Endoplasmic Reticulum (ER). This folded membrane forms sacs to store proteins or other substances. It creates a vast surface area where the manufacture of proteins and new membranes can take place. Endoplasmic reticulum is a folded mass of membranes made of the same phospholipids found in the plasma membrane. There are two types of ER smooth (without ribosomes) and rough (with ribosomes)