Endoplasmic reticulum is a complex membrane lined network of flattened sacs, tubules and vesicles that runs throughout the cytoplasm of eukaryotic cells from plasma membrane to nuclear envelope. The ER was first noted by Porter, Claude and Fullman in 1945. It was named ER by Porter in 1953. It is absent in Prokaryotes but present in all the eukaryotes except germinal cells and mature mammalian erythrocytes.
In mature cells, ER occurs in two forms: Rough (RER) and Smooth(SER)
Functions of RER1. Smooth Endoplasmic Reticulum is formed from RER through loss of ribosomes.
2. It possess ribophorins for holding ribosomes over it.
3. Luminal side of RER possess enzymes for processing polypeptide synthesised by attached ribosome.
4. RER has a large surface area which provides proper space to ribosomes for their activity without interference from others.
5. Secretory, lysosomal and membrane proteins formed by ribosome attached to RER enter its lumen for intracellular and extracellular transport.
6. Zymogens of lysosome enzymes are synthesised by RER.