Polysaccharides are long polymers of monosaccharides and their derivatives. Unlike proteins or nucleic acids, these polymers can be either linear or branched, and they can contain only one type of monosaccharide (homopolysaccharides), or more than one (heteropolysaccharides). Polysaccharides can also be roughly divided into groups according to their two main functions: energy storage and their contributions to structural components of cells.
Starch is the main energy reserve in plants; glycogen is the main energy reserve in animals. Starch is a homopolysaccharide and has two forms: amylopectin and α -amylose. In nature, starch is approximately 10 to 30 percent α -amylose. Alpha-amylose is a linear chain polymer composed of glucose residues in α (1→4) linkages. Its molecular weight varies from several thousand to more than one million grams (2,205 pounds) per mole. In contrast to amylopectin, which comprises 70 to 90 percent of natural starch, α -amylose is a branching polysaccharide. Although amylopectin, like α -amylose, is composed entirely of α -glucose, its α -glucose residues are joined not only in α (1→4) linkages but also at α (1→6) branch points. Branches occur at every twelve to thirty residues along a chain of α (1→4) linked glucoses. As a result, amylopectin has one reducing end and many nonreducing ends. Amylopectin and α -amylose are broken down by the enzyme amylase. In animals, salivary α -amylase begins the digestion process in the mouth. Pancreatic α -amylase continues the process in the intestine.
Glycogen is the energy storage carbohydrate in animals. Glycogen is found mainly in the liver (where it is responsible for up to 10 percent of liver mass) and skeletal muscle (1 to 2 percent of skeletal muscle mass). Like amylopectin, it consists of α -glucose residues in α (1→4) linkage, with α (1→6) branch points. However, glycogen branches more abundantly than amylopectin, with branches at every eight to twelve residues. As a result, it has many more nonreducing ends. Glycogen is broken down at these nonreducing ends by the enzyme glycogen phosphorylase to release glucose for energy. Having many reducing ends, glycogen is more readily broken down in cases in which an animal needs a sudden burst of energy.
The primary structural homopolysaccharides are cellulose and chitin. Cellulose, a major component of plant cell walls, is the most abundant natural polymer on Earth. It is responsible for much of the mass of wood. Cotton is almost pure cellulose. Like α -amylose, cellulose is a linear polysaccharide composed entirely of glucose. However, in cellulose the glucose residues occur in β (1→4) linkage rather than α (1→4) (see Figure 1). This change in linkage has profound effects on the chemical and structural properties of cellulose. The glucose molecules in cellulose are alternately inverted (every other one inverted) such that each chain has a highly extended and rigid conformation. In addition, individual cellulose strands can form hydrogen bonds with one another to provide additional strength. Most animals, including humans, lack the enzymes necessary to dissolve α (1→4) linkages and so cannot digest cellulose. The animals that can (such as ruminants) do so via a symbiosis with bacteria that secrete cellulose-degrading enzymes.
The second most abundant polymer on Earth is chitin. Chitin comprises much of the exoskeletons of crustaceans, insects, and spiders, as well as the cell walls of fungi. Structurally, chitin is very similar to cellulose, except that its basic monosaccharide is N-acetylglucosamine. Chitin, like cellulose, has its repeating units joined in β (1→4) linkages.
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