What are the three groups of polysaccharides?

December 19, 2025 •

3 min read

Polysaccharides, long chains of carbohydrate molecules, perform essential functions in all living organisms, from providing energy to forming protective structures. These vital macromolecules are generally categorized into three primary groups based on their function and composition: storage, structural, and heteropolysaccharides.

Quick Summary

This article outlines the three main classifications of polysaccharides. It details the characteristics of storage polysaccharides, such as starch and glycogen, which serve as energy reserves. The discussion covers structural polysaccharides like cellulose and chitin that provide rigid support. It also examines heteropolysaccharides, which are composed of multiple monosaccharide types.

Key Points

Three Main Groups: The three groups of polysaccharides are storage polysaccharides (e.g., starch and glycogen), structural polysaccharides (e.g., cellulose and chitin), and heteropolysaccharides (e.g., hyaluronic acid and heparin).
Energy Storage: Storage polysaccharides are readily broken down to provide energy and are characterized by alpha-glycosidic bonds and a branched or coiled structure.
Structural Support: Structural polysaccharides provide strength and rigidity to organisms, featuring linear chains with beta-glycosidic bonds that form strong fibers.
Diverse Monomers: Heteropolysaccharides are composed of different types of monosaccharide units and are involved in specialized functions like lubrication and anticoagulation.
Biological Importance: Polysaccharides are essential macromolecules, serving as energy reservoirs, providing structural frameworks for cells and organisms, and playing roles in cell communication and tissue function.
Key Examples: Key examples for each group include plant starch and animal glycogen for storage, and plant cellulose and arthropod chitin for structural purposes.
Digestibility Differences: The differing glycosidic bonds determine function; alpha-bonds in storage polysaccharides are easily digestible, while beta-bonds in structural ones like cellulose are not.

Introduction to Polysaccharides

Polysaccharides, also known as glycans, are complex carbohydrates consisting of long chains of monosaccharide units linked by glycosidic bonds. These polymers are crucial for life, playing roles that range from energy storage to creating the rigid frameworks of plant cell walls. While the constituent monosaccharide units are often the same (leading to homopolysaccharides like starch), some complex polysaccharides are built from different types of monosaccharides (known as heteropolysaccharides). Understanding the classification of these macromolecules is key to grasping their diverse roles in biochemistry and nutrition.

The Three Major Functional Groups

Polysaccharides are most commonly categorized into three functional groups based on their primary role in an organism: Storage, Structural, and Heteropolysaccharides.

Group 1: Storage Polysaccharides

Storage polysaccharides serve as a readily accessible energy reserve for organisms. They are typically composed of alpha-glucose monomers linked together, allowing for easy enzymatic hydrolysis to release glucose.

Starch: The primary energy storage in plants and the most common carbohydrate in human diets. Stored in plant seeds, tubers, and roots, it consists of amylose (unbranched) and amylopectin (branched).
Glycogen: The main energy storage in animals and fungi, often called "animal starch". It is stored primarily in the liver and muscles and is highly branched for rapid glucose release.

Group 2: Structural Polysaccharides

Structural polysaccharides provide strength, rigidity, and protection. They often feature beta-glucose linkages, forming long, straight chains that align to create strong fibers through hydrogen bonding.

Cellulose: The most abundant organic molecule on Earth, forming plant cell walls. Its linear beta-glucose chains create microfibrils providing structural support. Humans cannot digest it; it functions as dietary fiber.
Chitin: A nitrogen-containing polysaccharide providing structural support. It makes up the rigid exoskeletons of arthropods and the cell walls of fungi. The nitrogen enhances its strength.

Group 3: Heteropolysaccharides

Heteropolysaccharides are made of two or more different monosaccharide units. They perform specialized functions like being components of connective tissue, signaling, and lubrication.

Hyaluronic Acid: Found in animal connective tissues and skin, composed of D-glucuronic acid and N-acetyl-glucosamine units. It lubricates joints and helps tissue repair.
Heparin: A blood anticoagulant composed of various monosaccharide units.

Comparison of Storage and Structural Polysaccharides


Feature	Storage Polysaccharides (Starch, Glycogen)	Structural Polysaccharides (Cellulose, Chitin)
Primary Function	Energy storage	Structural support and protection
Linkage Type	Primarily alpha-glycosidic bonds	Primarily beta-glycosidic bonds
Molecular Structure	Branched (glycogen, amylopectin) or helical (amylose)	Linear, unbranched chains
Solubility in Water	Generally less soluble, stored compactly	Insoluble, forms strong fibers
Digestibility	Easily digested by enzymes (e.g., amylase)	Generally indigestible by most organisms
Hydrogen Bonds	Internal hydrogen bonds cause coiling and compactness	Extensive hydrogen bonds between parallel chains for strength

The Role of Polysaccharides in Nature and Commerce

Beyond their biological roles, polysaccharides are vital to industries. Cellulose is used for paper and textiles, while starches serve as thickeners. Their unique properties, determined by composition and bonding, make them versatile materials.

Conclusion

The three main groups of polysaccharides—storage, structural, and heteropolysaccharides—are essential for life. Storage polysaccharides like starch and glycogen provide energy, while structural ones such as cellulose and chitin offer support. Heteropolysaccharides perform specialized tasks. The specific function of each polysaccharide is dictated by its structure, including the types of monosaccharides and glycosidic bonds.

Frequently Asked Questions

The primary function of storage polysaccharides, such as starch in plants and glycogen in animals, is to serve as a readily available energy reserve.

Both starch and glycogen are storage polysaccharides made of glucose. The main difference is that starch is stored by plants, while glycogen is stored by animals, and glycogen is more highly branched than starch, allowing for more rapid energy release.

Humans cannot digest cellulose because they lack the necessary enzymes to break the beta-glycosidic linkages that hold the glucose monomers together in its long, fibrous chains.

Cellulose is the main component of plant cell walls, and is the most abundant organic molecule on Earth. It is found in wood, cotton fibers, and the fibrous parts of all plants.

Heteropolysaccharides are polysaccharides composed of two or more different types of monosaccharide units. They often serve specialized functions, such as acting as components of connective tissue.

Examples of heteropolysaccharides include hyaluronic acid, found in connective tissues and joints, and heparin, which acts as a blood anticoagulant.

Chitin is a structural polysaccharide that contains nitrogen. It is found in the exoskeletons of arthropods, such as insects and crustaceans, and in the cell walls of fungi.