Chemistry of cell wall and mechanism of biochemical degradation during ripening

Chemistry of cell wall

Cell wall is the outer rigid covering of a cell. It is primarily made up of celluloses, hemicelluloses pectins and lignins. Cellulose, a polysaccharide of glucose unit is the major polysaccharide present in biosphere and is not digested in human gut. The hemicelluloses are a heterogeneous group of polysaccharide that contains numerous kinds of hexose and pentose sugars and in some cases residues of uronic acid. These polymers are classified according to the types of sugar residue predominating and are individually referred to as xylans, arabino-galactans, glucomannans, and so on. Pectin is the polymer of galactouronic acid with certain degree of methylation. However, purified pectins have covalently associated with pectin are rhamnose, arabinose and xylose. Lignin, another important constituent of cell wall is the three dimensional polymer comprised of phenyl propane units such as syringaldehyde and vanillin, and these are linked through aliphatic three carbon side chain. Lignification of cell wall results in the rigidity of the cell. The relative proportion and contents of cell wall constituents vary considerably among species, with maturity at harvest, and with elapsed time after harvest. The components of cell wall are the major component of “Dietary fibre”. The biological availability of some nutrients such as protein can be reduced by these constituent, but there is considerable evidence for the beneficial role played by fiber in health and disease.

 

Biochemical changes in cell wall constituents

Carbohydrates are the biggest victim of biochemical changes that takes place in cell wall during the entire life cycle of vegetative cells. The catabolism of cell wall polysaccharides is the major reaction involving during fruit ripening and softening. Pectin undergoes enzymatic degradation on the onset of ripening. Two chief pectin degrading enzymes identified pectin methyl esterase and pectin hydrolase. Pectin hydrolase of polygalacturonase is present in few fruits like tomato, pear, pineapple and avocado, are responsible for the hydrolysis of polymers of galactose. On the other hand, the commonly occurring pectin esterases are responsible for the removal of methyl group from the polymer. Methylation of pectin via S-adenosylmethionine also appears to contribute to the softening of ripening fruits. Cellulase activity increases during ripening of tomato fruit, it does not appear to have a detectable effect on softening. Enzymes act on cell wall polysaccharides to produce sugars in fruits during ripening. In some vegetative cells, the post harvest biosynthesis of lignin leads to toughening. This is seen in case of asparagus.

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