Carbohydrate molecules aren't only used as energy molecules and they're not only used to actually provide the matrix around the cell structure and integrity, but they can also be used to actually modify the properties and increase the functionality of protein molecules. So the process by which we covalently attach a carbohydrate component onto a protein molecule is known as protein glycosylation, and this type of molecule is known as the glycoprotein. Now, previously we also discussed proteoglycans and we said that proteoglycans are also an example of biological molecules that contain a protein component as well as a sugar component. So what exactly is the difference between a proteoglycan and a glycoprotein? Well, in the case of glycoproteins, the sugar component makes up a much smaller percentage by mass of the biological molecule than compared to the proteoglycan. And this is the main difference between these two types of biological molecules.