Cellulose, 1—»4-/3-links structure

The whole of a multi-cellular organism is contained by outer cell layers, which are described in biology texts, and maintained by connective tissue. Connective tissue is a novel, external biopolymer structure of multi-cellular organisms found within their new extracellular, circulating fluid compartments (see Section 8.9). As mentioned there, the main connective tissues, covalently cross-linked structures, are (1) those of plants, celluloses (polysaccharides), often cross-linked by lignin (2) those of lower animals and insects, mixed cross-linked polysaccharides and... 

Alginates, found in cell walls of some marine algae and also formed by certain bacteria, consist in part of a linear i-l,4-linked polymer of D-mannuronate with a cellulose-like structure. Alginates also contain... 

Both cellulose I structures have similar, complex hydrogen bonding systems that contain the 03—H- 05 hydrogen bond that is very typical for p-(l—>4)-linked molecules... 

Enzymes that can be harnessed for the breakdown of hemicellulose in cereal crops, and crop fiber biomass are becoming increasingly important because of their pivotal role in the utilization of these renewable energy sources. Hemicelluloses (xylans, aiabinoxylans) are widely found as stmctmal components in plant cell walls, where they cross-link with lignin and are extensively hydrogen-bonded to cellulose [1]. Structurally, xylans are heteropolysaccharides consisting of a linear P-d-(1— 4)-linked xylopyranoside backbone that. 

Figure 7.1 Typical chemical structure of celluloses. The structure of cellulose is composed of glucose monomers linked by (5-glycosidic bonds. Strong hydrogen bonds link glucose units between different polymer chains, leading to crystalline structures that are resistant to water.

Due to its cross-linked structure, lignin can be extracted from wood only by breaking up the initial network and a deterioration of its structure. Presently, industrial lignins (>50 x 10 tons per annum) are species exclusively obtained from a chemical treatment used in the manufacture of paper pulp or cellulose fibers. 

True xylans are relatively rare. The xylan from esparto grass gives only D-xylose on hydrolysis, and it has been shown to be a chain of p-D- i - 4)-linked D-xylopyranose units. It has a DP of about 75 and has a single (i 3)-linked branch point. The xylan backbone resembles cellulose in structure. Chemically combined D-glucuronic acid is the biochemical precursor of the D-xylose units forming the xylan. 

Cellulose consists of several thousand o-glucose units linked by l- 4-/3-glyco-side bonds like those in cellobiose. Different cellulose molecules then interact to form a large aggregate structure held together by hydrogen bonds. 

Glucose molecules can link together into chains, with each ring tethered to the next by a bridging oxygen atom. In one form, this is cellulose, the stiff material that gives the stalks of plants and the trunks of trees their structural strength. Chitin, a variation on cellulose, is an even stiffen material that forms the exoskeletons of crustaceans such as crabs and lobsters. 

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