Which of the following are polysaccharides?

Polysaccharides are the most abundant carbohydrates found in nature, forming long, complex molecules from smaller monosaccharide units. This guide clarifies which molecules are polysaccharides by examining prominent examples and explaining their vital biological roles for energy storage and structural support.

Quick Summary

This guide defines polysaccharides and explores prominent examples, including starch, glycogen, cellulose, and chitin, highlighting their functions in various organisms.

Key Points

• Polysaccharides Defined: These are complex carbohydrates (glycans) made from long chains of monosaccharide units joined by glycosidic bonds.

• Energy Storage Polysaccharides: Starch in plants and glycogen in animals serve as energy reserves, storing glucose in a compact form for later metabolic use.

• Structural Polysaccharides: Cellulose and chitin provide structural support, with cellulose forming plant cell walls and chitin constituting arthropod exoskeletons and fungal cell walls.

• Homopolysaccharides vs. Heteropolysaccharides: Classification is based on monomer composition; homopolysaccharides (like starch) contain one type of monomer, while heteropolysaccharides (like hyaluronic acid) have multiple types.

• Digestibility Depends on Linkage: Human enzymes can digest the alpha-glycosidic bonds in starch but not the beta-glycosidic bonds in cellulose, making cellulose indigestible dietary fiber.

• Structural Features Dictate Function: Branched polysaccharides are ideal for energy storage (quick access), while linear polysaccharides are best for structural rigidity (strong fibers).

What Defines a Polysaccharide?

A polysaccharide, also known as a glycan, is a large carbohydrate molecule made up of many smaller monosaccharide units linked by glycosidic bonds. Unlike monosaccharides and disaccharides, polysaccharides are typically insoluble in water and lack sweetness due to their size. Their primary functions in biological systems are energy storage and providing structural support. Polysaccharides are categorized as either homopolysaccharides (composed of a single type of monosaccharide) or heteropolysaccharides (composed of multiple types).

Homopolysaccharides

Homopolysaccharides are formed from repeating units of a single monosaccharide type. Key examples include:

• Starch: The energy storage carbohydrate in plants, made of glucose units in both linear (amylose) and branched (amylopectin) forms.
• Glycogen: The main energy storage polysaccharide in animals and fungi, a highly branched glucose polymer allowing for rapid glucose release.
• Cellulose: The primary structural component of plant cell walls, a linear glucose polymer with beta linkages indigestible by humans.
• Chitin: A structural polysaccharide in arthropod exoskeletons and fungal cell walls, composed of N-acetylglucosamine units.

Heteropolysaccharides

Composed of two or more different monosaccharide units, heteropolysaccharides contribute to complex structures. Examples include:

• Hyaluronic Acid: A lubricant and shock absorber in connective tissues.
• Heparin: An anticoagulant found in various tissues and blood.
• Pectin: A gelling agent in plant cell walls used in food production.

Comparison of Key Polysaccharides

Here's a comparison of some common polysaccharides:

Feature	Starch	Glycogen	Cellulose	Chitin
Organism	Plants	Animals, Fungi	Plants, Algae	Arthropods, Fungi
Function	Energy Storage	Energy Storage	Structural Support	Structural Support
Monomer	Glucose	Glucose	Glucose	N-acetylglucosamine
Structure	Branched & Linear	Highly Branched	Linear	Linear
Linkage	α-glycosidic bonds	α-glycosidic bonds	β-glycosidic bonds	β-glycosidic bonds
Digestibility (Human)	Digestible	Digestible	Indigestible	Indigestible

Functions in Biological Systems

Polysaccharides play vital roles in biological systems based on their structure. Storage polysaccharides like starch and glycogen provide easily accessible energy reserves. Structural polysaccharides such as cellulose and chitin offer rigidity and protection, forming plant cell walls and animal exoskeletons, respectively.

Polysaccharides and Human Digestion

The type of glycosidic linkage in a polysaccharide determines if humans can digest it. Enzymes like amylase can break the alpha linkages in starch and glycogen. However, humans lack the enzyme cellulase to break the beta linkages in cellulose, making it indigestible dietary fiber.

Conclusion

Polysaccharides are essential complex carbohydrates involved in energy storage and structural support across diverse organisms. Examples such as starch, glycogen, cellulose, and chitin highlight the variety in their composition and function. Understanding these molecules is key to grasping fundamental biological processes. Many biomolecules fall into the polysaccharide category. For more information, consult resources like Biology LibreTexts.

Frequently Asked Questions

What is the difference between a polysaccharide, a disaccharide, and a monosaccharide?

A monosaccharide is a single sugar unit (e.g., glucose). A disaccharide consists of two monosaccharides linked together (e.g., sucrose). A polysaccharide is a large polymer of many monosaccharides, making it a complex carbohydrate.

Are starch and glycogen the same thing?

No, both are glucose polymers used for energy storage, but starch is the storage form in plants, while glycogen serves this function in animals. Glycogen is also more highly branched than starch, which allows for faster energy release in animals.

Why can't humans digest cellulose?

Humans cannot digest cellulose because it is made of glucose units linked by beta-glycosidic bonds. Our digestive system does not produce the necessary enzyme, cellulase, to break these specific bonds.

What are some common food sources of polysaccharides?

Common food sources rich in polysaccharides include potatoes, rice, pasta, and bread (containing starch). Dietary fiber, found in fruits, vegetables, and whole grains, is primarily composed of cellulose.

What are the main functions of polysaccharides in living organisms?

Polysaccharides primarily serve two functions: energy storage (e.g., starch in plants, glycogen in animals) and structural support (e.g., cellulose in plant cell walls, chitin in arthropod exoskeletons).

Is chitin a protein?

No, chitin is a polysaccharide, meaning it is a carbohydrate polymer. While it contains nitrogen and provides structural support, like some proteins (e.g., keratin), it is chemically composed of repeating N-acetylglucosamine units, not amino acids.

Why aren't polysaccharides sweet like simple sugars?

Polysaccharides are not sweet because their large and complex molecular structure prevents them from binding to the sweet taste receptors on our tongues, which are designed for smaller, simple sugar molecules.