Secondary walls

Structure of the Cell Wall. The iaterior stmcture of the ceU wall is shown in Figure 6. The interfiber region is the middle lamella (ML). This region, rich in lignin, is amorphous and shows no fibnUar stmcture when examined under the electron microscope. The cell wall is composed of stmcturaHy different layers or lamellae, reflecting the manner in which the cell forms. The newly formed cell contains protoplasm, from which cellulose and the other cell wall polymers are laid down to thicken the cell wall internally. Thus, there is a primary wall (P) and a secondary wall (S). The secondary wall is subdivided into three portions, the S, S2, and layers, which form sequentially toward the lumen. Viewed from the lumen, the cell wall frequendy has a bumpy appearance. This is called the warty layer and is composed of protoplasmic debris. The warty layer and exposed layer are sometimes referred to as the tertiary wad. 

Fig. 6. Interior stmcture of the ceU wad of Scotch pine, where S = secondary wall, P = primary wad, and ML = middle lamella. Chemical composition of ced wad lignin, 28.0 wt % cedulose, 40.3 wt % and hemicedulose, 28.7 wt %. Extractives, not shown, ate 3.0 wt %.

The preparation of PCNTs from benzene has been reported by Endo et al. [17-20], The process is essentially similar to that used for VGCFs described above, but without secondary wall-thickening phase (see Fig. 2). Thus PCNTs can be regarded as the precursor for VGCFs, but having a much smaller nanometer-scale... 

The components of the plant cell wall (8-21) are the middle lamella (intercellular substance), the primary wall, and the secondary wall. The middle lamella is the pectic layer between cells and holds adjoining cells together as do membrane carbohydrates. The primary wall is thin (1-3 pm) and flexible containing cellulose, hemicelluloses, pectins, and glycoproteins. This wall provides mechanical strength, maintains cell shape,... 

The secondary wall is located inside the primary wall and can possess pits and sometimes three distinct layers, Sl5 S2, and S3. The secondary wall can contain 25% lignin, and cellulose is more abundant than in primary walls. Support and resistance to decay are the main functions of the primary wall. 

Schlerenchyma Thick cell walls containing lignin-nonextensible secondary walls, lack protoplasts at maturity, living or dead at functional maturity Strength and supporting elements of plant parts, scattered throughout plant (also see conducting tissue)... 

Tracheids Long tapering cell with lignified, secondary wall thickenings or pitted secondary walls lack protoplasts at maturity more primitive (less specialized) type of cell than vessel members widespread in vascular plants Conduct water and mineral salts water moves upward from tracheid to tracheid through pits... 

The outer secondary cell wall (SI) is comparable in thickness to the primary wall and consists of four to six lamellae which spiral in opposite directions around the longitudinal axis of the tracheid. The main bulk of the secondary wall is contained in the middle secondary cell wall (S2), and may be as little as 1 fim thick in early woods and up to 5 fim in summer wood. The microfibrils of this part of the wall spiral steeply about the axial direction at an angle of around 10 to 20°. The inner secondary wall (S3), sometimes also known as the tertiary wall, is not always well developed, and is of no great technological importance. 

To visualize whether or not wood can be delignified by laccase III, ultraviolet photomicrographs (280nm) were taken before and after treatment of 0.5 mjLt cross-sections of red pine with laccase III (Figure 11). After enzyme treatment areas having an absorbance less than 0.2 in the secondary wall, and an absorbance less than 0.4 in the middle lamella, appeared. Each part of the secondary wall, middle lamella, and cell junction was subjected to ultraviolet microscopy, and absorption curves in the 240-300nm region were determined. The absorption curves of three samples after treatment... 

Lignin is found in plant cell walls of supporting and conducting tissue, mostly the trac-heids and vessel parts of the xylem. It is largely found in the thickened secondary wall but can occur elsewhere close to the celluloses and hemicelluloses. 

S2 thickening the other was secondary wall lignilication which proceeded after S2 thickening. 

Figure 6. Incorporation of tritiated phenylalanine into the compound middle lamella lignin and the secondary wall lignin determined by counting the silver grains. Symbols are as follows 0, compound middle lamella 0> secondary wall.

Figure 12. A tracheid after S3 stage. Cytoplasm is filled with s-ER s. Radioactivity is distributed on the s-ER s and secondary wall.

In woody gymnosperms, there are significant differences in the distribution, reactivity and physical properties of protolignins found in the compound middle lamella and the secondary wall (1-3). Additionally, variations between lignins in vessels and fibers have also been noted (3). All of these... 

Figure 5. A schematic representation of the process of deposition of cell wall components and the heterogeneous formation of protolignin macromolecule. ML, middle lamella CC, cell corner P, primary wall CML, compound middle lamella S1 S2, and S3, outer, middle, and inner layer of secondary wall H, G, and S,p-hydroxy-, guaiacyl-, and syringylpropane units.

Terminal complex consolidation has also been reported in vascular plants as loosely aligned files of rosettes associated with secondary wall formation (13,14,34,35). Similar rosette files were also observed during primary wall formation in rapidly elongating regions of Avena coleoptiles... 

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