Structural Deterioration of the Produce: The Breakdown of Cell Wall Components - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 164

Table 8.1 Common polymers of primary cell wall and their structures

Polymer Molecular structure

Cellulose (1 - D -glucan chains held together with hydrogen bonding, forming

very long crystalline microfibrils
Cross section contains 36 glucan chains

→ 4) β

Xyloglucan Backbone similar to cellulose, i.e., (1 - D -glucan

Regular substitution on three out of four consecutive glucose residue with
α

→ 4) β

- D -xylose
Xylose occasionally extended with

β α

- D -galactosyl- - L -fucose or

α

- L -arabinose in some species
The reducing end of unsubstituted glucose residues is susceptible to
cleavage by Trichoderma endo-(1

- D -glucanases (EGases)
producing similar amounts of heptasaccharide (Glc4.Xyl3) and
nonasaccharide (Glc4.Xyl3.Gal.Fuc) xyloglucan subunit
oligosaccharides

→ 4) β

Glucomannan Backbone contains regions of (1 → 4) β - D -glucan and (1 → 4) β - D -mannan

in nearly similar amounts galactomannan
Occasionally terminal has a side chains of single unit of α - D -galactose

Glucuronoarabinoxylan Backbone of (1 → 4) β - D -xylan

Side chains of single unit of nonreducing terminal α - L -arabinose and
α - D -glucuronic acid

Homogalacturonan Made of long chains of (1 → 4) α - D -galacturonic acid

Initially highly methyl-esterified

Rhamnogalacturonan I (RG-I) Made of alternating α - D -rhamnose and α -d-galacturonic acid residues;

long side chains of either unbranched (1 → 4) β - D -galactan or branched
α - L -arabinans or type I arabinogalactans attached to the rhamnose
residues

Rhamnogalacturonan II (RG-II) Backbone made of (1 - D -galacturonic acidlike homogalacturonan;

complex side chains of different types of neutral sugar. A minor cell
wall component. RG-II monomers can dimerize together as boron
diesters and may affect the cell wall porosity

→ 4) α

Structural proteins Four different types including expansin; some are heavily glycosylated

cellulose. These models have provided useful starting points to understand ripening-
associated changes in cell wall, that lead to change in fruit texture (Vincken et al., 2003;
Brummell, 2006; Vicente et al., 2007).
Cellulose microfibrils are composed of unbranched (1,4)-linked - D -glucan and syn-

thesized in the plasma membrane by large hexameric complexes. Long crystalline ribbons
of about 3-5 nm width of cellulose microfiber, formed by extensive hydrogen bonding
among several parallel glucans chains, provide cell wall mechanical strength and resistance
to enzymatic attack. The matrix glycans (hemicelluloses) are neutral or weakly acidic,
composed mainly of neutral sugars, and do not contain galacturonic acid (GalA). The ba-
sic structure of hemicellulose is similar to cellulose, but does not form microfibrils due to
its branching and other sugar modifications. Hemicelluloses are synthesized in the Golgi
apparatus and deposited to the wall surface by transport through vesicles. Xyloglucan and
arabinoxylan predominate among the various components of hemicellulose. The xyloglu-
can backbone is similar to that of cellulose but has numerous regularly spaced xylose
side chains, some extended with either Gal-Fuc or Ara. The arabinoxylan backbone is

α