The ER can be classified in two functionally distinct forms: smooth endoplasmic reticulum (SER) and rough endoplasmic reticulum (RER). The morphological distinction between the two is the presence of protein-synthesizing particles, called ribosomes, attached to the outer surface of the RER. The functions of the SER, a meshwork of fine tubular membrane vesicles, vary considerably from cell to cell, one important role being the synthesis of phospholipids and cholesterol, which are major components of the plasma and internal membranes. The RER is generally a series of connected flattened sacs. It plays a central role in the synthesis and export of proteins and glycoproteins and is best studied in secretory cells specializing in these functions. The many secretory cells in the human body include liver cells secreting serum proteins (e.g., albumin), endocrine cells secreting peptide hormones (e.g., insulin), pancreatic acinar cells secreting digestive enzymes, and cartilage cells secreting collagen.