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Structure and Function of Homopolysaccharides

In this structure and function of homopolysaccharides post we have briefly explained about cellulose, chitin, starch, glycogen, and xylan homopolysaccharide.

Polysaccharides (glycans) are long chains of monosaccharides. Each monosaccharide is connected together via glycosidic bonds to form the polymeric structure known as homopolysaccharide. Polysaccharides are the largest component of biomass. It is estimated that more than 90% of the carbohydrate mass in nature is in the form of polysaccharides.

A typical homopolysaccharide is defined to have only one type of monosaccharide units in the chain; whereas, a heteropolysaccharide is composed of two or more types of monosaccharides. In both types of polysaccharide, the monosaccharide can link in a linear fashion or they can branch out into complex formations.

Structure and Function of Homopolysaccharides

Cellulose

Cellulose is one of the most abundant biomaterials on the earth. It is generally synthesized by plants, but it is also produced by some bacteria. Cellulose is a tough, fibrous, and water insoluble homopolysaccharide, primarily found in cell walls of plants. It plays an integral role in keeping the structure of plant cell walls stable.

Homopolysaccharide

Chemical Structure of Cellulose

Cellulose is a homopolysaccharide of β-D-glucose joined by β-1,4 linkages. Like amylose, cellulose molecule is linear, unbranched homopolysaccharide, consists of 10,000- 15,000 thousand β-D-glucose units joined by glycosidic linkage. The β configurations of cellulose allow the formation of very long, straight chains which is optimal for construction of fibers having high tensile strength.

Most animal cannot use cellulose as a fuel source because they lack an enzyme to hydrolyse the β-1,4 linkages. However, cellulose is still an important constituent of our diet as a component of dietary fibers. Termites readily digest cellulose (and therefore wood), because their intestinal tract harbours a symbiotic microorganism (Trichonympha), that secretes cellulose, which hydrolyses the β-1,4 linkages. Wood-rot fungi and bacteria also produce cellulose.

Chitin

Is chitin a polysaccharide? Yes! Chitin is one of the most important biopolymers in nature. It is mainly produced by fungi, arthropods and nematodes. Chitin is the principle component of the hard exoskeletons of nearly a million species of arthropods, insects, lobsters and crabs. It is the second most abundant polysaccharides next to cellulose present in the biosphere. In insects, it functions as scaffold material, supporting the cuticles of the epidermis and trachea.

Homopolysaccharide

Chemical Structure of Chitin

Chemically, chitin is a linear homopolysaccharide polymer composed of N-acetyl-β-D-glucosamine residues as monomer linked by glycosidic bond (β-1,4 linkages). The only chemical difference from cellulose is the replacement of the hydroxyl group at C2 position with an acetylated amino group. Chitin polymers tend to form microfibrils (also referred to as rods or crystallites) of ∼3 nm in diameter that are stabilized by hydrogen bonds formed between the amine and carbonyl groups.

Starch

Starch is a homopolysaccharide comprised of glucose monomer units joined by glycosidic linkage. Starch is the most important storage polysaccharide or nutritional reservoir of plant cells. The starch molecules are present inside the plant cells and exist as large cluster or granules. More than half of the carbohydrates ingested by humans are in the form of starch.

The starch molecules are heavily hydrated because they have many exposed hydroxyl groups available to make hydrogen bond with water molecule. Starch exists in two forms (amylose and amylopectin), both are made up of glucose monomers.

Structure and Function of Homopolysaccharides

(i) Amylose

It is an unbranched type of starch, consists of α-D-glucose monomers joined by α-1, 4 glycosidic linkages. Amylose contain is single chain of several thousand D-glucose residues and length of the chain can vary and hence the molecular weight of amylose can be from few thousand to more than a million Daltons (50-5000 units of glucose).

Homopolysaccharide

Chemical Structure of Amylose

(ii) Amylopectin

It is the branched type of starch. The α-D-glucose monomers units are linked in a chain similar to amylose by glycosidic bond (α-1, 4). The branching is originated from the chain by (α-1, 6) glycosidic linkage on an average after every 24-30 residues. The molecular weight of amylopectin can be upto 200 million Daltons or contains upto 106 monosaccharide units of α-D-glucose.

Chemical Structure of Amylopectin

Glycogen

Glycogen is main storage polysaccharide molecule of animal cells. Structurally, glycogen is like amylopectin, the only difference between them is the extent of branching. The glycogen is extensively branched compared to amylopectin and with every 8-12 residue the new branch emerges from the glycogen chain.

Homopolysaccharide

Chemical Structure of Glycogen

Glycogen is the polymer of α-D-glucose connected by glycosidic linkage (α-1, 4). The branching point has (α-1, 6) linkage. Each glycogen molecule has one reducing end and many non-reducing ends. Glycogen is present in all cell types and functions as glucose reserve. Glycogen is abundantly present in liver and muscle cells and stored as large granules. These large granules are comprised of cluster of smaller granules containing glycogen molecules with an average molecular weight of several million Daltons. These granules also contain enzymes responsible for synthesis and degradation of glycogen.

Branched sites allow several sites for simultaneous synthesis and degradation. Branching speeds up the process of degradation. Branching makes glycogen an efficient way to store glucose because it provides compactness as well as the accessibility to enzymes. Because glycogen is insoluble and contributes little to the osmolarity of the cytosol. If all glycogen stored in liver cells are converted to glucose the osmolarity will change to 0.4M. At this osmolality the cell will rupture because of osmotic entry of water inside the cell.

The glucose concentration in blood is approx. 5mM therefore the uptake of glucose inside the cell will be difficult due to higher concentration of glucose inside the cytosol (0.4M). Due to these reasons the glucose is stored inside the cells as glycogen. When cells need glucose they enzymatically extract glucose monomers from glycogen without changing the overall osmolality of the cells.

Xylan

Homopolysaccharide

Chemical Structure of xylan

Xylans are a class of polymers with a structure similar to cellulose, but with xylose as the repeating unit. The most basic representative contains only D-xylose with -1-4 linkages and is found in plant walls. Several heteropolysaccharides have xylan backbones and various saccharides as branches. In plant cell walls, xylans are frequently associated with cellulose.

Further Readings

Reference

Structure and Function of Homopolysaccharides