Some of the essential functions of Endoplasmic Reticulum are as follows:
Endoplasmic reticulum is capable of continual reorganization. Rough ER can convert to smooth ER and vice versa, depending upon the metabolic requirements of the cell. Like the cell as a whole, the endoplasmic reticulum is a dynamic, not a static system. Functions of reticulum depend upon its location and cell type. Following are the important functions of endoplasmic reticulum:
1. Mechanical support:
Endoplasmic reticulum, along with system of microtubules and microfilaments provide mechanical support for the colloidal areas of the cytoplasm.
2. Enzymatic activities:
Membranes of reticulum provide an increased surface for various synthetic and metabolic activities within cell. Enzymes attached to its surface will act more efficiently than when dissolved in cytoplasm. A number of important enzymes are found in the ER membrane, viz., stearases, NADH-cytochrome c-reductase, NADH diaphorase, glucose-6- phosphatase, Mg++ activated ATPase, nuclcotide diphosphatase, etc.
3. Transport of synthetic products:
ER functions as an intracellular transport system, both for nascent proteins and at least sometimes for lipids. Vacuolar system of reticulum helps in collection of synthetic products of cell. These products accumulate in reticular vacuoles and later on are transported to other parts of cell or for release from the cell in secretion. Perhaps the osmotic properties of the membranes are significant for this activity.
4. Storage of metabolites:
Endoplasmic reticulum plays important role in the conservation of metabolic products such as proteins and sometimes lipids.
5. Formation of nuclear membrane:
Membranes of reticulum are source of origin of some other membranes. Nuclear membrane also develops from reticulum during cell division.
6. Intracellular impulse conduction:
Specialised modification like sarcoplasmic reticulum transmits impulses intracellular.
7. Protein synthesis:
Caro and Pallade (1964) have produced morphological evidence that granular (ribosomes) endoplasmic reticulum is the site of protein synthesis. This suggests that ER may provide a pathway for messenger RNA to move from nucleus to the cytoplasmic ribosomes. This synthesized protein later on becomes concentrated above Golgi apparatus.
8. ATP synthesis:
The ER is also a site of ATP synthesis in the cell. This ATP is utilized as a source of energy for the intracellular transport of materials or in RNA metabolism involving ribosomes.
9. Synthesis of glucose and glycogen:
By experiments on liver cells, it has been proved that ER is associated with the synthesis, storage and metabolism of glucose and glycogen. Membranes of smooth ER in mouse liver cells function in glycogenolysis. Smooth, tubular ER of interstitial cells shows the ability to catalyze the oxidation of progesterone to testosterone and acetic acid. This oxidation involves four enzymes found in ER.
10. Synthesis of lipids:
Agranular endoplasmic reticulum is related with the synthesis and metabolism of the lipids (Christensen 1961 and Claude 1968). Agranular ER is found in abundance in those cells in which active lipid metabolism occurs.
11. Agranular ER also synthesizes non-protein substances such as the cholesterol, glycerides, hormones (testosterone and progesteron), etc. In retinal epithelial cells agranular ER also synthesizes certain visual pigments from vitamin A.
12. In muscle cells, there are large amounts of smooth ER and it is known as sarcoplasmic (sarco = muscle) reticulum. It’ is involved in the concentration of calcium ions by a process requiring the utilization of ATP (energy-requiring). The calcium ions are stored in the sarcoplasmic reticulum and released after the muscle stimulation by nerve impulse, hormones or other means. The release of Ca++ leads to muscular contraction.
13. In plant cells, during cell division, ER fragments form the new nuclear envelopes of the daughter nuclei.