The endoplasmic reticulum is a system of interconnected tubes and pouches inside a cell, like a hollow funnel cake. There are two different specialized areas of the endoplasmic reticulum (ER), each with different functions. One is called the smooth ER and the other is called the rough ER. The two are connected, but produce different types of molecules for the cell. The smooth ER makes the lipid molecules that are the stuff of cell membranes, while the rough ER makes the protein machines of the cell.
The endoplasmic reticulum is a system of interconnected tunnels. The tunnel walls are membranes that prevent water from freely moving in or out. Membranes are made of oily molecules called phospholipids. The special thing about phospholipids is that they form sheets -- membranes -- that separate water, salts, sugars and proteins on the outside from those that are on the inside. The rough ER is called rough because it has many protein machines called ribosomes that are attached to its surface. The smooth ER is called smooth because it does not have ribosomes on it and looked smooth to the person who first observed it under the microscope.
The smooth ER has the special job of making lipids, or oily molecules, for the cell. There are two main types of lipids that come from the smooth ER. One is the phospholipids, which are molecules that have an electrically charged head and a water-fearing tail. This north pole and south pole nature is why phospholipids arrange themselves as membranes and make up the membranes of a cell. The other type of lipid that comes for the smooth ER is cholesterol. In addition to being part of cell membranes, cholesterol is modified by the smooth ER to make androgens and estrogens, the male and female sex steroids.
The rough ER has the job of making lots and lots of proteins. Ribosomes are the machines that make new proteins. Many ribosomes can be found on the outer surface of the rough ER because they are inserting the new proteins that they are making into the rough ER. Ever wonder how proteins on the surface of the cell can be stuck in the cell membrane? Well, those proteins got shoved into the membrane of the rough ER while their ribosomes were making them and inserting them into the rough ER’s inner chamber. The rough ER will eventually ship that part of its membrane to the surface of the cell, which is how that protein ends up on the surface of the cell.
The rough ER makes proteins and ships them to the rest of the cell or out of the cell. However, it does this with the help of another organelle called the Golgi apparatus. The Golgi is also a system of tubed membrane tunnels. If the rough ER is the front desk of a UPS shipping office, the Golgi is the backdoor. The rough ER sends the proteins that were inserted into its inside or its membrane to the Golgi. The Golgi adds sugar molecules to these proteins, which serve as address labels. The Golgi then ships these proteins in small pouches called vesicles. The pouches deliver proteins to places throughout the cell or out of the cell.
