Endoplasmic Reticulum | Rough ER and Smooth ER | ScienceMonk

The endoplasmic reticulum is an organelle found in eukaryotic cells that form an interconnected network of cisternae.

A cell is the smallest structural and functional unit of the body. Every cell is enclosed by a cell membrane. Which separates the extracellular material, from the intracellular material; maintaining the integrity of a cell. The nucleus is the control centre of the cell which contains threads of chromatin enriched with deoxyribonucleic acid (DNA).



 

Endoplasmic Reticulum

 

The cytoplasm is the medium for a chemical reaction that provides a platform upon which other organelles can operate. Each type of organelle plays a specific role in the function of the cell. These organelles include mitochondrion, ribosomes, endoplasmic reticulum, Golgi apparatus, and lysosomes.

What is the Endoplasmic Reticulum?

Endoplasmic Reticulum (ER) being the largest membrane-delineated intracellular compartment in eukaryotic cells, forms an interconnected network of flat, membrane-enclosed sacs that are known as cisternae. It has a surface area up to 30 times of plasma membrane.

It was observed first by Garnier in 1897. Later, the word “reticulum”, was given by Porter in 1953 to describe this fabric of membranes.



Structure of Endoplasmic Reticulum

The ER consists of the nuclear envelope and peripheral ER, which includes smooth tubules and rough sheets. The nuclear envelope is made up of the inner nuclear membrane (INM) and outer nuclear membrane (ONM), sharing a common lumen with the peripheral ER.

There are hundreds of nuclear pores spanning the outer nuclear membrane and inner nuclear membrane on the nuclear envelope that allow the transport of molecules including RNAs and proteins. The nuclear envelope is connected to sheets that make up part of the peripheral ER.

Sheets are flattened made up of two lipid bilayers with an intervening lumen. These sheets are in a stacked conformation; connected via a twisted membrane with helical edges. Cisternae are a tubular, three-dimensional polygonal network that is held through the cytoskeleton.

They are around 50nm in diameter in mammals whereas 30nm in diameter in yeast. The phospholipid membrane encloses the cisternal space (lumen) which is continuous with the perinuclear area.



Endoplasmic Reticulum
Chemical Composition of Endoplasmic Reticulum

ER membranes contain high lipid content with protein which includes phospholipids, phosphatidylinositol, neutral lipids, sulfolipids, cholesterol, and some phytosterols.

ER membrane proteins that have been isolated enzymes like cytochrome p450 and its subgroups, electron transport protein complexes like Cytochrome C reductase, Cytochrome b5 reductase. Glucose-6 phosphatases in ER are common as well.

Furthermore, they exhibit chemical heterogeneity at their cytoplasmic and lumen surfaces. They do have resident proteins, which perform functions such as protein folding, protein modification, and protein transport.



Biogenesis

One school of thought says that the membranes develop from plasma membranes. They say that the layers develop from plasma membranes by inward invagination and growth. Other viewpoints out that ER develops from the outer nuclear membrane.

Thus, both these views point out that ER develops from both plasma membranes and the outer nuclear membrane. It is also accepted that both plasma membranes and nuclear membranes are derived from endo-membranes.

Types of Endoplasmic Reticulum: Rough ER and Smooth ER

Rough Endoplasmic Reticulum (rough ER) and Smooth Endoplasmic Reticulum (smooth ER).
Both are present in the cells of plants and animals. Although they appear as if separate, they are sub-compartments of the same organelle.

Cells that specialized in the production of proteins tend to have a larger amount of rough ER whereas cells producing lipids and steroid hormones have a greater amount of smooth ER.



  • Rough Endoplasmic Reticulum(RER)

RER is composed of exceedingly convoluted but flattish sealed sacs that are contiguous with the nuclear membrane. It is called the ‘rough’ endoplasmic reticulum because it is studded with ribosomes on its outer surface.

They are called membrane-bound ribosomes and are attached to the outer cytosolic side of the ER. RER is found throughout the cell although the density is quite higher near the nucleus and the Golgi apparatus.

Ribosomes on the RER are membrane-bound and are responsible for the assembly of many proteins, which is called translation. Specific cells of the pancreas and digestive tract produce a high volume of protein serving as enzymes.

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Many of the proteins that are created in the cells of the pancreas and the digestive tract and act as digestive enzymes. It is in the lumen of the rough ER that where proteins are folded to produce the highly significant biochemical architecture that provides recognition and other linking sites.

Proteins are subjected to regulation, and any of the proteins that are found to be incorrectly formed or misfolded are thus further rejected. These rejects are stored either in the lumen or sent for recycling.

A type of emphysema (which a lung problem) is caused by the ER regulatory system continually rejecting misfolded protein. The protein is wrongly folded as a result of obtaining an altered genetic message. And, the protein required is never exported from the lumen of the rough ER.

Cystic fibrosis is caused by a missing single amino acid, phenylalanine, in a particular position. The very exacting service provided by the regulatory section determines the error and rejects the protein retaining in a lumen of RER.



  • Smooth Endoplasmic Reticulum (SER)

Smooth Endoplasmic Reticulum (SER) is tubular than RER and forms an interconnecting network sub-compartment of ER. It is found throughout the cytoplasm in almost even distribution. It is not studded with ribosomes on the outer surface thus appear smooth.

It works exclusively in the manufacturing of lipids and in some cases to the metabolism of them and their associated products. In liver cells, for example, smooth ER enables glycogen stored in granular form on the outer surface of SER to get broken down into glucose.

It is also involved in the production of steroid hormones in the adrenal cortex and endocrine glands. It also plays a part in detoxifying many organic chemicals by converting them to safer water-soluble products.

A more considerable amount is found in liver cells where It is present to detoxify products of natural metabolism and to endeavour to detoxify overloads due to excessive alcohol drinking and also barbiturates from a drug overdose.

To resolve this, SER can double its surface area within a few days, returning to its standard size. The calcium ions are released from the smooth endoplasmic reticulum which triggers a contraction of muscle cells.

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