Yet another important cell organelle, the Endoplasmic Reticulum is really fascinating. We often talk about nucleus of a cell, the DNA, the RNA and ribosomes, the cell membrane and more.
The attention that the Endoplasmic Reticulum gets is somewhat less that the other organelles of a cell. This isn’t fair because the Endoplasmic Reticulum is a highly specialized and complex organelle. It has a wide range of functions.
In this article on Endoplasmic Reticulum facts, we are going to talk about its structure, its types and eventually read about its functions. We promise, it is going to be an interesting read. So, have some patience and read on…
Before we start, a quick look at this table will allow you to get some handy information that you can refer to whenever needed:
|True membrane bound||Yes|
|Present in||Eukaryotic cells (both plants and animals) except Red Blood Cells in mammals and spermatozoa|
|Absent in||Prokaryotic cells|
|Types||Smooth and Rough|
|Observed by||Albert Claude, Ernest Fullam and Keith Porter|
Now, let’s start…
Endoplasmic Reticulum Facts: 1-5 | What is Endoplasmic Reticulum?
1. It is an organelle of a cell. It is found only in Eukaryotic Cells. Eukaryotic Cells are types of cells that have a nucleus. So, Endoplasmic Reticulum is found in both animal and plant cells. In mammals however, this organelle is not present in Red Blood Cells and Spermatozoa.
2. Why such a name for the organelle? The name Endoplasmic Reticulum has two parts. The first part is derived from the term Endoplasm, which is the inner part of the Cytoplasm of a cell.
3. The second part, that is, Reticulum actually means ‘a fine network’. So, Endoplasmic Reticulum actually means a fine network present in the inner part of the Cytoplasm. The question is, ‘network of what?’
4. It is basically a network of Cisternae, vesicles and tubules. Cisternae are flattened sac-like structures that are enclosed in double membranes. Vesicles are transport sacs that operate inside a cell (intracellular). Tubules are minute or very small tubes.
5. Nearly 50% of the total Eukaryotic Cell surface is made of this particular organelle. It is the largest organelle in the cell.
Endoplasmic Reticulum Facts: 6-10 | Types and Structure of Endoplasmic Reticulum
6. When we speak of the structure of Endoplasmic Reticulum, we speak about the types first. However, if we are to speak about general structure then the Endoplasmic Reticulum is a network of Cisternae, Vesicles and Tubules.
7. This intricate network is very close to the nuclear membrane and from there, the network spreads out into the cytoplasm. The Endoplasmic Reticulum is sheathed in a membrane, which is a phospholipid membrane.
8. The double-membraned sac-like structures called Cisternae have tiny spaces (just like space inside a sac). These spaces are referred to as Cisternal Space or Endoplasmic Reticulum Lumen.
This Cisternal Space or Endoplasmic Reticulum Lumen is continuous with the Perinuclear Space. (The Perinuclear Space in turn is the space between two lipid bilayers of the nuclear membrane.
The two layers are called the Inner Nuclear Membrane and the Outer Nuclear Membrane. The space between the Inner and Outer Nuclear Membranes is called the Perinuclear Space.)
9. Points 6 to 8 give the general structure of the Endoplasmic Reticulum. However, in order to understand the structure of the organelle in details, we need to know that there are two morphological variants or forms of this organelle. They are:
- Smooth Endoplasmic Reticulum (often referred to as sER)
- Rough Endoplasmic Reticulum (often referred to as rER)
10. Both the sER and rER have the Cisternal Space or Endoplasmic Reticulum Lumen (ER Lumen). The ER Lumen is filled with a fluid, which makes up 10% of the total cell volume. The ER Lumen is kept separate from the Cytoplasm by the phospholipid membrane.
Endoplasmic Reticulum Facts: 11-15 | Structure of Rough Endoplasmic Reticulum
11. The Rough Endoplasmic Reticulum also goes by the following name:
- rER or RER
- Rough ER
- gER or GER
- Granular ER
12. The rER gets its name because of the rough appearance it gets due to Ribosomes (protein-producing organelles) studded on its outer surface.
The sites where these Ribosomes attach with the outer surface of the Rough Endoplasmic Reticulum are known as Translocons (singular: Translocon). These Ribosomes are not permanent structures on the rER. They keep on getting attached and released as and when required.
13. This Rough Endoplasmic Reticulum or Granular Endoplasmic Reticulum is held it is place by the Cytoskeleton. The Cytoskeleton is noting but the skeleton of the cell and it is so called because it remains embedded in the cell’s Cytoplasm. The Cytoskeleton is made of proteins.
14. The Granular or Rough Endoplasmic Reticulum’s membranes are double membrane sheets that are located not only near the Outer Nuclear Membrane but is also continuous with the Outer Nuclear Membrane or the Outer Nuclear Envelope.
15. Apart from these important points (11 to 14), the structure of the rER is same as the general structure of the Endoplasmic Reticulum we have described earlier. It is nothing but a network of Cisternae, Vesicles and Tubules.
Endoplasmic Reticulum Facts: 16-20 | Structure of Smooth Endoplasmic Reticulum
16. The Smooth Endoplasmic Reticulum or sER is no different when it comes to structure of Endoplasmic Reticulum. Yes, it is also a network of Cisternae, Vesicles and Tubules.
17. There are however a couple of differences between the structure of sER and rER. The sER does not have Ribosomes attached to its external surface. So, it doesn’t have a rough appearance. It has a smooth appearance and hence the name, Smooth Endoplasmic Reticulum.
18. The other difference between the sER and rER is that Smooth Endoplasmic Reticulum appears like a group of small tubules because they have more tubular appearance compared to the Rough Endoplasmic Reticulum.
19. Here is something really interesting. In the sER, one can find small areas that are partially smooth and partially rough. These areas are known as Transitional Endoplasmic Reticulum or Transitional ER.
20. The Transitional ERs are basically exit sites from where transport vesicles (carrying proteins and lipids manufactured by ER) move out of ER and move towards the Golgi Apparatus.
Endoplasmic Reticulum Facts: 21-25 | Sarcoplasmic Reticulum
21. What is this Sarcoplasmic Reticulum? Well, it is actually a variant of the Smooth Endoplasmic Reticulum. However, it is not present in all cells. It is present only in the muscle cells known as myocytes. It is abbreviated as SR.
22. The Sarcoplasmic Reticulum is a tubules’ network extending throughout muscle cells. This tubules’ network or the Sarcoplasmic Reticulum wraps around the myofibrils (that are basically contractile units found in myocytes).
23. It is interesting to know that though the Sarcoplasmic Reticulum wraps around the myofibrils, the SR is never in direct contact with the myofibrils.
24. Unlike the Endoplasmic Reticulum (sER and rER), the Sarcoplasmic Reticulum doesn’t produce anything. It is rather a storehouse where positively charged Calcium ions (Ca2+) are stored.
25. The SR is responsible for releasing Calcium ions in the Cytoplasm of striated (striped) muscle cells (known as the Sarcoplasm) when stimulation of muscle fibers take place.
Endoplasmic Reticulum Facts: 26-30 | Functions of Rough Endoplasmic Reticulum
26. The primary job of the Rough Endoplasmic Reticulum is that of protein synthesis and protein maturation. Synthesis and maturation of only those proteins take place that are eventually sent to either Plasma Membrane (also known as Cell Membrane) or endocytic vesicles or secretory vesicles.
27. What happens is that when a Ribosome produces a protein, the first segment of the peptide chain thus produced will have what is known as ‘Signal Sequence’. This first segment will then bind itself with SRP or Signal Recognition Particle found in the Cytoplasm. SRP is a small protein or RNA complex. This is where the Ribosome will stop production of protein (through translation) for sometime.
28. Now, the whole of the Ribosome with the peptide chain (whose Signal Sequence is in turn, attached with SRP) will dock on the outer surface of the rER on specific sites called Translocons. These Translocons have SRP receptors. So, the SRP attached to the peptide chain’s Signal Sequence will now attach with the SRP receptor on the rER’s Translocon.
29. At this stage, the Ribosome that attached with the rER will continue protein production through translation of information found in mRNA. As the translation keeps happening, the protein or the peptide chain gradually enters into the Lumen of the rER.
30. Once the peptide enters the Lumen, the Signal Sequence is cut off. Immediately after that, the Lumen of the rER starts post-translation modifications on the peptide chain.
Endoplasmic Reticulum Facts: 31-35 | Functions of Rough Endoplasmic Reticulum
31. Once inside the Lumen, the first processing that happens is that carbohydrates are attached to the peptide in a process known as Glycosylation. Carbohydrates are attached to specific amino acids in the peptide chain.
32. This Glycosylation serves two purposes. They are:
- Assisting in correct folding of the proteins in 3-dimensional structures.
- Assisting in properly targeting the protein to the desired location.
33. Once Glycosylation is completed, the next step kicks in. This step is known as Protein Folding. Inside the Lumen, there is predefined set of chemical conditions that accomplishes the process of folding the proteins using Chaperone Proteins (proteins designed for folding and unfolding as well as assembly and disassembly of molecular structures) and folding enzymes.
34. The chemical conditions assist in folding proteins with proper conformation. Improper folding of proteins is not desirable because in such a case, the immune system of the body will look at them (the improperly folded proteins) as ‘foreign structures’ and attack them, leading serious problems.
35. If the Lumen of the rER folds a protein incorrectly, the protein will be retained in the Granular Endoplasmic Reticulum and it will then be degraded.
Endoplasmic Reticulum Facts: 36-40 | Functions of Smooth Endoplasmic Reticulum
36. The Smooth Endoplasmic Reticulum or sER has a completely different function. It is the site where lipids are synthesized, carbohydrates (sugars) are metabolized and various toxins and drugs are detoxified.
37. The synthesis of lipids takes place on the interface where the membrane of the sER meets the Cytoplasm.
The initial material that is required for lipid synthesis remains embedded in the membrane of the sER. Other building materials and enzymes required for building the lipid are present in the Cytoplasm.
They continually work in the initial material until the lipid that is desired is produced.
38. Once the desired lipid is produced, it has to be transported to the desired location such as the Plasma Membrane. Two theories have been proposed on how that happens.
First theory states that a sER membrane patch gets pinched off to form a vesicle that transports the lipid to the desired location. The other model says that there are water-soluble proteins that remove the lipid from the membrane of the sER and releases it to the targeted location.
39. Other functions of the sER include:
- Production of steroids (produced by sER present in endocrine gland and adrenal gland).
- Production of membrane phospholipids and cholesterol required for building membrane formation.
- It also helps with carbohydrate metabolism.
- It also helps in membrane synthesis and repair.
- It helps as a facilitating organelle for glycogen to glucose conversion in Liver.
- In muscle cells, it (Sarcoplasmic Reticulum) helps in cell signaling using stored Calcium ions.
40. Detoxification of drugs and bodily-produced metabolic wastes take place in sER of the Liver cells. The sER of Liver cells have special enzymes capable of making drugs soluble in lipid and metabolic wastes soluble in water so that they can be expelled from the body quite easily.
That concludes our list of Endoplasmic Reticulum facts. In case you think that we have missed out something or if you think adding some extra points will be helpful, feel free to contact us either through our contact form or through the comments section. We will modify our Endoplasmic Reticulum facts sheet accordingly.
Image Credit: LadyofHats | Public Domain