What are Polysaccharides and What are Their Different Types?
Just like disaccharides and monosaccharides, Polysaccharides also play important roles in our lives. To know more about them in detail, we have come up with this article. In this article, you will learn about the different types of Polysaccharides and their structure.
You must have already heard about simple and complex carbohydrates. Starch is the most common type of complex carbohydrate which is present in the food you consume. So, these complex carbohydrates consist of severely branched molecular structures. They are nothing but the Polysaccharide itself. They are complex carbohydrates formed with a chain of monosaccharides. The bonds that keep the chain together are glycosidic. Some common Polysaccharides examples are starch, glycogen, and cellulose. Homopolysaccharides and Heteropolysaccharides are the two fundamental types of Polysaccharides. In this article, you can learn about Polysaccharides in detail, their structure, types, and examples and so on.
What is a Polysaccharide?
Polysaccharides are nothing but complex carbohydrates, formed with a chain of monosaccharides. Glycosidic linkage is what keeps the chain of monosaccharides bonded together. You can say that a molecule of a Polysaccharide has a certain number of sugar molecules that come together to form a larger molecule. You can also call them Glycans. Polysaccharides can also be defined as the long polymers of carbohydrates that are made up of repeating mono- or di-saccharide units joined by glycosidic connections (e.g., glucose, fructose, galactose). They might be linear or very branching in structure. Storage Polysaccharides like starch and glycogen, as well as structural Polysaccharides like cellulose and chitin, are just a few examples. More than 10 monosaccharide units make up a Polysaccharide. Personal preferences differ on how large a carbohydrate must be to be classified as Polysaccharides or oligosaccharides. Polysaccharides are classified into two parts, which are Homopolysaccharides and Heteropolysaccharides. Below you can learn more about the same.
Types of Polysaccharides
Homopolysaccharide: In this type, molecules get formed with a single type of monosaccharides. You can determine a general theme for such molecules by studying their monosaccharide units. When Homopolysaccharide gets formed using nothing except glucose molecules, then you can call them The roots of Polysaccharide that lucans. If it uses galactose molecules while forming, then you may call it Galactus. In the topics below, you can find plenty of explanations about Glucans.
Heteropolysaccharide: These are Polysaccharide molecules consisting of more than a single kind of monosaccharides. Hyaluronic acid, heparin, and chondroitin sulfate are some common examples of Heteropolysaccharides.
Structure of Polysaccharides:
Following is how a typical Polysaccharide structure looks –
Every Polysaccharide gets formed with the same process. In which the monosaccharides form a chain using glycosidic bonds. These bonds have oxygen molecules bridging the two carbon rings. The bond gets created when the carbon of one molecule loses the hydroxyl group, and the hydroxyl group of a different monosaccharide loses the hydrogen. An oxygen molecule connects two carbon rings in these glycosidic linkages. Since two molecules of hydrogen and one molecule of oxygen get expelled, it becomes a dehydration reaction. In other words, the bond is produced when a hydroxyl group is removed from one molecule's carbon, and the hydrogen is removed from another monosaccharide's hydroxyl group. The obtained structure of molecules tells you about the various properties and structures of the final Polysaccharide. A Polysaccharide utilized for energy storage will allow simple access to the constituent monosaccharides, but a Polysaccharide used for support will typically be a lengthy chain of monosaccharides forming fibrous structures.
Polysaccharide Classification
Depending on the monosaccharide components, Polysaccharides can be Homopolysaccharides or Heteropolysaccharides. A Homopolysaccharide (also known as homoglycan) has only one form of monosaccharide, whereas a Heteropolysaccharide (also known as heteroglycan) contains multiple types of monosaccharides.
Polysaccharides are classed as storage or structural Polysaccharides based on their function. Polysaccharides that are employed for storage are known as storage Polysaccharides. Plants, for example, store glucose in the form of starch. Glycogen is the storage form for simple sugars in animals. Carbohydrates with a structural purpose are known as structural Polysaccharides. celluloses, which are polymers of repeating glucose units connected by beta-linkages, are found in plants. Chitin is produced by certain animals and is used as a structural component of exoskeletons, for example.
Polysaccharide Examples
Now that you have learned what a Polysaccharide and its structure is, it’s time to learn about Polysaccharide examples. Naturally, there are three main Polysaccharides. You come across them every day in your life, and they are starch, glycogen, and cellulose. Below you can read about them in detail.
Starch
You can find starch in all photosynthetic plants. The roots and seeds of the plant have more starch in general. When plants synthesize glucose, excessive glucose gets stored in the form of starch. It consists of a linked glucose molecule, and that’s why it’s a glucan.
The molecular formula for starch is (C6H10O5)n. Here, the ‘n’ indicates the number of linked molecules. Below is a typical structure of starch.
You can find starch in the seeds of plants as granules. Upon heating those granules in water, you get a colloidal suspension. Further, you get two components from the process, Amylose and Amylopectin.
Glycogen
Glycogen is a glucan, which exclusively contains D-glucose units. It acts as a reserved source of carbohydrates for plants as well as animals. Below you can see the structure and function of glycogen.
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The structure of glycogen is quite similar to amylopectin. However, the branching takes place more frequently in a glycogen molecule. As you can see, it is indeed a larger molecule. Glucose molecules are small in size, and they can diffuse out of a cell membrane. Since glycogen is bigger, it doesn’t diffuse out of the cell membrane. It serves the essential function of storing glucose within cells.
Cellulose
Cellulose is a crucial structural component of the cell walls of plants that are photosynthetic. It is fibrous in nature and highly insoluble in water too. Keep in mind that cellulose is a glucan. The D-glucose units have connections in the (1 – 4) fusion. It is a beta linkage, and it’s indeed different from the glycogen.
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The structure of the cellulose –OH group points outwards from the chain structure. When two chains come in contact with each other, they get stacked on each other because of the hydrogen bonding between hydroxyl groups. In the end, you obtain an insoluble fibrous structure that is perfect for the functions of cellulose in the cell walls.
Differences and Similarities between Chitin and Cellulose
Both cellulose and chitin are long-chain polymers of thousands of glucose monomers strung together in long strands. The side-chains connected to the carbon rings of the monosaccharides are the only difference between the two Polysaccharides. The glucose monosaccharides in chitin have been changed to include a group with additional carbon, nitrogen, and oxygen. A dipole is created by the side chain, which promotes hydrogen bonding. Cellulose is known to form hard materials but chitin can form even harder materials.
Long, linear chains form from both Polysaccharides. Long fibers are formed by these chains and are deposited outside of the cell membrane. The fibers weave into a complicated structure that is maintained in place by hydrogen bonds between side chains thanks to the help of certain proteins and other factors. As a result, simple glucose molecules that were previously used for energy storage can be transformed into molecules with more structural stiffness. The monosaccharides used change relatively slightly between structural and storage Polysaccharides. Instead of a structural Polysaccharide, glucose molecules can branch and store many more bonds in a smaller space by modifying the shape of the molecule. The configuration of the glucose used is the only difference between cellulose and starch.
FAQs on Polysaccharides
1. What are polysaccharides and what are some common examples?
Polysaccharides are large, complex carbohydrates, also known as glycans, formed by the linking of many smaller monosaccharide units through glycosidic bonds. They are essentially polymers of sugars. Key examples found in nature include starch and cellulose in plants, and glycogen in animals.
2. What is the fundamental difference between homopolysaccharides and heteropolysaccharides?
The fundamental difference lies in their constituent monosaccharide units. A homopolysaccharide is composed of only a single type of monosaccharide unit repeated throughout its chain (e.g., starch is made only of glucose). In contrast, a heteropolysaccharide is made of two or more different types of monosaccharide units (e.g., hyaluronic acid, found in connective tissues).
3. What are the main functions of polysaccharides in living organisms?
Polysaccharides serve two primary functions in living organisms:
- Energy Storage: They act as a compact, long-term reserve of energy. For example, starch in plants and glycogen in animals and fungi are broken down into glucose when energy is required.
- Structural Support: They provide rigidity and shape to cells and organisms. For example, cellulose forms the strong, fibrous framework of plant cell walls, and chitin forms the hard exoskeleton of arthropods.
4. How is the glycosidic linkage formed in a polysaccharide?
A glycosidic linkage is formed through a dehydration synthesis (or condensation) reaction. In this process, a hydroxyl (-OH) group from one monosaccharide and a hydrogen atom from another are removed, forming a molecule of water (H₂O). This allows a covalent bond to form between the two monosaccharide units, linking them via an oxygen atom (C-O-C bridge).
5. Why can humans digest starch but not cellulose, even though both are polymers of glucose?
The difference is due to the type of glycosidic bonds. Starch is composed of glucose units linked by α-1,4 glycosidic bonds, which human digestive enzymes like amylase can easily break down. Cellulose, on the other hand, has β-1,4 glycosidic bonds. Humans lack the specific enzyme, cellulase, required to hydrolyze these β-linkages, making cellulose indigestible dietary fibre.
6. How does the structure of glycogen make it suitable for rapid energy release in animals?
Glycogen’s suitability for energy storage comes from its highly branched structure. This branching creates numerous terminal ends on a single molecule. When the body needs energy quickly, enzymes can act on many of these ends simultaneously, rapidly hydrolyzing them to release a large amount of glucose into the bloodstream. This is far more efficient than releasing glucose from a single end of a linear molecule.
7. Are polysaccharides considered reducing or non-reducing sugars?
Polysaccharides like starch and cellulose are generally considered non-reducing sugars. A reducing sugar must have a free anomeric carbon with a hydroxyl group that can be oxidized. While a polysaccharide chain has one reducing end, the vast majority of its monosaccharide units have their anomeric carbons tied up in glycosidic bonds. Therefore, the overall molecule exhibits negligible reducing properties.
8. What key structural feature distinguishes storage polysaccharides from structural polysaccharides?
The key feature is the geometry of their glycosidic bonds, which dictates their three-dimensional shape.
- Storage Polysaccharides (e.g., starch): Use α-glycosidic linkages, which cause the polymer chain to form a loose, helical structure. This is compact and ideal for packing glucose units for storage.
- Structural Polysaccharides (e.g., cellulose): Use β-glycosidic linkages, which result in long, straight, and unbranched chains. These linear chains can align parallel to each other, forming strong intermolecular hydrogen bonds that create rigid, stable fibres for structural support.
