The nuclear envelope joins with the membrane of the endoplasmic reticulum (ER). It is a system of complex network spread throughout the cell.
These are present in Eukaryotic cells & are of two types: RER & SER
On the outside surface of the RER are ribosomes.
The main function of RER is to keep together and transport the proteins made on the ribosomes.
Instead of simply diffusing away into the cytoplasm, newly made proteins enter into RER. Here, they fold into three-dimensional shape.
A mature cell that makes and secretes large amounts of protein such as one that makes digestive enzymes — has RER that occupies as much as 90 per cent of the total volume of the cytoplasm.
RER is also a storage unit for enzymes and other proteins.
Small vesicles containing newly synthesized proteins pinch off from the ends of the RER and either fuse with the Golgi complex or pass directly to the cell surface membrane.
ER with no ribosomes attached is known as SER.
Smooth ER is not involved in protein synthesis but is the site of steroid (lipid hormone) production. It also contains enzymes that detoxify, or make harmless, a wide variety of organic molecules, and it acts as a storage site for calcium in skeletal muscle cells.
The channels having material present in them which is separated from the cytoplasmic materials by the spherical or tubular membranes is called cisternae.
- R provides mechanical support tothe cell.
In some cells SER transmit impulses. For example nerve and muscle cells.
Both RER and SER are involved in the transportation of materials from one part of the cell to the other.
Difference between Smooth Endoplasmic Reticulum and Rough Endoplasmic Reticulum
|Smooth Endoplasmic Reticulum (SER)||Rough Endoplasmic Reticulum (RER)|
|1. It does not bear ribosomes over the surface of its membranes.||1. It possesses ribosomes attached to its membranes.|
|2. It is mainly formed of vesicles and tubules.||2. It is mainly formed of cisternae and a few tubules.|
|3. It is engaged in the synthesis of glycogen, lipids and steroids.||3. It takes part in the synthesis of proteins and enzymes.|