If all polypeptides initially begin in the free ribosomes, how exactly do the free ribosomes know to attach onto the membrane bound organelle our endoplasmic reticulum. So basically, the polypeptides contain a special type of sequence of amino acids known as the signal sequence. And it's the signal sequence, as we'll see in just a moment, that directs the free ribosomes to actually bind onto the membrane of the endoplasmic reticulum. Now, the process by which our growing polypeptide chain is transported from the cytoplasm and onto the membrane of the endoplasmic reticulum along with the ribosome is known as translocation. And this is what we're going to discuss in this lecture. So let's begin by looking at the following diagram.