What is the Endoplasmic Reticulum?
The endoplasmic reticulum, sometimes abbreviated as ER, is an important organelle found in most eukaryotic cells. It resembles a large network of folded membranes and tubes that surrounds part of the nucleus. The endoplasmic reticulum is always closely associated with the nucleus because it is responsible for manufacturing proteins based on the genetic “instructions” contained in the nucleus.
There are two types of endoplasmic reticulum: rough and smooth. The rough endoplasmic reticulum appears “rough” because it is covered in attached ribosomes. Rough endoplasmic reticulum is positioned right up against the nucleus, so that mRNA copied from the DNA in the nucleus can easily pass to ribosomes on its surface.
From there, a ribosome begins to translate the mRNA into a chain of amino acids in the internal space of the rough endoplasmic reticulum. When the amino acids have built into complete proteins, part of the endoplasmic reticulum’s membrane will pinch off to form a vesicle. A vesicle is somewhat like an envelope used to send proteins to different locations in a cell. Proteins built in the rough endoplasmic reticulum are sent to the Golgi body or the directly to the cell membrane.
The smooth endoplasmic reticulum does not have any attached ribosomes and is not involved in protein synthesis. Instead, the smooth endoplasmic reticulum helps to create lipids and steroids. A special kind of smooth endoplasmic reticulum, called the “sarcoplasmic reticulum”, is also able to store ions that may be needed by the cell at a later time.
Almost all eukaryotic cells have an endoplasmic reticulum, with the exception of red blood cells and sperm cells. However, the size of a cell’s endoplasmic reticulum varies greatly depending on the cell’s job. Cells that need to create large numbers of proteins, such as liver or pancreas cells, usually have a very extensive endoplasmic reticulum.
Endoplasmic Reticulum. (2008). Encyclopedia Brittanica. Retrieved from