How can a polysaccharide play a structural role

Storage Starch is used as a storage polysaccharide in plants. In animals , the structurally similar glucose polymer is the more densely branched glycogen and is sometimes called animal starch. Structural Chitin and cellulose are examples of structural polysaccharides. Cellulose is used in the cell walls of plants and other organisms and is said to be the most abundant organic molecule on earth.

Chitin is found in arthropod exoskeletons and in the cell walls of some fungi. What roles do polysaccharides play in living things? Almost all carbohydrate polymers with degrees of polymerization DP less than 20 are soluble in water [ 7 ].

Solubility decreases with the increase of molecular weight. For example, the amylose and amylopectin in starch are reluctant to dissolve in cold water due to high molecular weight, while maltodextrin starch after chain cleavage by acid or enzyme with the DP value less than 20 demonstrates very good solubility in cold water.

The dissolution rate of polysaccharide samples is also highly affected by the molecular weight and molecular weight distribution. Higher molecular weight usually leads to lower dissolution rate, as disentanglement from the particle surface and subsequent diffusion to the bulk solution of large molecules take a longer time compared to that of small molecules.

It has also been reported that samples with high polydispersity dissolved about twice as fast as monodisperse ones of the same Mn [ 1 ]:.

Charged polysaccharides are referred to polysaccharides that carry charged groups in the molecules, which include both negatively acidic polysaccharides and positively charged polysaccharides. The charged groups help with the solubility of polysaccharides, which is achieved by 1 increasing the molecular affinity to water and 2 preventing the intermolecular association due to the electrostatic effects posed by the charged group.

Acidic polysaccharides are polysaccharides containing carboxyl groups e. The acidic group may be free or as a simple salt with sodium, potassium, calcium, or ammonium or naturally esterified with methanol. Therefore, most of the natural occurring pectin is readily soluble in water due to the charged group, although high in molecular weight.

It also should be noticed that adding salt or reducing pH value could shield the charged effect, which leads to gelation under some circumstances. For example, high methyl ester pectin gel at pH 3. Low methyl ester pectin can react with calcium ions to form gel, even under relative high pH environment.

Therefore, when dissolving the pectin into water, it is essential to avoid the gelling condition; similar to other hydrocolloids, the dissolution usually needs high shearing mixing [ 8 ]. Pectic polysaccharides from American ginseng. Adpated from Guo, et al. Adopted from Cui et al. As one typical positively charged polysaccharide Figure 4 , chitosan is derived from the deacetylation of chitin. The positively charged groups come from protonation of its free amino groups, which is the key to its water solubility.

Chitosan is insoluble in neutral and basic environments due to the lacking of a positive charge. However, in acidic environments, protonation of the amino groups increases the degree of water solubility. Following this property, chitosan has been widely used for drug delivery, e. Structural feature of chitosan. The linear polysaccharides with highly regular conformation that can form crystalline or partial crystalline structures are mostly insoluble in water, while branching structure could increase the solubility for two reasons: 1 the branching structure could weaken the intramolecular interaction due to the steric effects, which prevent the intermolecular association, and 2 the highly branched structure could also decrease the excluded volume when compared to polysaccharides with same molecular weight, which potentially increases the critical concentration and therefore improves the water solubility.

However, it can be modified by decreasing the Mw and introducing either charged sodium carboxymethyl cellulose CMC or branching groups methyl cellulose MC , hydroxylpropyl cellulose HPC , hydroxylpropyl methyl cellulose HPMC to increase the solubility Figure 5. Schematic chart for cellulose derivatives. Starch contains both amylose and amylopectin. Amylopectin exhibits better solubility than amylose due to the highly branched structure, although the latter has relative low molecular weight amylose, 10 5 ; amylopectin, 10 7 —10 9.

According to the structure and solubility difference, amylose and amylopectin can be separated from each other in starch granules according to the following procedure: firstly, starch granules are completely dispersed in hot water or aqueous dimethyl sulfoxide; amylose then can be precipitated by the addition of butanol as a crystalline complex due to the linear structure after cooling.

Afterward, amylopectin can be recovered from the supernatant by lyophilization [ 13 ]. Guar gum and locust bean gum both belong to the galactomannan family Figure 6 , while the degree of branching for guar gum galactose to mannose is higher than locust bean gum galactose to mannose about , which could easily prevent strong cohesion of the main backbones of different neighboring molecules, so that no extensive crystalline regions of guar gum can be formed, while locust bean gum is easy to form gel due to the naked region of the molecules, which favors the formation of junction zone [ 14 ].

Schematic chart for galactomannan structure. Xylans of all higher plants possess 1—4 linked D-xyl P residues as the backbone, substituted by various degrees with sugar units including arabinose, xylose, and glucuronic acid 4-O-methyl. However, with the increase of degree of substitution such as arabinose arabinoxylan as shown in Figure 7 , its solubility dramatically increased [ 15 ]. Schematic chart of arabinoxylan.

A: T-Ara P. Polysaccharides are very large, high molecular weight biological molecules that are almost pure carbohydrate. They are constructed by animals and plants from simpler, monosaccharide molecules, by joining together large numbers of the simpler molecules using glycosidic bonds -O-.

In some of the largest polysaccarhide structures there can be 10, individual units joined together. There is a large diversity of polysaccharide form; they can differ in the type of sugar, the connections between the sugars and the complexity of the overall molecule.

Sometimes known as glycans , there are three common and principal types of polysaccharide, cellulose, starch and glycogen , all made by joining together molecules of glucose in different ways. This molecule is synthesized, stored, modified and used as a building material by plants. It is certainly the most abundant of all the polysaccharides. A similar molecule called glycogen is found in animal cells that need to store a lot of energy, like muscle cells. Glycogen is a polymer of alpha-D-glucose, with frequent branches off of carbon six.

Since glycogen is even more dense than starch, it's a more efficient form of energy storage for organisms that move around. The important thing to remember about polysaccharides is the relationship between their structure and function. Polysaccharides generally perform one of two functions: energy storage or structural support.

Starch and glycogen are highly compact polymers that are used for energy storage. Cellulose and chitin are linear polymers that are used for structural support in plants and animals, respectively.