* Rough ER: This part of the ER has ribosomes attached to it. Ribosomes are the sites of protein synthesis.

* Smooth ER: This part of the ER is responsible for lipid (fat) synthesis, detoxification, and other metabolic processes.

While the ER is the primary site of fat and protein production, other organelles also play a role:

* Golgi apparatus: Processes and packages proteins and fats made by the ER.

* Mitochondria: Provides the energy necessary for the synthesis of fats and proteins.

So, the ER is the workhorse of the cell when it comes to making fats and proteins, but it works in collaboration with other organelles.