What are the roles of endoplasmic reticulum and ribosomes in protein synthesis?
Ribosomes are the organelles for protein synthesis. When ribosomes translate proteins on surface of endoplasmic reticulum (ER), such proteins are directly delivered within the lumen of ER.
ER with ribosomes scattered on surface is called rough ER (=RER). Its lumen gets supply of proteins. These proteins are packaged, modified and delivered via Golgi vesicles in secretory granules while some other proteins are designated to end up in lysosomal granules.
Proteins delivered in RER are thus supplied to other cell organelles (not delivered in cytoplasmic pool), or secreted outside cell.