All living organisms are composed of cells and cell products. Each eukaryotic cell contains several cytoplasmic organelles. Among the organelles, endoplasmic reticulum performs as intracellular transport system for various substances and also helps to exchange the materials between nucleus and cytoplasm. Prokaryotic cells and RBC (red blood cell) do not have any endoplasmic reticulum.
Endoplasmic reticulum is the interconnected system of membrane-bounded tubules and vesicles which form irregular reticulum or network in the cytoplasmic matrix. All nucleated animal and plant cells contain endoplasmic reticulum. This membranous system extends throughout the cytoplasm from nuclear membrane to plasma membrane. According to some scientists, endoplasmic reticulum is originated by the invagination of nuclear membrane but some suggested that they grow through the expansion of the pre-existing membrane.
Physical Structure of Endoplasmic Reticulum
Morphologically, endoplasmic reticulum may happen in the three structures:
Cisternae: They are long, flattened, unbranched membrane-bound tubule with a diameter of 40-50 µm. They arranged parallel to each other to form lamellae. They are coated with ribosome.
Vesicles: They are oval or rounded membrane-bound isolated vacuolar structure with a diameter of 25-500 µm.
Tubules: They are long and branched structures with a diameter of 40-90 µm which form the reticular system in the cytoplasm along with cisternae and vesicles. Like the unit membrane, it is made up of lipo-protein.
Types of Endoplasmic Reticulum
There are two types of Endoplasmic reticulum such as Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER). Rough endoplasmic reticulum is also known as granular endoplasmic reticulum because it is attached with ribosomes while smooth endoplasmic reticulum is also known as agranular endoplasmic reticulum because they do not have attached ribosome with them.
Chemical Composition of Endoplasmic Reticulum
Chemically, it is made up of the following components:
Functions of Endoplasmic Reticulum
Endoplasmic Reticulum (ER) performs the following functions: