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It is an extensive network of membrane-bounded spaces or channels that occur throughout the cytoplasm mainly endoplasm. It is absent in prokaryotes and presents in all eukaryotic cells except RBC, germinal cells. It is well developed in metabolically active cells like pancreatic cells, liver cells.
On the basis of presence or absence of ribosomes ER is categorized into two types;
|Rough ER||Smooth ER|
|It contains ribosome on the membrane.||It does not contain any ribosome.|
|It lies towards the nuclear membrane.||It lies towards cell membrane.|
|It mainly consists of cisternae.||It mainly consists of tubules.|
|It occurs in protein synthesizing cells.||It occurs in lipid synthesizing cells.|
The endoplasmic reticulum is composed of three kinds of structures
Cisternae are elongated, flattened unbranched membrane-bounded sac-like structure which is placed one above another in a parallel bundle. It occurs mainly towards the nuclear membrane.
Vesicles are oval or rounded membrane-bounded vacuole-like structure which is isolated in the cytoplasm.
They are elongated, branched tube like structure which joins with cisternae and vesicles to form a network.