Polysaccharides are large carbohydrate molecules created by linking many smaller sugar units together. These complex carbohydrates provide both energy storage and structural support in biological systems. Starch, glycogen, and cellulose are the three most common examples of these multi-unit molecules. While they all share the simple sugar glucose as their building block, differences in how these units are connected fundamentally change their properties and biological functions.
The Fundamental Difference in Chemical Structure
The core distinction between these three polysaccharides lies in the specific arrangement of the glucose units. Starch and glycogen are both composed of \(\alpha\)-glucose, linked primarily by the \(\alpha\)-1,4-glycosidic bond. This linkage causes the polymer chain to coil into a helical structure, allowing the molecules to be compact within the cell.
Cellulose is built from \(\beta\)-glucose units connected by \(\beta\)-1,4-glycosidic bonds. This change in bond orientation forces every alternating glucose unit to flip 180 degrees, resulting in a linear, unbranched chain. The linear chains allow for extensive hydrogen bonding between adjacent chains, facilitating tight packing into strong microfibrillar fibers.
The branching pattern is the second major structural difference, distinguishing glycogen from starch and cellulose. Starch is a mixture of two polymers: unbranched amylose and moderately branched amylopectin, which has \(\alpha\)-1,6-glycosidic bonds roughly every 25 to 30 glucose units. Glycogen is significantly more branched, with \(\alpha\)-1,6-glycosidic bonds appearing every 8 to 12 glucose units. This high degree of branching creates multiple terminal ends, adapting glycogen for rapid breakdown and glucose release.
Contrasting Biological Roles
The structural differences—the helical versus linear shape and the degree of branching—directly determine the biological role each polysaccharide plays. Starch and glycogen are optimized for energy storage, serving as reserves that can be quickly broken down to release glucose. The coiled, open structure of these \(\alpha\)-linked polymers allows digestive or metabolic enzymes easy access to the bonds that must be broken.
Glycogen’s highly branched structure provides a large number of accessible ends for simultaneous enzyme action. This specialization is beneficial for the rapid energy demands of active animals. Starch, with its less frequent branching, functions as a slower, more sustained energy reserve for plants.
Cellulose’s linear, tightly packed structure suits its function as a structural support element. Extensive hydrogen bonding between parallel cellulose chains creates strong, water-insoluble microfibrils. These microfibrils provide the rigidity and tensile strength necessary for plant cell walls, allowing the plant to maintain its shape and grow upright.
Origin and Storage Locations
The biological source and location of these polysaccharides reflect the needs of the producing organisms. Both starch and cellulose are plant-based, with cellulose forming the primary component of the plant cell wall. Cellulose is the most abundant organic compound on Earth, present in all plant matter, including wood and cotton.
Starch is the main energy storage molecule for plants, accumulating in granules within structures like seeds, roots, and tubers (e.g., potatoes and grains). This localized storage provides a food source for the plant during periods of low photosynthesis, such as winter or night.
Glycogen, often referred to as “animal starch,” is the primary energy reserve in animals, including humans. It is predominantly stored in two locations. In the liver, it is used to maintain stable blood glucose levels for the entire body. In the skeletal muscles, it provides an immediate fuel source for muscle contraction.
Impact on Human Digestion
The difference in chemical structure directly impacts human digestion and nutrition. The human body produces specific enzymes, collectively called amylases, which recognize and break down the \(\alpha\)-1,4-glycosidic bonds in starch and glycogen. This enzymatic action breaks these large molecules into absorbable glucose units, providing a direct caloric source.
In contrast, humans lack the necessary enzyme, cellulase, required to hydrolyze the \(\beta\)-1,4-glycosidic bonds found in cellulose. Since the digestive system cannot break these \(\beta\)-linkages, cellulose passes through the small intestine virtually intact. It therefore functions as dietary fiber rather than as a caloric source.
The non-digestible nature of cellulose is beneficial, as it contributes bulk to the stool and helps regulate bowel movements, supporting a healthy digestive tract. Although the body cannot digest cellulose, some gut bacteria in the large intestine possess the cellulase enzyme. They ferment a small amount of the fiber, yielding short-chain fatty acids that provide a minor amount of energy and offer health benefits.