Extracellular and cell surface macromolecules derived from a class of glycoproteins of high molecular weight occurring primarily in the matrix of connective tissue and cartilage. Proteoglycans are composed of a protein core with sites for the attachment of one or more polysaccharide chains, particularly glycosaminoglycan; they assemble polysaccharides rather than proteins in their side chains. PGs function in cell adhesion, growth, and organization of the extracellular matrix.