Rough ER is found throughout the cell but the density is higher near the nucleus and the Golgi apparatus. Ribosomes on the rough endoplasmic reticulum are called 'membrane bound' and are responsible for the assembly of many proteins. This process is called translation.
The smooth ER also regulates and releases calcium ions and processes toxins. It is described as 'smooth' to distinguish it from rough ER, which has ribosomes for protein synthesis on its surface. Each organelle in the cytoplasm of a cell is responsible for performing a certain function.