Endoplasmic Reticulum

Endoplasmic reticulum (ER) is a series of membrane-bound chambers within a cell which play an important part in the synthesis of lipids and proteins which are destined to be inserted into 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 excretion.

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. The amino acid chain which these ribosomes build is injected into the ER, as shown in this diagram.

Rough endoplasmic reticulum

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 malformed 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 malformed 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.