelation (high-methoxy pectin gels with sugar/acid).
Synthesized in Golgi, deposited in primary walls/middle lamella of dicots; less in grasses.
Option Analysis
Option Monomer Forms Pectin? Explanation 1 Glucose No Glucose polymerizes to cellulose (β-1,4) or starch (α-1,4/α-1,6); structural plant polysaccharide, not acidic pectin. 2 Glucuronic acid No Present in xyloglucans, hemicelluloses; oxidized glucose with C-6 carboxyl, but pectin backbone is galacturonic (galactose-derived). 3 Galacturonic acid Yes Core HG unit: galactose oxidized at C-6 to uronic acid; polymerized via α-1,4 linkages; methyl esters common in native pectin. 4 Iduronic acid No Rare plant sugar; found in animal GAGs (heparin, dermatan sulfate); L-iduronic differs stereochemically from D-galacturonic. Biochemical Role
Pectin demethyl-esterified by pectin methylesterases enables Ca²⁺ cross-bridges (“egg-box” model) for wall strengthening. Commercial uses: jams (high-methoxy), low-sugar gels (low-methoxy). Degrades via polygalacturonases in ripening/pathogenesis. Key for plant physiology/NEET botany.
