The electron microscope reveals an extensive membrane system in the cytoplasm called Endoplasmic reticulum (ER). It was first reported by Porter in 1945. This continuous membrane system joins the nuclear membrane on one end and the cell membrane on the other.
All cells do not possess E.R. It is generally absent in egg and ambryonic cells. Generally, ELECTRONS is fully developed in cells actively engaged in protein and hormone synthesis.
E.R. is composed of three different types of structure. These are cisternae, vesicles and tubeless.
(a) Cisternae: These are long, flat and unbranched plates or lamellae arranged in parallel rows.
(b) Vesicles: Usually round or ovoid sacs. They often occur isolated in the cytoplasm.
(c) Tubules: They are irregularly branched tube-like structures having a diameter of 50-100µ. These structures are surrounded by this unit membrane of 50-60 thickness and their lumen is filled with the secretory products of the cell.
Types of ER:
Two types of ER, such as smooth walled and rough walled, have been recognized. They may be present in the same or different types of cells.
(i) Smooth Endoplasmic Reticulum (SER): The surface of this type of reticulum is smooth as ribosomesr not attached. Smooth ER is present in cells, which are actively engaged in steroid synthesis, carbohydrate metabolism, pigment production etc.
(ii) Rough Endoplasmic Reticulum: The rough ER have ribosomes attached throughout the surface. These are present in cells, which are active in protein synthesis.
The major functions are the following:
(a) Common to both ER.
(i) Forms the skeletal framework.
(ii) Active transport of cellular materials.
(iii) Metabolic activities due to presence of different enzymes.
(iv) Provides increased surface area for cellular reactions.
(v) Formation of nuclear membrane during cell division.
(b) Function of Smooth ER.
(i) Lipid synthesis
(ii) Glycogen synthesis
(iii) Steroid synthesis like cholesterol, progesterone, testosterone, etc.
(c) Function of Round ER
(i) It provides site for protein synthesis.
(ii) It helps in transport of proteins.