Cartilage, a “connective tissue,” is composed of cells (chondrocytes), protein fibers (chiefly collagen) and clusters of complex molecules called “proteoglycans.” A proteoglycan consists of a long protein (polypeptide) with many side chains attached to it. (The structure of proteoglycan looks somewhat like a test-tube brush.) The attached side chains are polysaccharides-chiefly chondroitin sulfate and keratin sulfate. Glucosamine is a key component of keratin sulfate. Glucosamine is also a component of hyaluronic acid, another glycosaminoglycan found in cartilage and other connective tissues. Hyaluronic acid forms the backbone for the proteoglycan clusters. Glycosaminoglycans have a strong attraction for water. The water-holding ability of proteoglycan clusters to hold water gives cartilage its strong, sponge-like quality. It also allows nutrients to flow into cartilage and wastes to flow out.