The simplest GAG is hyaluronan (hyaluronic acid, hyaluronate). It is the simplest, because it does not contain sulphated sugars, or complex arrangements of a number of different disaccharide units, it has the longest chain (around 25,000 non-sulphated disaccharide units), it is not covalently linked to other proteins to make proteoglycans, and it is not made in the cell and released by exocytosis, but it is made by an enzyme complex at the cell surface. Because it is so large, it has a molecular weight of 8 x 106 and just fits into a cube of 300nm3, once it is hydrated with water. This means a small amount of GAG absorbs and swells up a large volume, so this GAG is a useful space-filler.