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Polysaccharides

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Polysaccharides

A polysaccharide (poly- = “many”) refers to a long chain of monosaccharides linked by glycosidic bonds. The chain may be branched or unbranched, and it may contain different types of monosaccharides. The molecular weight may be 100,000 daltons or more depending on the number of monomers joined. Primary examples of polysaccharides include starch, glycogen, cellulose, and chitin.

Starch

Starch is the stored form of sugars in plants. It consists of a mixture of amylose and amylopectin (both polymers of glucose). Plants are able to synthesize glucose. Sometimes, they synthesize excess glucose, beyond their immediate energy needs. In such cases, plants store the excess glucose as starch in different parts, including roots and seeds. The starch in the seeds provides food for the embryo as it germinates. In fact, it can also act as a source of food for humans and animals.

Enzymes, such as salivary amylases, break down the starch that humans consume into smaller molecules, such as maltose and glucose. The cells can then absorb the glucose.

In general, α 1-4 or α 1-6 glycosidic bonds join glucose monomers together to form starch. The numbers 1-4 and 1-6 refer to the carbon number of the two residues that join to form the bond.

Amylose is a form of starch. As you can see in the image below, unbranched chains of glucose monomers (only α 1-4 linkages) form amylose. On the other hand, amylopectin, another form of starch, is a branched polysaccharide (α 1-6 linkages at the branch points).

Amylose and amylopectin are two different forms of starch. Amylose consists of unbranched chains of glucose monomers connected by α 1,4 glycosidic linkages. Amylopectin consists of branched chains of glucose monomers connected by α 1,4 and α 1,6 glycosidic linkages. Because of the way the subunits join, the glucose chains have a helical structure. In general, glycogen (not shown) is similar in structure to amylopectin but more highly branched. Image Attribution: OpenStax Biology

Glycogen

Glycogen is the storage form of glucose in humans and other vertebrates. It consists of monomers of glucose. Glycogen is the animal equivalent of starch. As a matter of fact, it is a highly branched molecule usually stored in liver and muscle cells. Whenever blood glucose levels decrease, glycogen is broken down to release glucose in a process known as glycogenolysis.

Cellulose

Cellulose is the most abundant natural biopolymer. The cell wall of plants is consists mostly of cellulose. This provides structural support to the cell. Wood and paper are mostly cellulosic in nature. Cellulose is consists of glucose monomers linked by β 1-4 glycosidic bonds (see image below).

In cellulose, glucose monomers are linked in unbranched chains by β 1-4 glycosidic linkages. Because of the way the glucose subunits join, every glucose monomer flips relative to the next one. This results in a linear, fibrous structure. Image Attribution: OpenStax Biology

As you can see in the image above, every other glucose monomer in cellulose flips over, and the monomers pack tightly as extended long chains. This gives cellulose its rigidity and high tensile strength—which is so important to plant cells.

Human digestive enzymes cannot break down the β 1-4 linkage. However, herbivores such as cows, koalas, and buffalos can, with the help of the specialized flora in their stomach, digest plant material that is rich in cellulose and use it as a food source. In these animals, certain species of bacteria and protists reside in the rumen (part of the digestive system of herbivores) and secrete the enzyme cellulase.

The appendix of grazing animals also contains bacteria that digest cellulose, giving it an important role in the digestive systems of ruminants. Cellulases can break down cellulose into glucose monomers. The animal can then use the glucose as an energy source. Termites are also able to break down cellulose because of the presence of other organisms in their bodies that secrete cellulases.

Carbohydrate functions in different animals

Carbohydrates serve various functions in different animals. Arthropods (insects, crustaceans, and others) have an outer skeleton, called the exoskeleton, which protects their internal body parts (as you can see in the bee in the image below).

Insects have a hard outer exoskeleton made of chitin, a type of polysaccharide. Image Attribution: Louise Docker / OpenStax Biology

This exoskeleton is made of the biological macromolecule chitin, which is a polysaccharide-containing nitrogen. It consists of repeating units of N-acetyl-β-d-glucosamine, a modified sugar. In fact, chitin is also a major component of fungal cell walls. Actually, fungi are neither animals nor plants and form a kingdom of their own in the domain Eukarya.