Wood Science. Wood Cell Structure. Chemical Components of Wood. Chemical Components of Wood. Chemical Components of Wood. Chemical Components of Wood

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1 Wood Science Chapter 3 The Woody Cell Wall Wood Cell Structure Wood is a very complex material structurally, made up of tiny cells, each of which has a precise structure of tiny openings, membranes, and intricately layered walls. A practical knowledge of wood structure is important for knowing how to convert wood into products. Wood is composed principally of carbon, hydrogen, and oxygen Carbon is the dominant element by weight. Element Carbon Hydrogen Oxygen Nitrogen Ash % Dry Weight > These elements are combined into organic polymers: Cellulose Hemicellulose Lignin Wood Type Cellulose Hemicellulose Lignin (% of dry weight) Hardwood Softwood Think of cell walls of wood as reinforced structures: Cellulose contributes reinforcing fibers (aggregated into crystals and microfibrils) Hemicellulose surrounds cellulose microfibrils Lignin binds and provides rigidity as well as plastic nature Basic components of the cell wall are the result of photosynthesis: 6CO 2 + 6H 2 O? C 6 H O 2 Other sugars are also produced Inorganic compounds play a role Common elements are Ca, K, Mg (Mn, Si) 1

2 Cellulose (C 6 H 10 O 5 ) principal chemical constituent of the cell walls of the higher plants, a complex carbohydrate occurring in the form of polymer chains. Accounts for ~1/3 of all plant biomass (Chemically) very stable material, forms highly-ordered crystalline structures Cellulose Glucose molecules are joined together end to end, with elimination of a water molecule from each chemical linkage. This long-chain polymer has a degree of polymerization (n) which may be as large as 10,000, such that: (C 6 H 10 O 5 )n Molecule still too small to be seen with an electron microscope, with the largest cellulose molecules of about 10 microns. Cellulose Cotton is 99% pure cellulose, and fine writing papers have a high cellulose content. Though it is a carbohydrate with much food value as sucrose, cellulose cannot be digested by humans and most animals because digestive enzymes cannot hydrolyze the linkages between glucose units. Ruminants maintain intestinal colonies of microorganisms that produce cellulase enzymes, which convert cellulose to glucose. Termites have similar internal microflora. Hemicellulose Group of carbohydrates found in the cell wall in more or less intimate association with cellulose; sometimes defined as those less-resistant substances in the cell wall which though insoluble in hot water can be removed with either hot or cold dilute alkalies or readily hydrolyzed into sugars and constituent acids by means of hot dilute acids. produced from other products of photosynthesis: 6-carbon sugars (glucose, galactose, mannose) 5-carbon sugars (xylose and arabinose) sugar derivitives (glucuronic acid) branched polymers with low DP (~200) contributes to bonding between cellulose and lignin Hemicellulose Xylans (hardwoods) Formed by polymerization of the anhydro forms of xylose, arabinose, and 4-methylglucuronic acid Galactoglucomannans (softwoods) Formed by polymerization of residues of galactose, glucose, mannose. Lignin A complex and high molecular weight polymer built upon phenylpropane units. Though composed of C, H, and O, lignin is not a carbohydrate but is phenolic in nature. It occurs between and within cell walls: Between binding agent holding cells together. Within gives rigidity to the cell. Lignin helps reduce dimensional change with moisture-content Lignin adds to wood s toxicity, making wood resistant to decay and insect attack Wood without lignin would be cotton. 2

3 Lignin is produced from hydroxyphenylpropane monomers. Complex structure within a woody cell. Exact configuration is unknown. Can t be extracted without destroying the integrity of the structure Lignin Lignin Lignin is light colored in its native form, but treatments to remove it cause it to darken. Chemical pulps that contain lignin must be bleached to make them white. Newsprint manufacture mechanically separates fibers, but leaves lignin in liquor. When newsprint is exposed to air and/or sunlight, it yellows and browns. Newsprint has low strength because the lignin causes the fibers to be stiff and to not bind together very well. Mobile home-wall strength is affected by disruptions of domestic tranquility The Cell Wall Primary Cell Wall initial layer of the cell wall; formed during or following cell division and later modification during the postcambial differentiation of the cell. Secondary Cell Wall that portion of the cell wall formed after the cell enlargement has been completed. Cell Wall Development After formation, the cell enlarges and the cell wall gradually thickens as biopolymers (cellulose, hemicellulose, and lignin) are progressively added to the inside (lumen side) of the wall. Eventually the protoplasm is lost and the wall thickens as successive arrangements of biopolymer assemblies are laid down. Cell Wall Development Stages of Development: a. The new cell has only an ultra-thin primary wall b and c. The cell enlarges and then wall the wall thickens as secondary wall (S) forms inside of primary (P). d. Wall continues to thicken with a buildup of deposits. 3

4 Cell Wall Structure Cell walls are composed of microfibrils. Microfibril bundles of cellulose polymer chains and associated polysaccharides of other types that are united at some regions in highly ordered crystalline lattices know as crystallites and are less highly ordered in the zones between the crystallites (amorphous regions). The secondary wall is a laminated filamentary composite, with cellulose providing the structurally reinforcing network. These filaments are embedded in a matrix composed of hemicelluloses and lamellar lignin sheets bound together through chemical bonds. The secondary wall is composed of three distinct layers. S1 the first few (4-6) microfibrils (or lamellae), spiraled around the cell interior with the long axes of the microfibrils nearly perpendicular to or degrees from the long axis of the cell. S2 The next layer forms, with the spiraling angle changing to degrees. It is much thicker, from cells in earlywood to 150 or more in latewood. This layer has the greatest effect on how the cell behaves. S3 The last several layers are arranged similar to S1, with a degree angle to the long axis. The Pits Concentration of lignin is highest in S2 layer because bulk of the material is there - this layer is actually highly cellulosic Compound middle lamella is really thin, but contains ~10% of the cell s lignin - high concentration! 4

5 Wood Pits Pit a recess in the secondary wall of a cell, together with its external closing membrane; open internally to the lumen. Normally, pit placement in one cell is exactly matched by the position of pits in adjoining cells, called Pit Pairs. This provides the primary avenues of cell lumen to cell lumen transport. Pit Parts Pit Aperture opening of a pit into the cell lumen to the chamber; pit orifice. Pit Membrane that portion of the compound middle lamella which closes a pit cavity externally Pit Types Simple Pit a pit in which the cavity becomes wider, or remains of constant width, or only gradually narrows toward the cell lumen, during the growth in thickness of the secondary wall. Typically Parenchyma Cells Pit Types - Bordered Bordered Pit Pit with an overhanging margin, or a pit in which the cavity becomes abruptly constricted during the thickening of the secondary wall. Typically nonparenchyma cells Margo the outer ring of the pit membrane lying between the torus and the pit boarder in bordered pits of conifers. Torus central thickened portion of the pit membrane of a bordered pit. Bordered Pits Bordered Pits 5

6 Half-Bordered Pits Half-Bordered Pit a pairing of a bordered pit and a simple pit in adjacent cells. Often times when a parenchyma cell contacts a conductive cell. Bordered pits in hardwoods do not have a torus and there is no dissolution of portions of the primary wall. Pits Pits There is no evidence to suggest that pit regions substantially reduce fiber strength in cell walls. The local variation in microfibers around the pits can affect directional shrinkage properties of wood. Spiral Thickening ridges on the inner face of the secondary cell wall in the form of single or multiple helices around the cell axis, characteristic of Douglas-fir and yew and many hardwood species. Spiral Thickening Spiral Thickening Wood Anatomy GeeWiz for a 1 in. Cube of Pine Wood Has 5 million longitudinal tracheids, each ~ 3.5 mm long If LT s laid end-to-end, distance covered would be about 10 miles Approximately 300 million bordered pits [bordered pits connect longitudinal tracheids] Approximately 1 billion cross-field pits [cross field pits-passageways between longitudinal tracheid and ray parenchyma] 6