Any substance that swells in water to form gels or sticky solutions. Structurally they are mostly complex, highly branched polysaccharides, although a few gums with relatively simple structures are known.
Three main classes of gum are recognized. Acidic polysaccharides of glucuronic or galacturonic acids form glassy, hard gums; these are often produced by plants as a result of injury. Examples include gum arabic and tragacanth. In algae another class of acidic gums are found in which the acidity is due either to sulphate acid ester groups or to uronic acids. Examples are agar, alginic acid, and carrageenin. The third class contains gums that are obtained from seeds, examples being the seeds of the carob tree (Ceratonia siliqua). Natural gums are increasingly being replaced by synthetic substitutes in the manufacture of adhesives.