The long chains of sugars found in nature are formally known as polysaccharides, which translates literally to “many sugars.” These large carbohydrate molecules are polymers built from hundreds or thousands of smaller sugar units, called monosaccharides. They are joined together through a specific type of covalent bond called a glycosidic linkage.
The complex architecture of these chains allows them to serve diverse and important functions in living organisms, ranging from storing energy to forming rigid support structures. Their properties are determined by the type of monosaccharide used, the length of the chain, and the pattern of branching within the molecule.
The Short Chains of Sugar
The fundamental unit of all carbohydrates is the monosaccharide, or a single sugar molecule. Common examples include glucose, the primary fuel for human cells, as well as fructose and galactose. These simple sugars are readily absorbed by the body and represent the most basic form of carbohydrate energy.
When two monosaccharide units bond together, they form a disaccharide, or a double sugar. Sucrose (common table sugar) is a disaccharide made by linking one glucose molecule to one fructose molecule. Another familiar disaccharide is lactose (milk sugar), which consists of a glucose molecule bonded to a galactose molecule.
Both monosaccharides and disaccharides are sweet-tasting and easily dissolved in water, providing immediate sources of energy. The long chains of polysaccharides are constructed by joining many of these single sugar units into extended, often branched, structures.
Storage and Structural Roles of Polysaccharides
Polysaccharides are functionally separated into two main categories: energy storage and structural support. In plants, the primary energy storage polysaccharide is starch, a blend of two glucose polymers: the unbranched amylose and the highly branched amylopectin. Plants store starch in seeds, roots, and tubers as a long-term energy reserve.
The equivalent storage polysaccharide in animals and humans is glycogen, sometimes referred to as “animal starch.” Glycogen is a polymer of glucose, but it is much more extensively branched than plant starch. This dense branching allows for rapid breakdown and release of glucose when energy is needed, primarily in the liver and muscle cells.
Structural polysaccharides are built for strength and rigidity. Cellulose is the most abundant structural polysaccharide, forming the strong cell walls of plants. It is a straight, unbranched chain of glucose units linked together, allowing the chains to align and form strong, fibrous structures.
Another structural example is chitin, which provides the tough, protective material for the exoskeletons of insects and crustaceans, as well as the cell walls of fungi. Chitin is chemically related to cellulose but is made of a modified glucose unit, lending it immense tensile strength.
Digestion and Dietary Importance
Polysaccharides derived from plants, particularly starch, are the major source of energy in the human diet. The body breaks down digestible starches using specialized enzymes, such as amylase, found in saliva and the small intestine. Amylase cleaves the bonds holding the glucose units together, breaking the large polysaccharide down into its monosaccharide components.
The resulting glucose is then absorbed into the bloodstream, where it is transported to cells for immediate energy or stored as glycogen. The speed of this breakdown is influenced by the starch’s molecular structure, with more branched forms like amylopectin being digested more quickly.
Other polysaccharides, primarily cellulose and certain plant cell wall components, are non-digestible by human enzymes and are classified as dietary fiber. The human body lacks the specific enzymes required to break the chemical bonds in cellulose. This indigestible material travels through the small intestine intact, adding bulk to the stool and promoting regularity. Fiber also serves as a food source for beneficial gut bacteria in the large intestine, which ferment it to produce short-chain fatty acids that support intestinal health.