Endoplasmic reticulum (EN-doh-PLAZ-mik reh-TIH-kyoo-lum) is a network of sac-like structures and tubes in the cytoplasm (gel-like fluid) of a cell. Proteins and other molecules move through the endoplasmic reticulum. The outer surface of the endoplasmic reticulum can be smooth or rough. The rough endoplasmic reticulum has many ribosomes on its outer surface and makes proteins the cell needs. The smooth endoplasmic reticulum makes other substances that the cell needs, such as lipids (fats) and carbohydrates (sugars). The endoplasmic reticulum is a cell organelle.