There are two types endoplasmic reticulum. Smooth endoplasmic reticulum is a smooth cellular organelle found near the nucleus of the cell. Rough endoplasmic reticulum is rough because it is covered in ribosomes.
Smooth Endoplasmic Reticulum Function
Rough Endoplasmic Reticulum Function
RNA is synthesized into proteins by ribosomes connected to the rough endoplasmic reticulum.
In this electron microscope image, tiny dots are ribosomes connected to the rough endoplasmic reticulum.