Smooth Endoplasmic Reticulum- Structure and Functions

Endoplasmic Reticulum (ER)

  • In 1897, Garnier observed the Endoplasmic Reticulum (ER) with a light microscope.
  • Ergastoplasm was the term coined by him.
  • In 1945 Porter first reported the Endoplasmic Reticulum.
  • It was discovered independently by Porter and Thompson.
  • It is a continuous membrane system.
  • At one end, it connects the nuclear membrane.
  • At the other end, it connects the cell membrane. 
  • The intracellular space is divided into two parts. i.e inside the endoplasmic reticulum and another one is the rest of the cytoplasm.
  • Depending upon the cell, its extend may vary from one to another.
  • The spaces or the channel lined with membrane are present in the endoplasmic reticulum.
  • Such spaces consist of the endoplasmic matrix. It is the fluid and is different from the cytoplasmic matrix. 
  • The cytoplasmic matrix is present outside the reticulum.
  • The thickness of an endoplasmic reticulum membrane is about 50-60 A.
  • Generally, it comprises about 30-60 percent of the membrane system in the cell. So compared to the external surface it increases the internal surface about 30 to 40 times. 
  • The Endoplasmic reticulum is of two types: Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER)
  • The Rough endoplasmic reticulum consists of ribosomes whereas it is absent in the smooth endoplasmic reticulum.
  • Since doesn’t have ribosomes, it is also called the agranular endoplasmic reticulum. 
  • The Smooth endoplasmic reticulum is made up of vesicles and tubules.
  • The Smooth endoplasmic reticulum is more abundant near the plasma membrane and may be connected to the plasma membrane.
  • In the metabolically active cells such as pancreatic cells and liver, it is present quite extensive.
  • In the storage cells, it is present simply. In the adipose cell, it is found as tubules.
  • In the spermatocytes, it is reduced. It is found in the form of few vesicles. 
  • The Endoplasmic reticulum is not present in all cells. 
  • It is absent in:
    • Prokaryotic cell
    • Egg
    • Mature erythrocyte
    • Embryonic cell
  • The SER is present in liver cells which stores the glycogen. Similarly, it is also found in interstitial cells, adrenal cortical cells of the kidney, adipose cells, muscle cells, and the retinal cell of an eye. 
  • The Smooth endoplasmic reticulum consists of the enzyme glucose-6-phosphatase. It converts glucose-6-phosphate to glucose. 
  • In the sebaceous gland, there is the presence of plenty of smooth endoplasmic reticulum.
  • It is also found in leucocytes.
  • It is also believed that the sphaerosomes are originated from the smooth endoplasmic reticulum.
Smooth Endoplasmic Reticulum- Structure and Functions
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Structure of Smooth Endoplasmic Reticulum (smooth ER)

Different structures are present in the Endoplasmic reticulum. They are discussed below:

  • Cisternae
  • Vesicles
  • Tubules

a. Cisternae

  • The Smooth endoplasmic reticulum primarily consists of cisternae. 
  • It is of 3 dimensions.
  • They are arranged in parallel rows and are long, flat, and unbranched.
  • They are like plates or lamellae. 
  • Its diameter is 40-50nm. 
  • In mammals, their diameter is 50nm as in yeast their diameter is 30nm.
  • Different protein helps in the stabilization of the structure. They are:
    • Reticulons, DP1
    • REEPs (Receptor expression enhancing protein).
  • They either give shape to the membrane through the process of oligomerization or the proteins may bend the membrane using the structural elements. 
  • These proteins are essential for the presence of tubular cisternae.
  • If in case there is deletion or suppression of this protein, it may lead to an abundance of sac-like structure in the endoplasmic reticulum.
  • It may lead to the condition where tubules will be absent completely.
  • They are parallel to each other and are found in bundles.
  • They are present in such cells that are active in the synthetic activity.

b. Vesicles

  • Their shapes are as round and ovoid sacs. 
  • Its diameter is 25 to 500nm.
  • They seem like small vacuoles. 
  • They are also known as microsomes.
  • In the cytoplasm, they occur as the isolated part while some vesicles are found attached to other components of ER.

c. Tubules

  • Their structure is just like the tube. 
  • Some may be regular or irregular and branched or unbranched.
  • They form a network-like structure along with the cisternae and vesicles. They are generally present towards the peripheral parts of the cell.
  • The thin unit membrane surrounds the tubules and the secretory products of the cell fill the lumen
  • They have a diameter of 50-100n.

Note

  • From the sides of the existing structures, the new tubules may bud off in the case of the smooth endoplasmic reticulum. 
  • Some tubules may fuse which is with the help of GTP hydrolysis.
  • The presence of the actin and the microtubule cytoskeleton determines the extent of the smooth end endoplasmic reticulum.
  • Using the motor proteins, the endoplasmic reticulum tubules, they can slide along the cytoskeletal framework. 
  • Its structure is significant in two types of cells. They are muscle cells and neurons.
  • In the neuro, along with the entire cell, there is a continuous network. It is due to the interaction with actin and microtubules.
  • It is present in the dendritic spines and axons.
  • It has spread across the synapse.
  • It has got association with the mitochondria at the synapse.
  • It is found that the association between the mitochondria and smooth ER has remained intact even when there is depolymerization of a cytoskeleton and where there are major morphological changes in the endoplasmic reticulum network of tubules.
  • The Sarcoplasmic reticulum is one of the smooth endoplasmic reticulum in the muscle cell. 

Functions of Endoplasmic Reticulum (ER)

  • To carry out the different cellular reactions, it provides an increased surface area.
  • The Endoplasmic reticulum contains different enzymes like ATPase, reductase, dehydrogenase, phosphatase, etc which perform various synthetic activities. It also performs metabolic activities as well.
  • During cell division, it forms the nuclear membrane.
  • Different types of cellular materials can be actively transported.
  • It provides mechanical support by forming the skeletal framework. So, it is also called the endoskeletal of the cell.
  • Since there is the presence of different enzymes, a metabolic activity can be performed.
  • The endoplasmic reticulum consists of the membrane, through which the exchange of molecules occurs.
  • It occurs by the process of osmosis, diffusion, and active transport. 
  • Like the plasma membrane, the endoplasmic reticulum membrane has proteins like permeases. It also has carriers proteins for active transport.
  • The Endoplasmic reticulum acts as the circulatory and transport system.

 Functions of the Smooth Endoplasmic Reticulum (smooth ER)

  • Inside the cells of the adipose tissue, it synthesizes the fats. 
  • It is involved in the synthesis and lysis of glycogen. The hydrolysis of the glycogen is known as glycogenolysis.
  • Enzymes like glycosomes are present in it.
  • In the retinal cell, it involves in the formation of the visual pigments by using vitamin A.  It involves the synthesis of ascorbic acid.
  • Synthesis of steroid hormones and sterols. . It includes steroids like cholesterol, progesterone, and testosterone.
  • Detoxification of chemicals takes place with the help of cytochrome P-450.
  • Through the smooth endoplasmic reticulum, the synthetic products of the rough endoplasmic reticulum can pass to the Golgi complex.
  • The intercellular calcium ions are stored and released from it. It occurs during the muscle contraction of the sarcoplasmic reticulum.

References and Sources

  • https://biologydictionary.net/smooth-endoplasmic-reticulum/ 
  • https://www.britannica.com/science/smooth-endoplasmic-reticulum
  • https://www.biologydiscussion.com/cell-membrane/endoplasmic-reticulum-structure-types-and-function-of-endoplasmic-reticulum-biology/1819 
  • https://www.biologydiscussion.com/cell/endoplasmic-reticulum/endoplasmic-reticulum-er-discovery-types-and-functions/70515 
  • https://www.slideshare.net/bikashsingh2510/endoplasmic-reticulum-30480224
  • https://www.thoughtco.com/endoplasmic-reticulum-373365

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