Directionality of the fibre c-axis of cellulose crystallites in microfibrils of Valonia ventricosa
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Microfibril
Lamella (surface anatomy)
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Characteristics of the deposition of cellulose microfibrils during formation of polylamellate walls and the arrangement of cortical microtubules in the tip-growing bipolar cells of Chamaedoris orientalis were examined by replica preparation methods and indirect immunofluorescence microscopy. The polylamellate wall is made up of two or three kinds of wall lamella which differ in terms of the orientation of microfibrils. Individual lamellae were periodically initiated one after another from the pole that was situated exactly at each growing apex of the cell and they were deposited basipetally. The orientation of microfibrils in each lamella was constant during deposition. Microfibrils in different lamellae were deposited at the same time through the cell wall but the timing of the deposition was staggered between neighboring lamellae so that the microfibrils would not be interwoven. By contrast, cortical microtubules were persistently arranged longitudinally all over the cell and no focal points to which they converged helically were visible, even around the cell apices. The mechanisms that regulate the formation of the polylamellate wall are discussed and a model for interpreting the involvement of the cortical microtubules in such mechanisms is proposed.
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A fine structure of cell wall lamellae in a coenocytic green algaBoergesenia forbesii was examined by electron microscopy. The wall has a polylamellate structure containing cellulose microfibrils 25 to 30 nm in diameter. The outer surface of the cell was covered by a thin structureless lamella, underneath which existed a lamella containing randomly-oriented microfibrils. The major part of the wall consisted of two types of lamellae, multifibrillar lamella and a transitional, matrix-rich one. In the former, microfibrils were densely arranged more or less parallel with each other. In the transitional lamella, existing between the multifibrillar ones, the microfibril orientation shifted about 30° within the layer. The fibril orientation also shifted 30° between adjacent transitional and multifibrillar layers, and consequently the microfibril orientation in the neighboring multifibrillar layers shifted 90°. It was concluded that the orientation rotated counterclockwise when observed from inside the cell. Each lamella in the thallus wall become thinner with cell expansion, but no reorientation of microfibrils in the outer old layers was observed. In the rhizoid, the outer lamellae sloughed off with the tip growth.
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Plant cell
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This chapter describes plant cell walls as the best studied of the eukaryotic types, which consist of three basic layers: middle lamella, primary cell wall, and secondary cell wall. It clarifies that plant cells are glued together by a middle lamella, which separates their walls, and within the middle lamella is a primary cell wall. It also examines cellulose, which is the major carbohydrate of most plant cell walls and is the most abundant carbohydrate on earth. The chapter discusses algal cell walls that display an incredibly diverse composition and contain cellulose microfibrils and mannose or xylan-rich carbohydrates. It refers to filamentous fungal cell walls which are composed mainly of the polysaccharides glucans, chitin, and glycoproteins.
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Abstract Cellulose is a polysaccharide consisting of a linear chain of β (1→4) linked D ‐glucose and it is the most abundant polymer on earth. As a major structural component of the cell wall, cellulose accounts for about one‐third of plant mass. The regulation of cellulose biosynthesis is essential to plant development. Cellulose is synthesised by the cellulose synthase (CESA) complex in the plasma membrane. This article reviews the composition and regulation of the cellulose synthase complex with a focus on the role of cytoskeleton in higher plants. In this article, the evolving views in the field of cellulose biosynthesis are discussed and the unresolved questions, such as in vitro cellulose synthesis, structure of CESA and mechanism underlying microtubule–microfibril alignment hypothesis, are highlighted. Key Concepts: Cellulose is a polysaccharide consisting of a linear chain of β (1→4) linked D ‐glucose. Cellulose is synthesised by large membrane‐bound protein complexes known as cellulose synthase complexes (CSCs). Microtubule–microfibril alignment hypothesis states that there is a causal link between the orientation of cortical microtubules and nascent microfibrils. Primary cell wall surrounds all plant cells and is formed during cell division and expansion. Secondary cell wall is formed between the primary cell wall and the plasma membrane in cells that are subject to mechanical stress.
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Texture (cosmology)
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The cell walls of Fusarium sulphureum have a microfibrillar component that is randomly arranged. X-ray-diffraction diagrams of the microfibrils are consistent with a high degree of crystallinity and show that they are chitin. The chitin microfibrils of the peripheral walls envelop the hyphal apex and extend across the septae. During the first 8 h in culture, the conversion of conidial cells to chlamydospores is evidenced by a swelling of the cells and the original microfibrils remain randomly arranged. Within 24 h new wall material is deposited as the cells expand and the wall thickens. The new microfibrils are indistinguishable from those of the original conidial cells.After 3 days in culture, the chlamydospores are fully developed and have the characteristic thick wall which is a continuous layer of randomly arranged microfibrils. Chlamydospores maintained in a conversion medium for 8 days have microfibrils identical with those in 3-day-old cultures; thus a further change in the microfibril orientation did not occur during that period.Alkaline hydrolysis of the walls removes most of the electron-dense staining constituents from the inner wall layer and leaves the outer wall layer intact. This treatment also reveals some of the wall microfibrils. An additional treatment of the walls with HAc/H 2 O 2 completely removes the wall components that react positively to heavy metal stains. The results are discussed in relation to the structure of other fungal cell walls.
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The cell wall of growing plant tissues has frequently been interpreted in terms of inextensible cellulose microfibrils ‘tethered’ by hemicellulose polymers attached to the microfibril surface by hydrogen bonds, with growth occurring when tethers are broken or ‘peeled’ off the microfibril surface by expansins. This has sometimes been described as the ‘sticky network’ model. In this paper, a number of theoretical difficulties with this model, and discrepancies between predicted behaviour and observations by a number of researchers, are noted. (i) Predictions of cell wall moduli, based upon the sticky network model, suggest that the cell wall should be much weaker than is observed. (ii) The maximum hydrogen bond energy between tethers and microfibrils is less than the work done in expansion and therefore breakage of such hydrogen bonds is unlikely to limit growth. (iii) Composites of bacterial cellulose with xyloglucan are weaker than pellicles of pure cellulose so that it seems unlikely that hemicelluloses bind the microfibrils together. (iv) Calcium chelators promote creep of plant material in a similar way to expansins. (v) Reduced relative ‘permittivities’ inhibit the contraction of cell wall material when an applied stress is decreased. Revisions of the sticky network model that might address these issues are considered, as are alternatives including a model of cell wall biophysics in which cell wall polymers act as ‘scaffolds’ to regulate the space available for microfibril movement. Experiments that support the latter hypothesis, by demonstrating that reducing cell wall free volume decreases extensibility, are briefly described.
Xyloglucan
Microfibril
Hemicellulose
Breakage
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Microfibril
Secondary cell wall
Immunogold labelling
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The discovery of xyloglucan and its ability to bind tightly to cellulose has dominated our thinking about primary cell wall structure and its connection to the mechanism of cell enlargement for 40 years. Gene discovery has advanced our understanding of the synthesis of xyloglucan in the past decade, and at the same time new and unexpected results indicate that xyloglucan’s role in wall structure and wall extensibility is more subtle than commonly believed. Genetic deletion of xyloglucan synthesis does not greatly disable cell wall functions. Nuclear magnetic resonance studies indicate that pectins, rather than xyloglucans, make the majority of contacts with cellulose surfaces. Xyloglucan binding may be selective for specific (hydrophobic) surfaces on the cellulose microfibril, whose structure is more complex than is commonly portrayed in cell wall cartoons. Biomechanical assessments of endoglucanase actions challenge the concept of xyloglucan tethering. The mechanically important xyloglucan is restricted to a minor component that appears to be closely intertwined with cellulose at limited sites (‘biomechanical hotspots’) of direct microfibril contact; these may be the selective sites of cell wall loosening by expansins. These discoveries indicate that wall extensibility is less a matter of bulk viscoelasticity of the matrix polymers and more a matter of selective control of slippage and separation of microfibrils at specific and limited sites in the wall.
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Secondary cell wall
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