Now, right outside the nucleus of most eukaryotic cells is a network of membraneous faults known as the endoplasmic reticulum, or simply the Er. Now, the membrane of the endoplasmic reticulum is a bilayer. It's a phospholipid bilayer that separates the cytosol portion of our cell with the inside space of our Er. Now, the space the fluid inside our endoplasmic reticulum is known as the Er lumen, or this is thermal space. Now, the Er can basically be subdivided into two regions. We have the smooth Er and we have the rough Er.