Structurally, the endoplasmic reticulum is a network of membranes found throughout the cell and connected to the nucleus. There are two basic types of ER. Both rough ER and smooth ER have the same types of membranes but they have different shapes. Rough ER looks like sheets or disks of bumpy membranes while smooth ER looks more like tubes. Rough ER is called rough because it has ribosomes attached to its surface. Rough ER (RER) is very important in the synthesis and packaging of proteins. Smooth ER (SER) acts as a storage organelle. It is important in the creation and storage of lipids and steroids.