What is the primary difference between a glycoprotein and a proteoglycan?
In glycoproteins, the protein component is synthesized first, then a nascent carbohydrate group is covalently attached to the protein. For proteoglycans, the carbohydrate was generated first, then a newly-made peptide attached.
Glycoproteins are found only in the membrane and proteoglycans are found only in the cytosol.
Glycoproteins have a higher protein content by mass, being typically 50%+ of the total mass. Proteoglycans have a much higher carbohydrate content, with carbohydrates comprising typically 95%+ of the total mass.
There is no difference; the two terms are simply different terms for the same type of biomolecule.