These proteins are known as integral proteins, or they are destined to leave our cell altogether. Now, the membrane of the rough Er is actually physically connected to the membrane of the nucleus known as the nuclear membrane or the nuclear envelope. And for this reason, we see that the perinuclear space, the space between the double layer of our nuclear envelope and the Er lumen, is actually physically connected as well. And that makes sense, because inside our nucleus, in a region known as the nucleolus, we basically synthesize the ribosomal RNA subunits that are needed to create the ribosomes that end up in the rough endoplasmic reticulum. Now, once the proteins in the rough endoplasmic reticulum are synthesized on the cytosol side, we basically take those synthesized proteins, we place them into the Er lumen, and then they travel through the rough Er lumen and eventually into the smooth Er lumen. So now let's discuss the smooth endoplasmic reticulum.