Polysaccharide: Videos & Practice Problems

Polysaccharides are complex carbohydrates composed of long chains of monosaccharides linked by glycosidic bonds. They are formed through a series of condensation reactions where monosaccharides, such as glucose, are joined together by the removal of water molecules. The glycosidic bonds can be either alpha or beta, influencing the structure and digestibility of the polysaccharide. Examples include starch, cellulose, and glycogen. Starches are energy stores in plants, cellulose provides structural support, and glycogen serves as an energy reserve in animals.

Alpha and beta linkages refer to the orientation of the glycosidic bonds between monosaccharides in polysaccharides. Alpha linkages (α) occur when the OH group on the carbon-1 of one sugar is below the plane of the ring, making the bond easier to digest. Beta linkages (β) occur when the OH group is above the plane, making the bond more difficult to digest. For example, amylose and glycogen have alpha linkages, making them easier to digest, while cellulose has beta linkages, making it more rigid and harder to break down.

In plants, polysaccharides primarily serve as energy stores and structural components. Starches like amylose and amylopectin store energy, while cellulose provides structural support in cell walls. In animals, polysaccharides like glycogen serve as energy reserves. Glycogen is stored in the liver and muscles and can be broken down into glucose when energy is needed. The structural polysaccharides in animals are less common, but chitin in the exoskeletons of arthropods is an example.

Cellulose and starch are both polysaccharides composed of glucose units, but they differ in their glycosidic linkages and structure. Cellulose has β(1→4) linkages, forming straight, rigid chains that provide structural support in plant cell walls. These β linkages make cellulose difficult to digest. Starch, on the other hand, consists of α(1→4) linkages in amylose and both α(1→4) and α(1→6) linkages in amylopectin, resulting in a more branched and flexible structure that is easier to digest and serves as an energy store in plants.

Glycogen is considered the animal equivalent of starch because it serves a similar function in energy storage. Like starch, glycogen is a polysaccharide composed of glucose units. However, glycogen is more highly branched, with α(1→4) linkages and α(1→6) linkages occurring approximately every 10 glucose units. This branching allows for rapid release of glucose when energy is needed. Glycogen is stored in the liver and muscles, providing a readily available energy source for animals, similar to how starch functions in plants.