Polysaccharides are complex carbohydrates, or polymeric sugar molecules, made of many monosaccharide units linked by glycosidic bonds. They serve vital roles, such as energy storage in plants (starch) and animals (glycogen), and structural support in plants (cellulose). The process of digestion, known as enzymatic hydrolysis, breaks these complex structures down into their component simple sugars.
The Digestive Journey of Polysaccharides
The human digestive system is finely tuned to break down most dietary polysaccharides, but the process is not uniform for all types.
The Mouth: First Step of Breakdown
Digestion begins in the mouth, where chewing mechanically breaks down food into smaller pieces. Salivary glands release an enzyme called salivary amylase, which starts the chemical breakdown of starch into smaller polysaccharides and the disaccharide maltose. However, this is a preliminary step, as salivary amylase is inactivated by the acidic environment of the stomach.
The Small Intestine: The Main Event
Most carbohydrate digestion, and the bulk of polysaccharide breakdown, occurs in the small intestine. Here, the food is mixed with pancreatic amylase, which continues to break down starch into smaller oligosaccharides and maltose. The intestinal wall's cells then produce specific enzymes to further hydrolyze these intermediate sugars. These enzymes include:
- Maltase: Breaks down maltose into two glucose molecules.
- Lactase: Breaks down lactose (a disaccharide) into glucose and galactose.
- Sucrase: Breaks down sucrose (table sugar) into glucose and fructose.
Breakdown of Key Polysaccharides
The specific products of polysaccharide breakdown depend on the type of carbohydrate consumed.
Starch: Plant-based Energy
Starch, a primary energy store in plants found in foods like potatoes and grains, is composed of long chains of glucose units. Amylase enzymes in the mouth and small intestine break down these chains, first into shorter chains and disaccharides (like maltose), and finally into individual glucose molecules, which are then absorbed.
Glycogen: Animal Energy Storage
Glycogen is a highly branched polysaccharide that serves as an energy reserve in animals, stored primarily in the liver and muscles. When the body needs glucose for energy, enzymes like glycogen phosphorylase break down glycogen into individual glucose units.
Cellulose: Indigestible Fiber
Cellulose, which forms the structural component of plant cell walls, is another polysaccharide made of glucose units. The key difference is the type of glycosidic bond linking the units. Humans lack the enzyme cellulase needed to break these specific bonds, so cellulose passes through our digestive system undigested. It functions as dietary fiber, aiding in regular bowel movements.
The Monosaccharides: The Final Products
Ultimately, the process of digestion yields absorbable monosaccharides, or simple sugars, which are the fundamental building blocks of all carbohydrates. The three primary monosaccharides are glucose, fructose, and galactose. Once absorbed into the bloodstream from the small intestine, they are transported to the liver. The liver then converts fructose and galactose into glucose, which is the main form of carbohydrate used by the body for energy.
Polysaccharide Digestion and its Products
| Polysaccharide | Primary Digestive Enzyme(s) | Breakdown Products (Intermediate) | Final Absorbed Monosaccharide(s) |
|---|---|---|---|
| Starch (Amylose & Amylopectin) | Amylase (salivary & pancreatic) | Oligosaccharides, Maltose | Glucose |
| Glycogen | Glycogen Phosphorylase | Glucose-1-phosphate | Glucose |
| Cellulose | Humans lack cellulase | N/A (undigested) | N/A (passes as fiber) |
Conclusion
In summary, the answer to what polysaccharides get broken down into is simple: monosaccharides. This conversion happens through a multi-step process of enzymatic hydrolysis, beginning in the mouth and culminating in the small intestine. Digestible polysaccharides like starch and glycogen are transformed into glucose, providing the body with vital energy. Meanwhile, indigestible polysaccharides like cellulose provide crucial dietary fiber, showcasing the diverse fates of these complex carbohydrates in the human body. Understanding this fundamental biochemical process is key to grasping how our bodies derive energy from the food we eat. For further insight into the chemical reactions, explore resources like Khan Academy's overview on hydrolysis.
