These are unbranched polysaccharide chains, which are made up of long chains of repeating disaccharide units. One sugar residue in the pair is always an amino sugar (N-acetylglucosamine, or N-acetylgalactosamine) – hence the name aminoglycan. The other sugar residue is usually glucoronic or iduronic acid. The most important thing to know about glycosaminoglycans is that they are: 1) highly negatively charged, so they attract lots of cations (i.e. sodium ions), which in turn causes lots of water to be sucked into the matrix; 2) inflexible, as the long sugar chains cannot fold up like proteins, so they remain extended and 3) they are strongly hydrophilic. These properties mean that the GAG’s form a matrix like a sponge, that sucks in lots of water, like a porous hydrated gel, and they are good at resisting compressive forces. This is particularly important for example in cartilage, a form of connective tissue, which is found in joints which have to withstand large compressive forces as we walk around. The extracellular matrix consists of 10% GAG’s and most of the rest is water. This also makes nice big spaces for diffusion of molecules, and cells to move around in.