Cell Microfibrils

Frey-Wyssling A (1954) The fine structure of cell microfibrils. Science 119 80... 

The microfibrils iu vegetable fibers are spiral and parallel to one another iu the cell wall. The spiral angles iu flax, hemp, ramie, and other bast fibers are lower than cotton, which accounts for the low extensibiUty of bast fibers. 

Microfibrils are laid down as ribbons having their flat faces parallel to the cell periphery. HemiceUuloses and lignin are deposited between the microfibrils... 

Fig. 3. A cross-section of a nearly square cellulose microfibril, with the individual molecular chains shown as rectangles. Also shown are the one- and two-chain unit cells of la and ip. This view of the microfibril is parallel to the long axis. The chains are arranged so that the edges of the crystal correspond...

The superimposition of diffraction spots from both phases gives the previously reported pattern that was thought to require an eight-chain unit cell. In the la stmcture, because of its one-chain unit cell, all chains must have parallel packing. Since the la and ip stmctures exist in the same microfibril of cellulose, the chains in the ip stmcture should also be parallel. 

The filaments of all plant fibers consist of several cells. These cells form crystalline microfibrils (cellulose), which are connected together into a complete layer by amorphous lignin and hemi-cellulose. Multiple layers stick together to form multiple layer composites, filaments. A single cell is subdivided into several concentric layers, one primary and three secondary layers. Figure 5 shows a jute cell. The cell walls differ in their composition and in the orientation of the cellulose microfibrils whereby the characteristic values change from one natural fiber to another. 

Xylans as true homopolymers occur in seaweeds of the Palmariales and Nemaliales, however, their backbone consists of Xylp residues linked by -(1 3) (Type X3, Fig. la) or mixed -(1 3, 1 -> 4)-glycosidic linkages (Type Xmy Fig. lb). They are assumed mainly to have a structural function in the cell-wall architecture, but a reserve function cannot be ruled out [4]. From the microfibrils of green algae (Siphonales) such as Caulerpa and Bryop-sis sp., X3 was isolated and the structure confirmed by methylation analysis, C-NMR spectroscopy [7], as well as by mass spectrometry of enzymically released linear oligosaccharides up to a degree of polymerization (DP) of... 

Fig 1. Electron micrograph of a platinum/carbon replica prepared by the fast-freeze, deep-etch, rotary-shadow replica technique printed in reverse contrast. Cell walls of onion parenchyma have an elaborate structure with many thin fibres bridging between thicker cellulosic microfibrils. Scale bar represents 200nm. 

Replicas of the tomato cell walls are very similar to those of onion parench5una cell walls but replicas of the DCB-adapted walls did not show the structure of the walls clearly. The principle components of the adapted walls are shorter thinner fibres which seemed to form a gel-like structure with little evidence of long cellulosic microfibrils characteristic of the unadapted cells. It is possible that such a gel will bind water more strongly and reduce the amount of etching that takes place, resulting in a less well-defined replica (2). 

If cellulose exists in the cell wall as a network within a pectic matrix, the pectin that is within about 2nm of the cellulose network maybe on or near exposed surfaces of cellulose microfibrils. Both the gel and the eggbox pectins are represented in this low mobility spectrum. 

There are a number of possible locations within the cell wall for the pectin further away from cellulose. If there are covalent links between pectins and xyloglucans (16), then pectic chain segments close to these links would appear in the region sharing the same mean mobility characteristics as cellulose. The majority of the pectic molecule, diverging from the microfibrils would appear in the region with greater mean mobility. 

The intermediate-mobility pectin can exist in any space in the cell wall more than 2nm away from cellulose microfibrils. It could therefore be in the middle lamella, cell comers or between layers of microfibrils in addition to the above proposal. The pectin seen in this part of the spectram are probably a heterogeneous mixture from a number of locations. 

Raman microscopy cellulose microfibrils in cell walls and distinguishing crystalline and noncrystalline inclusions Analysis of bioaccumulations in plant vacuoles ... 

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. 

Figure 2.5 The molecular architecture of the cellulose molecule showing its relationship to the microfibrils and to the total cell wall.

For a detailed description of the ultrastructure of wood and the cell wall, the reader is referred to the comprehensive texts listed above. Briefly, the cell wall of wood is composed of a number of discernable layers (Figure 2.2). These are divided into the primary (P) and secondary (S) layers the secondary layer is further subdivided into the Sj, S2 and S3 layers. The primary layer is the first to be laid down when the cell is formed and is composed of microfibrils, which have an essentially random orientation that allows for expansion of the cell to occur as cell growth takes place. The secondary layer is subsequently formed, with each of the sub-layers exhibiting different patterns in the way the microfibrils are oriented, as illustrated in Figure 2.2. Of these, the 83 layer occupies the... 

The space between the microfibrils is occupied by the hemicelluloses and by lignin. However, the incomplete filling of the intermicrofibrillar region results in the existence of what are usually referred to as micropores (or microvoids) in the cell wall. These have diameters of the order of nanometres and thus technically should be referred to as nanopores, but since the term micropores is the most commonly used in the literature, it will be used throughout this book. 

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