Endoplasmic Reticulum: Structure and function

Endoplasmic Reticulum Definition
“Endoplasmic Reticulum is a complex network of tubular membranes exclusively present in the cytoplasm of the eukaryotic cell.”

What is the Endoplasmic Reticulum?
The endoplasmic reticulum transpires in two forms: a type with a ribosome-studded surface and another with a smooth surface. The latter is called the smooth endoplasmic reticulum, and the former is called the rough endoplasmic reticulum. These membranes form continuous folds, eventually joining the outer layer of the nuclear membrane. Except for sperm cells and red blood cells, the endoplasmic reticulum is observed in every other type of eukaryotic cell.

Endoplasmic Reticulum Diagram
The below diagram shows the variants of the endoplasmic reticulum:

Rough ER
Smooth ER
Rough endoplasmic reticulum has ribosomes embedded within its structure, giving a “rough” appearance. The smooth endoplasmic reticulum does not have these ribosomes, hence appearing “smooth.”
Structure of Endoplasmic Reticulum
The structure of the endoplasmic reticulum is shaped like a sac. Since ER is of two types, each has its own distinguishing features:

Rough Endoplasmic Reticulum Structure
The rough endoplasmic reticulum is named so because of its appearance.
It is a series of connected flattened sacs having several ribosomes on its outer surface, hence the name.
It synthesizes and secretes proteins in the liver, hormones and other substances in the glands.
Rough ER is prominent in cells where protein synthesis happens (such as hepatocytes)
Smooth Endoplasmic Reticulum Structure
The smooth endoplasmic reticulum, on the other hand, does not have ribosomes.
The smooth endoplasmic reticulum has a tubular form.
It participates in the production of phospholipids, the chief lipids in cell membranes and are essential in the process of metabolism.
Smooth ER transports the products of the rough ER to other cellular organelles, especially the Golgi apparatus.
Functions of Endoplasmic Reticulum
As stated above, the endoplasmic reticulum is categorised into two types, and both these types of ER perform specific functions:

Smooth Endoplasmic Reticulum Function:
Smooth ER is responsible for the synthesis of essential lipids such as phospholipids and cholesterol.
Smooth ER is also responsible for the production and secretion of steroid hormones.
It is also responsible for the metabolism of carbohydrates.
The smooth ER store and releases calcium ions. These are quite important for the nervous system and muscular systems.
Rough Endoplasmic Reticulum Function:
The majority of the functions of rough ER is associated with protein synthesis.
The rough endoplasmic reticulum also plays a vital role in protein folding.
Also ensures quality control (regarding correct protein folding).
The second most important function after protein synthesis and protein folding is protein sorting.
Further Reading:

Difference Between Smooth Endoplasmic Reticulum And Rough Endoplasmic Reticulum
Endomembrane System
Frequently Asked Questions
1. What is endoplasmic reticulum?
The endoplasmic reticulum is a tubular network of membranes found within the cytoplasm of the eukaryotic cell.

2. List the types of endoplasmic reticulum.
The endoplasmic reticulum is classified into two types:

Smooth endoplasmic reticulum
Rough endoplasmic reticulum
3. List the functions of the endoplasmic reticulum.
The endoplasmic reticulum performs the following functions:

It is responsible for the production and secretion of steroid hormones.
It is also responsible for the synthesis of essential lipids such as phospholipids and cholesterol.
It is responsible for the metabolism of carbohydrates.
ER releases calcium ions, which are necessary for the nervous system and muscular system.