Cellulose hemicelluloses and

Chemical Constituents of Cell Wall. Variation in chemical composition across the cell wall is also shown in Figure 6. The principal constituents of cellulose, hemicellulose, and lignin are present throughout the cell wall but in different proportions. Cellulose is not present in the interfiber middle lamella, which is virtually all lignin. The layer is essentially all carbohydrates (qv), especially hemiceUuloses, having Uttie or no lignin. 

The primary walls of growing plant cells are composed of 90% carbohydrate and 10% protein (51). Carbohydrate in the primary wall is present predominantly as cellulose, hemicellulose, and pectin. The pectic polysaccharides, are defined as a group of cell wall polymers containing a-l,4-linked D-galactosyluronic acid residues (62,76). Pectic polysaccharides are a major component of the primary cell waU of dicots (22-35%), arc abundant in gymnosperms and non-graminaceous monocots, and are present in reduced amounts (-10%) in the primary walls of the graminaceae (27,62). 

Plant cell walls provide the obvious functions of stmctural support and integrity and can vary tremendously in size, shape, composition and stmcture depending on cell type, age and function within the plant body. Despite this diversity, plant cell walls are composed of only three major classes of polysaccharides cellulose, hemicellulose and pectins. Pectins, or polyuronides, are imbedded throughout the cell wall matrix and are particularly abundant in the middle lamella region. Pectins generally account for 10-30% of the cell wall dry weight and... 

Since many years, pectolytic enzymes have been widely used in industrial beverage processing to improve either the quality and the yields in fruit juice extraction or the characteristics of the final product [1,2]. To this purpose, complex enzymatic mixtures, containing several pectolytic enzymes and often also cellulose, hemicellulose and ligninolytic activities, are usually employed in the free form. The interactions among enzymes, substrates and other components of fruit juice make the system very difficult to be investigated and only few publications are devoted to the study of enzymatic pools [3-5], An effective alternative way to carry out the depectinisation process is represented by the use of immobilized enzymes. This approach allows for a facile and efficient enzymatic reaction control to be achieved. In fact, it is possible to avoid or at least to reduce the level of extraneous substances originating from the raw pectinases in the final product. In addition, continuous processes can be set up. 

Plant cell walls are constructed from cellulose, hemicelluloses, and pectins with varying amounts of lignin, tannins, gums, proteins, minerals,... 

Wood is a composite material that is made, up basically of a mixture of three main constituents, cellulose, hemicellulose, and lignin (see Textbox 54), all of them biopolymers synthesized by the plants, which differ from one another in composition and structure (see Textbox 58). The physical properties of any type of wood are determined by the nature of the tree in which the wood grows, as well as on the environmental conditions in which the tree grows. Some of the properties, such as the density of wood from different types of trees, are extremely variable, as can be appreciated from the values listed in Table 71. No distinctions as to the nature of a wood, whether it is a hardwood or a softwood, for example, can be drawn from the value of its specific gravity. 

Wood consists mainly of cellulose, hemicelluloses and lignin in various proportions. The amounts and compositions of these component groups depend primarily on the wood species [38]. 

Figure 1.15 gives an overview of the main constituents of non food biomass. There are three components Cellulose, hemicellulose and lignin. Cellulose and hemicellulose are built form sugar-type monomers, but their cost-effective isolation through enzymatic depolymerization remains a challenge. 

Lignocellulose is the fibrous material that forms the cell wall of a plants architecture . It consists of three major components (Fig. 2.1) cellulose, hemicellulose and lignin [3, 14-16]. It contrasts with the green parts of the plants and the seeds, which are rich in proteins, starch and/or oil. 

Fourth lesson - combination of different compounds in unique macrostructure provides unique performance properties. Starch is used extensively in nature to store carbon and energy. Starch is readily digested and must be protected from degradation by a resistant coating, for example, a seed (e.g. com, wheat or rice) or a skin (e.g. potato). Woody materials such as trees, soft plants and grasses are composed of a complex combination of aliphatic and aromatic compounds (cellulose, hemicellulose and lignin). 

Many of the physical, chemical and biological properties of wood can be understood by referring to the polymeric chemical constituents. In many cases of wood modification, these polymeric components are modified to some extent. The three structural polymeric components of the wood cell wall are cellulose, hemicellulose and lignin. There are many excellent texts describing the structure and function of these components, and only a brief account is given here. 

Ramiah, M.V. and Goring, D.A.I. (1967). Some dilatomeric measurements of the thermal decomposition of cellulose, hemicellulose and lignin. Cellulose Chemistry and Technology, 1(3), 277-285. 

Agricultural residues (stem, leaves, etc.) currently left in the fields after harvesting are made of cellulose, hemicellulose, and lignin. They are not competing with the feedstock for the food industry. 

Miller, R. S. and Bellan, J. (1997) A generalized biomass pyrolysis model based on superimposed cellulose, hemicellulose and lignin kinetics. Comb. Sci. and Techn., 126, 97-137. 

The basic structure of all wood and woody biomass consists of cellnlose, hemicelluloses, lignin and extractives. Their relative composition is shown in Table 2.4. Softwoods and hardwoods differ greatly in wood stmctnie and composition. Hardwoods contain a greater fraction of vessels and parenchyma cells. Hardwoods have a higher proportion of cellulose, hemicelluloses and extractives than softwoods, but softwoods have a higher proportion of lignin. Hardwoods ate denser than softwoods. 

Biorefinery includes fractionation for separation of primary refinery products. The fractionation refers to the conversion of wood into its constituent components (cellulose, hemicelluloses and lignin). Processes include steam explosion, aqueous separation and hot water systems. Commercial products of biomass fractionation include levulinic acid, xylitol and alcohols. Figure 3.3 shows the fractionation of wood and chemicals from wood. 

For monitoring the extent of polysaccharide hydrolysis, l.c. methods that sepeu ate and analyze the non-fermentable oligosaccharides (d.p. 3-30) derived from cellulose, hemicellulose, and pectins are useful, and have already been described (see Section III,l,c). For determination of the monosaccharide composition of completely hydrolyzed, plant polysaccharides, l.c. is especially useful and has been applied to the compositional analysis of hydrolyzed plant fiber,wood pulps,plant cell-walls,and cotton fibers.In these representative examples, the major sugars of interest, namely, glucose, xylose, galactose, arabinose, and mannose, have traditionally been difficult to resolve by l.c. The separa-... 

Complex pyrolysis chemistry takes place in the conversion system of any conventional solid-fuel combustion system. The pyrolytic properties of biomass are controlled by the chemical composition of its major components, namely cellulose, hemicellulose, and lignin. Pyrolysis of these biopolymers proceeds through a series of complex, concurrent and consecutive reactions and provides a variety of products which can be divided into char, volatile (non-condensible) organic compounds (VOC), condensible organic compounds (tar), and permanent gases (water vapour, nitrogen oxides, carbon dioxide). The pyrolysis products should finally be completely oxidised in the combustion system (Figure 14). Emission problems arise as a consequence of bad control over the combustion system. 

Extraction of the cellulase system. The culture of SSF from each flask (originally 5 g of substrate) was mixed well with more water to bring the final weight of the mixture (mycelium plus unutilized lignin, cellulose, and hemicelluloses) to 100 g. Tween 80 was added at a rate of 0.1%. The mixture was shaken for 0.5 h and centrifuged. The supernatant was used for enzyme determination. We estimated that about 7% to 10% cellulases remained adsorbed on the residues (mycelium and unutilized cellulose, hemicelluloses, and lignin) when the residues were suspended in water and Tween 80 as before and the supernatant was tested for cellulase titer. 

Reactive organic chemicals can be bonded to cell wall hydroxyl groups on cellulose, hemicelluloses, and lignin. Much of our research has involved simple epoxides (1 3) and isocyanates (4), but most of our recent effort has focused on acetylation. Acetylation studies have been done using fiberboards (5f6), hardboards (7 11) particleboards (12-20), and flakeboards (21-23), using vapor phase acetylation (8,2 257, liquid phase acetylation (, ), or reaction with ketene (28). 

Cellulose is found in nature in combination with various other substances, the nature and composition of which depend on the source and previous history of the sample. In most plants, there are three major components cellulose, hemicelluloses, and lignin. Efficient utilization of all three components would greatly help the economics of any scheme to obtain fuel from biomass. Hemicelluloses, lignocellulose and lignin remaining after enzymatic degradation of the cellulose in wood would require chemical or thermal treatment - as distinct from biochemical - to produce a liquid fuel. 

Plant cell walls are complex, heterogeneous structures composed mainly of polymers, such as cellulose, hemicelluloses, and lignins. In spite of several decades of research, cell wall assembly and the biosynthesis and ultimate biodegradative pathways of individual polymers are still far from being fully understood. One simple example will suffice Even today, no enzyme capable of catalyzing cellulose formation in vitro has been obtained. 

Cellulose, hemicelluloses, and lignin are the main components of cell walls in woody plants. For a long time, these plant polymers have stimulated the interest of many plant botanists and biochemists in terms of their biosynthetic pathways, functional interrelationships, and anatomical distribution. 

Charring is known to preserve aspects of the physical structure of wood, seeds, and fruit (42). Srinivasan and Jakes (43) have shown that in charring some aspects of the physical shape of Indian hemp fiber are retained. In the carbonization of wood, Ercin et al (44) report the loss of cellulose, hemicellulose and lignin infrared absorbance bands in the range of 1300-1000 cm-1 and the appearance of two new bands at 1250 cm 1 attributed to the asymmetric C-O-C and at 1450 cm-1 attributed to aliphatic C-H bending. 

Cellulosic materials are quite variable from source to source, not only in cellulose, hemicellulose, and lignin content but also in the crystallinity of the cellulose. As a consequence, each natural substrate would be expected to have its own unique set of process conditions to optimize glucose yield and minimize secondary product contamination. The next section on kinetics of acid hydrolysis will examine this point. 

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