Polysaccharide

Sugar consisting of many entities. Polysaccharides function as the energy depots in the cells and as structural elements both in cell walls and in stems and leaves. As opposed to the simple polysaccharides used by plants to store energy—for example, in the form of glycogen and starch—the polysaccharides used by algae as structural elements are more complex and heterogeneous. On the one hand, plants make use of a class of polysaccharides, called pectins, that can be used as thickeners. Algae, on the other hand, make use of polysaccharides that are distinctly their own: alginate, carrageenan, and agar. They compose what are known as soluble dietary fibers, which can absorb water in the stomach and intestines and form gels. Insoluble dietary fibers are also polysaccharides—for example, cellulose. See also carbohydrates.