What are Polysaccharides?
Polysaccharides are
long chains of monosaccharides linked by glycosidic bond.
(Poly: many, Saccharides:
sugar units)
Polysaccharides broadly classified
into
- Homopolysaccharides: made up of repeating units
of single monosaccharide type (e.g., starch, cellulose both made up of
repeating units of glucose)
- Heteropolysaccharides: made up of different types of monosaccharides (e.g., hemicellulose, agar)
Homopolysaccharides
further classified into
- Storage polysaccharides: Any polysaccharide that serves as a form of stored energy in living organisms.
- Structural polysaccharide: is any type of polysaccharide that forms the structural framework of a cell.
1. Starch: Storage food in plants, starch is a homopolysaccharide made up of glucose monomers linked by α-1,4 and α-1,6 glycosidic bonds. Its semi-crystalline structure allows for efficient energy storage and controlled release in the form of glucose. (In Rice, potato etc)
2. Glycogen: Storage food in animals. Glycogen is also a homopolysaccharide made up of glucose monomers. Unlike starch, glycogen is highly branched, with its glucose units linked by both α-1,4 and α-1,6 glycosidic bonds, as well as additional α-1,6 linkages at branching points. (In Liver, muscle cells)
3. Inulin is made up of fructose monomers linked by β-2,1 glycosidic bonds. inulin is not readily digested by humans, making it a type of dietary fiber. However, it's a favourite food for certain gut bacteria, which ferment it into beneficial short-chain fatty acids that promote gut health. In Jerusalem artichokes or chicory root, bananas, onion etc
4. Dextran: Produced by some bacteria and fungi. A
homopolysaccharide made of glucose units linked by α-1,6 glycosidic bonds.
- Unlike starch or glycogen, dextran has a linear structure with few, if any, branches.
- Used as a blood plasma expander and a thickening agent in food and cosmetics.
Structural
polysaccharide
5. Cellulose is the most abundant organic molecule
on Earth. This tough homopolysaccharide, is composed of glucose units linked by
β-1,4 glycosidic bonds, forms the main structural component of plant cell
walls. Its strong hydrogen bonds and linear chains give it remarkable strength
and rigidity, providing support and stability to plants. Wood, paper, cotton
etc
6. Lignin: is a complex polymer made up of
phenylpropanoid units which are formed from hydroxyl- and methoxy-substituted
phenylpropane units. It acts as a glue that strengthens and reinforces the plant
cell walls, especially woody plants.
7. Chitin: It is a homopolysaccharide made of
N-acetylglucosamine monomers linked by β-1,4 glycosidic bonds. This fibrous and
tough material forms the exoskeletons of insects, crustaceans, and fungal cell
wall.
Heteropolysaccharides:
8. Agar: Obtained from red algae mainly of the genus Gracilaria and Gelidium. It is heteropolysaccharide made up of agarose, a neutral polymer, and agaropectin, a charged, sulfated polymer. Agarose is a linear polysaccharide mostly galactose that are linked in a straight chain without any branches. Agarose gives agar the gelling ability. Agaropectin is a branched polysaccharide with a mix of galactose, other sugars, and sulfate groups. Agaropectin plays a crucial role in texture and stabilizing the agarose gel.
9.Gums: Diverse heteropolysaccharides, often extracted from plants like Acacia ( Gum Arabica) and xanthan gum. Their composition varies, but typically includes a mix of monosaccharides like galactose, arabinose, and rhamnose.
10. Hemicellulose: Hemicellulose is a heteropolysaccharide that is often formed by five or six monomeric residues; some of them are glucose, galactose, xylose, arabinose, mannose, and glucoronic acid. Hemicellulose acts like a glue, binding cellulose fibers together and offering additional rigidity to plant tissues.
11.Pectin: Heteropolysaccharide primarily composed of methylated polygalacturonic acid units linked 1→4-glycosidic bonds and other sugars such as rhamnose, arabinose, galactose, and xylose. Abundant in apple and citrus fruits.
- N-acetylglucosamine (GlcNAc): This modified sugar provides the backbone of the chain.
- Another sugar: Depending on the specific GAG type, this can be glucuronic acid, iduronic acid, galactose, or N-acetylgalactosamine.
- Play crucial roles in cell signaling, lubrication and structure.
- Hyaluronic acid: Found in joints, skin, and eyes, it provides cushioning and lubrication.
- Heparin: This anticoagulant, crucial for blood clotting, is found in blood vessels and lungs.
- Chondroitin sulfate: Key in cartilage and bone, it provides structure and shock absorption.
- Keratan sulfate: Present in the cornea and brain, it contributes to cell signaling and development.
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