Endoplasmic reticulum (ER) is one of the organelle that makes up the endomembrane system of eukaryotes. The ER is of two types viz: rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).
The RER is called "rough" because of the ribosome organelles that are attached to its surface. The attachment of this ribosomes (site of protein synthesis) also defines its function. The RER is principally involved in the production of protein. Hence, the Rough Endoplasmic Reticulum functions mainly in the production of membrane and proteins secreted by the cell.
All the cells in a person's body have the same DNA and the same genes. However, the difference between cells in different tissues and organs is that the "expression" of the genes differs between cells. Expression means that the message from the DNA is being copied and made into protein.
