The endoplasmic reticulum is a type of cell organelles which forms a series of flattened membrane-bound sacs in the eukaryotes. It serves various functions in the cell. Most importantly it folds, modifies and transports proteins. All eukaryotic cells contain endoplasmic reticulum. There are two types of ER: rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum or (SER).
Rough Endoplasmic Reticulum
Rough endoplasmic reticulum (RER) is named for its rough appearance due to the ribosomes attached on its surface. Its is adjacent to the cell nucleus. The membrane of rough ER is continuous with the outer membrane of the nuclear envelope. The ribosomes present on its surface are specialized for making specific proteins which have a signal sequence which help them direct towards the endoplasmic reticulum for processing. The proteins synthesized by the rough ER have specific destinations. For example some proteins remain within the ER while others go to the Golgi apparatus where they are packaged and then transported to various other destinations either within the cell membrane or to the exterior of the cell. Proteins which are synthesized for transport to the Golgi apparatus are moved to the lumen of th ER where they are folded, modified and assembled.
The rough endoplamic reticulum can send signals to the nucleus when there is some problem with the protein synthesis and modification. A signaling mechanism known as Unfolded Protein Responce UPR is activated when more than necessary proteins are accumulated in the lumen of ER. The UPR triggers a another mechanism to control the synthesis of proteins and speeds up the folding and modification of proteins in the lumen. If the UPR fails, apoptosis or programmed cell death occurs.
Smooth Endoplasmic Reticulum
Smooth endoplasmic reticulum is the other type of ER which is not associated with ribosomes. The synthesis of lipids such as cholesterol and phospholipids takes place in the smooth ER. These lipids are involved in the formation of new cell membrane. In some cells smooth ER synthesizes steroid hormones from the cholesterol. In liver cells it helps in the detoxifying the drugs and harmful chemicals.
Sarcoplasmic reticulum is a specialized type of smooth ER which is found in myocytes or muscle cells. It is involved in the regulation of calcium ion concentration in the muscle cells. The sarcoplasmic reticulum plays an important role in excitation-contraction coupling.