Agar is made up of two polysaccharides, agaropectin and agarose. As with carageenan, the backbone of these polysaccharides is the sugar galactose.
Agar is most familiar as the gelatinous growth medium used in petri dishes to culture bacteria. But it also finds uses in cooking as a gel for jellies and fillings. It is a soluble fiber, and as such finds uses as a laxative and diet aid.
As a growth medium, agar has nice properties, such as a high melting point (85 degrees Celsius), and a lower solidifying point (40 degrees). It can stay gelled at incubating temperatures.
Purified agarose is used as a medium for electrophoresis, the separation of proteins or DNA fragments by size using an electric field.