The rough endoplasmic reticulum is the site of protein synthesis and the smooth endoplasmic reticulum is the site of lipid synthesis. what is the third compartment of the endomembrane system and what is that compartment's function? a. golgi apparatus; finishes the processing of the proteins and sorts them for export out of the cell b. lysosomes; sacs which export the proteins and lipids out of the cell c. ribosomes; the place of protein transcription d. vesicles; packages of proteins surrounded by cell membranes that sort the proteins and lipids for packaging e. none of the above is correct.