Garnier for the first time had described them as filamentous structure, ergastoplasm. They were first discovered by Claude (1951) under the electron microscope. The term ‘endoplasmic reticulum‘ was first coined by Porter et.al in 1945. E.R. is abundant in liver, pancreas and other actively synthesizing cells. E.R. is absent in RBC, ova, embryonic cells and prokaryotes.
There are two types of endoplasmic reticulum:-
- Smooth Endoplasmic Reticulum
- Rough Endoplasmic Reticulum
Difference between SER and RER
- Ribosomes are present only over the surface of rough endoplasmic reticulum.
- SER is mainly made of vesicles and tubules while RER is made of cisternae and tubules.
- SER involved in the synthesis of glycogen, lipids, and steroids while RER engaged in the synthesis of proteins and enzymes.
- RER helps in the formation of the lysosomes through the Golgi apparatus and SER gives rise to spherosomes.
- Narrow pores are present below the RER, they are the passage of newly synthesized polypeptides into ER channels. These pores are absent in SER.
- RER is often internal and connected with nuclear envelope while is peripheral and may be related to plasmalemma.
- In SER ribophorins are absent but present in RER and function for providing attachment to ribosomes.
- RER may develop from the outer membrane of nuclear envelope and SER may form by loss of ribosome in RER.
However, rough ER may be converted into smooth ER and vice versa, depending upon the metabolic requirements of the cell.
Characteristics of endoplasmic reticulum
- E.R. occurs in three forms cisternae, vesicles, and tubules.
- E.R. originates from the ground substance (hyaloplasm) or infoldings of plasma membrane or evaginations of the nuclear membrane. However, exact nature of origin is unknown.
- E.R. present in retinal cells is called myeloid bodies.
- Those present in muscle cells are called as sarcoplasmic reticulum.
- Microsomes are the fragments of endoplasmic reticulum containing ribosomes.
- They contain a large amount of RNA and phospholipids.
- The high rate of protein synthesis is seen here and re also involved in the synthesis of triglycerides and steroids.
Functions of endoplasmic reticulum
- Endoplasmic reticulum provides mechanical support to the cytoplasm.
- It has a significant role in the intracellular transport and storage of proteins and sometimes lipids.
- It synthesizes lipids and lipoproteins.
- Smooth ER is involved in glycogenolysis.
- It forms other cellular organelles such as Golgi bodies, peroxisomes, and glyoxysomes.
- It creates nuclear envelope at the end of the telophase stage of the cell division.