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. And let's begin by discussing what the rough Er is and what the functions of the rough Er are. So the membraneous folds found closest to the nucleus of our cell contain ribosomes embedded on the cytosol side of our endoplasmic reticular membrane. These ribosomes function to synthesize proteins that are ultimately either embedded into the cell membrane.