All living organisms found on earth are made up of cells. They are the building blocks of life. Cells are categorized into two types- Eukaryotic cell and Prokaryotic cell. The major difference between these two types of cells in the presence of cell organelles. The former cell has a number of cell organelles present in it whereas the latter lacks these cell organelles. The Endoplasmic Reticulum is one such cell organelle found in every Eukaryotic cell excluding some exceptions like RBCs. It performs a variety of functions in a cell and has two parts as Rough endoplasmic reticulum and Smooth endoplasmic reticulum.
ER was discovered by Keith Porter and Claude. The word 'reticulum' in ER means ‘network’ and this word was added in its name by Porter. Its discovery was made possible due to the invention of the electron microscope.
Endoplasmic Reticulum is a tubular, membraneous cell organelle seen in continuity with the nuclear membrane of the nucleus in the cell. All the membranes of the ER form a network and are interconnected with each other.
Endoplasmic Reticulum is a single membraned organelle and is also a part of the Endomembrane system of the cell. The network formed by it is highly complex.
Types of ER
On the basis of appearance and function, Endoplasmic Reticulum is of two types-
Smooth Endoplasmic Reticulum (SER)
Rough Endoplasmic Reticulum (RER)
Difference Between SER and RER is as Follows
Rough Endoplasmic Reticulum
It is found in close proximity to the nuclear membrane. It bears a number of Ribosomes on itself. This is the reason why they are called Rough ER.
Due to the presence of small granular ribosomes that are present on the surface of ER they are termed as Rough ER.
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Structure of RER
RER has a number of flattened tubules that are joined with each other. They are highly dense near to the nuclear membrane. These are highly convoluted tubules.
Functions of Rough Endoplasmic Reticulum
RER helps in the formation of proteins in the cell. The presence of ribosomes is actually responsible for the synthesis of the proteins by the RER.
Thus RER is found in excess in those cells where protein synthesis takes place in a large amount.
Smooth Endoplasmic Reticulum
RER continues towards the periphery of the cell to form the SER. Ribosomes are absent on the surface of SER that is why it has a smooth appearance. Thus it's called Smooth Endoplasmic Reticulum.
Structure of SER
Unlike RER, SER is a meshwork of fine tubules. It has vesicles also. It is less extensive than RER.
Functions of SER
Smooth endoplasmic reticulum performs the function of the synthesis of lipids. Thus they are present in the cells of those organs where steroid secretions take place like an ovary.
SER cannot synthesize proteins as it lacks ribosomes which are considered as the sites of protein synthesis.
SER also helps in the process of detoxification in the liver.
Overall Functions of Endoplasmic Reticulum
The functions of SER and RER jointly give a complete idea of the functions performed by the ER as a whole.
These functions are as follows-
ER helps in membrane biogenesis.
This process refers to the formation of the cell membrane. It is widely known that the cell membrane is made up of lipids and proteins. These lipids and proteins are actually provided by the combined work of RER and SER.
ER performs many mechanical functions of the cell thus providing the cell with mechanical support.
The large surface of the ER helps in the exchange of various materials in the cell by diffusion and active transport.
It also helps in the circulation of substances within the cell.
It is also responsible for the formation of certain cell organelles like lysosomes.
The endoplasmic reticulum of muscle cells is known as Sarcoplasmic reticulum. They are considered as the storehouse of calcium ions.
1. What do you understand by the Endomembrane System?
The Endomembrane system consists of 4 cell organelles. These are-
All these organelles are different from each other structurally as well as are functionally distinct from each other but still, they are dependant on each other. Thus they form an endomembrane system. Their functions are linked to each other and are coordinated. This can be understood as the ER helps in the formation of the Golgi apparatus.ER transports its synthesized substances to the Golgi which modifies them. Lysosomes are formed by both Golgi and ER. Vacuoles found in plant cell stores extra substances. Thus all these organelles are interlinked in some or the other way.
2. Differentiate Between Rough Endoplasmic Reticulum and Smooth Endoplasmic Reticulum.
The Difference Between SER and RER are as follows
RER-Rough Endoplasmic Reticulum
SER-Smooth Endoplasmic Reticulum
They are also known as granular ER.
They are also called Agranular ER.
They have ribosomes on its surface.
They lack ribosomes.
They are found near the nucleus of the cell.
They are found away from the nucleus in the cell.
Their function is to synthesize proteins.
Their function is to synthesize lipids.
They are abundant in cells where protein synthesis takes place like plasma cells and acinar cells.
They are abundant in those organs which facilitate steroid and lipid secretion.
They seem to be formed by the nuclear membrane.
They are a continuation of RER.
They are involved in the formation of other organelles like the Golgi apparatus.
They help in the formation of spherosome.