And if the polypeptide chain actually contains the signal sequence, what happens is a special complex of molecules known as the signal recognition particles or SRP, which are basically a complex of RNA and protein molecules, will bind onto the signal sequence. And once they bind onto the signal sequence, they will basically transport this entire complex, known as the SRP ribosome complex, onto the membrane of the endoplasmic reticulum. So once again, polypeptides variedestine for secretion or to be embedded in the cell membrane, begin synthesis in the free ribosome. However, shortly after the synthesis actually begins, it stops, because the ribosomes are transported into the cytoplasmic side of the endoplasmic reticulum's membrane and the polypeptide synthesis then resumes and the growing polypeptide chain extends into the er lumen. Now, the question is, what exactly is the mechanism by which the ribosome knows to migrate onto the endoplasmic reticulum? So this should be not ribosome, but endoplasmic reticulum.