The endoplasmic reticulum (ER)
is an organelle (a specialized cell part) that appears in all eukaryotic cells. (Eukaryotic organisms include all living things except for bacteria and cyanobacteria, the latter widely known as blue-green algae.) The ER is made of a series of membranes that extend throughout the jelly-like cytoplasm. The ER's main function is to manufacture and transport material to other locations.
There are two regions in the ER, with differing structures and functions:
- Rough ER is coated with ribosomes, so it looks bumpy. It consists of stacks of flattened sacs involved in the production and export of proteins, glycoproteins, and hormones. In cross section, rough ER looks like a series of maze-like channels.
- Smooth ER, without ribosomes, produces lipids and steroids. It also has enzymes that are involved in the breakdown of toxins, drugs, and toxic byproducts from cellular reactions. Smooth ER also stores steroids and ions for the cell. It looks like a network of smooth tubes.