The Endoplasmic Reticulum (ER)

When eukaryotic cells are examined under the electron microscope, they reveal a network, or reticulum, of very small tube-like structures spread throughout the cytoplasm. This network is called the endoplasmic reticulum, usually shortened to ER.

How the ER organises the cell interior

By forming this network, the ER splits the space inside the cell into two separate compartments:

• The luminal compartment, which is the space enclosed inside the ER itself.
• The extra-luminal compartment, which is the surrounding cytoplasm outside the ER.

Two types of ER

The outer surface of the ER may or may not carry ribosomes, and this gives rise to two forms:

• Rough endoplasmic reticulum (RER) — ER that has ribosomes attached to its outer surface, which gives it a rough appearance. RER is commonly seen in cells that are actively engaged in making and secreting proteins. It is extensive in such cells and stays continuous with the outer membrane of the nucleus.
• Smooth endoplasmic reticulum (SER) — ER that lacks ribosomes on its surface, and therefore looks smooth.

What each type does

The two forms of ER are specialised for different tasks: …