What does the endoplasmic reticulum (ER) do?

The endoplasmic reticulum (ER) is the largest organelle in a cell. It consists of an intricate network of membranes located within the cytoplasm of the cell and performs multiple functions. The primary role of the endoplasmic reticulum is to synthesize proteins that allow the rest of the cell to function. It also helps with storing calcium and metabolizing lipids, and is the site of the biogenesis of cellular membranes.     

The endoplasmic reticulum or ER is only found in the eukaryotic cells. It may be rough or smooth. The outer surface of the rough ER has ribosomes attached to it. These are tiny, round organelles whose main function is to produce the proteins. The smooth ER, which does not have ribosomes attached to it, is responsible for producing other vital substances needed by the cell. Together the rough and smooth endoplasmic reticulum produces all the protein and other substances that the rest of the cell needs to function optimally.