Bulk of the cytoplasmic compartment of a cell is occupied by complex , continuous channel system in the form a network of closed membrane bound and intercommunicating branched tubules, vesicles and flattened sacs ,This system was discovered by porter (1945) and named endoplasmic reticulum, by him in 1953. It is distinguished into two part rough or granular endoplasmic reticulum and smooth or agranular endoplasmic reticulum ,At place tubules of the ER may be seen connected with plasma membrane and nuclear envelope.
The wall of endoplasmic reticulum is a unit membrane structurally resembling the plasma membrane .Its outer surface faces the cytosol and the inner one faces the lumen of the reticulum, which contains a watery fluid , The roughness of the RER is because of numerous minute particles, called ribosomes, attached to its outer surface.
Functions of endoplasmic reticulum:
1. It provides mechanical support to the cytoplasm as an accessory to the cytoskeleton,
2. It acts as a continuous intracellular transport system between nucleus and cytosol and between the cytosol and extracellular medium, exchanging materials with the cyotosol by osmosis, diffusion and active transport .
3. The RER collects and stores, in the fluid of its cavity, the proteins synthesized by its surface bound ribosomes. Many of these proteins are first processed by Means of enzymes and , then transported to the cytosol for use by various other organelles.
4. The SER is rich in other important metabolic enzymes which bring about synthesis of triglycerides for storage such as in tissues, of phospholipids and cholesterol in all cells for generating new units membrane required for cell repair growth and division , and of steroid hormones inc certain endocrine glands.
5. In liver cells, the SER is involved insynthesis of lipoproteins carbohydrade metabolism synthesis of glycogen , its storage and its subsequent breakdown into glucose .
6. Its vesicles reform the nuclear membrane at telophase stage of cell division , This membrane dissolves at the earlier stages of division .
7. In muscle cells. It serves for intracellular conduction of impulses and release and withdraw of calcium ions.
8. In lymphocytes , plasma cells and liver cells etc. SER is involved in detoxification of endogenous and exogenous antigens.