Endoplasmic Reticulum | Definition, Function, & Location

Electron microscopic studies of eukaryotic cells reveal the presence of a network of reticulum of tiny tubular structures scattered in the cytoplasm that is called the endoplasmic reticulum (ER).

Ultrastructure: The endoplasmic reticulum is composed of the following three kinds of structures, viz. cisternae, tubules and vesicles.

1. Cisternae : The cisternae are long, flattened, parallel, sac-like, interconnected structures. These are found in cells which are actively involved in protein synthesis. The cisternae usually occur in those Cells which have synthetic roles, e… cells of pancreas and brain. They are usually associated with large subunit (60 S) of ribosomes.
2. Tubules : The tubules are branched or unbranched structures forming the reticular system alongwith the cisternae and vesicles. They are free of ribosomes and are common in cells involved in lipid and sterol synthesis.
3. Vesicles : The vesicles are oval, membrane bound vacuolar structures. They are also free of ribosomes. They are abundant in the pancreaic cells and these are the only ER structures found in spermatocytes.

ER divides the intracellular space into two distinct compartments:

1. Luminal compartment : It is the internal space enclosed by ER membrane.
2. Extra luminal compartment : It is the space, present outside the ER in the cytoplasm.

Types of Endoplasmic Reticulum

On the basis of presence or absence of ribosomes on the surface of endoplasmic reticulum, it is of two types:

(I) Smooth endoplasmic reticulum (SER) :

The endoplasmic reticulum which is free of ribosomes is known as SER. When it is observed under the electron microscope it appears as smooth tubular structures. The muscle cells are also rich in smooth type of endoplasmic reticulum which is known as sarcoplasmic reticulum.

Function of SER:

1. It is specialised in the synthesis of lipids and steroids,
2. Detoxification of drugs,
3. Associated with muscle contraction by release and uptake of Ca* ions.
4. Synthetic products of RER pass onto Golgi complex through SER.

Rough endoplasmic reticulum (RER) :

The endoplasmic reticulum bearing ribosomes on its surface is called RER,it gives a rough granular appearance under the electron microscope. They are extensive and continuous with the outer membrane of the nucleus.

RER contains two types of glycoproteins i.e., Ribophorin-I and Ribophorin-II for the attachment of 60 S subunit of 80 S ribosome.

Function of RER:

1. Ribosomes are the site of protein synthesis and thus, RER are present in the cells which are actively involved in the protein synthesis and secretion.
2. It provides precursors of enzymes for the formation of lysosomes in Golgi complex.
3. It gives rise to SER.