The cell's delivery system is called the endoplasmic reticulum. This structure is a system of tubules, sacs and membranes that may be interconnected. The endoplasmic reticulum is continuous with the membrane of the nucleus as well as the membrane surrounding the cell.
Two types of endoplasmic reticulum exist in a cell: rough and smooth. Ribosomes, structures that make proteins, cling to the rough endoplasmic reticulum, giving it a lumpy appearance. The rough endoplasmic reticulum, or RER, consists mainly of disk-like sacs. The smooth endoplasmic reticulum, or SER, lacks ribosomes on its surface and is composed of mostly tubules.
The SER makes and transports lipids, or fats, out of the cell. The RER transfers proteins from the ribosomes to the outside of the cell or to another structure called the Golgi apparatus to be processed further.