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2.6.1.2. Structural homopolysaccharides

Structural homopolysaccharides

As has already been said, in this type of polysaccharides the O-glucosidic bonds are difficult to hydrolyze because they are β-glucosidic, which allows them to constitute biological structures resistant to alteration.

Cellulose

Cellulose is a polysaccharide with a skeletal function typical of plants. It is the main element of the cell wall, which surrounds the cell, and persists after its death. Vegetable fibers (cotton, flax, hemp, esparto grass, etc.) and the interior of the tree trunk (log or wood) are mainly formed by cellulosic walls of dead cells.

Cellulose is a polymer of β-D-glucopyranose linked by β bonds (1→4). Two glucoses form a cellobioseEach polymer has 150 to 5,000 cellobiose molecules, and an average molecular weight of 800,000. These polymer chains are unbranched, but they can be arranged in parallel by joining via hydrogen bonding, in highly ordered crystalline aggregates.

Humans cannot digest cellulose because our digestive enzymes cannot break the β bond. Many microorganisms and some invertebrates (Lepisma saccharina or silverfish, and Teredo navalis or wood-boring mollusk) are capable of secreting the enzyme cellulase, which can break the β bond. Xylophagous insects, such as termites, and ruminant herbivores (cow, sheep, goat, camel) can take advantage of cellulose because symbiotic microorganisms  in the digestive tract produce cellulase. Non-ruminant herbivores, such as horses, have cellulose residues in their feces.

The pulp is an important nutrient for many animals, although most species require the help of organisms synbiotics to produce cellulase, the enzyme capable of breaking the β glycosidic bond .

 

Chitin

It is a structural polysaccharide, a fundamental component of the exoskeleton of arthropods ( insects , crustaceans , etc.). It is also part of the cell coatings of fungi. The constituent monomer is a derivative of glucose (N-acetyl-β-D-glucosamine). The union between them is made by β bonds (1 → 4), which gives rise to a linear chain, similar to cellulose . It is also not digestible by animals.