TLDR;
This session is the third and final part of the carbohydrate series, focusing on polysaccharides. Polysaccharides are classified based on function (structural or storage) and composition (homo- or heteropolysaccharides). The session details the types, structures, and functions of various polysaccharides like starch, glycogen, inulin, cellulose, chitin, and pectin, as well as heteropolysaccharides found in bacterial cell walls and animal extracellular matrices.
- Polysaccharides are long chains of carbohydrate molecules with thousands of monosaccharide units.
- They are classified as structural or storage polysaccharides based on their function.
- They are also classified as homo- or heteropolysaccharides based on their composition.
Introduction [0:16]
The session begins with a welcome and a request for viewers to like and share the session. It is announced as the third part of the carbohydrates series, focusing on polysaccharides, following previous discussions on monosaccharides and disaccharides.
Unacademy Announcements [1:10]
The instructor shares the successful results of IIT JAM 2025, highlighting that 29+ top 50 rankers were from Unacademy. An invitation is extended to students preparing for the 2026 examination to join Unacademy for comprehensive preparation. Viewers are encouraged to install the Unacademy app, follow the instructor, and attend free sessions, including a biochemistry PYQ session. The Unacademy Telegram channel is promoted for updates, PDFs, and important information related to IIT JAM, GAT-B, and other exams. A new animal biology course by Ruchita Mam is also announced. Information about subscription offers, including saving up to 60% using the code UNLOCK, and a chance to win up to 90% scholarship through a weekly test is provided.
Polysaccharides Overview [5:32]
Polysaccharides are classified based on their function and composition. Functionally, they are either structural or storage polysaccharides. Compositionally, they are either homopolysaccharides (made of one type of monomer) or heteropolysaccharides (made of multiple types of monomers). Polysaccharides are long chains of carbohydrate molecules, containing thousands of monosaccharides, and are considered complex biomacromolecules.
Storage Polysaccharides: Starch [10:38]
Starch is a homopolymer composed of D-glucose units and serves as the storage form of glucose in plants. It is commonly found in foods like wheat, rice, and potatoes. Starch has a helical arrangement and consists of two main structures: amylose and amylopectin. Amylose is a simple, linear chain with alpha 1-4 linkages, while amylopectin is a branched starch with both alpha 1-4 linkages in the linear chains and alpha 1-6 linkages at branching points, which occur every 25 to 30 residues. The enzyme amylase can break down starch.
Amylose and Amylopectin [15:39]
Amylose features alpha 1-4 glycosidic bonds in a linear chain, while amylopectin includes both alpha 1-4 glycosidic bonds in its linear segments and alpha 1-6 glycosidic bonds at branching points. Starch is the preferred energy storage form in plants, synthesized and stored in plastids.
Storage Polysaccharides: Glycogen [19:45]
Glycogen, also known as animal starch, is the primary storage form of glucose in the liver and muscles. It is composed of repeating glucose units and stored in the cytosol of liver and muscle cells. The liver breaks down glycogen to release glucose into the bloodstream when blood glucose levels are low, a process called glycogenolysis. Glycogen has alpha 1-4 linkages in its linear chains and alpha 1-6 linkages at branching points, with branching occurring every 1 to 12 residues.
Glycogen Synthesis and Breakdown [24:39]
When glucose levels are high, insulin is released, promoting glucose absorption and glycogenesis (glycogen formation). When glucose levels are low, glucagon is released, activating glycogenolysis (glycogen breakdown). GLUT4, an insulin-mediated glucose transporter, is essential for glucose absorption. Diabetes results from a lack of insulin, preventing GLUT4 expression and leading to high blood glucose levels.
Storage Polysaccharides: Inulin [29:10]
Inulin is an energy storage form found in plant tubers, composed of fructose units with beta 2-1 linkages. Humans cannot easily digest inulin, so it acts as a standard for renal clearance, specifically to measure the glomerular filtration rate (GFR). Inulin is a linear polymer with a terminal glucose residue and fructose units linked by beta 2-1 bonds.
Inulin and Gut Health [32:55]
Humans cannot derive energy from inulin because they lack the enzymes to digest it. However, beneficial gut bacteria can use inulin as a prebiotic, supporting gut health. The gut bacteria play an important role in the immune function.
Structural Polysaccharides: Cellulose [35:17]
Cellulose is the most abundant biomolecule and a structural component found in the cell walls of plants and in the cell walls of certain fungi called Oomycetes. It is composed of repeating D-glucose units linked by beta 1-4 glycosidic bonds and has a sheet-like structure. Cellulose is synthesized by the enzyme cellulose synthase, located in the plasma membrane of plant cells. Animals lack cellulase and cannot break down cellulose.
Cellulose Digestion in Herbivores [39:27]
Herbivores rely on symbiotic bacteria in their guts to break down cellulose. These bacteria possess the necessary enzymes to degrade cellulose, providing nutrients to the host animals.
Structural Polysaccharides: Chitin [42:44]
Chitin is found in the cell walls of fungi and the exoskeletons of insects. It is composed of N-acetyl-D-glucosamine units linked by beta 1-4 glycosidic bonds and forms sheet-like structures.
Structural Polysaccharides: Pectin [44:07]
Pectin is a homopolymer composed of D-galacturonic acid with alpha 1-4 linkages, forming sheet-like structures. It is found in the middle lamella region of plant cell walls. Pectin is degraded by the enzyme pectinase, also known as polygalacturonidase, which hydrolyzes the middle lamella, leading to fruit softening.
Pectin and Fruit Softening [46:53]
To slow fruit ripening, polygalacturonidase can be inhibited using antisense technology, such as siRNA and miRNA. This technique was used in the development of the Flavor Saver tomato, which had a longer shelf life but was too firm for cooking.
Heteropolysaccharides: Bacterial Cell Wall [50:01]
Heteropolysaccharides play structural roles in bacterial cell walls and the extracellular matrix of animal cells. Bacterial cell walls are made of peptidoglycan, composed of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) residues. NAM residues are linked to a pentapeptide composed of D-amino acids. The linkage between NAG and NAM is a beta 1-4 glycosidic bond, which can be broken by lysozyme.
Peptidoglycan and Antibiotics [52:47]
Transpeptidase, an enzyme responsible for cross-linking peptidoglycans, is essential for rigid cell wall formation. Beta-lactam antibiotics, like penicillin, inhibit transpeptidase, preventing cross-linking and weakening the bacterial cell wall.
Heteropolysaccharides: Extracellular Matrix [55:18]
Heteropolysaccharides are also found in the extracellular matrix (ECM) of animal cells. Examples include heparin, keratan sulfate, chondroitin sulfate, dermatan sulfate, and hyaluronic acid. These glycosaminoglycans (GAGs) have various functions, such as lubrication (hyaluronic acid), tensile strength (chondroitin sulfate), skin-related roles (dermatan sulfate), anticoagulation (heparin), and maintaining corneal transparency (keratan sulfate).
Lectins and Affinity Chromatography [57:38]
Lectins are proteins or glycoproteins found in plants, animals, and microorganisms with a high affinity for carbohydrates. They can be used in affinity chromatography to separate carbohydrates.
Conclusion and Announcements [58:36]
The session concludes with a summary of the discussed topics and a reminder of the homework question about which cells secrete insulin and glucagon. The instructor reiterates the availability of Unacademy's courses for IIT JAM 2026 preparation, including live sessions for animal biology, immunology, and mathematics. Information about subscription offers, the UNLOCK code for a 60% discount, and the upcoming scholarship test is repeated.
