So what exactly is a proteoglycan? Well, a proteoglycan is basically a protein molecule that has attached to some type of glycosaminoglycan or many glycosaminoglycans. In fact, usually the glycosaminoglycan is the predominant component of the proteoglycan. And usually 95% of the proteoglycan by mass consists of glycosaminoglycans and only 5% consists of that protein component. Now, what exactly are the functions of proteoglycans. Well, proteoglycans, these protein, sugar, biomolecules have four important functions which I've listed on the board and the first two functions we're going to focus on in this lecture.