Pectic polysaccharides constitution

The plant cell wall contains different types of polysaccharides, proteins (structural glycoproteins and enzymes), lignin and water, as well as some inorganic components (1, 14-16). The plant cell suspensions, however, grow as a population of cells with a primary cell wall(17). The main components of these walls are cellulose-free polysaccharides and pectic polysaccharides in particular, which constitute 1/3 of their dry weight. (18). Some fragments, e g. methanol, acetic, ferulic and p-cumaric acids, are connected with the pectic polysaccharides by ester bonds with the carboxylic and hydroxylic groups. 

It should be determined whether any of these factors is large enough to constitute a significant interference in the planned experiment. See also the comments on NDF below. An NDF method which includes pectic polysaccharides, P-glucans and fructans has been published by Hall et al. (1997). 

Recognizing the need for a more uniform, polysaccharide material for detailed constitutional studies, O Dwyer prepared two hemicellulose fractions from beechwood previously extracted with water and 0.2% sodium hydroxide. As in previous work, the hemicelluloses were extracted from beechwood sawdust with 4% aqueous sodium hydroxide. Pectic materials were precipitated with lime-water, and a fraction designated hemicellu-lose-A was obtained by acidifying the solution with acetic acid. To the filtrate, from hemicellulose-A, was added 95 % ethanol to precipitate hemi-cellulose-B. After further purification, both of these hemicelluloses were subjected to chemical study. Hemicellulose-A ([ccjn —107° in 1 % sodium hydroxide) was observed to be soluble in cold water when freshly prepared, but it was insoluble after being dried. The preparation did not reduce... 

Pectins are probably the most complex polysaccharides known, in terms of their chemistry and are certainly so in terms of their biosynthesis. Classically they were regarded as al,4-galacturonans, with various degrees of methyl esterification, and the terms pectic acid and pectinic acid referred to the non-esterified and partially esterified forms respectively. A third term protopectin , was used of insoluble pectin that could not be extracted from plant cell walls by hot solutions of chelating agents. It was considered that these three classes of pectin constituted a pectic triad . This view is now known to be erroneous, but it is still frequently put forward, especially in botanical texts. Consequently any discussion of the synthesis of pectins must be prefaced by a description of their chemistry, as it is now understood. 

Plant cell wall polysaccharides are the main organic compounds found in nature. They are divided into three groups cellulose, hemicelluloses, and pectic substances (IJ)- Cellulose is a linear and long homopolymer consisting of 1,4-linked P-D-glucopyranosyl residues. Its main function is to ensure the rigidity of the plant cell wall. Hemicelluloses constitute the second most abundant plant material after cellulose. They are highly hydroscopic and have an influence on the flexibility of cell walls. 

---