Polysaccharides

Polysaccharides are complex carbohydrates made up of multiple sugar (monosaccharide) units linked together. They are essential macromolecules in biology and serve various functions in living organisms. Polysaccharides play a crucial role in energy storage, structural support, and cellular recognition. Let us have a glance over the topic of polysaccharides.

What is Polysaccharides?

A polysaccharide is a type of carbohydrate, specifically a complex carbohydrate, that consists of multiple monosaccharide (simple sugar) units linked together. These long chains of sugar molecules can be quite large and complex. Polysaccharides serve various important functions in living organisms, such as energy storage, structural support, and cellular recognition.

Polysaccharides have diverse functions in living organisms, including energy storage, structural support, and as components of important molecules like DNA and RNA. They are made up of long chains of monosaccharides (simple sugars) linked together by glycosidic bonds, and the specific arrangement and type of monosaccharides determine the properties and functions of the polysaccharide.

Common examples of polysaccharides include starch, glycogen, cellulose, and chitin.

Characteristics of Polysaccharides

Characteristics of polysaccharides are given below:

• Polysaccharides consist of hydrogen, oxygen and carbon. The ratio of hydrogen to oxygen is 2:1.
• High molecular weight carbohydrates.
• They are non-sweet.
• Insoluble in water.
• Hydrophobic nature.
• No crystal formation on desiccation.
• Compact inside the cells and osmotically inactive.

Types of Polysaccharides

Polysaccharide are categorized in two ways:

• Homopolysaccharides.
• Heteropolysaccharides.

Homopolysaccharides or Homoglycans

Homopolysaccharides, also known as homoglycans, are a specific class of polysaccharides composed of a single type of monosaccharide repeated throughout their structure. In other words, all the monosaccharide units in a homopolysaccharide are the same. This uniformity in monosaccharide composition distinguishes homopolysaccharides from heteropolysaccharides, which consist of more than one type of monosaccharide.

Some common examples of homopolysaccharides include

• Cellulose: Cellulose is a homopolysaccharide composed entirely of glucose units linked together by β-1,4-glycosidic bonds.
• Amylose: Amylose is a component of starch and is also a homopolysaccharide consisting of glucose units linked by α-1,4-glycosidic bonds
• Chitin: Chitin is a homopolysaccharide found in the exoskeletons of arthropods (e.g., insects and crustaceans), as well as in the cell walls of fungi.

Heteropolysaccharides or Heteroglycans

Heteropolysaccharides, also known as heteroglycans, are a class of polysaccharides composed of more than one type of monosaccharide, and they can have a diverse range of sugar units in their structures. Unlike homopolysaccharides, where all monosaccharide units are the same, heteropolysaccharides contain a variety of monosaccharides with different structures and properties.

Some examples of heteropolysaccharides include:

• Hyaluronic Acid: Hyaluronic acid is a heteropolysaccharide found in connective tissues, synovial fluid in joints, and the extracellular matrix. It consists of repeating units of two different monosaccharides: N-acetylglucosamine (GlcNAc) and glucuronic acid.
• Heparin: Heparin is a heteropolysaccharide found in the granules of mast cells and basophils.
• Glycosaminoglycans (GAGs): Glycosaminoglycans are a group of heteropolysaccharides found in the extracellular matrix and connective tissues.
• Peptidoglycan: Peptidoglycan is a heteropolysaccharide found in the cell walls of bacteria.

Clinical Importance of Both Homoglycans and Heteroglycans

Polysaccharides are plentiful natural polymers found in plants, animals and microorganisms with exceptional properties and important roles to sustain life.

• They are rich in nutritional value and the positive effects on our immune and digestive functions and detoxification system.
• Polysaccharides are used as demulcents, in drug formulations, for dental-impression materials, dusting powders, hemostatics, and for the treatment of mild intestinal disorders.
• They can also be used to distinguish normal from malignant cells, in cell surface investigations, radioimmunoassay, and in targeting drugs to specific tissues.

Structure of Polysaccharides

The process of formation of polysaccharides is the same as that of monosaccharides via glycosidic bonds. These glycosidic bonds comprise an oxygen molecule bridging two carbon rings. The bond is formed when a hydroxyl group is lost from the carbon of one molecule, while the hydrogen is lost by the hydroxyl group of another monosaccharide. Because two molecules of hydrogen and one of oxygen are ejected, the reaction is a dehydration reaction.  The structure of the molecules being combined resolutes the structure and properties of the resulting polysaccharides.

Polysaccharides typically contribute to structural support and protection and serve as nutrient and energy stores. During polysaccharide formation, at each condensation, a molecule of water is released which reduces the bulk and makes polysaccharides almost insoluble thus making them ideal for storage and as structural components. Being large-sized they are unable to pass through the cell membrane. The passage is allowed through an active process. Unlike sugars, polysaccharides are not sweet. 

Functions of Polysaccharides

Function of polysaccharide’s are given below:

• Support: Polysaccharides are supportive to the cells. The composition of the cell walls of plants is polysaccharides cellulose, which provides support to the plant’s cell wall. Chitin plays a crucial role in providing support to the extracellular matrix around the cells in insects and fungi.
• Energy Source: Polysaccharides are broken down for the formation of energy in the cells.
• Storage: The food is stored in the form of starch in plants and glycogen in animals.
• Messenger: Several polysaccharides become covalently bonded with lipids and proteins to form glycolipids and glycoproteins. These glycolipids and glycoproteins are used to send messages or signals within and between the cells.

Important Polysaccharides

Polysaccharides play several important roles in living organisms and have various functions in biological processes. Here are some of the key importance of polysaccharides:

• Energy Storage: Polysaccharides serve as a storage form of energy in organisms.
• Structural Support: Polysaccharides contribute to the structural integrity of cells and tissues.
• Lubrication and Cushioning: Polysaccharides like hyaluronic acid are found in synovial fluid in joints and the extracellular matrix.
• Extracellular Matrix: Polysaccharides are important components of the extracellular matrix, which provides structural support to tissues and helps regulate cell behavior, cell signaling, and tissue repair.

Heteropolysaccharides in Archaebacteria

• Like Eubacteria, Archaea contain a cell wall made up of various polysaccharides and glycoconjugates. There is the absence of peptidoglycan in archaea, but they still form rigid cell boundaries that confer resistance to high internal osmotic pressure.
• They have D-type of amino acids instead of L-amino acids.
• In some archaeal species, the S-layer can be further supported by polysaccharides, by a second S-layer sheet, or by additional surface glycoproteins.

FAQ’s – Polysaccharides

1. How do polysaccharides serve as an energy storage mechanism in organisms?

Starch in plants and glycogen in animals are both polysaccharides used for storing energy. When needed, these polysaccharides can be broken down into individual glucose molecules, which can be readily used for energy production.

2. How do polysaccharides differ from monosaccharides?

Polysaccharides are composed of multiple monosaccharide units linked together, whereas monosaccharides are single sugar molecules. Polysaccharides are larger and more complex than monosaccharides.

3. What are some common examples of polysaccharides?

Examples of polysaccharides include starch and cellulose in plants, glycogen in animals, chitin in arthropods and fungi, hyaluronic acid in connective tissues, and various glycosaminoglycans in the extracellular matrix.

4. How does glycogen differ from starch in terms of structure and function?

Glycogen is highly branched and serves as an energy storage molecule in animals, primarily in the liver and muscles. Starch is a mixture of amylose (a linear polysaccharide) and amylopectin (branched) and serves as a storage form of energy in plants.

5. What is the function of hyaluronic acid in the human body?

Hyaluronic acid is involved in lubricating joints, reducing friction, and providing cushioning in the extracellular matrix. It is found in connective tissues and synovial fluid.