The ribosome free part of the endoplasmi reticulum that takes part in the synthesis of various lipids (e.g., for building membranes such as phospholipids), fatty acids and steroids (e.g., hormones), and also plays an important role in carbohydrate metabolism, detoxification of the cell (enzymes in the smooth ER detoxify chemicals), and calcium storage. It is well developed in cells that produce steroid hormones and in liver cells. It also is a large transporter of nutrients found in the cell.
The portion of the endoplasmic reticulum studded with ribosomes that manufactures and transports proteins destined for membranes. These ribosomes insert proteins into the lumen of the ER where the ER helps modify and fold the protein.