The endoplasmic reticulum, or ER, is a cellular network of membranes that form tube-like chambers called "cisternae." It has a wide range of functions, from the synthesis of lipids and proteins to detoxifying drugs and poisons. The ER membrane is actually continuous with the nuclear envelope, and folds back and forth across itself to form the cisternae. The space thus created is called the "cisternal space." It is so important to cellular function that it typically makes up over half of the total membrane of the cell.
There are two types of ER, smooth and rough. Rough ER is studded with ribosomes, which gives it a rough appearance when viewed by the electron microscope. Smooth ER lacks ribosomes. The two types are not separate from each other, but are different regions of the same membranous network. Ribosomes are the site of protein manufacture by means of translation of messenger RNA from the cell nucleus.
The functions of the ER depend on the type of cell it is a part of. For example, in a liver cell, the smooth ER plays a key role in detoxifying drugs and poisons. In fact, frequent use of drugs such as sedatives or alcohol cause the cell to increase the amount of smooth ER present to more efficiently metabolize the substance. This, in turn, leads to an increase in tolerance to the substance, requiring higher levels to achieve the same effect. In a muscle cell, the smooth ER plays a major role in contraction by means of a calcium pump that passes calcium into the cisternal space. When the muscle is stimulated, the calcium rushes back across the ER membrane to trigger contraction.
In cells that produce specialized proteins, the rough ER is one site of manufacture. A typical example is the production of insulin. The islet cells of the pancreas produce insulin and release it into the bloodstream. The ribosomes on the rough ER of these cells create the insulin protein chain one amino acid at a time. As the protein is being assembled, it is passed into the cisternal space where it begins to fold into its functional shape. Once it has been assembled and folded, the insulin protein is packaged into a transport vesicle that buds off of the ER membrane. This transport vesicle then travels to the Golgi apparatus where it undergoes further processing before being transported out of the cell.
Almost all of a cell's lipids, such as phospholipids and cholesterol, are created in the ER. In a series of steps, the lipids are assembled, inserted into the ER membrane, and then packaged in transport vesicles or attached to special transfer proteins that deliver them to their destination.
The importance of the vast membranous network of the endoplasmic reticulum should not be underestimated. It is a vital organelle of every eucaryotic cell. It produces almost all of the body's lipids, and many of its proteins, and also functions to detoxify drugs and poisons. Without it, higher life forms would not be possible.