Middle lamella composition

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 %.

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. 

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... 

The plant cell is surrounded by a cell wall, which determines many features of the plant. The outer layer of the cell wall are called middle lamella because it contains heavy layer of pectin (a polygalacturonan) that serves as the glue to hold one plant cell firmly to an adjacent cell. The inner layer of the wall is cell membrane. The cell membrane is completely different from the cell wall in form, composition, and function. Whereas the wall is a rigid, relatively thick structure, the cytoplasmic membrane is thin (approximately 75 A) and flexible. The membrane is composed of protein and lipid, whereas the wall is carbohydrate in nature. The wall provides support, whereas the membrane regulates the movement of substances into and out of the cell. 

The cell wall is built up by several layers, namely (Figs. 1-12 and 1-13), middle lamella (M), primary wall (P), outer layer of the secondary wall (SJ, middle layer of the secondary wall (S2), inner layer of the secondary wall (S3), and warty layer (W). These layers differ from one another with respect to their structure as well as their chemical composition. The microfibrils wind around the cell axis in different directions either to the right (Z helix) or to the left (S helix). Deviations in the angular directions cause physical differences and the layers can be observed in a microscope under polarized light. 

In addition to UV microscopy, there are other microscopic techniques for investigating the topochemistry of lignin in wood. These methods include fluorescence and electron microscopy The combination of these techniques with UV microscopy should give the best results However, when a composite technique is employed, precautions must be taken. For example, in measurements of lignin concentration by bromination/UV microscopy, a correction factor is needed to account for the difference in the reactivities of middle lamella and secondary wall lignins toward bromine (Saka et al, 1982) (see Chap, 4.4). 

Within the cell wall, each distinct region see Figure 3, Chapter 11) has a distinct composition and attributes. For a typical softwood, the middle lamella and primary wall are mostly lignin (8.4% of the total weight) and hemicellulose (1.4%), with very little cellulose (0.7%). The Si (outer) layer... 

The whole pectin fraction usually contains one or both types of pectinic acid and arabinogalactans. Hemicellulose is a complex mixture of arabino-galactan, xylans, sometimes some glucomannan or galactoglucomannan, and callose. Its composition is variable with the source. The cellulose fraction, not only contains p-l,4-glucan, but almost always has small amounts of inseparable glucomannan. The primary cell wall is relatively richer in hemicellulose than the secondary wall, while the latter contains a good deal of cellulose. Pectins are found in primary walls and probably comprise much of the middle lamella. 

Lewin [95], in his review, also highlighted several important features of the chemistry of substances in the middle lamella of jute and flax. His observations regarding flax are discussed briefly along with some particular features of the chemical composition of specific fibers in the following text. 

Chemical composition of coir fiber reported by Varma et al. [178] is given in Table 8.3. [124] had suggested the lignin content to be about 35%, most of it in the middle lamella and the primary wall as such, it is said to protect the cellulose from chemical and physical attack [139]. An estimated moisture content of 20% has also been reported [109]. Timell [172], based on extractive-free material, gives the carbohydrate content of hemp and jute products, as shown in Table 8.4. 

The swelling of bast and leaf fibers in alkaline solutions is a complex phenomenon. Their structure is more complicated than in the case of pure cellulose. Different components of the middle lamella react in different ways, and rates, in relation to each other and to the cellulose. In addition, the composition of the fiber changes with the time of treatment. A part of the... 

The amount and composition of lignins vary among taxa, cell types, and individual cell wall layers and middle lamella, and are influenced by developmental and environmental conditions [18]. 

Several layers have to be distinguished in the wall thickness, not only by the macromolecular composition but also by the microflbril orientation, i.e., the angle between the fibril orientation and the longitudinal axis of the cell. Between two adjacent cells lies a highly lignified region called middle lamella. Both middle lamella and adjoining primary walls are sometimes referred to as compound middle lamella. The secondary cell wall is laid down after cell extension is... 

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