Endoplasmic Reticulum, also known as ER, is the largest, membrane intracellular organelle that can be found in eukaryotic cells as prokaryotic cells lack any type of membrane bound organelles.
It is known for being a highly dynamic organelle that emanates from the nuclear envelope towards the plasma membrane.
Essentially, the Endoplasmic Reticulum is the main transportation of the eukaryotic cell. Though it has many other important roles, like protein folding, transportation is its main function.
It is a specific type of organelle that is made up of two different subunits – rough endoplasmic reticulum and smooth endoplasmic reticulum.
Both rough ER and smooth ER share a lot of the same proteins as each other and both work to synthesise certain lipids and cholesterol.
Though, different types of cells contain different levels of the two types of ER, depending on the function of the cell. RER is mainly found near the nucleus of the cell, while SER is more towards the cell membrane or plasma membrane within the cell.
The outer face of the RER is dotted with ribosomes, which are the site where protein synthesis takes place. You will find RER especially present in cells like hepatocytes.
The SER lacks the same ribosomes and mainly functions in lipid synthesis but not metabolism, the production of steroid hormones or detoxification.
The lumen is enclosed in the membrane of the endoplasmic reticulum and it is completely separate from the cytoplasm.It is important to note though, that some studies have recently shown that the lumen may have some connectivity to the nucleus.
The Cytoplasm – is the internal environment of the cell apart from the nucleus.
The endoplasmic reticulum plays various roles within the cell, these functions range from protein synthesis and transport to the metabolism of carbohydrates. Now depending on the particular cell, the ER may make up as much as 50% of the cell’s volume.
The Location And Structure Of ER
As was already mentioned in this article, the endoplasmic reticulum is the largest organelle in most eukaryotic cells.
Though it is made up of a single, continuous membrane system, ER is usually actually divided into 3 main parts; the nuclear membrane, tubular network and cisternae.
The actual structure of ER is a series of membranes, these membranes are the cisternae. The sac-like structures are held together by something called the cytoskeleton.
The phospholipid membrane encloses the lumen, which is continuous with the perinuclear space but separate from the cytosol.
The functions of the endoplasmic reticulum can be looked at as the synthesis and export of proteins and membrane lipids, but these functions vary between ER and cell type and cell function.
The quantity of both rough and smooth endoplasmic reticulum in a cell can slowly change over time from one type to the other, depending on the changing needs of the metabolic activities of the cell.
This transformation can include embedding of new proteins in the membrane as well as structural changes. Changes in protein content may occur without noticeable structural changes.
The endoplasmic reticulum has many general functions, including the folding of protein molecules in sacs called cisternae and the transport of synthesised proteins to the Golgi apparatus. Rough endoplasmic reticulum is also involved in protein synthesis.
Often there will be many proteins, like glycoproteins, that will move across the endoplasmic reticulum membrane.
These proteins are transported by the ER all over the cell and are marked with a specific tag called a signal sequence.
Within the polypeptide chain that is formed, there will be a few amino acids, these amino acids work as tags and are removed when the polypeptide eventually reaches its intended destination.
Other peptides also reach the ER via a specific membrane, which is embedded with complex proteins.
These various proteins that are destined for a place that is outside the endoplasmic reticulum are packed into transport vesicles and moved along the cytoskeleton toward their destination.
In human fibroblasts, the ER is always co-distributed with microtubules and the depolymerisation of the latter causes its co-aggregation with mitochondria, which are also associated with the ER.
The ER also has another very important function as it is a part of a protein sorting pathway. It is essentially the eukaryotic cell’s transportation system.
The majority of the proteins that reside there are retained within through a retention motif. This ‘motif’ is made up of 4 different amino acids at the end of a protein sequence.
The Nuclear Envelope
There are 2 specific lipid bilayers that are within and if fact, make up the inner and outer nuclear membrane.
Because of this continuity of the endoplasmic reticulum from the nuclear envelope, most scientists will agree that in eukaryotic organisms, the endoplasmic reticulum has been present since the very beginning – the development from prokaryotic or more primitive cells.
The Protein linkers maintain the morphology of the nuclear membrane. They are located in the perinuclear space and maintain a close distance of approximately 50 nm between the outer and inner nuclear membrane.
Because of this, the protein linkers play a frankly essential role in maintaining the morphology of the nuclear membrane/envelope.
Now excluding these protein linkers, the complex structure of the nuclear envelope is also enhanced by various other molecules.
For example, interactions observed between the proteins of the inner nuclear membrane and chromatin as well as lamin of the outer nuclear membrane and the nuclear pores have been proven to be absolutely necessary for the maintenance of the nuclear membrane.
Though the nuclear envelope does play an incredibly important role by creating a barrier between the inner part of the nucleus and the cytoplasm, this is present in eukaryotic cells.
It consists of hundreds of little pores through which many different molecules, for example proteins, are transported.
This will allow the various molecules, like those protein molecules we mentioned, to diffuse out of the nucleus and into the cytoplasm. Here they will help to build and with the development/building of different parts of the cell.
The rate of diffusion is almost always largely dependent on the size of the specific molecule.
For example, whereas molecules that measure up to 9nm in diameter can simply diffuse through the nuclear pore complexes, those with a diameter of about 39nm have to be actively transported (in or out of the nucleus).
It is important to note that the nuclear membrane is composed of nuclear pore complexes and the transport system between the nucleus and the cytoplasm is referred to as nucleocytoplasmic transport.
Cisternae (ER Sheets)
The cisternae, also known as the endoplasmic sheet, is the section of the endoplasmic reticulum that makes up part of the peripheral ER.
While observing the cell under microscopic investigations, the cisternae appear as a series of stacked flat sheets. In cells, they are particularly prominent around the nucleus as they radiate from the nuclear membrane/envelope.
The nuclear envelope/nuclear membrane and the endoplasmic sheets are also composed of two lipid bilayers as well as a lumen.
The structure of this portion of the endoplasmic reticulum, including the curved regions at the membrane edges, is maintained by several proteins. A good example of this is the CLIMP63, a type II integral membrane protein.
Through interactions achieved between charged coiled-coil domains, the CLIMP63 forms oligomers capable of acting as bridges between the membrane sheets.
This, in turn, maintains a specific luminal distance. In the cells of animals, this distance is about 50nm.
The increased depletion of this protein has been shown to reduce this distance to about 30 nm, proving that it still plays a role in maintaining the structure of endoplasmic sheets.
Endoplasmic Reticulum Tubules
When compared to the other portions of the endoplasmic reticulum, ER tubules are extremely dynamic structures that are often rearranging and interconnecting themselves.
They are cylindrical shapes with aa approximate diameter that ranges between 30 and 100 nm in diameter.
Because of the interconnection that takes place at the three-way junctions, ER tubules are quite loosely packed in what resembles polygonal arrays in the cytoplasm.
With regards to location, ER tubules radiate from both the nuclear membrane and the cisternae. From here, they spread throughout the cytoplasm (to different parts of the cell).
As compared to the sheets of the endoplasmic reticulum, ER tubules have a higher membrane curvature as well as surface area to volume ratio.
Unlike the rough endoplasmic reticulum, they have fewer ribosomes. As such, they are largely composed of the smooth endoplasmic reticulum.
The area of the endoplasmic reticulum that is enclosed by the ER membrane. As such, it’s an extensive/elongated area located within the membranes of the ER.
In eukaryotic cells, the lumen is found throughout the cell wherever the ER spreads. Given that the ER radiates from the nuclear membrane, studies have shown the internal compartment of the nucleus to be in continuity with the lumen of the endoplasmic reticulum.
The Difference Between RER And SER
The proximity of the rough ER to the cell nucleus gives the ER unique control over protein processing.
The rough ER is able to rapidly send signals to the nucleus when problems in protein synthesis and folding occur and thereby influences the overall rate of protein translation.
When misfolded or unfolded proteins accumulate in the ER lumen, a signalling mechanism known as the unfolded protein response (UPR) is activated.
The response is adaptive, such that UPR activation triggers reductions in protein synthesis and enhancements in ER protein-folding capacity and ER-associated protein degradation. If the adaptive response fails, cells are directed to undergo apoptosis.
Smooth ER, by contrast, is not associated with ribosomes, and its functions differ. The smooth ER is involved in the synthesis of lipids, including cholesterol and phospholipids, which are used in the production of new cellular membranes.
In certain cell types, smooth ER plays an important role in the synthesis of steroid hormones from cholesterol. In cells of the liver, it contributes to the detoxification of drugs and harmful chemicals.
The sarcoplasmic reticulum is a specialised type of smooth ER that regulates the calcium ion concentration in the cytoplasm of striated muscle cells.
Protein Synthesis And Processing
Protein synthesis is one of the major functions of the endoplasmic reticulum. Typically, translation of proteins starts in the cytoplasm.
However, some of these proteins are taken to the endoplasmic reticulum (through the translocon) where they undergo folding before being transported to the appropriate destination.
Apart from simply processing proteins, the endoplasmic reticulum is also known as the site for quality control.
As such, it ensures that only proteins that are properly processed are transported to the appropriate destination. Here, then, proteins are retained until they attain the right conformation.
Apart from being the major site for protein synthesis, the endoplasmic reticulum is also the site of lipid biogenesis (biosynthesis of cholesterol and phospholipids).
Here, lipid components are first moved to the ER-Golgi intermediate compartment where they are biochemically modified.
Structural lipids are mostly synthesised in the endoplasmic reticulum (ER), from which they are actively transported to the membranes of other organelles.
Lipids can leave the ER through vesicular trafficking or non-vesicular lipid transfer and, curiously, both processes can be regulated either by the transported lipid cargos themselves or by different secondary lipid species.
For most structural lipids, transport out of the ER membrane is a key regulatory component controlling their synthesis.
Distribution of the lipids between the two leaflets of the ER bilayer or between the ER and other membranes is also critical for maintaining the unique membrane properties of each cellular organelle.
Calcium storage is one of the functions commonly attributed to the endoplasmic reticulum (ER) in nonmuscle cells. Several recent studies have added support to this concept.
Analysis of reticuloplasm, the luminal ER content, has shown that it contains several proteins (reticuloplasmins) which are prospective calcium storage proteins.
One of these, calreticulin, is also present in the sarcoplasmic reticulum (SR). In sea urchin eggs, a calsequestrin-like protein has been clearly localised to the ER.
The recent demonstration that the IP3 receptor, which has similarities with the calcium release channel in the SR is also localised in the ER membrane suggests that calcium stored in the ER is important for intracellular signalling.
The alternative view, that the physiologically important calcium store is a specialised organelle, the calciosome, is not supported by these observations.
Recent evidence also suggests that ER calcium might be important in ER structure and in the retention of the luminal ER proteins.
Aside from various synthetic functions, the endoplasmic reticulum also acts as the transport system for various molecules. For instance, from the cytosol, the endoplasmic reticulum is responsible for the transport of proteins to the Golgi apparatus.
As mentioned, the endoplasmic reticulum spreads from the nuclear membrane to other parts of the cell (towards the plasma membrane). This allows it to transport various molecules from one point to the desired destination.
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