SER Full Form

What is the full form of SER?

SER is an abbreviation for smooth endoplasmic reticulum. The smooth endoplasmic reticulum is a network of microscopic disk-like tubular membrane vesicles in the cytoplasm of eukaryotic cells that are important in the synthesis and storage of lipids such as cholesterol and phospholipids, which are used in the construction of new cellular membranes. The smooth endoplasmic reticulum (SER), the second most prevalent kind of endoplasmic reticulum, can be distinguished from the rough endoplasmic reticulum (RER) by the absence of ribosomes. The RER's outer surface is related to protein-synthesising ribosomes, which give the membrane its "rough" appearance. Both plant and animal cells contain SER.

This Story also Contains

1. What is the full form of SER?
2. Smooth Endoplasmic Reticulum
3. Location and Presence of the Smooth Endoplasmic Reticulum
4. Discovery of Smooth Endoplasmic Reticulum
5. Structure of the Smooth Endoplasmic Reticulum
6. Sarcoplasmic reticulum: A type of SER
7. Functions of the Smooth Endoplasmic Reticulum

Smooth Endoplasmic Reticulum

SER is the ribosome-free tubular endoplasmic reticulum. It exists in eukaryotic cells and is involved in the production of lipids, the metabolism of carbohydrates, the regulation of calcium concentration, and the detoxification of drugs. The ER is one of the most visible organelles in a eukaryotic cell. The endoplasmic reticulum is made up of cisternae, which are interconnected networks of flattened sacs. The ER membranes can potentially extend into the cell membrane and are joined to the outer nuclear envelope. The SER is crucial to numerous metabolic processes. For instance, it creates steroid, phospholipid, and lipid molecules that are found in plasma membranes. As a result, cells that emit these substances, such as testes, ovaries, and skin oil glands, overproduce the smooth endoplasmic reticulum.

Location and Presence of the Smooth Endoplasmic Reticulum

Almost all eukaryotic cells include the smooth endoplasmic reticulum. However, other eukaryotic cells, including adult RBCs, embryonic cells, and eggs, do not possess SER. On the other hand, some specialised cells have a lot of SER. Sebaceous glands, gonadal cells that produce steroid hormones (such as Leydig cells in the testis and follicular cells in the ovary), hepatocytes in the liver, and cells of striated muscles are examples of cells with abundant SER.

Discovery of Smooth Endoplasmic Reticulum

Despite being one of the largest and most crucial organelles in terms of function, ER was one of those organelles that took a while to be discovered. According to reports, the French cytologist Charles Garnier and colleagues first observed ER using light microscopy in 1902. However, their contemporaries did not often agree with their observations. After another 50 years passed, two scientists were finally able to demonstrate the endoplasmic reticulum's existence. This was made possible by the development of electron microscopy. In 1953, the American cell biologists Keith R. Porter and George E. Palade discovered an organelle that resembled a net inside the cytoplasm, which they termed the "endoplasmic reticulum." Early in the 1960s, the endoplasmic reticulum's structure, make-up, and function in regulating calcium and muscle contraction were all identified. The importance of ER in protein synthesis was discovered in 1971 by American cellular and molecular researcher Günter Blobel.

Structure of the Smooth Endoplasmic Reticulum

The smooth endoplasmic reticulum is distinguished from the rough endoplasmic reticulum by the absence of ribosomes on its surface. Smooth ER is found throughout the cytoplasm and is attached to the nuclear envelope. It is made up of a membrane-enclosed network of vesicles and tubules that are arranged in a reticular pattern to provide a larger surface area for the activity or storage of important enzymes. Vesicles are rounded or oval elements that resemble vacuoles and are dispersed throughout the cytoplasm.

In comparison to the RER, the SER's tubules have a wider range of diameters, are commonly connected to one another by branches, and are arranged into an extremely compact three-dimensional reticulum. SER can also appear as spiral or concentric arrays of strongly fenestrated cisternae in steroid-secreting cells. The lumen, which is surrounded by a phospholipid membrane, is the inside of the smooth endoplasmic reticulum.

Sarcoplasmic reticulum: A type of SER

The sarcoplasmic reticulum is a special kind of smooth endoplasmic reticulum that is seen in muscle cells. The sarcoplasmic reticulum, often known as SR, is a highly specialised type of smooth endoplasmic reticulum that controls intracellular calcium homeostasis. The sarcoplasmic reticulum is a network of tubules that runs throughout the muscle cells. Each segment forms a cuff-like structure around a myofibril, the fine contractile fibres that run the length of the striated muscle cell, wrapping around it without coming into contact with it.

The sarcoplasmic reticulum is a network of tubules that extends throughout muscle cells and wraps around the myofibrils. Its name literally translates as "net-like structure present inside the muscle fibres.” Controlling intracellular Ca²⁺ concentrations is crucial for muscular contraction. The majority of the calcium needed for muscle contraction is stored in the sarcoplasmic reticulum, which serves as a large chamber inside the cell.

Based on its roles, the sarcoplasmic reticulum is separated into two domains: the longitudinal sarcoplasmic reticulum and the junctional sarcoplasmic reticulum.

Around each myofibril, the longitudinal sarcoplasmic reticulum is a network of many linked tubules. The terminal cisterna is a single, dilated sac-like development that is formed when the longitudinal tubules connect at their terminals. Both the longitudinal and junctional sarcoplasmic reticulum (SR), which are routinely aligned with particular areas of the sarcomeres, exhibit a particular spatial arrangement with regard to the myofibrils.

The entrance doors to the sarcoplasmic reticulum are known as SERCA, and each cisterna contains groups of structures known as ryanodine receptors that serve as exit doors. In order to cause muscle contraction, the calcium stored in the sarcoplasmic reticulum is released by ryanodine receptors. During muscle relaxation, calcium is returned to the sarcoplasmic reticulum with the aid of SERCA, a pump that moves calcium ions from the cytoplasm into the sarcoplasmic reticulum.

Functions of the Smooth Endoplasmic Reticulum

The SER is essential for cellular metabolism. According to the cell type, several metabolic processes involving SER are involved, and they are numerous in some cell types that largely rely on SER functions.

Lipid synthesis, carbohydrate metabolism, control of intracellular calcium concentration, and drug detoxification are among the main duties of SER. Numerous enzymes involved in the manufacture of lipids, particularly phospholipids and steroids, can be found in the sER.

There is a significant quantity of SER in the cells that secrete these substances, including those in the ovaries, skin oil glands, and testes.

SER is critical for maintaining healthy cholesterol levels. Even though the endoplasmic reticulum produces very little cholesterol, the majority of the molecular machinery that controls cellular cholesterol homeostasis is found there.

Transferring molecules made in the rough ER to the Golgi complex also involves the smooth endoplasmic reticulum.

1. Lipid synthesis

The main location for lipid synthesis is the smooth endoplasmic reticulum, notably at the membrane contact sites (MCS). MCS are locations where ER membranes come into close contact with other cytoplasmic organelles, allowing for the transfer of molecules. These organelles include the Golgi apparatus, mitochondria, lysosomes, peroxisomes, endosomes, chloroplasts, and plasma membrane.

The vast majority of cellular lipids are synthesised by endoplasmic reticulum (ER)-specific enzymes found in these locations of SER. Therefore, the SER has a crucial part to play in maintaining the proper balance of various lipid classes and categories, which in turn affects the cellular lipid biomass. Signals from both inside and outside the cell are used to activate these ER-localised enzymes that are present in SER. The cellular lipids produced by the sER give the cell the membrane lipids it needs to undergo changes in shape and structure dependent on growth and differentiation. They also support the cell's overall membrane homeostasis.

Phospholipid production happens at the membrane-contact point between the ER and mitochondria. In order to safeguard the cell and the organelles, phospholipids largely form barriers in cellular membranes. Additionally, they have a role in the control of cellular activities linked to synaptic transmission, immunological surveillance, and cellular development. Additionally, phospholipids are crucial for non-vesicular lipid transfer between the ER and other organelles.

A family of waxy lipids known as ceramides is essential to the cell's structure. They are also essential for cell motility, adhesion, cell cycle, and cellular signaling. Ceramides are produced in the SER. As a part of the membrane of the transport vesicle, they are carried from the SER to the Golgi.

Enzymes necessary for the production of sterols and steroids are also found in SER. Due to their lipophilicity and inability to be stored in vesicles from which they may quickly diffuse, steroid hormones produced in the smooth endoplasmic reticulum are only produced as precursors when necessary. The sex hormones of vertebrates and the numerous steroids secreted by the adrenal glands are among the steroids generated by the SER in animal cells. The smooth endoplasmic reticulum is abundant in the cells that produce and secrete these hormones, such as the cells in the testes and ovaries.

2. Carbohydrate metabolism

In eukaryotes, glucose serves as the primary energy source. In the absence of glucose, organisms have developed the ability to synthesise glucose from various non-carbohydrate substrates. Gluconeogenesis is the process of producing glucose from non-carbohydrate precursors such as pyruvate, oxaloacetate, succinate, lactate, etc. These non-carbohydrate precursors go through a number of processes where they are changed from one intermediate chemical to another while being assisted by a number of enzymes.

The final molecule created in the majority of tissues is glucose 6-phosphate; no free glucose is produced. Since glucose-6-phosphate cannot diffuse outside of the cell, it is kept there as storage. This opens the door for effective glucose regulation in turn. To hydrolyze glucose 6-phosphate into glucose, a specialised enzyme called glucose 6-phosphatase is produced by the smooth endoplasmic reticulum. Only the liver and kidney tissues, which are essential for maintaining blood glucose homeostasis, have this enzyme. The smooth endoplasmic reticulum in these tissues is where glucose-6-phosphate is first carried out before being transformed into glucose by the glucose-6-phosphatase enzyme.

3. Regulation of calcium concentration

Additionally, calcium ions are kept in the smooth endoplasmic reticulum. For instance, the sarcoplasmic reticulum, a distinct variety of smooth endoplasmic reticulum, plays a crucial role as an intracellular calcium buffer zone in muscle cells. Calcium ions rush through the endoplasmic reticulum membrane into the cytosol in response to a nerve impulse that stimulates the muscle cells, causing the contraction of the muscle cells. Other cell types respond to calcium ions produced from the smooth endoplasmic reticulum by secreting vesicles containing freshly made proteins, among other things.

4. Drug detoxification

A large family of enzymes called cytochrome P450s is found in the smooth endoplasmic reticulum membrane. These SER enzymes aid in the detoxification of medicines and toxins, particularly in liver cells. Drug molecules are typically given a hydroxyl group during smooth ER detoxification in order to make them more soluble and easier to remove from our bodies. Drugs that are processed by the smooth endoplasmic reticulum in this way include the sedative phenobarbital and other barbiturates.

Barbiturates, alcohol, and many other drugs can speed up the detoxification process by causing the smooth endoplasmic reticulum and the detoxification enzymes it contains to proliferate. In turn, this raises the drug's tolerance, requiring higher dosages to provide a certain effect, like sedation. The proliferation of the SER in response to one medicine can also raise the need for a greater dosage of other drugs because some of these detoxifying enzymes found in the smooth endoplasmic reticulum have broad activities.