Endoplasmic reticulum, abbreviated ER, is an organelle in living cells that makes proteins and lipids (fats). An organelle is a small, organlike structure within a cell that has a specialized function. The endoplasmic reticulum is found in eukaryotic cells, cells with a nucleus. It is sandwiched between the nucleus and another organelle called the Golgi apparatus. This position helps the endoplasmic reticulum carry out its many important functions.
There are two kinds of endoplasmic reticulums: the rough ER and the smooth ER. Although they have different names, these two are connected and are considered to be different parts of the same organelle.
The rough endoplasmic reticulum consists of a collection of saclike structures. These structures resemble long, thin balloons stacked one upon another. The sacs are linked, forming a continuous interior space called the lumen. The long, balloonlike parts are dotted with structures called ribosomes. These structures are what give this part of the ER its rough appearance.
The smooth endoplasmic reticulum is made up of tubelike structures that do not carry ribosomes. The tubes of the smooth ER are connected to one another as well as to the sacs of the rough ER.
The ribosomes of the rough endoplasmic reticulum produce many of the proteins needed by cells to carry out daily activities. Protein production begins in the cell’s nucleus. Genes stored in the nucleus contain the chemical instructions for making proteins. These instructions are copied from genes into the structure of a molecule called messenger RNA (ribonucleic acid). The messenger RNA can pass out of the nucleus, carrying the instructions to ribosomes in the ER and elsewhere. Ribosomes assemble proteins according to the instructions.
ER ribosomes secrete the proteins directly into the lumen, where they are modified and folded. Most proteins made in the ER are transported to the Golgi apparatus for sorting and processing. From there, they may be packaged into bubblelike structures called transport vesicles. Transport vesicles can be used to move proteins around within the cell or to dump them outside of it.
The smooth endoplasmic reticulum does not make proteins. It makes other materials such as lipids and cholesterols that are needed by the cell. Another important function of the smooth ER is to break down and remove drugs and other compounds that might be toxic to the cell. In certain cells, such as liver cells, the smooth ER also plays a critical role in converting stored molecules of glucose, a type of sugar. The smooth ER converts glucose into a form that can be transported out of the cell and deposited into the bloodstream.
