Polysaccharides are the large biological molecules that belong to the category of carbohydrates. They are the biological polymers made up of hundreds or thousands of monosaccharide subunits attached via glycosidic bonds. According to the definition, polysaccharides are the carbohydrates that contain more than ten monosaccharide subunits. Upon degradation, they yield individual monosaccharides. Commonly occurring polysaccharides are starch, glycogen, cellulose, and hemicellulose. In this section of notes, we will try to understand the structure, properties, and functions of polysaccharides using these examples.
StarchIt is a common storage polysaccharide present in plants. It is also known as amylum. StructureStarch is a polysaccharide made up of glucose subunits that are linked via glycosidic bonds. The glucose subunits combine to form two polymer components that together form starch. These polymer components of starch differ in their structure and properties. These include; AmyloseAmylose make up of around 20-30% of the total starch. The structure and properties of amylose are mentioned below. StructureAmylose is made up of long chains of glucose molecules bound together via alpha 1-4 glycosidic linkages. This means that all the glucose molecules in long chains are in alpha orientation with the first carbon of one molecule attached to the fourth carbon of the next molecule via glycosidic bonds. The number of the glucose subunits range from 300 to 3000 or even more. The long chain of amylose can fold onto itself or can bind with other hydrophobic compounds. When the chain of amylose folds onto itself, a spiral-shaped structure is formed known as a helix. The helix is kept in shape by the hydrogen bonds between the hydrogen atoms and the hydroxyl groups of glucose molecules. The amylose chain can also bind hydrophobic molecules using hydrogen bonds. This is the way amylose is bound to amylopectin in starch. PropertiesAmylose component of starch has the following properties;
AmylopectinAmylopectin makes around 70-80% of total starch. Its structure and properties are discussed below. StructureAmylopectin is also a polymer of glucose molecules linked together via alpha 1-4 glycosidic bonds just like amylose. The main difference between the structure of amylose and amylopectin is of branching. The amylopectin component of starch shows extensive branching. A branch breaks away from the original chain of glucose molecules after every 24 to 30 subunits. The glucose molecule at each branch point is attached to the original chain via an alpha 1-6 glycosidic bond. the rest of the glucose subunits in the branch are attached via the same alpha 1-4 glycosidic bonds as in the original chain. Each polysaccharide branch in amylopectin also contains 24 to 30 glucose subunits. PropertiesThe properties of amylopectin are also different from amylose.
Both these components are joined together to form starch. SourcesStarch is the main storage carbohydrate in plants. They store glucose in the form of starch granules. It is present in grains, roots, tubers, fruits, etc. SynthesisPlants synthesize starch from glucose molecules that are made by the process of photosynthesis. The alpha 1-4 glycosidic bond is formed between the glucose molecules by starch synthase enzyme. Another enzyme called starch branching enzyme adds the alpha 1-6 glycosidic bond at branch points. All this process uses energy in the form of ATP. DegradationStarch undergoes hydrolysis into glucose molecules. In the human digestive system, starch is digested by amylase enzyme into smaller molecules. It yields maltose and glucose molecules. Maltose is further digested by maltase to form glucose. UsesFollowing are the uses of starch;
GlycogenGlycogen is the storage polysaccharide in animals. It is also known as animal starch. StructureGlycogen is a branched polymer of glucose molecules. Its structure is similar to that of amylopectin. The individual glucose molecules in the chain are linked via alpha 1-4 glycosidic bonds. Branching takes place after every 8 to 12 subunits with each new branch containing 8-12 glucose molecules. Each branch is linked to the main chain via the alpha 1-6 glycosidic bond. All the glucose molecules have alpha orientation. Each branch has a free glucose molecule at its end known as the reducing end. The glycogen molecules are arranged in the form of granules. These granules have a protein at their core known as glycogenin protein. Properties
SourcesGlycogen is animal in origin. Humans and other animals store glucose in the form of glycogen. It is present abundantly in the muscles and liver of animals. However, in small amounts, it is present in all animal cells. SynthesisGlycogen is synthesized in our bodies in a process known as glycogenesis. It is an energy-consuming process in which extra glucose molecules in cells are stored in the form of glycogen. An enzyme called the glycogen synthase makes alpha 1-4 glycosidic bonds between the glucose molecules to make straight chains of glucose. Another enzyme called glycogen branching enzyme removes a chain of six to seven glucose molecules from the reducing end of the original chain and adds this chain to a non-reducing glucose molecule by alpha 1-6 glycosidic bond. New glucose molecules are added to the reducing ends of both ends and the process repeats. Remember that the end of the chain containing the last added glucose molecule is called the reducing end. DegradationThe breakdown of glycogen occurs by a process known as glycogenolysis. The process begins at the non-reducing end of the chain. This means that the glucose molecule added at the beginning of the process is cleaved first. The enzyme involved in this process is known as glycogen phosphorylase. This enzyme cleaves the alpha 1-4 glycosidic bonds and releases glucose molecules in the form of glucose-1-phosphate. A special enzyme known as glycogen debranching enzyme is required for breaking the alpha 1-6 glycosidic bonds. Importance
CelluloseIt is the most abundant polysaccharide present in nature. Cellulose is a structural polysaccharide that is only present in plants. It is not present in animal cells. StructureLike the other two polysaccharides discussed above, cellulose is also made up of glucose subunits. However, it differs with respect to the glycosidic bonds present among the glucose subunits. In cellulose, the glucose subunits are linked together via beta 1-4 glycosidic bonds. All the glucose molecules in cellulose are in beta orientation meaning that the OH groups of the anomeric carbons (carbon number 1) are oriented upwards. In order to make the beta 1-4 glycosidic bonds, the successive glucose molecules should be arranged opposite to each other because the OH group on carbon 1 is above the ring while that on the carbon 4 is below the ring. Thus, every second glucose molecule in the structure of glucose is inverted at 180 degrees. Unlike starch and glycogen, the chains of glucose subunits do not show branching in cellulose. The glucose chains also do not form a ring or any helical structure. Rather, these chains are arranged parallel to each other in cellulose. The glucose chains in cellulose are held side by side through hydrogen bonds that are formed between the hydrogen atoms and the abundant hydroxyl groups attached to each carbon of glucose. In this way, these chains from strong microfibrils within cellulose. In living structure, these microfibrils of cellulose are arranged in the form of a polysaccharide matrix. PropertiesFollowing are the properties of cellulose;
SourcesCellulose is a polysaccharide that is present in cell walls of plants. Humans consume cellulose in the form of green leafy vegetables that are abundant in fiber. It is also present in cell walls of algae and some fungi. Cellulose is also a component of biofilms secreted by some bacteria. DegradationAs mentioned in the properties, humans cannot digest cellulose. We do not have the enzyme required to break the beta 1-4 glycosidic bond between two glucose molecules. Therefore, the cellulose we consume in the form of green leafy plants is excreted in feces without digestion. However, cellulose can be digested in herbivores like cows, horses, sheep, goats, etc. because the symbiotic bacteria in these animals release the enzyme that can break the beta 1-4 glycosidic bonds. Importance
HemicelluloseHemicellulose is another important polysaccharide present in cell walls of plants. It is a heteropolymer meaning that it is made up of more than one type of monosaccharide subunits. In addition to glucose, it also contains arabinose, xylose, mannose, galactose, and some other monosaccharide. Hemicellulose is an amorphous solid having shorter chain lengths as compared to that of cellulose. It also shows branching of chains. It is not resistant to hydrolysis. It can yield monosaccharide subunits even in dilute acidic or basic solutions. Its main function is to cross-link the microfibrils made up of cellulose and glycoproteins. Polysaccharides are the biopolymers made up of repetitive monosaccharide subunits. They differ greatly in their structure, properties, and functions. Polysaccharide that are important include starch, glycogen, cellulose, and hemicellulose. All of polysaccharides are non-reducing sugars. Starch is storage polysaccharide in plants made up of two components, amylose and amylopectin.
Starch is the main carbohydrate source of animals as it is present in every staple food. Glycogen is storage polysaccharide in animals, made of glucose chains that show branching after every 8 to 12 subunits.
Cellulose is a structural polysaccharide present in the plant cell walls.
Hemicellulose is another polysaccharide present in cell walls of plants.
Frequently Asked QuestionsWhat are polysaccharides?Polysaccharides are the polymers of carbohydrates. They are large molecules that are made by combining hundreds or thousands of carbohydrate molecules. What are some examples of polysaccharides?Some examples of polysaccharides include starch, glycogen and cellulose. Starch is a storage polysaccharide found in plant cells while glycogen is a storage polysaccharide present in animal cells. Cellulose is present in the cell wall of plant cells. What are the functions of polysaccharides?Polysaccharides act as storage molecules in both animal and plant cells. They also provide structural support such as cellulose and have a role in cell signalling. Can humans digest polysaccharides?Humans can digest and break down starch. However, cellulose cannot be digested in humans because of lack of cellulase enzyme. References
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