Endoplasmic Reticulum
The large, folded membrane surrounding the nucleus is the endoplasmic reticulum (reticulum = “network”), or ER. The phospholipid bilayer membrane is continuous with the nuclear envelope. The endoplasmic reticulum has two distinct regions: the rough endoplasmic reticulum, or RER, where the membrane is covered inbound (covered) with ribosomes, and the smooth endoplasmic reticulum, or SER, which lacks bound ribosomes.