Endoplasmic‌ ‌Reticulum‌

The endoplasmic reticulum (ER) is a large organelle composed of membranous sheets and tubules that start near the nucleus and stretch throughout the cell. The endoplasmic reticulum produces, packages, and secretes many of the products a cell creates.

What is Endoplasmic Reticulum?

The Endoplasmic reticulum (ER) is one of the important organelles in the cell. Whereas, the nucleus’s function is to serve as the cell brain, the ER acts as a manufacturing and packaging system. The Golgi apparatus, ribosomes, mRNA, and tRNA work closely with ER.

Endoplasmic Reticulum Structure is a network of membranes distributed in the cell and connected to the nucleus. The membranes vary slightly from cell to cell, and the size and shape of the ER are dictated by the activity of a cell. 

For example, prokaryotes or red blood cells do not have an ER of any kind. Cells which synthesize and release many proteins will require a large amount of ER. For good examples of cells with large ER structures, you might look at a cell from the pancreas or liver.

Before going in to detailed descriptions, first, checkout endoplasmic reticulum diagram:

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Structure of the Endoplasmic Reticulum

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The ER membrane system can be anatomically divided into two structures – cisternae and sheets.

Cisternae are structurally tubular and form a three polygonal-dimensional network. In mammals, they are about 50 nm in diameter, and in yeast, they are 30 nm in diameter.

On the other hand, ER sheets are membrane-enclosed, two-dimensional flattened sacs, which extend throughout the cytoplasm. They are often associated with ribosomes and special proteins called translocons, which are necessary for the translation of proteins within the RER.

Types of Endoplasmic Reticulum 

The endoplasmic reticulum (ER) provides around 50 percent of the total membrane surface in an animal cell. The organelle called 'endoplasmic reticulum' exists in plants and animals and is a very important site for the processing of lipids (fats) and other proteins. Some of these items are made for other organelles and are exported to them.

The endoplasmic reticulum is categorized into two types: rough endoplasmic reticulum and smooth endoplasmic reticulum.

In both animal and plant cells, the endoplasmic reticulum is seen.

Cells that are specialized in protein production will continue to have more rough ER, while cells that contain lipids (fats) and steroid hormones will have a more smooth ER.

Rough Endoplasmic Reticulum

Rough ER (RER) has also been listed in the ribosomes portion and is very important in protein synthesis and packaging.

Ribosomes are attached to the ER membrane, making it "rough". Also, the RER is attached to the nuclear envelope surrounding the nucleus. This direct relation between perinuclear space and ER lumen enables molecules to pass across both membranes. The protein synthesis process starts when mRNA passes from the nucleus to a ribosome on the surface of the RER.

As the ribosome builds the chain of amino acids, the chain is forced into the RER's cisternal space. When protein production is completed and these are collected, the RER pinches off a vesicle. The vesicle, a small bubble of the membrane, can travel into the cell membrane or the Golgi apparatus. Some of the proteins are used in the cell and others are sent out into the space between cells.

Smooth Endoplasmic Reticulum

The smooth Endoplasmic reticulum (SER) acts as a storage organelle. It is essential for lipid and steroid formation and storage. Steroids are a kind of ringed organic molecule, which is used in an organism for many purposes. They are not just about building up a weight lifter's muscle mass. Your body's cells which release oils also have more SER than most cells. A variant of the SER is the sarcoplasmic reticulum (SR). It can store several ions in a solution that will be required by the cell at a later time. When a cell has to do something quickly, scanning the atmosphere for extra ions that may or may not float around doesn't make any sense. Hence, it stores ions required for the process.

Ultrastructure of Endoplasmic Reticulum

Endoplasmic reticulum structures are surrounded by a thick membrane.

The membrane also consists of three layers — outer and inner thick layers consist of protein molecules, and phospholipids are the two mid-thin and translucent layers.

The endoplasmic reticulum membrane of the plasma membrane, nuclear membrane, and Golgi complex membrane is continuous. Endoplasmic reticulum lumen serves as a path for secretory products, and in it, Palade (1956) found secretary granules.

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Endoplasmic Reticulum Functions

• It is primarily responsible for transportation to another organ of proteins and other carbohydrates, including lysosomes, Golgi apparatus, plasma membrane, etc.

• They also provide cellular reactions with increased surface area.

• They aid in nuclear membrane formation during cell division.

• They play an important role in forming the skeletal structure.

• They play a crucial role in protein, lipid, glycogen, and other steroid synthesis, such as cholesterol, progesterone, testosterone, etc.

Key Points

• If the endoplasmic reticulum ( ER) has ribosomes attached to it, it is known as rough ER; if not, it is known as smooth ER.

• Proteins formed by rough endoplasmic reticulum are intended for use outside the cell.

• Smooth endoplasmic reticulum functions include carbohydrate, lipid, and steroid hormone synthesis; alcohol detoxification and poison detoxification; and calcium ion storage.