Endoplasmic reticulum is the network of flattened sacs and tubules which perform different functions in the cell. Two regions in the endoplasmic reticulum differ from each other in terms of structure and function. One of the areas is known as rough endoplasmic reticulum since its ribosomes are attached to the membrane’s cytoplasmic side. The other region is known as smooth endoplasmic reticulum because it does not possess attached ribosomes. The smooth endoplasmic reticulum is a tubules network whereas the rough endoplasmic reticulum is a series of flattened sacs. The area inside endoplasmic reticulum is called the lumen. The endoplasmic reticulum extends from the cell membrane to the cytoplasm ad forms a continuous connection with the nuclear envelope. The lumen and the area inside the nuclear envelope are the parts of the same compartment.