Endoplasmic reticulum (ER)

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 (RER)
• Smooth ER (SER)

Structure of ER

The endoplasmic reticulum is composed of three kinds of structures

• Cisternae
• Vesicles
• Tubules

Cisternae

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

Vesicles are oval or rounded membrane-bounded vacuole-like structure which is isolated in the cytoplasm.

Tubules

They are elongated, branched tube like structure which joins with cisternae and vesicles to form a network.

Functions of ER

• It forms cell circulatory system and helps in transportation of materials inside the cytoplasm.
• It acts as cytoskeleton and gives mechanical support.
• It helps in compartmentalization of cytoplasm for synthetic activity.
• These cell organelles form primary lysosomes.
• It forms desmotubule in plasmodesmata between adjoining cells.
• It forms nuclear envelope during cell division.
• It helps in protein synthesis.(RER)
• It helps in lipid synthesis. (SER)
• It helps in glycogenolysis.(SER)