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Quick Notes on Endoplasmic Reticulum!
Structure of Endoplasmic Reticulum in Cell:
Structurally the endoplasmic reticulum (ER) consists of cisternae, tubules and vesicles (Fig. 2.42).
The cisternae are broad, flat, membrane-bound spaces arranged parallel to each other to form lamellae, which are connected with each other. The tubules are 50-1OOA in diameter. The vesicles or sacs are membrane- bound, isolated globose cavities. The membrane of ER has the typical three-layered unit- membrane structure. The endoplasmic reticulum divides the cytoplasm into compartments (Fig. 2.43).
There are two morphological types of ER — the rough or granular (ergastoplasm) and the smooth or agranular form. The rough structure is due to the presence of ribosomes on the membrane surface while the smooth ER are not covered with ribosomes. These are inter-convertible.
Function of Endoplasmic Reticulum in Cell:
The endoplasmic reticulum is the site of secretion of secretory proteins. Proteins are synthesized on the ribosomes and enter the cisternae through channels in the membrane. It is also associated with the synthesis and breakdown of fatty acids and glycogen. ER is the major site of synthesis of cholesterol and steroid hormones. It provides additional mechanical support to cytoplasm.
It also acts as an intracellular transport system for various substances. Watson (1959) has suggested that exchange between the nucleus and the cytoplasm takes place through nuclear openings which communicate with the endoplasmic reticulum. The ER also provides an extensive internal surface for exchange of material between the matrix of cytoplasm and the lumen of cisternae.
