Endoplasmic Reticulum (ER)

The endoplasmic reticulum (ER) is a large, dynamic structure that performs many intracellular functions such as calcium storage, protein synthesis, and lipid metabolism. It is often referred to as the transportation unit of a eukaryotic cell. Renal tubules, leaves, and nuclear envelope from the various domains of the endoplasmic reticulum (ER). One of the main functions of ER is to function as a site for protein synthesis for secreted and integral membrane proteins and as a subpopulation of cytosolic proteins. Ribosomes are transferred to the cytosol side of ER for protein synthesis. In this article, you will be studying the endoplasmic reticulum (ER), its functions, identifying the characteristics of the endoplasmic reticulum (ER) and knowing why liver cells have a lot of smooth ER.

What is the Endoplasmic Reticulum (ER)?

All eukaryotic cells consist of the endoplasmic reticulum, and ER is one of the most prominent organelles—it has more than 50% of the total membrane content. 

It is the continuation of the outer layers of the nuclear membrane, which consists of sac-shaped sheets, more commonly known as cisternae. This flat and sac-shaped sheet can be found in either one of the two forms: membranes that have ribosomes on their surface (or are bound by ribosomes/rough ER) or membranes on which no ribosomes are present (smooth ER). 

ER performs various functions, such as synthesising lipids and proteins, protein folding, and protein chaperoning.

Garnier observed the endoplasmic reticulum under a light microscope and termed it the Elastoplast in 1897. Using an electron microscope, the apical membrane of the endoplasmic reticulum was first seen in 1945. Then, in 1953, Porter used the term endoplasmic reticulum, which means “network,” to describe the tissue of this membrane.

The endoplasmic reticulum is a combination of two forms with a ribosome-laden surface, and another one is devoid of ribosomes and thus has a smooth surface. 

The first is the rough endoplasmic reticulum (RER), and the latter is the smooth endoplasmic reticulum (SER). These membranes form continuous creases and eventually come in contact with the outer layer of the nuclear envelope. 

Except for red blood and sperm cells, the endoplasmic reticulum is found in all other eukaryotic cell types.

Endoplasmic Reticulum: can be classified into two types:

1. Rough ER
2. Smooth ER

Smooth Endoplasmic Reticulum (SER)

The Smooth ER is involved in lipid metabolism, which acts as the storage cell for calcium. This is crucial in the muscle cells that require Ca2+ ions for contraction. Phospholipids and cholesterol are synthesised with the help of Smooth ER. It is often located near the outer shell of the cell.

SER aids in synthesising lipids and steroids and helps detoxify from medicine. The synthetic product of the rough endoplasmic reticulum is passed to the Golgi complex via the smooth endoplasmic reticulum and helps in muscle contraction. 

Rough Endoplasmic Reticulum (RER)

The RER is generally seen near the nucleus. Its membrane-bound ribosomes are the reason which gives it the feature of a ‘rough’ appearance. These ribosomes create proteins that are moved into the organelle, and the proteins created are for the lumen of ER. 

These proteins contain a short signal from a few amino acids in their N-terminus and are firstly translated in the cytoplasm.

RER are sites of protein synthesis because they have the presence of ribosomes. Since they are sites of protein synthesis, they are found in sites actively involved in protein synthesis. They are not only synthesisers but are also present at the site of protein secretion. They are involved in the formation of the smooth endoplasmic reticulum. 

What does the Endoplasmic Reticulum (ER) do?

The endoplasmic reticulum performs various functions, and some of them are mentioned below:

1. The endoplasmic reticulum helps in the movement of carbohydrates and proteins to other organelles.
2. It enables a wider area for cellular reactions and thus helps in the easy flow of proteins.
3. At the time of cell division, it helps form nuclear membranes. 
4. The endoplasmic Reticulum helps infusion lipids, proteins, glycogen and steroids like progesterone, testosterone, and cholesterol.

After having good knowledge of its functions, let’s identify the characteristics of the endoplasmic reticulum.

Characteristics of the Endoplasmic Reticulum (ER)

Below mentioned are some primary ER characteristics: 

1. It is a type of organelle which is composed of two subunits: 

• The Rough Endoplasmic Reticulum (RER)
• The Smooth Endoplasmic Reticulum (SER)

1. It is an organelle found in eukaryotic cells of both animals and plants.
2. The endoplasmic reticulum forms an interconnected network of flat membranes known as Cisternae.
3. The Endoplasmic Reticulum was first observed under a light microscope by Garnier, who termed it the ergastoplasm in 1897.

Why do liver cells have a lot of smooth ER?

Hepatocytes contain more smooth ER than any other cell in the body. This is because the liver is responsible for gluconeogenesis and bile synthesis. In addition, smooth ER is involved in the production of steroids and lipids, which are essential for the formation of bile for fat digestion. 

Conclusion

The endoplasmic reticulum is a network of intracellular membranes to which proteins and other molecules move. The endoplasmic reticulum can be smooth or coarse, and its function is generally to produce proteins for the rest of the cell to function. Liver cells are responsible for the fusion of gluconeogenesis and bile juice, because of which they have smooth ER.