Microscopic studies of Eukaryotic cells reveal the presence of a network of reticulum of tubular structures scattered in the cytoplasm that is called the endoplasmic reticulum (ER).
Components of Endoplasmic Reticulum:
- Cisternae
- Vesicles
- Tubules
Structure of these three are:
- Cisternae – These are long flattened and unbranched units are arranged in stacks.
- Vesicles – These are oval membrane-bound structures.
- Tubules – These are irregular, often unbranched tubes bounded by a membrane. Tubules may be free or associated with cisternae.
The structure of the Endoplasmic Reticulum is like the Golgi body but in E.R cisternae, vesicles and tubules are isolated in the cytoplasm and these do not form complex.
Golgi body is a localized cell organelle while the endoplasmic reticulum is widespread in equal in the cytoplasm. E.R is often termed as “system of membranes”
E.R divide the intracellular space into two distinct compartment that is luminal (inside E.R) and extraluminal (cytoplasm) compartments.
80s ribosomes binds by their larger subunit with the help of two glycoproteins (Ribophorin Ⅰ and Ribophorin Ⅱ on the surface of Rough E.R)
| Rough E.R (R.E.R) | Smooth E.R (S.E.R) |
| These are granular | These are agranular |
| Mainly composed of cisternae. | Ribosomes and Ribophorins absent |
| Abundantly occurs in the cell which are actively involved in protein synthesis and secretion Examples: Liver, Pancreas, Goblet Cells | Mainly composed of tubules. Abundantly occurs in cells concerned with glycogen and lipid metabolism. In animal cells, liquid-like steroidal hormones are synthesized in S.E.R. Examples: Adipose tissues, Interstitial cells, Muscles, Glycogen storing in liver cells and adrenal cortex. |
Modifications of Endoplasmic Reticulum:
- Sarcoplasmic Reticulum (S.R): These smooth endoplasmic reticulum occur in skeletal and cardiac muscles. Sarcoplasmic Reticulum stores Ca+2 and energy-related compounds required for the contraction of muscles.
- Microsomes: These are pieces of E.R with associated ribosomal particles. These can be obtained by fragmentation and high-speed centrifugation of cells. They do not exist as such in a living cell. Scientists used microsome for the study of in-vitro protein synthesis.
- Ergastoplasm:- Ribosome with small parallel cisternae of E.R than called Ergastoplasm. In nerve cell, it is called Nissl’s bodies.
- Myeloid Body: It is specialized smooth endoplasmic reticulum, which pigmented epithelial cells of the retina in the eye. It is a light-sensitive structure and involved in pigment migration.
Functions of Endoplasmic Reticulum:
- Mechanical support:- Microfilaments, Microtubules and E.R forms endoskeleton cell.
- Intracellular exchange of materials:- Endoplasmic reticulum forms an intracellular conducting system for the cell to communicate for any change. Transport of materials in the cytoplasm from one place to another may occur through the E.R. At some places E.R is also connected to the plasma membrane so, E.R can secrete the material outside the cell.
- Rough E.R:- Provides sites for the protein synthesis, because rough E.R., has ribosome on its surface.
- Lipid synthesis:- Lipid (cholesterol and phospholipids) synthesized by the granular portion of E.R. (Smooth E.R).
- Cellular metabolism:- The membrane of the reticulum provides an increased surface for metabolic activities within the cytoplasm.
- Formation of the nuclear membrane:- Fragmented vesicle of the disintegrated nuclear membrane and E.R. elements arranged around the chromosomes to form a new nuclear membrane during the cell division process.
- Formation of lysosomes, Golgi body and some Micro bodies.
- Detoxification:- Smooth E.R concerned with detoxification of drugs, pollutants and steroids. Cytochrome P450 in E.R. act as an enzyme which functions in the detoxification process.
- E.R provides the precursor of secretory material to Golgi body.
