The endoplasmic reticulum (often abbreviated as ER) is a network of tunnels which extend away from the nucleus. There are two types of ER, named for their appearance under a microscope.
Rough endoplasmic reticulum (image courtesy of Nanoworld)
Rough ER contains ribosomes, whereas smooth ER does not. Proteins are usually synthesized at these ribosomes and then pass through the ER to other organelles within the cell. Most often, the ER leads the proteins to another set of organelles called the Golgi bodies.
The ER is also responsible for synthesizing new cell membrane should the cell require it. This is because the enzymes which create the lipids in the cell membrane are primarily found in the ER, usually because enzymes, like any other proteins, are synthesized by the ribosomes in the rough ER. However, instead of leaving the ER to the Golgi bodies, the enzymes attach to the walls of the ER.
The Golgi bodies, which are also known as the Golgi apparatus or the Golgi complex, are stacks of membranous pouches with many small vesicles bubbling off or fusing with the membrane. In general, the Golgi bodies act as a transport station, packaging materials (such as the proteins from the ER) into vesicles and sending them off either to other organelles or for excretion (removal) from the cell. Sometimes, the Golgi bodies make minor changes to the proteins which they receive from the ER before they are released into the cell.
