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      Figure 2.2 Figure illustrates different polysaccharides molecules in the algal cell wall. (A) is alginate, having β (1–4) linked D-mannuronic acid subunits with epimerically attached α-L-guluronic acid at C-5 position. (B) Fucoidan is sulfated α (1–3) (1–4)-linked un sulfated and 2-sulfated-α-L-fucose residues. (C) Sulfated galactans which composed of 3-sulfated with 4-linked α-D(L)-galactopyranosyl β-D-sand subunits [62, 63].

Red algae glycans are structurally different among the species depends upon harvesting season and location. Functionally, the algae produce sulfated polysaccharides for the adaptation to high salinity marine environment and it’s also found in the other marine organisms like, angiosperms and invertebrates. During high saline condition it for 3D hydrogel network that can retain a large amount of water, thus protect the macroalgal from desiccation. Moreover, the attached sulfated group have natural tendency to retain the divalent metals such as, Ca²⁺ and Mg²⁺ [33].

      2.3.3 Fucoidans

      Fucoidans is also the group of sulfated hetero- and homopolysaccharides especially in the cell wall of brown algae. Compositionally, simple fucoidans is homofucans which composed of only L-fucose residues (Figure 2.2C). Some studies have been also reviewed that fucoidans is fucose containing heteropolysaccharides with complex structures. The group of fucoidans polysaccharides are ranging from the compounds having high amount of uronic acids and low amount of fucose sugar. Further, the fucoidans also have mannose, xylose, galactose, uronic acid along acetyl factionalized. Sulfated homofucans is exclusively composed of sulfated fucopyranose with small content >10% of other sugars such as, D-mannose, D-xylose, D-galactose and very small content of uronic acid. Other hand, the heterogenous sulfated fucans is consist of sargassans and ascophyllans with small amount of fucose. In another heterogenous fucans polysaccharides have galactose and fructose with equivalent amount, thus known as galactofucans and fructofucans, respectively. Although, the fucoidans polysaccharides have not been found in other algal species and vascular plants, but fucan sulfates present in some marine echinoderms but its unlike that of algal fucoidans [32, 38].

      Functionally, fucoidans help in algal cell wall formation by supporting their structures. It also contributes in gametes formation from reproductive organs and extrusion spores. The presence of sulfonate group in fucoidans polysaccharides, it contributes exchanging of divalent cations such as, Ca²⁺, Mg²⁺, Na⁺, etc. with environments by adjusting the algae with high concentration of salts and toxic effects of the heavy metals. It also plays pivotal role in zygote morphogenesis of fucoid algae [39].

2.4 Fungal Cell Wall Polysaccharides

Fungal cell wall has dynamic structure which is essential for morphogenesis, viability and act as barrier to protect the fungi form harsh environmental conditions. The fugal cell wall secretes enzymes which degrade the host cell/tissues for invasion to obtain nutrient that required for their growth. More interestingly, the fungal cell wall has environmental sensors to detect the environmental condition and thus fungi resist itself to high osmotic pressure and toxic compounds. Structurally, the fungal cell wall has two layers. The inner layer is consisting of β (1–3), (1–6) chitin and glucan which bound to other polysaccharides and proteins. The outer layer is made of carbohydrate matrix ad it considered to be highly variable among the different fugal species (Figure 2.3). The fungal cell wall has role in adherence to host tissue, stimulation of host immunity and rescue the fungi from host immune defense. Further, the fungal cell act as target of choice for the designing of antifungal therapeutics and diagnostic tools. The fungal cell has approximately 90% of polysaccharides [40–43], which have briefly discussed below.

      The illustration depicts the cell walls of plant cell. The plant cell wall composed of cellulose microfibrils that have been cross linked with each other through glycans, pectin and other substances.f

      Figure 2.3 Describes the structural polysaccharides of fungal cell wall, which includes; alpha/beta glucan, that combined with Chitin and chitosan to form fungal cell wall [65].

      2.4.1 Glucan

      Glucans is of the excessively found in fungal cell wall, which is made of several glucose units. It named due their specific linkages and the carbon atoms that involve in their bonding. On the basis of their function and characteristics, glucan have two different types, alpha (α) glucan and beta (β) glucan. Alpha glucan has amorphous structure, water soluble and act as energy reservoir such as, starch of the plants and algae, glycogen of the fungi and animals. But the fungal α-glucan has microfibrillar structure and insoluble in water. Microfibril formed due to the bonding of several glucan chains with each other through hydrogen bonding. While β-glucan have crystalline structure thus insoluble in water. Plant cellulose is the type of β-glucan and different from that of the fungi which is β (1–3) linked. Glucan combined with chitin and form the structural component of the fungal cell wall. It excessively found in all fungal phyla but absent in Microsporidia [42, 44].

      2.4.2 Chitin and Chitosan

      Fungi cell wall is also composed of chitin that binds to the glucan to impart rigidity. Chitin is a polymer which is composed of β (1–4) linked N-acetylglucosamine. It is synthesized from the substrate known as UDP-N-acetylglucosamine by an enzyme called chitin synthase. Beside the fungal cell wall, chitin is also found in crustaceans and insects.

Chitosan is also found in the inner layer of fungal cell wall. But mostly chitin is deacylated to form chitosan. Structurally, chitosan is composed of β (1-4) N-acetylglucosamine and more than 50% glucosamine. Both chitin and chitosan provide structure and support to cell wall. The ratio of chitin/chitosan varies

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