Fibre cells

The paper-making properties of all of these fibres are quite different from each other and also from wood. This is mostly due to the differing morphology and to some extent the differing chemistry of the fibre cells. The photomicrograph (Figure 1.2), shows a comparison between various non-woody fibre types. 

The overall composition of plant fibre cells in terms of carbon, hydrogen and oxygen is variable and dependent on the degree of lignification. For wood it is approximately 50% carbon, 6% hy-... 

A very large number of changes take place within the whole fibre suspension during the refining process and a precise interpretation of the influence of the process upon mechanical and other sheet properties therefore becomes extremely difficult. The more important effects can be summarised as follows. There is a shortening of the fibres arising from a cutting effect and part of the fibre cell wall is... 

The anatomical unit of muscle is an elongated cell called a fibre. Each individual fibre cell consists of myofibrils which are bundles of contractile protein filaments composed of actin and myosin (Figure 7.1). Differences in structure indicate that muscles have evolved to perform particular functions. Although the structure of fibres, myofibrils and filaments of actin and myosin, is similar in all muscle types, their arrangement, action and control allow identification of three tissue types ... 

The muscle is composed of subunits called fascicles. The entire muscle and subunits with several fascicles are often surrounded by layers of connective tissue or fatty tissue. Fascicles are bundles of individual muscle fibres. Each fibre is one elongated cell that may extend for the length of the muscle. Each muscle fibre cell is segmented into distinct sectional bands. In contrast to most other tissues, cells of skeletal musculature have several nuclei. Within each muscle cell are numerous myofibrils, which also extend for the length of the muscle cell. Sarcomeres are the basic contractile subunit of myofibrils. 

Fig. 2. Macroscopic and microscopic structure of muscle (a) Entire muscle and its cross-section with fatty septa, (b) Fascicle with several muscle fibres (cells). A layer of fat along the fascicle is indicated, (c) Striated myofibre corresponding with one single muscle cell containing several nuclei. The lengths of a myofibre can be several tens of centimetres, (d) Myofibril inside a myocyte. It is one contractile element and contains actin and myosin and further proteins important for the muscular function, (e) Electron myograph of human skeletal muscle showing the band structure caused by the contractile myofilaments in the sarcomeres. One nucleus (Nu) and small glycogen granules (arrow, size <0.1 pm) are indicated.

Klerx, J. P. A M., Jansen Verplanke, C., Blonk, C. G, and Twaalfhoven, L. C (1988) In vitro production of monoclonal antibodies under serum-free conditions using a compact and inexpensive hollow fibre cell culture unit. J Immunol Methods 111, 179-188. 

Jensen CG, Watson M. 1999. Inhibition of cytokinesis by asbestos and synthetic fibres. Cell Biol Int 23(12) 829-840. 

Liver cell hydrops Liver cell hydrops is characterized by swollen hepatocytes, which contain a lot of liquid, but are generally free of fat. Hydropic liver cells may be 2 to 4 and even 10 times the size of normal hepatocytes. Cell hydrops is completely reversible when abstention is maintained. These cells usually die when alcohol intake is continued and are subsequently removed by macrophages and leucocytes. This leads to an alteration in the cytoskele-ton. Hydropically degenerated liver cells and hyaline liberated from Mallory bodies produce a leucocytic inflammatory reaction. Unless it is compensated by regenerative processes, this cellular deficit results in defective healing by fine-fibred, cell-poor fibrils. 

Humidity and Heat. Moisture is crucial to the normal behaviour of cellulosic fibres. Under moderate conditions (relative humidity 45-65%) water is readily absorbed through the network of pores running through a fibre cell, it coats cellulose crystallites and acts as a plasticiser of the amorphous regions, disrupting inter-chain hydrogen bonds. Without this bound water the fibre would be permanently brittle, with an effective glass transition point way above room temperature. 

The electrical activity produced by either the depolarization or the repolarization of myocardial tissue, specifically the nerve fibre cells, may be identified conveniently by the help of suitable electrodes and this may be plotted as a graph showing intensity (mV millivolts) along the Y-axis and time (seconds) along the X-axis, as shown in Fig. 12.1(a), also known as the electro-cardiogram (ECG). 

In addition to the fibrillar morphology of the fibre cell wall, the fibres are characterized by capillaries, voids, and interstices providing the cellulose fibres a highly porous character. The pore size ranged from 5 up to 30 nm and the pore volume fraction attained 1-3% for cotton and wood pulp. However, the total pore volume and pore size distribution are very sensitive to pretreatments. Mercerization leads to a decrease in pore diameter and an enhancement of micropore surface, while enzyme treatments enlarge the existing pores [4]. 

A recent cytochemical study by Waterkeyn (1981) has shown that callose always forms the innermost layer of the secondary wall of cotton fibre cells, just outside the plasmalemma. Hence the deposition of cellulose must occur outside this layer and cellulose molecules must either pass through it, or be constructed at its outer edge. No evidence was obtained to show whether the callose was or was not converted into cellulose, but Waterkeyn favoured the view that callose represents a permanently restored interface across which cellulose molecules are matured and organised to form the secondary wall. A careful distinction was drawn between the physiological and traumatic depositions of callose and there is, indeed, no evidence that the two processes are very closely related. 

Within the stem there are a number of fibre bundles, each containing individual fibre cells or filaments. The filaments are made of cellulose and hemicellulose, bonded together by a matrix of lignin or pectin. 

Coir fibre is obtained from the hard internal shell and the outer coat of coconuts. The individual fibre cells are narrow and hollow and the thick walls are made of cellulose. It is thick, strong and has high abrasion resistance. Mature brown coir fibre contains more lignin and less cellulose than fibres such as flax and cotton and is therefore stronger but less flexible. Coir fibre is relatively waterproof and is one of the few natural fibres resistant to be damaged by salt water. 

Figure 19.1 Schematic structure of a natural fibre cell. Reproduced with permission from Reference [4].

Figure 19.2 Schematic structure of an elementary plant fibre (cell). The secondary cell wall, S2, makes up about 80 per cent of the total thickness. Reproduced with permission from Reference [5].

The outer cell wall is porous and contains almost all of the non-cellulose compounds, except proteins, inorganic salts and colouring matters and it is this outer cell wall that creates poor absorbency, poor wettability and other undesirable textile properties. In most applications, fibre bundles or strands are used rather than individual fibres. Within each bundle, the fibre cells overlap and are bonded together by pectins that give strength to the bundle as a whole. However, the strength of the bundle structure is significantly Iowct than that of the individual fibre cell and thus the potential of the individual fibres is not fully exploited. 

The hemicellulose molecules of the matrix phase in a cell wall are hydrogen bonded to cellulose and act as a cementing matrix between the cellulose microfibrils, forming the cellulose/hemicellulose network, which is thought to be the main structural component of the fibre cell. The hydrophobic lignins on the other hand act as a cementing agent and increase the stiffness of the cellulose/hemicellulose composite. 

In a novel study, Zou et al. [49] reported nanoscale structural and mechanical characterization of the cell wall of bamboo fibres. They reported the discovery of cobble-like polygonal cellulose nanograins with a diameter of 21-198 nm in the cell wall of bamboo fibres. These nanograins are basic building blocks that are used to construct individual bamboo fibres. Nanoscale mechanical tests were carried out on individual fibre cell walls by nanoindentation. It was found that the nanograin structured bamboo fibres are not brittle in nature but somewhat ductile. Figure 1.17a... 

Fig. 1.17 Cross-sectional micrographs of a phloem fibre cap in a vascular bundle of a bamboo culm, (a) Optical micrograph of a fibre cap. (b)-(d) AFM phase images of bamboo fibres. (e)-(j) AFM phase images of the nanoscale structure in the fibre cell wall [49]...

Natural fibres are classified into three main groups, namely, bast (or stem), leaf and seed (or fruit). Bast fibres such as jute, hemp, kenaf and flax are fibrous bundles found in the inner bark of the plant stem. The fibre bundles consist of filaments of fibre cells made up of mainly cellulose and hemicelluloses. The cementing material between the fibre bundles is lignin while the filaments are held together by pectins. These fibres are separated from the woody matter through a process of natural... 

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