Water, cell wall

Agar occurs as a cell-wall constituent of the red marine algae Rho ophyceae, from which it is extracted by hot water, and marketed as a dry powder, flakes, or strips. It dissolves in hot water and sets on cooling to a jelly at a concentration as low as 0-5%. Its chief uses are as a solid medium for cultivating micro-organisms, as a thickener, emulsion stabilizer in the food industry and as a laxative. 

Surfaces can be active in inducing blood clotting, and there is much current searching for thromboresistant synthetic materials for use in surgical repair of blood vessels (see Ref. 111). It may be important that a protective protein film be strongly adsorbed [112]. The role of water structure in cell-wall interactions may be quite important as well [113]. 

Silicon is important to plant and animal life. Diatoms in both fresh and salt water extract Silica from the water to build their cell walls. Silica is present in the ashes of plants and in the human skeleton. Silicon is an important ingredient in steel silicon carbide is one of the most important abrasives and has been used in lasers to produce coherent light of 4560 A. 

The anatomical stmcture of wood affects strength properties, appearance, resistance to penetration by water and chemicals, resistance to decay, pulp quabty, and the chemical reactivity of wood (5). To use wood most effectively requires a knowledge of not only the amounts of various substances that make up wood, but also how those substances are distributed ia the cell walls. 

In green wood, the cell walls are saturated, whereas some cell cavities are completely filled and others may be completely empty. Moisture ia the cell walls is called bound, hygroscopic, or adsorbed water. Moisture ia the cell cavities is called free or capillary water. The distiaction is made because, under ordinary conditions, the removal of the free water has Htde or no effect on many wood properties. On the other hand, the removal of the cell wall water has a pronounced effect. 

FIGURE 11.22 If the cell walls of bacteria such as Escherichia coli are partially digested and the cells are then osmotlcally shocked by dilution with water, the contents of the cells are extruded to the exterior. In electron micrographs, the most obvious extruded component is the bacterial chromosome, shown here surrounding the cell. (Dr. Gopal Murti/CNRI/Phototakr NYC)... 

Lysozyme, extracted from egg whites, is an enzyme that cleaves bacterial cell walls. A 20.0-mg sample of this enzyme is dissolved in enough water to make 225 mL of solution. At 23°C the solution has an osmotic pressure of 0.118 mm Hg. Estimate the molar mass oflysozyme. 

Xyloglucans are classified as gum when they are extractable with hot water from seed endosperm cell walls, such as the tamarind seed xyloglucan, and as hemicelluloses because they are alkali-extractable from the cell walls of vegetative plant tissues where they are closely associated with cellulose [2]. Also /3-glucans with mixed linkages appear under the name gum as well as hemicellulose in the literature. 

AGX are also the dominant hemicelluloses in the cell walls of hgnified supporting tissues of grasses and cereals. They were isolated from sisal, corncobs and the straw from various wheat species [4]. A more recent study on corncob xylans [30] showed the presence of a hnear, water-insoluble polymer... 

From recent literature it is known that the disintegration of lignified cell walls can be achieved by steam explosion treatments resulting in solubilization of partially depolymerized hemicelluloses [91,92]. The application of this method on wheat bran yielded feruloylated GAX with different feruUc acid content [93]. Partly depolymerized water-soluble, acetylated AGX was obtained from spruce wood by employing microwave treatment [94]. 

AG type II is most abundant in the heartwood of the genus Larix and occurs as minor, water-soluble components in softwoods. Certain tree parts of western larch (I. occidentalis) were reported to contain up to 35% AG [378]. The polysaccharide is located in the lumen of the tracheids and ray cells. Consequently, it is not a cell-wall component and, by definition, not a true hemicellulose. However, it is commonly classified as such in the field of wood and pulping research. This motivated us to include the larch AG in the review. 

This is a very widely available polymer, since it is the main component of the cell walls of all plants. It is a carbohydrate of molecular formula (C5H q05), where n runs to thousands. The cellulose monomer is D-glucose, and the cellulose molecules are built up from this substance, effectively by condensation and removal of the elements of water. 

Cell enlargement occurs when a demand for water is created by relaxation of the cell walls under the influence of turgor pressure and wall-loosening factors. Water enters the cell down a water potential gradient, extending the cell walls (Lockhart, 1965 Boyer, 1985 Tomos, 1985). 

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