What is the difference between rough and smooth ER in terms of their function?

The endoplasmic reticulum can either be smooth or rough, and in general its function is to produce proteins for the rest of the cell to function. The rough endoplasmic reticulum has on it ribosomes, which are small, round organelles whose function it is to make those proteins. Sometimes, when those proteins are made improperly, the proteins stay within the endoplasmic reticulum. They're retained and the endoplasmic reticulum becomes engorged because it seems to be constipated, in a way, and the proteins don't get out where they're suppose to go. Then there's the smooth endoplasmic reticulum, which doesn't have those ribosomes on it. And that smooth endoplasmic reticulum produces other substances needed by the cell. So the endoplasmic reticulum is an organelle that's really a workhorse in producing proteins and substances needed by the rest of the cell.

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The rough endoplasmic reticulum is a type of endoplasmic reticulum consisting of flattened sacs, studded with protein-synthesizing particles termed ribosomes on the outer surface.

• The rough endoplasmic reticulum is a part of the endomembrane system that is present in the cytoplasm of the cell.
• The organelle is involved in the synthesis, folding, modification, and transport of proteins to different organelles within the cell or outside of the cell.
• The name ‘rough’ ER is given due to the appearance of ribosomes on the surface as studs under the microscope.
• Found both in plant and animal cells, the RER membrane is continuous with the nuclear membrane.
• It is usually located near the Golgi apparatus, and the protein synthesized in the ribosomes on RER are packaged into vesicles and transported to the Golgi body.
• Rough ER is primarily made up of flattened sacs called cisternae with few tubules. The membrane is also provided with an essential protein complex termed translocon, which is vital for translation within RER.
• The ribosomes are attached to the endoplasmic reticulum with the help of the group of proteins, termed ribophorins.
• The structure of rough ER is based on the presence of cytoskeletal elements like microtubules, where changes in microtubules cause changes to the structure of RER.
• Besides, the ribosomes present on the rough ER often detach themselves and develop into individual cisternae.
• Within the lumen of the RER, newly formed proteins undergo slight modifications like cleavage of signal sequences and glycosylation. Some proteins might change their three-dimensional conformation within the lumen.
• Rough ER is closely associated with the formation of lysosomes.
• Also, rough ER also plays a significant role in quality control during the folding of the proteins where the ratio of sheets to tubules is changed when the quantity of unfolded proteins increases in the cell.
• Sometimes, apoptosis is initiated in the cell as a result of the increase in the content of unfolded proteins.
• Similarly, rough ER also contains multiple enzymes that are involved in RNA metabolism that function to bind and modify RNA.
• However, different diseases might result from the misfolding of proteins in RER. Disease like spondyloepimetaphyseal dysplasia is attributed due to the accumulation of misfolded collagen proteins in the RER.

Read Also: Endoplasmic Reticulum (ER)- Definition, Structure, Functions, and Diagram

Smooth Endoplasmic Reticulum (SER)

Smooth endoplasmic reticulum (SER) is a type of endoplasmic reticulum consisting of tubular vesicles that lack ribosomes on the outer surface and is involved in the synthesis and storage of lipids.

All living organisms found on earth are made up of cells. They are the building blocks of life. Cells are categorized into two types- eukaryotic cells and prokaryotic cells. The major difference between these two types of cells is in the presence of cell organelles. The former cell has several cell organelles present in it whereas the latter lacks these cell organelles. The endoplasmic reticulum is one such cell organelle found in every eukaryotic cell excluding some exceptions like RBCs. It performs a variety of functions in a cell and has two parts as rough endoplasmic reticulum and smooth endoplasmic reticulum.

What is an endoplasmic reticulum?

ER was discovered by Keith Porter and Claude. The word 'reticulum' in ER means ‘network’ and this word was added in its name by Porter. Its discovery was made possible due to the invention of the electron microscope. Endoplasmic Reticulum is a tubular, membranous cell organelle seen in continuity with the nuclear membrane of the nucleus in the cell. All the membranes of the ER form a network and are interconnected with each other. Endoplasmic Reticulum is a single membraned organelle and is also a part of the endomembrane system of the cell. The network formed by it is highly complex.

The endoplasmic reticulum is a cell organelle that is in charge of lipid production and protein processing. The endoplasmic reticulum (ER) is a network of linked membranous vesicles and tubules that manufacture lipids and modify proteins. The rough ER and the smooth ER, on the other hand, conduct these two roles in distinct sections of the ER. The lumen, or cisternal space, is the hollow section of the ER tubules. The ER membrane is a phospholipid bilayer packed with proteins that run parallel to the nuclear envelope.

Types of ER

Based on appearance and function, Endoplasmic Reticulum is of two types namely Smooth Endoplasmic Reticulum (SER) and Rough Endoplasmic Reticulum (RER). To understand the differences between the Smooth Endoplasmic Reticulum (SER) and Rough Endoplasmic Reticulum (RER) let us look into each subset individually. 

Rough  Endoplasmic Reticulum

It is found close to the nuclear membrane. It bears several ribosomes on itself. Due to the presence of small granular ribosomes that are present on the surface of ER  they are termed as rough ER. Ribosomes transport freshly generated proteins into the RER lumen, where proteins encounter structural changes like folding and side-chain addition. These altered proteins will be integrated into biomembranes, such as the ER membrane or that of various organelles, or released from the cell as protein hormones or enzymes. Phospholipids for cell membranes are also produced by the RER. If the phospholipids or changed proteins aren't meant to stay in the RER, they'll be transported to their final destination by transport vesicles that sprout from the membrane of the RER. The RER is common in cells that produce proteins because it is involved in altering proteins such as enzymes that will be secreted from the cell. An example of such a cell is hepatocytes.

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Structure of RER

RER has several flattened tubules that are joined with each other. They are highly dense near the nuclear membrane. These are highly convoluted tubules.

Functions of Rough Endoplasmic Reticulum

Since we have developed a basic understanding of the RER, let us look into the major functions performed by RER.

• RER helps in the formation of proteins in the cell. The presence of ribosomes is responsible for the synthesis of the proteins by the RER.

• Thus RER is found in excess in those cells where protein synthesis takes place in a large amount.

Smooth Endoplasmic Reticulum

RER continues towards the periphery of the cell to form the SER. Ribosomes are absent on the surface of SER which is why it has a smooth appearance. Thus it's called Smooth Endoplasmic Reticulum. Unlike RER, SER is a meshwork of fine tubules. It also consists of vesicles. It is less extensive than RER The smooth endoplasmic reticulum (SER) is connected to the RER.

Functions of SER

Let us look into some of the important functions of the SER mentioned below.

• The smooth endoplasmic reticulum performs the function of the synthesis of lipids. Thus they are present in the cells of those organs where steroid secretions take place like an ovary.

• SER cannot synthesize proteins as it lacks ribosomes which are considered as the sites of protein synthesis.

• SER also helps in the process of detoxification in the liver.

Overall functions of endoplasmic reticulum

The functions of SER and RER jointly give a complete idea of the functions performed by the ER as a whole. These functions are as follows-

• ER helps in membrane biogenesis.

• This process refers to the formation of the cell membrane. It is widely known that the cell membrane is made up of lipids and proteins. These lipids and proteins are provided by the combined work of RER and SER. 

• ER performs many mechanical functions of the cell thus providing the cell with mechanical support.

• The large surface of the ER helps in the exchange of various materials in the cell by diffusion and active transport.

• It also helps in the circulation of substances within the cell.

• It is also responsible for the formation of certain cell organelles like lysosomes.

Difference between RER and SER

Since we have understood the structure and functions let us look into the differences. The major differences are mentioned below in the tabular form.

Parameters 

Smooth Endoplasmic Reticulum (SER)

Rough Endoplasmic Reticulum (RER)

Ribosomes 

Ribosomes are absent or present at very low concentrations.

Ribosomes are present.

Organisation 

Contains vesicles and tubules.

Contains cisternae and a few tubules.

Formation\ synthesis 

Involved in the synthesis of glycogen, lipids, and steroids.

Involved in the synthesis of enzymes and proteins.

Connected to 

Connected to plasmalemma.

Connected to the nuclear envelope.

Ribophorins

Lacks the ribophorins

Ribophorins are present. Helps in the attachment of ribosomes to RER.

Storage 

Lipid storage 

Protein storage.

Fun Fact 

The endoplasmic reticulum of muscle cells is known as the Sarcoplasmic reticulum. They are considered the storehouse of calcium ions.

In the conclusion of the article, we have learned about the endoplasmic reticulum. We have also learned about the differences between RER and SER.

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