Polysaccharide

Polysaccharides are large compounds made by linking many monosaccharides together through glycosidic bonds. They are the most common type of carbohydrate found in food, and their structures can vary from straight chains to highly branched forms. Examples include storage types like starch, glycogen, and galactogen, as well as structural types like hemicellulose and chitin. The word "glycan" means the same thing as polysaccharide, though it is often used when talking about glycoconjugates—hybrid molecules that combine polysaccharides with proteins or lipids.

Polysaccharides are often not uniform, containing slight changes in their repeating units. They can be amorphous, like starch, or insoluble in water, like cellulose. Simple carbohydrates called monosaccharides have the general formula (CH₂O)ₙ, where n is three or more; examples include glucose, fructose, and glyceraldehyde. Polysaccharides, in contrast, have the general formula Cₓ(H₂O)ᵧ, where x and y are usually between 200 and 2500. When the repeating units are six-carbon monosaccharides—which is common—the formula simplifies to (C₆H₁₀O₅)ₙ, with n typically large. Polysaccharides are highly branched. Starch, for instance, holds iodine molecules in its helical sections, producing a dark blue color. Cellulose lacks these complex helices and does not trap iodine, so no color change occurs—this difference serves as a unique test for polysaccharides.

As a general rule, polysaccharides contain more than ten monosaccharide units, while oligosaccharides have three to ten units, though the exact cutoff varies by convention. Polysaccharides are important biological polymers. In living organisms, their roles are usually related to storage or structure. Starch, a glucose polymer, serves as a storage polysaccharide in plants, appearing as both linear amylose and branched amylopectin. In animals, the similar glucose polymer is glycogen, which is more densely branched and sometimes called "animal starch." Glycogen's structure allows it to be metabolized faster, suiting the active lives of moving animals. In bacteria, polysaccharides play a key role in multicellular behavior.

Cellulose and chitin are examples of structural polysaccharides. Cellulose is found in the cell walls of plants and other organisms and is the most abundant organic molecule on Earth. It is used in paper and textiles, and as a feedstock for rayon, cellulose acetate, celluloid, and nitrocellulose. Chitin has a similar structure but includes nitrogen-containing side branches, making it stronger. It is found in arthropod exoskeletons and the cell walls of some fungi, and has uses including surgical threads. Other polysaccharides include callose, laminarin, chrysolaminarin, xylan, arabinoxylan, mannan, fucoidan, and galactomannan.

Nutritional polysaccharides are common energy sources. Many organisms can easily break down starches into glucose, but few can metabolize cellulose. Some bacteria and protists can handle these carbohydrates, and ruminants and termites use microorganisms to process cellulose. Some polysaccharides are not very digestible, but as dietary fiber, they aid digestion. Soluble fiber binds to bile acids in the small intestine, reducing their entry into the body and lowering blood cholesterol. It also slows sugar absorption, reduces blood sugar response after meals, normalizes blood lipid levels, and, after fermentation in the colon, produces short-chain fatty acids with wide-ranging effects. Insoluble fiber is linked to a lower risk of diabetes, though the reason is unknown. Dietary fiber is considered important, and many developed countries recommend increasing fiber intake.

Starch is a glucose polymer with alpha-linkages between glucopyranose units. It consists of 15–20% amylose and 80–85% amylopectin. Amylose is a linear chain of several hundred glucose molecules, while amylopectin is branched and made of several thousand glucose units, with branches every 24–30 units. Starches are insoluble in water and can be digested by breaking alpha-linkages. Humans and other animals have amylases for this purpose. Potato, rice, wheat, and maize are major starch sources in the human diet, and starches are how plants store glucose.

Glycogen serves as secondary long-term energy storage in animal and fungal cells, with primary energy stores in adipose tissue. It is made mainly by the liver and muscles, but also by glycogenesis in the brain and stomach. Glycogen is similar to starch and sometimes called animal starch; it resembles amylopectin but is more extensively branched and compact. It is a polymer of α(1→4) glycosidic bonds with α(1→6)-linked branches. Glycogen appears as granules in the cytosol or cytoplasm of many cell types and plays an important role in energy metabolism.