Endoplasmic reticulum (ER) is an internal membrane that forms branching networks of many interconnected sacs & tubes. There are 2 types of ER: rough & smooth.
The outer side of the rough ER is studded with ribosomes. Rough ER stays closer to the nucleus & coordinates protein synthesis.
Smooth ER lacks ribosomes. It specializes in lipid synthesis, steroid hormone production, & detoxification.