Endoplasmic reticulum | Definition, Types, Discover, Structure, Function

Hello friends, in this article we will know what is endoplasmic reticulum, and will also learn about it like – Its Definition, Types, Structure, Discovered, functions etc. So let’s start without wasting time.

What is the Endoplasmic reticulum?

Endoplasmic reticulum definition –

The endoplasmic reticulum (ER) is a network-like structure of flattened tubes and sacs found inside the cytoplasm of eukaryotic cells. The endoplasmic reticulum plays an important role in the synthesis of proteins and lipids.

Endoplasmic reticulum discovered by Keith Porter.

Ultrastructure of Endoplasmic reticulum –

The endoplasmic reticulum forms an elaborate system of flattened tubules or channels in the ground substance of the cells. The channels are bounded by membranes that have the same basic structure as the plasma membrane.

In some cases, these extend from the cell membrane to the outer nuclear membrane and thus establish a continuous system of channels from the external environment to the nucleus.

The endoplasmic reticulum comprises three different components.

Vesicles – These are almost rounded spherical or ovoidal spaces that measure from 25 to 500 microns.

Cisternae – These are long and Flattened lamellar vesicles about 40-60 microns thick and usually disposed of in parallel Rows surrounding the nucleus in the forms of successive layers.

Tubules – They are in the form of irregularly branched tubules having a diameter of about 50-100 microns. These are most common in the cells that carry the synthesis of steroids and in the pigmented epithelial cells of the retina.

Endoplasmic reticulum diagram

endoplasmic reticulum
Endoplasmic reticulum

Types of Endoplasmic reticulum-

Two types of Endoplasmic Reticulum are found:

Agranular or Smooth endoplasmic reticulum (SER)

The membranes of this type of reticulum are without ribosomes. Smooth endoplasmic reticulum occurs in cells that do not synthesize protein like adipose cells glycogen storing cells and muscle cells.

Granular or Rough endoplasmic reticulum (RER) –

The membranes of rough endoplasmic reticulum are deposited with ribosomes. The smooth endoplasmic reticulum is found in all those cells that play an active role in protein synthesis.

What is the function of the Endoplasmic reticulum?

Common functions

Mechanical functions – The endoplasmic reticulum forms a skeletal Framework and Provides supplementary mechanical support to the colloidal matrix.

Exchange of material – The membranes of the Endoplasmic reticulum act as a segregation apparatus, i.e. These maintain Osmotic pressure within the cell. Isolate the material synthesised for delivery and regulate exchange between the inner compartments and the outer compartments or the compartments and cytoplasmic matrix.

Intracellular transport – The endoplasmic reticulum is said to act as a kind of circulatory system for the transport of various substances inside the cell cytoplasm. 

Specialised functions 

Intracellular transport –  Endoplasmic reticulum provides an increased Inner surface for various metabolic reactions and they themselves take an active part in them by means of attached enzymes.

Synthesis of secretory proteins –  The microsome function attached to the endoplasmic reticulum is intimately associated with protein synthesis.

Glycogen metabolism – In liver cells agranular endoplasmic reticulum is intimately associated with glycogen metabolism.

ATP synthesis – Membranes of the Endoplasmic reticulum are the site of ATP synthesis in the cell. ATP is used as a source of energy for all the intracellular metabolism and transport of material.

Formation of other cytomembranes – Endoplasmic reticulum takes part in the formation of other membranous structures of cell-like mitochondria and Golgi bodies. 

Transport of message from genetic material – Also provides passage for the message in the form of various RNAs from the genetic materials to pass from the nucleus to the various organelles in the cytoplasm thereby controlling the synthesis of proteins, fats, and carbohydrates.

What is annulate lamellae?

Usually, the membranes of the endoplasmic reticulum are continuous without any pores and pore complexes. But in some cells, some membranes of the Endoplasmic reticulum possess pores and pore complexes.

These represent and annulate lamellae. These were first described by Mc Culloch (1952).

The annulate lamellae are found in the cells of invertebrates in the immature oocytes And spermatocytes of vertebrates and the embryonic fetal cells which have a high rate of metabolism.

annulate lamellae
Formation of cisternae of endoplasmic reticulum from annulate lamellae

Annulate lamellae are double membrane sheets. about  2-12 lamellae are arranged to form stacks. structurally These are similar to the nuclear membrane. It is evidence that annulate lamellae arise from the nuclear membrane.

The outer membrane of the nuclear envelope forms finger-like processes which are pinched off into the cytoplasm in the form of vesicles Called blebs. This process is known as blebbing. These vesicles Migrate towards the Periphery of the cells in rows and fuse to form cisternae.

What is Microsome?

When the liver homogenates in 25M  fractionated by differential centrifugation the nuclear membrane mitochondria and cell debris spun out first.

The supernatant left is Spun at a very high speed, resulting in the separation of microsomal fraction or Microsome which settle down at the bottom of the tube.

In a typical  Liver cell, the microsomal fraction comprises membranes of the endoplasmic reticulum and Golgi body, and the ribosome is isolated as well as attached to the membranes. Such a microsomal fraction is known as heterogeneous.

But its composition varies. In cells where only agranular endoplasmic reticulum is found the microsomal Fraction comprises smooth membranes only but where the endoplasmic reticulum is granular, The microsomal fraction consists of ribosomes still attached to the membranes.

If the cells have only particles their microsomal fraction consists of particles (ribosome) alone.

Microsomes constitute about  15- 20% of the total mass of the cell. It consists of about 50-60% RNA, a few lipids, and a large number of enzymes. The enzymes are NADPH- cytochrome C reductase, NADPH diphosphatase, glucose- phosphatase, Mg++ activated ATPase, and UDP (uridine diphosphatase).

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