Discovery and Origin
Endoplasmic reticulum (also called ergastoplasm) was discovered by Porter et al. (1945) and the term ER was given by Porter (1953). According to different views, ER originates either from invagination of the plasma membrane or from the nuclear envelope.
Occurrence
Endoplasmic reticulum is present in all eukaryotic cells except ova, embryonic cells and mature erythrocytes. It forms more than 50% of the total membrane system of the cell. In muscle cells, it is known as sarcoplasmic reticulum.
Structure
Endoplasmic reticulum is a system of membrane‑bound channels and consists of three main components:
- Cisternae – flattened, parallel, interconnected sacs
- Tubules – branched tubular network
- Vesicles – small, round or oval sacs
Types of Endoplasmic Reticulum
1. Rough Endoplasmic Reticulum (RER)
RER bears ribosomes on its surface and therefore appears granular. Ribosomes are attached to ER by a glycoprotein called ribophorin (ribophorin I and II). RER contains minute pores through which newly synthesized polypeptides pass into the lumen for transport. RER is abundant in cells actively engaged in protein synthesis and secretion such as plasma cells, goblet cells, pancreatic cells and certain liver cells.
2. Smooth Endoplasmic Reticulum (SER)
SER lacks ribosomes and appears smooth. It plays an important role in the synthesis of glycogen, fats and sterols, and in detoxification of drugs and poisons. SER is well developed in liver cells, muscle cells, adipose tissue and steroid‑secreting cells.
Functions of Endoplasmic Reticulum
- Divides the cytoplasm into compartments for separation of biochemical activities
- Provides large surface area for enzymatic reactions
- Acts as a site for protein synthesis (RER)
- Functions as a transport and storage system
- Plays an important role in lipid metabolism and glycogen synthesis