A series of membrane bound chambers within a cell which plays an important part in the synthesis of lipids and proteins which are destined to be inserted in the cell membrane. There are two types of ER: smooth and rough.
Smooth ER is the site of synthesis and digestion of fatty acids and phospholipids. In the liver it is used to modify dangerous chemicals such as pesticides ready for
Rough ER is the site of manufacture of secretory proteins as well as proteins destined to be inserted in the cell membrane. It is rough because of the vast numbers
of ribosomes which stud its surface. As these ribosomes build an amino acid chain, it is injected through the ER into the vesicle, as shown in this diagram. Once the amino acid
chain enters the vesicle it is folded, often with the help of other enzymes.
There are several "quality control" mechanisms to ensure that incorrectly folded proteins do not leave the vesicle. There are special proteins called "chaperones" which stick to mis-formed proteins
and ensure they do not leave the ER. Life could not function well without these mechanisms. However genetic mutations can
cause some diseases which affect these quality control chaperones. For example cystic fibrosis is caused by a build up of
misformed proteins, together with their chaperones, inside ER vesicles.
The proteins made in the ER are packaged into vesicles and passed on the Golgi complex for further processing.