Function of Endoplasmic Reticulum

(1) Mechanical Support. The endoplasmic reticulum divides the fluid content of cells into compartments and thus provides additional mechanical support for the colloidal structure of cytoplasm.

(2) Protein synthesis. The RER is the site of secretion of secretory proteins. Proteins are synthesized on the ribosomes and enter the ER cisternae through channels in the membrane. In almost all studies ln almost all studies labeled secretory proteins leave the ER and enter the Golgi complex, from where they are secreted outside the cell.

(3) Glycogen synthesis and storage. Residual glycogen has been found associated with the ER in fasted animals. The enzyme glucose-6-phosphatase has been reported in the ER of rat liver cells. It has been linked to breakdown of glycogen (glycogenolysis).

(4) Lipid synthesis and storage. Electron microscope and autoradiographic studies of Steni and Stein suggested that the ER was the site of triglyceride formation. They injected radioactive labeled material into fasted and ethonal-treated rats.

(5) Synthesis of cholesterol and steroid hormones. Cholesterol is an important precursor of steroid hormones. The major site of cholesterol synthesis is the ER. In rats injected with radioactively labeled acetate, nearly all the labeled cholesterol is found in the microsomal fraction.

(6) Detoxification. The SER membranes have been shown to contain an enzyme system with detoxification properties. Administration or repeated doses of the drug Phenobarbital to rats resulted in increased activity of detoxification enzymes .This increased enzyme synthesis was blocked by protein synthesis inhibitors like actinomysin D and puromycin.

(7) Formation of micro bodies. Closely related with the ER are microbodies, which are small granular bodies filled with an electrondense substance and limited by a single membrane.

(8) Circulation and exchange. The ER acts as an intracellular transport system for various substances. The ER also provides an extensive internal surface for exchange of material between the matrix of the cytoplasm and the lumen of the cisternae. Substances may pass across the ER membranes by diffusion. as well as by active transport.

(9) Membrane flow. Transport of ions, molecules and particles into and out the cells may also take place through membrane flow. Thus substances like RNA and nuclear proteins may pass out from the nucleus outside the cell by the following route: Nuclear membranes → Pores → ER → Golgi complex → Plasma membrane → outside.

(10) Ionic gradients. The sarcoplasmic reticulum of striated muscle is involved in the concentration of calcium ions by an energy-requiring process utilizing ATP. The calcium ions are stored in the sarcoplasmic reticulum. When the muscle is stimulated by nerve impulse, hormones or other means, the calcium ions are released, leading to muscular contraction.