Polysaccharides

Polysaccharides, also called glycans, are made of hundreds of monosaccharide units linked into long chains, which may be branched or unbranched, all held together by glycosidic bonds. A polysaccharide is a true macromolecule built from repeated units of just one type of monomer (unlike, say, a protein, which mixes 20 different amino-acid monomers). Unlike the simple sugars they are built from, polysaccharides are insoluble in water and are not sweet. Depending on the biological role they play, polysaccharides fall into two functional groups: storage polysaccharides (such as starch in plants and glycogen in animals, which hold reserve glucose for later energy use) and structural polysaccharides (such as cellulose in plant cell walls and chitin in fungal walls and arthropod exoskeletons, which provide mechanical support). Beyond the main polysaccharides covered in this chapter, several other polysaccharide-type compounds occur biologically, summarised in the table below - including inulin (a fructose polymer, filtered by the kidney rather than metabolised), hyaluronic acid (gives cartilage and tendon their toughness and flexibility), agar (a mucopolysac …