(a) Schematic diagram of type I collagen fibril structure. Helical collagen molecules form from three polypeptide chains, and these associate laterally to form collagen fibrils with a characteristic banded structure. (Reproduced with permission from JPK Instruments AG, Berlin, Germany.) (b) Schematic diagram of proteoglycan aggregate macromolecule, depicting the glycosaminoglycan side-chains and the associated sulfated molecules (keratan sulfate and chondroitin sulfate). The sulfate groups are highly negatively charged and cause the aggrecan to spread out. (Reproduced with permission from themedicalbiochemistrypage LLC.)