So a free ribosome is a ribosome that synthesizes polypeptides that are destined to remain inside a cytoplasm. But the membrane bound ribosomes are those ribosomes that are responsible for forming proteins that are either secreted by the cell or which remain inside the membrane of that cell. Now, the question that we want to discuss in this lecture is the following. 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.