The fundamental roles of polysaccharides
Polysaccharides are large polymers, or macromolecules, made from many smaller monosaccharide units joined by glycosidic bonds. These complex carbohydrates play indispensable roles in the survival and functioning of nearly all living organisms. The three most biologically important polysaccharides—starch, glycogen, and cellulose—are all homopolymers of glucose, meaning they are composed exclusively of repeated glucose units. However, subtle differences in how these glucose units are linked together and arranged lead to profoundly different structures and functions. While starch and glycogen serve as crucial energy reserves, cellulose acts as a primary structural element.
Starch: The plant's energy storehouse
Starch is the chief storage polysaccharide in plants, allowing them to store excess glucose produced during photosynthesis. It is commonly found in large quantities in plant roots, seeds, and tubers, such as potatoes, corn, and wheat. Starch is a mixture of two different glucose polymers: amylose and amylopectin. Amylose is a linear chain of alpha ($α$)-glucose units with $α$-1,4 linkages, coiling into a helix. Amylopectin is highly branched, featuring $α$-1,4 linkages in its main chains and $α$-1,6 linkages at branch points, enabling quick enzymatic breakdown. Animals digest starch using amylase enzymes, releasing glucose for energy.
Glycogen: The animal's short-term energy reserve
Glycogen is the primary energy storage polysaccharide in animals, often called "animal starch". Similar to amylopectin but more highly branched, it has $α$-1,4 and frequent $α$-1,6 linkages, allowing rapid glucose release for energy demands. Glycogen is stored mainly in the liver to maintain blood sugar and in muscles for activity energy. Glycogenolysis breaks it down into glucose.
Cellulose: The structural foundation of plants
Cellulose is a structural polysaccharide forming rigid plant cell walls, and is the most abundant natural biopolymer. It is a linear, unbranched polymer of beta ($β$)-glucose with $β$-1,4 linkages. These linkages cause alternating glucose units to flip, creating straight chains that form strong hydrogen bonds and align into rigid microfibrils, giving cellulose high tensile strength and insolubility. Most animals, including humans, cannot digest cellulose due to lacking cellulase enzymes, so it acts as dietary fiber. Some herbivores have symbiotic microbes that digest it.
Comparing the three major polysaccharides
| Feature | Starch | Glycogen | Cellulose |
|---|---|---|---|
| Primary Function | Energy storage in plants | Energy storage in animals | Structural support in plants |
| Monomer | Alpha ($α$)-glucose | Alpha ($α$)-glucose | Beta ($β$)-glucose |
| Glycosidic Linkages | $α$-1,4 and $α$-1,6 linkages | $α$-1,4 and $α$-1,6 linkages | $β$-1,4 linkages |
| Branching | Moderately branched (amylopectin) and unbranched (amylose) | Highly branched | Unbranched, linear |
| Structural Shape | Helical (amylose) or amorphous (amylopectin) | Amorphous, densely granular | Linear, fibrous, and rigid |
| Digestibility by Humans | Easily digestible | Easily digestible | Indigestible; dietary fiber |
Conclusion
Starch, glycogen, and cellulose are homopolymers of glucose, demonstrating how a simple change in the type and arrangement of glycosidic bonds can create molecules with vastly different properties and biological functions. Their respective roles in energy storage and structural support are fundamental to the existence of both plant and animal life. Starch provides long-term energy for plants, glycogen serves as a rapidly accessible energy source for animals, and cellulose forms the sturdy framework of plant cell walls. Understanding these three polysaccharides is essential for comprehending basic life processes and the interconnectedness of biological systems.
Learn more
For a more detailed explanation of these polysaccharides and their chemical structures, visit the Chemistry LibreTexts project.
