Polysaccharides from plant cell-walls

The starches, the most important vegetable reserve carbohydrate and polysaccharides from plant cell walls, are discussed in greater detail on the following page. Inulin, a fructose polymer, is used as a starch substitute in diabetics dietary products (see p.l60). In addition, it serves as a test substance for measuring renal clearance (see p.322). 

Pectic polysaccharides from plant cell walls are rarely simple (1 4)-linked a-D-galacturonans but contain varying proportions of neutral monosaccharides. L-Rhamnose is the only neutral sugar present in the... 

Hemicellulose [9034-32-6] is the least utilized component of the biomass triad comprising cellulose (qv), lignin (qv), and hemiceUulose. The term was origiaated by Schulze (1) and is used here to distinguish the nonceUulosic polysaccharides of plant cell walls from those that are not part of the wall stmcture. Confusion arises because other hemicellulose definitions based on solvent extraction are often used in the Hterature (2—4). The term polyose is used in Europe to describe these nonceUulosic polysaccharides from wood, whereas hemicellulose is used to describe the alkaline extracts from commercial pulps (4). The quantity of hemicellulose in different sources varies considerably as shown in Table 1. 

To date, the structural features of pectic polysaccharides and plant cell walls have been studied extensively using chemical analysis and enzymatic degradation. In addition, research on isolation and physicochemical characterisation of pectin from citrus peels, apple peels, sunflower head residues and sugar beet pulp has been reported (2). However, the pectic polysaccharides extracted from wheat straw have only previously been reported by Przeszlakowska (3). The author extracted 0.44% pectic substances from Author to whom correspondence should be addressed. 

Hydrolysis. A range of hydrolytic procedures has been employed for the release of neutral sugars from plant cell walls. A high proportion of the neutral noncellulosic polysaccharides can be hydrolyzed quantitatively using 1M H2SO4 for2.5 hat 100°C (Selvendranetal., 1979),or 2M trifluoroacetic acid for 2 h at 120°C (Albersheim et al., 1967). 

Thus it is clear that pectic polysaccharides isolated from plant cell walls are a family of complex macromolecules. Recent work by Albersheim and associates suggests that the pectic polymers may not serve solely as structural polymers. Fragments of pectic polymers have been implicated in host-pathogen interactions (Darvill and Albersheim, 1984) and plant morphogenesis (Tran Thanh Van et al., 1985). Thus these polysaccharides still provide considerable challenges to both chemists, biochemists, and physiologists in determining their structures and functions. 

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. 

The amounts of carbohydrate polymers and lignin vary from one plant species to another. In addition, the ratios between various constituents in a single plant may also vary with age, stage of growth, and other conditions. However, cellulose is usually the dominant structural polysaccharide of plant cell walls (35-50%), followed by hemicellulose (20-35%) and lignin (10-25%). Average values of the main components in some lignocellulose wastes are shown in Table 8.1 [8]. 

A classic example of the creation of macrostructure from molecular organization is cellulose-based fibre materials. Cellulose is the dominant polysaccharide in plant cell walls and is often touted as being the most abundant biopolymer on earth. A basic cellulose unit, known as the elementary fibril, contains thirty-six l,4-(3-D-linked polyanhydroglucopyranose chains (Figure 12.3a), and may eventually be coated with non-cellulosic polysaccharides to form the cell wall microfibril. These microfibrils are then crosslinked by hemicelluloses/pectin matrixes during cell growth. The cellulose molecule is constrained to adopt... 

Metraux, J. P. (1982). Thin-layer chromatography of neutral and acidic sugars from plant cell wall polysaccharides. J. Chromatogr. 237 525-527. 

Dietary fibre is also a lot more difficult to handle experimentally than other nutrients. Attempts to purify it inevitably lead to significant changes in its physical properties, so studies where pure polysaccharides such as pectin, guar gum, cellulose, etc. have been fed should be interpreted with great caution. The behaviour of these substances when pure will be different from that when they form part of a plant cell wall. In addition, many so-called mild treatments, like washing in warm water prior to use in experiments, remove significant quantities of polysaccharides from the cell-wall matrix and so alter its likely effect. 

Historically, dietary fiber referred to iasoluble plant cell wall material, primarily polysaccharides, not digested by the endogenous enzymes of the human digestive tract. This definition has been extended to iaclude other nondigestible polysaccharides, from plants and other sources, that are iacorporated iato processed foods. Cellulose [9004-34-6] (qv) is fibrous however, lignin [9005-53-2] (qv) and many other polysaccharides ia food do not have fiberlike stmctures (see also Carbohydrates). 

Detergent Methods. The neutral detergent fiber (NDF) and acid detergent fiber (ADF) methods (2), later modified for human foods (13), measure total insoluble plant cell wall material (NDF) and the cellulose—lignin complex (ADF). The easily solubilized pectins and some associated polysaccharides, galactomaimans of legume seeds, various plant gums, and seaweed polysaccharides are extracted away from the NDF. They caimot be recovered easily from the extract, and therefore the soluble fiber fraction is lost. 

Figures 3 and 5 from Biochimie, vol 85, Perez S, Rodrigues-Carvajal MA, Doco T (2003) A complex plant cell wall polysaccharide rhamnogalactmonan 11. A structure in quest of a function. p 109-pl21...

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