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Cell structure of biomass (lignocellulose) Part VICell structure of biomass (lignocellulose) Part VI Cell structure of biomass (lignocellulose) Lignocellulose is another word for biomass that originates from plants. It generalises the structure of plants to the three main sugar-based polymeric structures; cellulose, hemicellulose and lignin. These three polymeric structures are mainly considered in most of the studies aiming for the understanding of decomposition mechanisms of woody and herbaceous biomass. In plant structures lignocellulose normally forms the most dominant group of constituents on a mass basis. Its main role is found in the cellular structure of plants and forms the foundation of cell walls and their mutual coherence. Lignocellulose provides mechanical strength and tenacity (toughness) to plant structures and so provides body and the opportunity to grow in height for optimal photosynthesis. A typical plant cell is structured as illustrated by Figure 2.1. A single cell is typically described by a primary and a secondary wall. Subsequently, three individual layers describe the secondary wall. Individual cells are connected through a gluey layer called the middle lamella.  Figure 2.1 Detailed impression of the structure of a cell wall, (a) part of the cell wall and middle lamella, primary wall and secondary cell wall, (b) macrofibril mutual structure, (c) microfibrill structure, (d) individual cellulose polymers including micelles, and (e) mutual coherence of individual cellulose polymers on a micro level (taken from: Raven and Eichhorn, 1999) The second layer of the secondary wall is the thickest one and is built from vertically oriented macrofibrils. The macrofibril is on its turn composed from microfibrils, which predominantly consist of evenly oriented cellulose molecules of certain length. The cellulose chains comprise amorphous parts, but also crystalline parts whereby subsequent cellulose molecules are connected. The polymeric composition of the different walls and layers varies strongly and each wall has different tasks. Figure 2.2 illustrates how the polymeric composition varies throughout the cell wall. The middle lamella predominantly contains lignin. Lignin acts as a binding agent and can be considered a glue to bind adjacent cells. Whilst the lignin fraction decreases cell inwards, the fraction of (hemi)cellulose increases. Cellulose meets a maximum content in the S2 of the secondary layer and hemicellulose in S3.   Figure 2.2 Distribution of lignocellulose within the three layered secondary wall (taken from Haygreen and Bowyer, 1987) The three-layered secondary cell wall mainly consists of cellulose and is very well organised by nature. The cellulose macrofibrils are embedded in a matrix of (disoriented) hemicellulose that bonds the macrofibrils mechanically, but also through hydrogen bonding. The cell wall has a ECN-C--05-067 15 repetitive pattern in which hemicellulose binds macrofibrils of a cell wall and lignin binds adjacent cells. The function of hemicellulose is often well illustrated by comparing its function to concrete in reinforced concrete. Without the concrete the iron rods loose their mutual coherence and orientation. Each layer of the three-layered cell wall has a different fibre orientation. The main body of the cells (S2) is a vertical oriented structure of fibres kept in a compact form by an outer husk (S1) and annular (inner) husk (S3) both with near perpendicular fibre orientation. The wood structure consists of many of these cellular units ???glued??™ together by the lignin-rich primary walls. The anisotropic nature of wood, the fibrous structure, is caused due to the differences in thickness and orientation of the different layers. The way cell walls are mechanically organised is copied multiple times from nature because of the high strength and tenacity it provides. Every type of green biomass has its own typical lignocellulose composition. Table 2.1 summarises the lignocellulose composition of biomass species used in the experimental programme of this work. Woody types of biomass are commonly divided into coniferous (larch) and deciduous (beech, willow) categories. Next to that a group of herbaceous species (straw) is commonly defined. Table 2.1 Lignocellulose composition of different biomass types used in this work and typical polymeric composition of deciduous and coniferous wood types (Holzatlas, 1974)
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