The rough endoplasmic reticulum (rough ER) is a complex membrane bound organelle and functions in the transport of the proteins made by ribosomes on its surface. Found in eukaryotic cells — the cells of plants, animals and humans — the rough endoplasmic reticulum changes with the needs of the cells. When the cell is actively making proteins, the rough ER can enlarge and become more complex. Ribosomes that dot the surface of the rough endoplasmic reticulum move onto the membrane as they begin to make protein and move off when they are done. The function of the rough endoplasmic reticulum is intimately bound to the production and transport of proteins in the cell.

Shaped like a three dimensional continuous maze, the rough ER is a system of membranes that extends from the nucleus of the cell to the cell membrane. During protein synthesis, the ribosomes on the rough ER are the site where messenger ribonucleic acid (mRNA) builds proteins according to the specific code it carries. As the protein is produced, it is moves into the interior of the rough ER where it awaits transport. Once the protein is complete, a bit of the rough ER membrane breaks off to form a vesicle around the protein. If the protein is needed within the cell, it may be transported in the vesicle to the Golgi apparatus or transferred to other organelles.

Proteins needed outside the cell travel through the rough ER to the cell membrane. Hormones are an example of the proteins that cells secrete via the rough ER. The membrane of the rough ER is continuous with the cell membrane, so the proteins can travel directly through the rough ER to the outside of the cell. Cells that are actively producing hormones may have an enlarged rough ER to accommodate the synthesis of these proteins.

Another function of the rough endoplasmic reticulum is modification of some of the proteins produced by the ribosomes. Once the protein enters the pathways in the interior of the rough ER, enzymes might add other molecules to the proteins. These altered proteins are then moved through the rough ER to where they are needed. Inside the rough ER, some proteins might be stored while others are sorted. Defective proteins are identified and stopped from further transport.

A second kind of endoplasmic reticulum is also found in eukaryotic cells. It lacks ribosomes along the outer surface and has a smooth appearance. This is the reason it is called smooth endoplasmic reticulum. Some of the main functions of smooth ER are the synthesis and movement of lipids and steroids.